Advances in Psychology Volume 65 Stimulus-Response Compatibility: An Integrated Perspective

STIMULUS-RESPONSE COMPATIBILITY An Integrated Perspective ADVANCES IN PSYCHOLOGY 65 Editors: G.E. STELMACH P.A. VRO...

Author: Robert W. Proctor

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STIMULUS-RESPONSE COMPATIBILITY An Integrated Perspective

ADVANCES IN PSYCHOLOGY

65 Editors:

G.E. STELMACH P.A. VROON

NORTH-HOLLAND AMSTERDAM * NEW YORK OXFORD * TOKYO

STIMULUS-RESPONSE COMPATIBILITY An Integrated Perspective

Edited by

Robert W. PROCTOR Department of Psychological Sciences Purdue University West Lafayette, Indiana, U.S.A.

T. Gilmour REEVE Motor Behavior Center Auburn University Auburn, Alabama, U.S.A.

1990

-

-

NORTH-HOLLAND AMSTERDAM NEW YORK OXFORD TOKYO

NORTH HOLLAND ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat 25 P.O. Box 21 1, 1000 AE Amsterdam, The Netherlands

Distributors for the United States and Canada: ELSEVIER SCIENCE PUBLISHING COMPANY, INC. 655 Avenue of the Americas New York, N.Y. 10010, U.S.A.

ISBN: 0 444 88092 5

0ELSEVIER SCIENCE PUBLISHERS B.V., 1990 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science Publishers B.V./ Physical Sciences and Engineering Division, P.O. Box 1991, 1000 BZ Amsterdam, The Netherlands. Special regulations for readers in the U.S.A. - This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the U.S.A. All other copyright questions, including photocopying outside of the U.S.A., should be referred to the copyright owner, Elsevier Science Publishers B.V., unless otherwise specified.

No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. pp. 3-30, 261-276: copyright not transferred. Printed in The Netherlands

V

FOREWORD T h e p r i n c i p l e o f compatibility in psychology a n d human f a c t o r s was developed as an o u t g r o w t h o f psycho-acoustic research b e g u n during World War I I . In 1946, I was called on t o leave m y position a t t h e U n i v e r s i t y o f Iowa (Head o f t h e V i o l i n Department in t h e School o f Music a n d D i r e c t o r o f t h e Psycho-Acoustic L a b o r a t o r y in t h e Psychology Department) t o d i r e c t psycho-acoustic research f o r t h e U n i v e r s i t y o f California's Division o f War Research in San Diego, California. At the Division, t h e war-time mission i n c l u d e d all aspects o f s o u n d Cavigation a n d r a n g i n g (sonar). Research a n d development were in p r o g r e s s o n u n d e r w a t e r acoustics, equipment development, a n d tactics. T h e missing research segment o f concern in t h e sonar system was t h e operator. Sonar i s an operator-equipment-environment i n t e r a c t i v e system used o n ships t o d e t e c t objects in t h e sea. In i t s a c t i v e mode o f operation, an u l t r a s o n i c p u l s e is t r a n s m i t t e d u n d e r w a t e r in a g i v e n d i r e c t i o n . Reflect i o n s f r o m solid objects a r e detected by t h e equipment, h e t e r o d y n e d t o 800 hertz, a n d presented as audio signals t o t h e o p e r a t o r . Progress of t h e signal t h r o u g h t h e w a t e r g i v e s r i s e t o r e v e r b e r a t i o n that, along w i t h a n y i n h e r e n t sea noise, causes masking o f t h e reflected signal. T h e sonar operator's t a s k s a r e t o d e t e c t t h e reflected signal in t h e noise, t h e n t o i d e n t i f y i t s p r o b a b l e source, a n d t o r e p o r t t h e source t o higher ship authority. Each o f these t a s k s involves v a r y i n g degrees o f u n c e r t a i n t y , a n d time is o f t h e essence in success t o w a r d e f f e c t i v e operations. I n t e n s i v e t r a i n i n g is r e q u i r e d t o accomplish t h e tasks. E f f e c t i v e performance by t h e sonar o p e r a t o r r e q u i r e s e a r l y detect i o n o f masked signals. Thus, t h e psycho-acoustic research a t t h e D i v i sion o f War Research focused o n signal detection. The results of this research generated a theory o f backward, o r residual, masking a n d were p u t t o use by t h e f l e e t worldwide. With t h e e n d o f World War II, t h e N a v y ' s Bureau o f Ships absorbed t h e Division o f War Research as t h e N a v y Electronics L a b o r a t o r y . The Laboratory's responsibilities were broadened t o i n c l u d e all electronic systems-related research a n d development, w i t h consequent b r o a d e n i n g of human f a c t o r s involvement. I was requested t o remain a n d develop a Human Factors Division. As indicated earlier, t h e t a s k o f d e t e c t i n g masked audio signals was d i f f i c u l t . However, no o t h e r modality o f signal d i s p l a y h a d been available on ships. Now as a peace-time research e f f o r t , it seemed desirable t o s t u d y a n d evaluate t h e addition o f a v i s u a l display, used simultaneously w i t h t h e a u d i t o r y display, in an e f f o r t t o enhance signal detection.'

ISee Small, A . M., & Gales, R . S . (1950). Recognition o f u n d e r w a t e r sounds. In U n d e r w a t e r acoustics research: A s u r v e y r e p o r t o n basic problems (pp. 123-1371, Washington, DC: Committee o n Undersea Warfare, National Research Council.

vi

Foreword

T h e o n l y available real-time visual d i s p l a y o f sonar signals was t h e cathode r a y t u b e w i t h A-scan. ( T h e 5 - s c a n t u b e was in i t s i n f a n c y . ) With A-scan, t h e light beam traces a horizontal l i n e f r o m l e f t t o right w i t h v e r t i c a l p e r t u r b a t i o n s , t h a t is, spatial excursions of t h e beam indic a t i n g t h e presence a n d level o f b a c k g r o u n d noise a n d signals, if present. Detection research was conducted w i t h s k i l l e d subjects, as i n e a r l i e r tests, but now u s i n g simultaneous bimodal d i s p l a y o f t h e t e s t materials. T h e r e s u l t s o f t h i s research on bimodal d i s p l a y were inconsistent, w i t h signal detection t h r e s h o l d s v a r y i n g f r o m p l u s o r minus t h r e e decibels, compared w i t h audio alone. In examining t h e t e s t p l a n a n d r e s u l t s f o r possible reasons f o r t h i s inconsistency, it was realized t h a t o u r d i s p l a y conditions i n v o l v e d "incompatible" a t t r i b u t e s . T h a t is, t h e c h a r a c t e r i s t i c o f t h e a u d i t o r y stimulus was variation in amplitude o f vibration, leading t o perception o f loudness changes, whereas t h e c h a r a c t e r i s t i c o f t h e c o n c u r r e n t v i s u a l stimulus was deviations f r o m t h e cathode r a y beam trace, leading t o perception of spatial position changes. Compatibility w o u l d require, therefore, t h a t comparable stimulus charact e r i s t i c s b e i n v o l v e d in t h e d u a l sensory display, r e q u i r i n g t h a t t h e v a r i able c h a r a c t e r i s t i c o f t h e v i s u a l stimulus should b e i n t e n s i t y , leading t o perception o f b r i g h t n e s s changes. Thus, t h e p r i n c i p l e o f compatibility was formulated. In 1951, I presented a p a p e r a t t h e Ergonomics Research Society in England on Compatibility as a Principle i n Multi-Sensory Displays. I recall t h a t Paul M. F i t t s , who was in attendance, p r o n o u n c e d formulation o f t h e compatibility p r i n c i p l e as a landmark o f g r e a t significance w i t h broad applicability. In t h e n e x t few years, h e a n d h i s students concent r a t e d on t h i s research area, a n d we met several times d i s c u s s i n g issues. T h e compatibility p r i n c i p l e soon f o u n d application in psychological a n d human-factors research a n d application i n v o l v i n g n o t o n l y stimulus-stimulus compatibility, but also stimulus-response a n d response-response compati bi Iit y .

T h e l i t e r a t u r e on compatibility effects is expanding, w i t h research a n d applications r e p o r t e d f r o m t h e U n i t e d States, Europe, a n d Asia. Among t h e areas of i n t e r e s t a r e information processing, t h e motor system, learning, language, memory, human performance, a n d person-machineenvironment design. The development of t h i s book is indeed timely, therefore, a n d I believe a v e r y s i g n i f i c a n t c o n t r i b u t i o n . I trust the reader w i l l understand, therefore, m y exhilaration w i t h these developments a n d m y expression of deep appreciation t o t h e e d i t o r s a n d a u t h o r s o f t h e p r e s e n t volume.

A r n o l d M. Small, S r . Professor Emeritus U n i v e r s i t y o f Southern California Los Angeles, California

vii

PREFACE T h e t e r m "stimulus-response compatibility" was popularized by t h e research o f Paul M. F i t t s a n d h i s colleagues, i n w h i c h assignments o f stimuli t o responses were manipulated. F i t t s adopted t h i s terminology f r o m a presentation by A r n o l d M. Small t o t h e English Ergonomics Research Society in 1951. Stimulus-response (S-R) compatibility r e f e r s t o t h e finding t h a t c e r t a i n mappings o f stimuli t o responses p r o d u c e f a s t e r a n d more accurate responding t h a n d o o t h e r s . Compatibility effects also a r e f o u n d f o r stimulus-stimulus ( S - S ) relations a n d response-response (R-R) relations. Since t h e i n i t i a l study, p u b l i s h e d by F i t t s a n d Seeger in t h e 1953 volume o f t h e Journal of Experimental Psychology, a wealth o f research on compatibility effects has been conducted. T h i s research has r a n g e d f r o m basic investigations in laboratory s e t t i n g s t o applied research in f i e l d settings. Because of t h e d i v e r s i t y of research on compatibility effects, p r o g r e s s t o w a r d a comprehensive, theoretical u n d e r s t a n d i n g o f t h e effects has been slow. Compatibility effects l o n g have been r e g a r d e d as t h e p r o v i n c e o f t h e areas o f human performance a n d human f a c t o r s . One reason f o r t h i s is t h a t many studies focused p r i m a r i l y on w h i c h o f several arrangements a n d assignments o f stimuli a n d responses was most compatible. However, w i t h t h e r i s e in p o p u l a r i t y o f c o g n i t i v e psychology o v e r t h e p a s t t w o decades, an increasing p r o p o r t i o n o f t h e research o n compatibility effects has been devoted t o d e f i n i n g t h e i r fundamental n a t u r e . T h e knowledge t h a t has been gained f r o m these studies indicates t h a t compatibility effects r e f l e c t basic c o g n i t i v e processes t h a t b e a r on a b r o a d r a n g e o f issues in c o g n i t i v e science. T h e recent research on compatibility effects f a l l s i n t o several b r o a d categories, i n c l u d i n g (a) mental representation, (b) psychophysiological indices a n d neurophysiological mechanisms, (c) motor performance, a n d (d) applications t o human f a c t o r s . T h e research o n representation a n d coding has established t h a t S-R compatibility effects r e f l e c t t h e n a t u r e o f t h e codes used t o t r a n s l a t e between stimuli a n d responses. F o r many situations, t h e coding is predominantly spatial. However, coding can o c c u r on t h e basis o f o t h e r salient features o f t h e stimulus a n d response sets. T h e research on neurophysiological bases a n d psychophysiological indices suggests t h a t c e r t a i n compatibility phenomena can b e related t o t h e s t r u c t u r e o f t h e b r a i n a n d t h a t psychophysiological measures can b e used t o discriminate effects t h a t appear t o b e similar when o n l y reaction times a r e examined. T h e research on motor performance has h i g h l i g h t e d t h e pervasiveness o f S-R compatibility effects a n d shown them t o b e independent f r o m t h e processes i n v o l v e d i n t h e i n i t i a t i o n a n d c o n t r o l o f movements. Finally, models developed f o r human-factors purposes show t h a t S-R compatibility effects r e f l e c t general aspects o f t h e humaninformation processing system. These models also show t h a t t h e r e l a t i v e compatibilities o f a l t e r n a t i v e assignments i n a v a r i e t y o f situations can b e p r e d i c t e d f r o m a common set o f processes.

viii

Preface

I n short, t h e p r o g r e s s made along several f r o n t s o f S-R compatibility research suggests t h a t we a r e o n t h e v e r g e o f a comprehensive, theoretical u n d e r s t a n d i n g o f compatibility phenomena. T o date, l i t t l e attempt has been made t o p r o v i d e a comprehensive o v e r v i e w t h a t integrates t h e views t h a t have been emerging f r o m t h e d i f f e r e n t lines o f research. T h e p u r p o s e o f t h e p r e s e n t volume is t o summarize t h e major f i n d i n g s w i t h i n each o f t h e categories o f research a n d t a k e an i n i t i a l step t o w a r d p r o v i d i n g an i n t e g r a t e d p e r s p e c t i v e . T h e theoretical issues addressed should b e o f i n t e r e s t t o researchers in perception, cognition, a n d action, as well as t o c o g n i t i v e scientists in general. Moreover, t h e p e r s p e c t i v e developed has applied implications t h a t should b e o f value t o humanfactors engineers. T h e p r e s e n t volume reflects t h e e f f o r t s o f numerous i n d i v i d u a l s . T h e most obvious a r e t h e c o n t r i b u t o r s , whom we would l i k e t o t h a n k f o r t h e i r excellent c h a p t e r s . Also, George E. Stelmach, t h e series co-editor, a n d K. Michielsen, o f N o r t h - H o l l a n d Publishers, p r o v i d e d encouragement f o r u s t o p r e p a r e t h e volume. O u r g r a d u a t e students, Daniel J. Weeks, Lanie Dornier, L a r r y P. Wiley, a n d M a r k A. Guadagnoli, p r o v i d e d h e l p f u l discussions a n d assisted t h e p r o j e c t i n various ways. B e t t y e Campbell a n d h e r staff, M a r k i e G a r d n e r a n d Bonnie Phillips, a t A u b u r n U n i v e r s i t y typed and printed the final manuscript. Janet S u g g assisted w i t h t h e placement o f f i g u r e s in t h e m a n u s c r i p t . Finally, t h i s p r o j e c t was s u p p o r t e d i n p a r t by a g r a n t f r o m t h e L i f e Sciences D i r e c t o r a t e o f the A i r Force O f f i c e o f Scientific Research (#AFOSR-88-0002). We would l i k e t o t h a n k A l f r e d R. Fregly, t h e manager o f t h e AFOSR p r o g r a m i n cognition, a n d John Jonides, consultant t o t h e program, f o r t h e s u p p o r t .

R o b e r t W. Proctor Purdue University T. Gilmour Reeve Auburn University

ix

CONTENTS Foreword

..............................................................

v

A r n o l d M. Small, S r . Preface

..............................................................

Contents

.............................................................

List of Contributors

..................................................

vii ix

...

xi11

P A R T I: ISSUES I N STIMULUS-RESPONSE C O M P A T I B I L I T Y 1. T h i n g s T h a t Go T o g e t h e r : A Review o f Stimulus-Response Compatibility a n d Related Effects ...............................

.3

Earl A . A l l u i s i a n d Joel S . Warm

2. T h e Effects o f an I r r e l e v a n t Directional Cue on Human Information Processing

.................................

.31

......................

.89

J. R i c h a r d Simon P A R T II: MENTAL REPRESENTATION

3 . Spatial Stimulus-Response C o m p a t i b i l i t y . . Carlo UmiltS a n d Roberto Nicoletti 4.

Spatial Coding a n d Spatio-Anatomical Mapping: Evidence f o r a Hierarchical Model o f Spatial Stimulus-Response Compatibility .............................. Gabriele Heister, Peter Schroeder-Heister, Walter H. Ehrenstein

5. Some Aspects o f Spatial Stimulus-Response Compatibility in A d u l t s a n d Normal C h i l d r e n

.117

and

. . . . . . . . . . . . . . . . . . .145

Elisabetta LBdavas

6.

T h e Salient-Features Coding P r i n c i p l e f o r Spatial- a n d Symbolic-Compatibility Effects

. . . . . . . . . . . . . . . . ... .163

T. Gilmour Reeve a n d Robert W. P r o c t o r PART 1 1 1 : 7.

PSYCHOPHYSIOLOGICAL INDICES A N D NEUROPHYSIOLOGICAL MECHANISMS

Stimulus-Response Compatibility Viewed f r o m a C o g n i t i v e Psychophysiological Perspective.. . . . . . . . . . . . . . . . . .. . ,183 Theodore R. Bashore

Contents

X

8. Cerebral Evoked Potentials: E a r l y Indexes o f Compatibility E f f e c t s .........................................

.225

R i c h a r d Ragot 9.

Psychological a n d Neurophysiological Factors in Stimulus-Response Compatibility ..............................

.241

John B r e b n e r

10. Attentional Processes in Spatial Stimulus-Response Compatibility

..................................................

261

Mieke Verfaellie, Dawn Bowers, a n d Kenneth M. Heilman PART

IV:

MOTOR PERFORMANCE

11. Stimulus-Response Compatibility a n d t h e Programming o f Motor A c t i v i t y : Pitfalls a n d Possible New Directions ......................................

.279

Howard N . Zelaznik a n d Elizabeth Franz 12. Response Selection a n d Motor Programming: E f f e c t s of Compatibility a n d Average V e l o c i t y ...........................

.297

Will A. C . S p i j k e r s 13.

Rapid Responses w i t h t h e L e f t or R i g h t Hand: ResponseResponse Compatibility Effects Due t o lntermanual Interactions ............................ . . . . . . . . . . . . . . . . . . . . . . .

311

H e r b e r t Heuer 14.

Perceptual-Motor Processing in Speech..

......................

.343

Peter C. Gordon PART

V: APPLICATIONS

TO HUMAN FACTORS

15. Stimulus-Response Compatibility a n d Human Factors ............. .... B a r r y H. Kantowitz, Thomas J. T r i g g s , a n d Valerie E. Barnes 16. T h e Mental Model i n Stimulus-Response Compatibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ray E. E b e r t s a n d Jack W. Posey 17. T o w a r d an Engineering Model o f Stimulus-Response Compatibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bonnie E. John a n d Allen Newell

389

Contents

xi

P A R T V I : A N I N T E G R A T E D PERSPECTIVE

18. Research on Stimulus- Response Compatibility : Toward a Comprehensive A c c o u n t . . ...........................

.483

Robert W. Proctor and T . Gilmour Reeve AUTHOR I N D E X SUBJECT I N D E X

......................................................

495

......................................................

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xiii

L I S T OF CONTRIBUTORS E a r l A. A l l u i s i

O f f i c e o f t h e S e c r e t a r y o f Defense (Research a n d Advanced Technology) T h e Pentagon Washington, D C

Valerie E. Barnes

Battelle Human A f f a i r s Research Centers Seattle, Washington

Theodore R. Bashore

Department o f P s y c h i a t r y Medical College o f Pennsylvania a t -EPPI Philadelphia, Pennsylvania

Dawn Bowers

Department of Neurology U n i v e r s i t y o f Florida Gainesville, Florida

John B r e b n e r

Department o f Psychology T h e U n i v e r s i t y of Adelaide Adelaide, South A u s t r a l i a

Ray E. E b e r t s

School of I n d u s t r i a l Engineering Purdue University West Lafayette, Indiana

Walter

H.

Ehrenstein

Elizabeth Franz

l n s t i t u t f u e r Arbeitsphysiologie an d e r U n i v e r s i t a e t D o r t m u n d Dortmu nd, West Germany Motor Behavior L a b o r a t o r y Department o f Physical Education, a n d Recreation Studies Purdue University West Lafayette, Indiana

Health

Peter C. Gordon

Department of Psychology Harvard University Cambridge, Massachusetts

Kenneth M. Heilman

Department of Neurology U n i v e r s i t y o f Florida and Gainesville Veterans Administration Medical Center Gainesville, Florida

Gabriele Heister

A b t e i l u n g Neuropsychologie Universitaetsspital Zuerich Zuerich, Switzerland

H e r b e r t Heuer

Fachbereich Psychologie der Philipps-Universitaet M a r b u r g , West Germany

List of Cpntributors

xiv

Bonnie E. John

Department o f Psychology Carnegie Mellon U n i v e r s i t y P i t t s b u r g h , Pennsylvania

B a r r y H. Kantowitz

Battelle Human A f f a i r s Research Centers Seattle, Washington

Elisabetta L i d a v a s

Dipartimento di Psicologia Universita di Bologna Bologna, I t a l y

Allen Newell

Department of Computer Science Carnegie Mellon U n i v e r s i t y P i t t s b u r g h , Pennsylvania

Roberto Nicoletti

Dipartimento di Psicologia Generale Universita di Padova Padova, I t a l y

Jack W. Posey

School o f I n d u s t r i a l Engineering Pu r d u e U n i v e r s i t y West Lafayette, Indiana

Robert W. Proctor

Department of Psychological Sciences Purdue University West Lafayette, I n d i a n a

Richard Ragot

C e n t r e National d e l a Recherche Scientifique UA654-LENA-Hopital d e la Salpetriere Paris, France

T . Gilmour Reeve

Motor Behavior Center Department o f Health a n d Human Performance Auburn University A u b u r n , Alabama

Peter Sch roeder- Heister

Zentrum Phi losoph ie u n d Wissenschaftstheorie Universitaet Konstanz Konstanz, West Germany

J. Richard Simon

Departments of Psychology a n d I n d u s t r i a l and Management Engineering U n i v e r s i t y o f Iowa Iowa C i t y , Iowa

A r n o l d M. Small;

Sr.

Will A. C . Spijkers

Human Factors Department I n s t i t u t e o f Safety and Systems Management U n i v e r s i t y of Southern California Los Angeles, California I n s t i t u t e f o r Psychology U n i v e r s i t y of Technology Aachen, West Germany

List of Contributors Thomas J . T r i g g s

Battelle Human A f f a i r s Research Centers Seattle, Washington

Carlo Umilt6

l s t i t u t o di Fisiologia Umana U n i v e r s i t s di Parma Parma, I t a l y

Mieke Verfaellie

Memory Disorders Research C e n t e r Boston Veterans Administration Medical Center a n d Boston U n i v e r s i t y School o f Medicine Boston, Massachusetts

Joel S. Warm

Department o f Psychology University of Cincinnati Cincinnati, Ohio

Howard N. Zelaznik

Motor Behavior L a b o r a t o r y Department o f Physical Education, Health, a n d Recreation Studies and Department o f Psychological Sciences Purdue Universty West Lafayette, Indiana

xv

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PART I ISSUES IN STIMULUS-RESPONSE COMPATI B I L l T Y T h e t w o chapters o f P a r t I p r o v i d e e x t e n s i v e reviews of research on stimulus-response compatibility a n d related compatibility effects. C h a p t e r 1, by A l l u i s i a n d Warm, takes a h i s t o r i c a l approach a n d l i n k s compatibility research t o t h e e a r l i e r studies o f population stereotypes. T h e c h a p t e r p r o v i d e s examples of v a r i o u s t y p e s o f compatibility effects a n d discusses possible mechanisms t h a t u n d e r l i e t h e effects. C h a p t e r 2, by Simon, describes his d i s c o v e r y o f what has come t o b e r e f e r r e d t o as t h e "Simon effect," w h i c h is t h a t reaction times t o symbolic stimuli a r e affected by t h e relation of an i r r e l e v a n t stimulus location t o t h e r e l e v a n t response location. T h e c h a p t e r also reviews t h e progression o f Simon's research, s t r e s s i n g t h e logic b e h i n d his experiments a n d t h e c r u c i a l findings. T h e chapters i n P a r t I p r o v i d e a good i n t r o d u c t i o n t o compatibility research, w i t h many o f t h e topics a n d issues t h a t a r e i n t r o d u c e d in these c h a p t e r s b e i n g p u r s u e d l a t e r i n t h e book.

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STIMULUS-RESPONSE COMPATIBILIN R. W. Proctor and T.G. Reeve (Editors Elsevier Science Publishers B. V. (dorth-Holland). 1990

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THINGS T H A T GO TOGETHER: A REVIEW OF STIMULUSRESPONSE C O M P A T I B I L I T Y A N D RELATED EFFECTS EARL A . A L L U l S l O f f i c e o f t h e S e c r e t a r y o f Defense (Research a n d Advanced Technology) JOEL S. WARM Department o f Psychology University of Cincinnati A p r i m a r y goal o f c o g n i t i v e psychology is t o broaden a n d deepen o u r u n d e r s t a n d i n g o f t h e phenomena a n d mechanisms o f human behavior. T h e operational t e s t o f t h e v a l i d i t y o f t h a t u n d e r s t a n d i n g lies in t h e successful application o f t h e p r i n c i p l e s d e r i v e d therefrom, f o r example, i n human-factors engineering. T h u s , c o g n i t i v e psychology a n d humanf a c t o r s e n g i n e e r i n g have a common b o n d - - t h e one produces t h e basic u n d e r s t a n d i n g a n d p r i n c i p l e s t h a t t h e o t h e r validates t h r o u g h successful applications. In these applications, a p r i m a r y goal o f human f a c t o r s e n g i n e e r i n g [,s t o enhance t h e design a n d i n t e g r a t i o n o f hardware, software, a n d

Iivewa r e ” components a n d interfaces i n t o e f f e c t i v e human -machine systems. T h e demands made on t h e liveware o r human-performance components i n such systems a r e generally viewed in stimulus-response (SR) terms. In t h i s context, t h e c o r r e c t paradigm involves n o t merely single stimulus a n d response p a i r i n g s , but r a t h e r sets o r alphabets o f stimuli a n d responses t h a t a r e matched o r coded i n t o S-R ensembles. It follows t h a t t h e t o p i c o f things t h a t go together--alphabets, S-R ensembles, optimum coding, S-R compatibility, a n d related effects--is, o r should be, c e n t r a l n o t o n l y t o t h e i n t e r e s t s of t h e c o g n i t i v e psychologist, but also t o t h e r e p e r t o i r e o f e x p e r t i s e o u t o f w h i c h t h e human-factors specialist c o n t r i b u t e s t o t h e design a n d use o f human-machine systems. T h a t t o p i c i s t h e focus o f t h i s c h a p t e r . Ensembles a n d C o m p a t i b i l i t y E f f e c t s In o r d e r t o have a usable code, it is necessary t o have a set o f discriminable stimuli. B u t t h a t is n o t s u f f i c i e n t . These stimuli must also b e a r r a n g e d i n t o an alphabet. T h e stimulus alphabet m u s t t h e n b e matched t o a response alphabet (an alphabet o f responses d r a w n f r o m some set o f discriminable responses) t o f o r m a code o r S-R ensemble.

T h e S-R ensemble must b e learnable a n d usable. T h a t is, t h e humans who c o n s t i t u t e t h e system’s liveware must b e able t o learn t o associate the symbols in t h e response alphabet w i t h those in t h e stimulus alphabet a n d t o d o so well enough t o p e r f o r m c o n s i s t e n t l y a n d c o r r e c t l y .

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I t has l o n g been established t h a t t h e ease a n d e x t e n t t o which these t w o objectives a r e attained is a f u n c t i o n o f t h e stimuli a n d responses used. T h i s dependence of performance o n t h e specific p a i r i n g s o f t h e stimulus alphabet w i t h a response alphabet (i.e., on t h e specific ensemble) i s what. F i t t s a n d Seeger (1953) called an S - R compatibility effect, a f t e r a 1951 suggestion o f A r n o l d Small's. It has likewise been long established t h a t S-R compatibility effects a r e r e l a t i v e l y l a r g e in magnitude. T h e y a r e especially l a r g e in comparison w i t h t h e effects e i t h e r o f variations in t h e amounts o f improvement p e r S-R ensemble w i t h practice, o r o f practice i t s e l f (Deininger & Fitts, 1955; F i t t s & Deininger, 1954). T h e effects are consistent a n d independent o f t h e performance measure employed. The e r r o r s associated w i t h a g i v e n S-R ensemble a r e positively correlated w i t h t h e reaction times ( R T s ) associated w i t h t h e same ensemble. Thus, an ensemble t h a t produces slow performance also tends t o produce inaccurate performance. Deininger a n d F i t t s (1955) observed t h a t although individuals slow down t o some degree while p e r f o r m i n g a p o o r l y encoded task, t h e y d o n o t slow down enough t o overcome t h e effects o f t h e poor encoding. Instead o f gaining accuracy a t t h e expense o f speed, t h e human sacrifices b o t h speed and accuracy in t h e process o f re-encoding t h e information. S-R compatibility effects a r e also r e l a t i v e l y stable, b u t t h e y a r e accentuated u n d e r conditions o f workload stress. For example, extraneous a c t i v i t y , such as a secondary task, affects performance least w i t h t h e most compatible ensembles a n d degrades performance most w i t h t h e least compatible p a i r i n g s (Garvey & Knowles, 1954).

In brief, t h e use o f highly compatible S - R ensembles tends t o optimize b o t h human performances a n d t h e system performances t h a t are dependent upon those human performances. Thus, t h e discovery a n d use of t h e mechanisms u n d e r l y i n g S-R compatibility effects is basic t o t h e development o f effective human-factors applications. Such applications would t e n d t o produce enhanced human-machine system performances t o t h e e x t e n t t h a t t h e system's Iiveware ( o r human-performance) components a n d interfaces were designed t o e x p l o i t t h e benefits o f u s i n g highly compatible S-R ensembles. Maximizing Compatibility In some o f t h e earliest studies, especially those u s i n g motorresponse alphabets, r e l a t i v e S-R compatibility appears t o approach a maximum (a) when t h e p a i r i n g s of stimulus and response alphabets correspond t o one another in a d i r e c t physical sense a n d (b) when t h e p a i r i n g s o f elements in t h e ensemble agree w i t h s t r o n g i n d i v i d u a l o r population stereotypes, where such stereotypes e x i s t (see Deininger & Fitts, 1955, pp. 319f). Operationally, such stereotypes a r e specified by tables o f t h e empirically determined frequencies w i t h which a set of permissible responses a r e made t o each stimulus in free-response situations where none o f t h e responses has been predesignated as "correct" o r "appropriate. Thus, correspondence a n d stereotypy a r e among t h e earliest o f i d e n t i f i e d mechanisms or techniques f o r maximizing compatibility. I'

Things That Go Together

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Correspondence Between Alphabets T h e emphasis o n physical correspondence between stimulus a n d response alphabets is p r o b a b l y a r t i f a c t u a l . T h e e a r l y generalizations w i t h t h i s emphasis a r e based n e a r l y e x c l u s i v e l y o n t h e r e s u l t s o f experiments t h a t focused o n perceptual-motor performances. Most o f t h e studies, a n d especially t h e e a r l i e r ones, used ensembles formed o f l i g h t - p a t t e r n e d stimulus alphabets p a i r e d w i t h m o t o r - p a t t e r n e d response alphabets. F o r example, Paul F i t t s a n d h i s associates g e n e r a l l y r e q u i r e d t h e i r subjects t o move a s t y l u s q u i c k l y in t h e d i r e c t i o n indicated by a coded visual stimulus (Deininger & Fitts, 1955; F i t t s & Deininger, 1954; F i t t s & Seeger, 1953). O n t h e o t h e r hand, G r a n t a n d h i s coworkers- r e q u i r e d k e y - p r e s s i n g responses t o light stimuli (Anderson, Grant, & Nystrom, 1954; M o r i n & Grant, 1955; N y s t r o m & Grant, 1954). Garvey and his colleagues also used p u s h - b u t t o n responses t o light stimuli ( G a r v e y & Knowles, 1954: G a r v e y & Mitnick, 1955; Knowles, Garvey, & Newlin, 1953) and, in some cases, also used a u d i t o r y stimulus symbols (Knowles e t al., 1953). A l l o f these studies indicate t h a t r e l a t i v e S - R compatibility t e n d s t o increase in p r o p o r t i o n t o t h e degree o f d i r e c t physical correspondence between t h e p a i r e d stimulus a n d response alphabets. T h i s observation can b e safely generalized t o all o t h e r ensembles employing similar t y p e s o f perceptual-motor performances. Even so, it is p r o b a b l y o v e r l y l i m i t i n g in i t s reference o n l y t o t h e p h y s i c a l aspects o f correspondence, especially when t h e emphasis i s n o t specifically on t h e motor aspects o f performance. T h e observation could, a n d p r o b a b l y should, b e broadened f u r t h e r t o i n c l u d e a t least t h e "dimensionality" aspects o f correspondence, a n d perhaps even t h e idea o f conceptual correspondence (see Alluisi, 1961). Conceptual correspondence: Dimensionality. A recommendation has been made elsewhere t o p r e s e r v e t h e multidimensional aspects o f a combination-stimulus alphabet by p a i r i n g it w i t h a conceptually matching multidimensional-response alphabet (Muller, Sidorsky, Slivinske, Alluisi, & Fitts, 1955, p. 90). T h e recommendation was based i n i t i a l l y o n an experiment by Slivinske, Bennet, a n d lrby (see M u l l e r e t al., 1955, pp. 74-83), but it is also s u p p o r t e d by t h e r e s u l t s o f o t h e r contemporary studies. F o r example, M o r i n a n d G r a n t (1955) measured R T w i t h t h r e e d i f f e r e n t S-R ensembles. A l l t h r e e ensembles employed t h e same stimulus alphabet o f 10 l i g h t s a r r a n g e d in a horizontal row, a n d t h e same response alphabet o f 10 k e y - p r e s s i n g b u t t o n s also a r r a n g e d in a horizontal r o w below t h e l i g h t s . However, t h e t h r e e S - R ensembles consisted o f d i f f e r e n t p a i r i n g s of t h e stimulus a n d response alphabets: (a) I n t h e direct-response ensemble, t h e k e y f o r a g i v e n light was d i r e c t l y below it; (b) in t h e reverse-response ensemble, t h e dimensionality was retained, but reversed, so t h e left-most k e y was p a i r e d w i t h t h e r i g h t - m o s t light, t h e r i g h t - m o s t k e y w i t h t h e left-most light, a n d so f o r t h ; a n d (c) in t h e randomly p a i r e d ensemble, t h e k e y s were randomly assigned t o t h e l i g h t s f o r each subject. Performance was b e s t w i t h t h e d i r e c t - r e s p o n s e ensemble, but n o t much worse w i t h t h e reverse-response ensemble. It was considerably degraded w i t h t h e random-response ensemble.

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Conceptual correspondence: L a t e r a l i t y . T h e r e s u l t s o f more recent studies a r e consistent w i t h t h e e a r l i e r f i n d i n g s a n d s u p p o r t as well t h e idea t h a t if conceptual correspondence i s maintained in t h e c o n s t r u c t i o n o f S-R ensembles, S-R compatibility effects w i l l t e n d t o b e maximized. F o r example, in another motor-response study, subjects pressed one o f t w o k e y s o n t h e i r l e f t o r right, w i t h t h e i r hands crossed o r uncrossed, when one o f t w o stimuli appeared on t h e l e f t o r right, o r above o r below a f i x a t i o n p o i n t (Wallace, 1971). Results showed t h a t S-R compatibility t e n d s t o b e maximized when t h e S-R ensemble i s c o n s t r u c t e d t o maintain correspondence o f ,:patiat laterality, t h a t is, o f t h e concepts o f "leftness" a n d " r i g h t n e s s . Thus, l e f t - k e y responses t o t h e l e f t stimulus, a n d r i g h t - k e y responses t o t h e right stimulus, p r o d u c e d t h e s h o r t e s t RTs, regardless o f t h e h a n d used o r w h e t h e r t h e hands were crossed o r not. O t h e r studies have also demonstrated t h i s t e n d e n c y f o r S - R compatibility t o b e maximized when t h e S - R ensemble is c o n s t r u c t e d w i t h alphabets t h a t a r e matched t o correspond in spatial l a t e r a l i t y . In one, similar t o t h e preceding, subjects c o u l d n o t see t h e k e y s o r t h e i r hands, a n d so depended solely on kinesthetic cues f o r response positioning (Wallace, 1972). In another, t h e S - R ensemble t h a t p r o d u c e d t h e s h o r t e s t R T s matched t h e l a t e r a l i t y o f an a u d i t o r y (ear-side) stimulus alphabet w i t h t h a t of a k e y - p r e s s i n g (key-side) response alphabet (Callan, Klisz, & Parsons, 1974). T h e effects were g r e a t e s t in t h e Donders' b - t y p e reactions, n e x t in t h e c-type, a n d least in t h e a - t y p e reactions. T h e effects o c c u r r e d regardless o f t h e a u d i t o r y tone used (high o r low) o r h a n d crossing. T h e importance o f conceptual correspondence in t h e c o n s t r u c t i o n o f S-R ensembles, as contrasted w i t h t h e more-limited c r i t e r i o n of p h y s i c a l correspondence, can also b e i n f e r r e d f r o m numerous o t h e r studies. In one example, eye-movement R T was measured t o monaurally presented tones where one f r e q u e n c y signaled a l o o k - l e f t command a n d a second f r e q u e n c y signaled a l o o k - r i g h t command. Oculomotor RTs were f a s t e r when t h e tone was p r e s e n t e d in t h e ear t h a t matched t h e look-direction command ( f r e q u e n c y ) o f t h e tone (Bertera, Callan, Parsons, E Pishkin, 1975). T h e r e a r e many o t h e r examples i n t h e l i t e r a t u r e , i n c l u d i n g t h e r e s u l t s o f e a r l i e r studies on t h e coding o f numerical information such as those n e x t discussed. Conceptual correspondence: Numerical codes. Studies o f coding numerical information f o r use w i t h vocal a n d motor responses show an advantage f o r t h e S - R ensemble o f vocal number-naming responses t o Arabic-numeral stimuli o v e r e i t h e r (a) motor k e y - p r e s s i n g responses t o these stimuli, o r (b) e i t h e r vocal o r motor responses t o o t h e r symbolic codes r e p r e s e n t i n g numerical information (see A l l u i s i & Muller, 1956, 1958). T h i s is shown in t h e data o f F i g u r e 1.

In t h e t w o experiments r e p o r t e d i n F i g u r e 1, t h e informationh a n d l i n g performance o f 10 subjects was measured w i t h conventional A r a b i c numerals a n d s i x o t h e r symbolic v i s u a l codes. Motor ( k e y p r e s s i n g ) responses were used in one experiment, a n d vocal (numbernaming) responses were used in t h e o t h e r . T h e symbolic codes included a set o f s t r a i g h t - l i n e symbolic A r a b i c numerals, t h r e e sets o f o r d e r e d

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Things That Go Together

STIMULUS ALPHABETS .

A - ARABIC NUMERALS

0 -SYMBOLIC ARABIC NUMER

0 -SIMPLE INCLINATION -

\I - CLOCK INCLINATION

x - BINARY INCLINATION O - ELIPSE-AXISRATIO

EB r8

EY r 8

Y

Y

3z B 2

3z 0

v)

z Y

z f

a

E

E 3

2

1

0 1

2

3

4

5

6

PRESENTATION RATE (BITS/SEC) F i g u r e 1. Rate o f information transmission as a f u n c t i o n o f t h e r a t e o f information presentation w i t h vocal (number-naming) a n d motor ( k e y p r e s s i n g ) responses t o seven, ten-element, symbolic visual codes. T h e solid diagonal l i n e represents t h e maximum possible transmission rate. Each data p o i n t is based on 1,000 stimulus presentations. F o r simplicity, t h e lines d r a w n r e p r e s e n t t h e data averaged o v e r t h e alphabets w i t h i n t h e t h r e e empirically e v i d e n t g r o u p i n g s o f t h e t w o numerical codes, t h e three inclination codes, a n d t h e s i n g l e codes of color a n d ellipse-axis ratio. (Revised f r o m A l l u i s i & Muller, 1956, 1958). symbols based on differences i n t h e visual inclination o f a l i n e (simple, clock, a n d b i n a r y ) , a set o f colors, a n d a set o f ellipses o f d i f f e r i n g axis ratios (five h o r i z o n t a l l y a n d f i v e v e r t i c a l l y o r i e n t e d ) . A l l symbolic alphabets were c o n s t r u c t e d o f symbols t h a t h a d been p r e v i o u s l y scaled a n d selected t o maximize d i s c r i m i n a b i l i t y . Each experiment employed b o t h

E.A. Alluisi and J.S. Warm

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self-paced a n d forced-paced rates o f information presentation, l a t t e r b e i n g v a r i e d f r o m 2 t o 6 bits/sec in unit steps.

with the

T h e data o f t h e forced-paced t r i a l s a r e shown i n F i g u r e 1. It is q u i t e a p p a r e n t t h a t t h e seven codes f a l l i n t o t h r e e g r o u p i n g s w i t h b o t h vocal a n d motor responses. T h e g r o u p i n g reflects how well t h e codes p o r t r a y ( a n d c o r r e s p o n d in t h e i r p o r t r a y a l o f ) t h e numerosity concept. T h e two numerical codes a r e s u p e r i o r t o t h e t h r e e inclination codes, a n d these a r e a l l s u p e r i o r t o t h e t w o remaining codes o f color a n d ellipse-axis ratio. Furthermore, t h e i n t e r a c t i o n o f stimulus codes w i t h response modes, i n d i c a t i v e o f v e r y s t r o n g S - R compatibility effects, is also q u i t e apparent a n d consistent w i t h p r i o r observations. Essentially similar S - R compatibility effects were f o u n d in t h e self-paced performances t h a t a r e n o t detailed here. Even w i t h t h e highly compatible number-naming responses t o t h e A r a b i c numerals, t h e importance o f t h e conceptual-correspondence aspect o f t h e S-R ensemble i t s e l f i s demonstrated in another s t u d y . Fitts and Switzer (1962) f o u n d t h a t t h e vocal "two" response t o t h e v i s u a l l y presented "2" in an ensemble composed o f t h e two-element v i s u a l alphabet, "1" a n d "2," p a i r e d w i t h t h e two-element vocal-response alphabet, "one" a n d "two," is made f a s t e r t h a n w i t h an ensemble consisting o f "2" a n d "7" p a i r e d w i t h "two" a n d "seven. " Conceptual correspondence: C e n t r a l processing. Wickens (1 987, in t h e r e c e n t l y p u b l i s h e d Handbook of Human Factors (Salvendy, 19871, discussed t h e concept o f "stimulus-central processing (SC) compatibility"--a concept t h a t h e based o n t h e notion t h a t t h e human has t w o d i f f e r e n t w o r k i n g memory systems f o r rehearsing o r t r a n s f o r m i n g v e r b a l a n d spatial t a s k s (see Baddeley & Hitch, 1974). He p o i n t e d o u t t h a t t h e p a r t o f c e n t r a l processing t h a t makes use of v e r b a l o r phonetic memory is more compatibly matched w i t h t h e a u d i t o r y stimuli a n d vocal responses, whereas t h e p a r t t h a t makes use o f spatial memory is b e t t e r matched w i t h spatial stimuli a n d responses (e.g., key-pressing responses t o l i g h t s ) . pp.

76-77),

T h e issue o f compatible modalities is more b r o a d l y addressed elsewhere (Wickens, 1984). T h e conclusion suggested i s t h a t p a i r i n g an a u d i t o r y - s t i m u l u s alphabet w i t h a speech-response alphabet is l i k e l y t o p r o d u c e a highly compatible ensemble f o r a v e r b a l t a s k . However, f o r a spatial task, a highly compatible ensemble i s more l i k e l y t o b e obtained k i t h t h e p a i r i n g o f a visual/spatial-stimulus alphabet w i t h a motor/ ( c o r r e s p o n d i n g l y s p a t i a l l - r e s p o n s e alphabet. T h e mediation is p r o v i d e d by t h e s h o r t - t e r m ( w o r k i n g ) memory system. T h i s suggests t h e use o f an S-C-R compatibility model t h a t p r e d i c t s t h a t human performance w i l l b e optimized when t h e stimulus alphabet (S), t h e c e n t r a l 'processing coding o f s h o r t - t e r m memory ( C ) , a n d t h e response alphabet ( R ) a r e compatibility matched. T h i s model is consistent w i t h t h e view t h a t it is r e a l l y t h e conceptual correspondence between stimulus a n d response alphabets, r a t h e r t h a n t h e physical correspondence, t h a t t e n d s t o optimize or maximize S-R compatibility. Of course, t h i s S-C-R approach t o optimizing S-R compatibility effects is essentially a special case o f t h e general stimulus-organism-response ( o r S - 0 - R ) orientation in experimental psychology ( c f . Woodworth, 1938; Woodworth & Schlosberg, 1954).

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Summary: Correspondence between alphabets. Thus, it appears t o b e reasonably well established t h a t S-R compatibility approaches a maximum when t h e p a i r i n g s o f stimulus a n d response alphabets c o r r e s p o n d t o one another i n a d i r e c t conceptual sense. I n t h e applicable cases, such as those t h a t employ motor responses t o stimuli w i t h d i f f e r e n t spatial locations, it is l i k e l y t h a t t h i s w i l l o c c u r when t h e alphabets c o r r e s p o n d t o one another in a d i r e c t p h y s i c a l sense as well. In selecting modalities f o r t h e stimulus a n d response alphabets, it i s p r u d e n t t o match them t o t h e c e n t r a l -processi ng , s h o r t - t e r m ( w o r k i n g ) memory, requirements o f t h e task. Population Stereotypes A n empirical t a s k is t y p i c a l l y used b o t h t o establish t h e existence o f a population stereotype a n d t o p r o v i d e i t s d e s c r i p t i o n . The task involves t h e d i s c o v e r y o r demonstration o f preferences, among subjects who a r e t h e members o r representatives o f some t a r g e t population, in t h e specific assignments o f response symbols t o stimulus symbols. As stated earlier, such stereotypes a r e specified operationally by tables o f t h e empirically determined frequencies w i t h w h i c h permissible responses a r e made o r assigned t o each stimulus symbol i n a free-response paradigm, :nd where "9,"" o f t h e responses has been predesignated as "correct" o r appropriate. Such preferences o r stereotypes a r e f o u n d f r e q u e n t l y , if n o t universally. T h e y may d i f f e r as a f u n c t i o n f; c u l t u r e o r p r a c t i c e . At times, t h e y a r e f o u n d t o b e c o n t r a d i c t o r y t o standard" design practices (see L u t z & Chapanis, 1955). Some contradictions may b e unavoidable, especially when c r o s s i n g c u l t u r a l lines. F o r example, t h e customary o r t y p i c a l room-light toggle-switch c o n t r o l is moved up t o a c t i v a t e a room light in t h e U n i t e d States, b u t down in t h e U n i t e d Kingdom. Stereotype8 in v e r b a l l e a r n i n g . Population stereotypes a r e used not o n l y in t h e s t u d y o f perceptual-motor skills, but also in t h e s t u d y o f language skills, where t h e y a r e b e t t e r known as association norms. There, inferences r e g a r d i n g t h e s t r e n g t h s o f i n d i v i d u a l stereotypes a r e f r e q u e n t l y made on t h e basis o f t h e s t r e n g t h of population stereotypes. Thus, if all o f t h e subjects in a sample f r o m some population d i s p l a y a single preference, it i s o f t e n assumed n o t o n l y t h a t t h e p r e f e r e n c e is u n i v e r s a l in t h e population, but also t h a t it is a v e r y s t r o n g p r e f e r e n c e in each i n d i v i d u a l member o f t h e population. The validity of that assumption is an empirical i s s u e - - i t may o r may n o t p r o v e t o b e t r u e - but, as F i t t s a n d Posner (1967) p o i n t e d out, "the rate a t which associations between p a i r s o f words a r e learned agrees closely w i t h predictions f r o m association norms" (p. 21). Such stereotypes r e p r e s e n t preferences embedded in l o n g - t e r m memory. T h e y a r e mediated by a central-processing mechanism. T h i s is n o t t h e same as t h e p r e v i o u s l y discussed mechanism o f s h o r t - t e r m ( w o r k i n g ) memory t h a t Wickens (1984, 1987) p r e s e n t e d as mediating t h e modality influences in maximizing S - R compatibility. T h e short-term memory mechanism deals w i t h t h e alphabet-correspondence issue; t h e l o n g - t e r m memory mechanism deals w i t h t h e issues o f preferences a n d stereotypes. In a normal adult, l o n g - t e r m memory can b e represented as including, in a f r e q u e n c y o r p r o b a b i l i t y sense, all t h e information t h a t

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has p r e v i o u s l y entered t h e person's c o g n i t i v e channel. Thus, f r e q u e n c y o f exposure, familiarity, appear t o b e important determiners o f stereotypes.

information-handling o r experience would

T h a t t h i s i s t h e case can b e i n f e r r e d f r o m t h e results o f t w o studies. In one t h a t dealt with aesthetic judgments, a multiple-R o f +.843 was f o u n d in p r e d i c t i n g r a n k - o r d e r preference o f appearance w i t h t h r e e frequency-based r a n k o r d e r i n g s o f English l e t t e r s ( A l l u i s i & Adams, 1962). In t h e second, a multiple-R o f +.672 was f o u n d in p r e d i c t i n g r a n k - o r d e r vocal d i s j u n c t i v e R T t o d i f f e r e n t English l e t t e r s ( c f . F i t t s 8 Switzer, 1962) w i t h t h e b e s t t w o o f several f r e q u e n c y and preferencebased p r e d i c t o r variables; namely, r a n k o r d e r f o r (a) f r e q u e n c y o f use in English a n d (b) preference o f appearance (Alluisi, 1963).

I t seems safe t o conclude t h a t Summary: Population stereotypes. where s t r o n g i n d i v i d u a l o r population stereotypes are found, a n d where t h e p a i r i n g s o f t h e elements in t h e stimulus and response alphabets a r e consonant w i t h t h e stereotypes, (a) t h e r e s u l t i n g S-R ensemble w i l l t e n d t o b e optimum, (b) S-R compatibility maximum, a n d (c) t h e r e s u l t i n g performances e i t h e r optimized o r maximized. Examples o f recognized population stereotypes relevant t o t h e design a n d use o f human-machine systems a r e l i s t e d by Woodson a n d Conover (1970, p. 1.301, a n d by Wickens (1987, pp. 89-90). T h e o t h e r side o f t h e coin is t h a t if s t r o n g i n d i v i d u a l o r population stereotypes e x i s t a n d if t h e y a r e violated i n t h e construction o f t h e S-R ensemble, human performances w i l l b e degraded r e l a t i v e t o what t h e y could have been--and p r o b a b l y t o levels below those t h a t were expected, predicted, o r planned in t h e design o f t h e workplace a n d system in which t h e ensemble is used. T h e human-factors specialist is well-advised t o follow t h e guidance o f V a n C o t t and Kinkade (19721, n o t o n l y in u s i n g t h e population stereotype ("what t h e operator expect:") i n designing control a n d display panels, b u t also in v e r i f y i n g w i t h operational personnel t o assure t h a t violation o f a population stereotype w i l l n o t b e incorporated" (p. 404) elsewhere in t h e design o f t h e system and i t s components.

S-R Compatibility Effects in Information H a n d l i n g T h e general topic o f S - R compatibility effects in information handling i s broad, has a relatively long history, and has been well covered elsewhere in many o f i t s aspects. In fact, a b r i e f section o n "Population stereotypes in responding t o directional cues" is included i n t h e chapter by F i t t s (1951, pp. 1306-1308) on "Engineering Psychology and Equipment Design," in t h e now classic Handbook of Ex,erimental Psychology (Stevens, 1951). More recently, sections on StimulusResponse Compatibility" appear in chapters on "Motor Control" (Keele, 1986, pp,. 30.8-30.100) in Boff, Kaufman, a n d Thomas (1986), on "Information Processing, Decision-Making, a n d Cognition" (Wickens, 1987, pp. 89-90) in Salvendy (19871, and on "Design f o r Action" in B a r b e r (1988, pp. 67-98). F u r t h e r , a section on "Compatibility," with the articulation o f a "Compatibility Principle" ("Minimize t h e amount o f information recoding t h a t w i l l b e necessary"), appears in "Software I n t e r f a c e Design" (Williges, Williges, & Elkerton, 1987, pp. 1419-1420).

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Also, e n t i r e t e x t s have been w r i t t e n on information t h e o r y a n d i t s applications in psychological research a n d t h e o r y , w i t h A t t n e a v e (1959), G a r n e r (1962), a n d Quastler (1955) b e i n g among t h e earliest. Other t e x t s have addressed psychological applications o f information theory; among them, B e r l y n e (1960), Broadbent (1958), F i t t s a n d Posner (1967), Luce (1960), a n d Senders (1958) span a r a n g e o f i n t e r e s t s p e r t a i n i n g t o t h i s topic. S t i l l o t h e r t e x t s have e i t h e r a m i n o r section (Dember & Warm, 1979, pp. 106-123), a c h a p t e r (Alluisi, 19701, o r a major section (Posner, 1986) devoted t o topics o f i n f o r m a t i o n - t h e o r y applications in psychological research a n d t h e o r y . These w o r k s a t t e s t t o t h e b r e a d t h o f impact o f i n f o r m a t i o n - t h e o r y concepts a n d measures in psychology. A l t h o u g h f e w o f them deal d i r e c t l y w i t h S-R compatibility effects, t h e y establish t h e foundation f o r t h e question, "How d o S-R compatibility effects influence human-information processing?" T a k e n together, these w o r k s suggest t h a t t h e influence is substantial. Information-Handling Rate Rates o f numerical information transmissions in forced-paced serial t a s k s have been measured in numerous experiments. In one (Alluisi, Muller, & Fitts, 1955, 19571, v i s u a l presentations o f t h e Arabic-numeral stimulus alphabet were matched t o t w o response alphabets, one vocal a n d t h e o t h e r motor. T h e highly compatible vocal response alphabet consisted of,,numbernaming w i t h t h e u;ual English names o f t h e numerals (i.e., one" f o r "1 "two" f o r "2, e t c . ) . T h e dimensionally compatible motor response alphabet consisted o f k e y - p r e s s i n g w i t h use o f a b a n k o f 10 keys, a r r a n g e d h o r i z o n t a l l y in t w o semicircles t o fit t h e n a t u r a l placement o f t h e 10 f i n g e r tips, numbered t o increase f r o m l e f t t o right (i.e., "1" f o r t h e l i t t l e f i n g e r on t h e l e f t hand, "2" f o r t h e r i n g f i n g e r , e t c . ) . ,I'

T h e experiment was designed p r i n c i p a l l y t o determine w h e t h e r t h e r a t e o f information h a n d l i n g in a forced-paced serial t a s k is a f u n c t i o n o f (a) t h e r a t e o f stimulus presentation (1, 2 , 3 stimuli/sec), (b) t h e u n c e r t a i n t y p e r stimulus (1, 2 , o r 3 bits/stimulus), o r (c) t h e j o i n t e f f e c t o f t h e t w o f a c t o r s expressed as t h e r a t e o f information presentation Each o f 10 h i g h l y p r a c t i c e d male subjects ( r a n g i n g f r o m 1 t o 9 bits/sec). responded t o 100 presentations of t h e stimuli in each o f t h e n i n e conditions w i t h b o t h o f t h e response alphabets. T h e r e s u l t i n g data a r e g i v e n in F i g u r e 2, in terms o f t h e rates o f information transmission. F i r s t , t h e interaction o f response mode w i t h t h e informationin vocal a n d motor presentation variables (i.e., t h e differences responses) is an S-R compatibility e f f e c t t h a t reflects t h e h i g h e r compatibility o f t h e vocal number-naming responses, o v e r t h e motor k e y p r e s s i n g responses, when p a i r e d w i t h t h e Arabic-numeral stimuli. It is n o t i n d i c a t i v e o f a n y general s u p e r i o r i t y o f vocal o v e r motor responses. Indeed, t h i s same k e y - p r e s s i n g motor-response mode is b e t t e r t h a n t h e number-naming vocal responses when p a i r e d w i t h spatial stimuli (rows o f l i g h t s ) t h a t c o r r e s p o n d p h y s i c a l l y t o t h e placement o f t h e k e y s (Muller, 1955). Thus, h i g h e r rates o f information transmission a r e obtained w i t h ensembles o f h i g h e r S-R compatibility.

E.A. Alluisi and J.S. Warm

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P r6 c

PRESENTATION RATE (BITS/SEC)

Figure 2. Rate of information transmission as a f u n c t i o n of t h e r a t e of information presentation w i t h vocal (number-naming) a n d motor ( k e y p r e s s i n g ) responses t o v i s u a l l y presented A r a b i c numerals a t t h r e e levels of stimulus complexity (1, 2 , 3 bits/stimulus) a n d t h r e e rates o f stimulus T h e solid diagonal l i n e represents t h e presentation (1, 2, 3 stimuli/sec). maximum possible transmission r a t e . Each data p o i n t is based o n 1,000 stimulus presentations. (Revised f r o m Alluisi, Muller, & Fitts, 1955, 1957) . Second, t h e data o f F i g u r e 2 also indicate t h a t o v e r these ranges o f stimulus complexities (1 t o 3 bits/stimulus) a n d presentation rates ( 1 t o 3 stimuli/sec), (a) increasing complexity a t a g i v e n stimulus-presentation r a t e produces an increased r a t e of information transmission generally, a n d especially so w i t h t h e highly compatible vocal response t o t h e A r a b i c numerals, b u t (b) increasing t h e r a t e o f stimulus presentation a t a g i v e n complexity generally produces a decrement, which is especially notable i n t h e less-compatible motor-response p a i r i n g s t o t h e A r a b i c numerals.

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S t a b i l i t y o f S-R C o m p a t i b i l i t y E f f e c t s S-R compatibility effects a r e q u i t e stable o v e r b o t h s h o r t a n d longt e r m practice. Indeed, t h e s t a b i l i t y o f t h e e f f e c t is among t h e earliestnoted c h a r a c t e r i s t i c o f t h e phenomenon ( D e i n i n g e r & Fitts, 1955; F i t t s & Seeger, 1953). T h i s s t a b i l i t y is also f o u n d in t h e data o f o t h e r studies. F o r example, A l l u i s i a n d M a r t i n (1957, 1958) f o u n d it in t h e persistence o f b e t t e r performances, as indicated by h i g h e r informationtransmission rates, obtained w i t h vocal (number-naming) responses o v e r motor ( k e y - p r e s s i n g ) t-fsponses t o t w o sets o f v i s u a l l y p r e s e n t e d A r a b i c numerals. One was a conventional" set o f AND-10400 numerals, a n d t h e o t h e r was a "symbolic" set based on an eight-element s t r a i g h t - l i n e m a t r i x . S h o r t - t e r m l e a r n i n g was measured t h r o u g h t h e responses o f 48 college students f r o m a pool o f volunteers. T h e g r o u p was randomly d i v i d e d i n t o t w o g r o u p s o f 24, one f o r t h e vocal responses a n d t h e o t h e r f o r t h e motor responses. Each subject responded during one session o f f i v e t r i a l s o f 200 stimulus presentations (100 each f o r conventional a n d symbolic A r a b i c numerals) on each o f 2 successive d a y s . Thus, when t h e 10 t r i a l s a r e p l o t t e d f o r each o f t h e f o u r combinations ( t w o stimulus t y p e s by t w o response modes), each data p o i n t represents 2,500 stimulus presentations. Results o f s h o r t - a n d l o n g - t e r m l e a r n i n g a r e essentially similar, as discussed below. L o n g - t e r m l e a r n i n g was measured similarly, but t h r o u g h t h e responses o f 10 o f t h e 48 subjects, a n d f o r a t o t a l o f 12, f i v e - t r i a l sessions. When t h e 12 sessions (blocks o f f i v e t r i a l s ) a r e p l o t t e d f o r t h e f o u r conditions, each data p o i n t also represents 2,500 stimulus presentations. A l t h o u g h t h e improvements in information-transmission rates ( i n bits/sec) r a n g e d f r o m about 30% t o 40%, on t h e average, o v e r t h e 12 sessions, t h e conventional A r a b i c numerals a r e c o n s i s t e n t l y s u p e r i o r t o t h e symbolic when p a i r e d w i t h vocal responses. N e i t h e r alphabet i s s u p e r i o r when p a i r e d w i t h motor responses. B o t h numeral t y p e s y i e l d b e t t e r performances w i t h vocal responses t h a n w i t h motor responses. Thus, t h e data show f o u r n o t q u i t e parallel lines, t h e bottom t w o o f which a r e i n t e r t w i n e d . I n t e r e s t i n g l y , t h e effects of p r a c t i c e a f t e r i n i t i a l learning, t h a t is, f r o m t h e 3 r d to t h e 12th session, a r e g r e a t e r w i t h t h e h i g h e r t h a n w i t h t h e lower-compatibility S-R ensembles. T h a t is, t h e t w o lines f o r t h e b e t t e r ensembles a r e diverging f r o m t h e t w o lines f o r t h e o t h e r (worse) ensembles ( A l l u i s i & Martin, 1958, p. 82). In contrast, F i t t s a n d Posner (1967, p. 24) show converging lines o f performance ( w i t h long-term practice--25 sessions spaced o v e r 3 months) in one high a n d one low compatibility condition f r o m t h e F i t t s a n d Seeger (1953) s t u d y . T h i s seeming contradiction o f f i n d i n g s c o u l d b e a reflection o f t h e subjects' attaining, o r a t least approaching, asymptotic levels o f performance in t h e h i g h - c o m p a t i b i l i t y condition w i t h t h e 3 months o f additional p r a c t i c e i n t h e l a t t e r s t u d y . Studies o f t h e i n t e r a c t i o n o f S - R compatibility a n d t h e r a t e o f gain of information suggest t h a t t h i s i s l i k e l y t o b e t h e case.

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S-R C o m p a t i b i l i t y a n d t h e Rate of Gain o f Information It was H i c k (1952) who i n i t i a l l y observed t h a t t h e r a t e o f gain o f information is a constant. T h a t is, a s t r a i g h t l i n e r e s u l t s when d i s j u n c t i v e R T is p l o t t e d as a function o f t h e amount o f information t r a n s m i t t e d ( H t ) p e r S-R event, a t least o v e r t h e r a n g e f r o m 1 t o 3 o r 4 bits/S-R e v e n t (but see Longstreth, 1988; L o n g s t r e t h & Alcorn, 1987; Longstreth, El-Zahhar, & Alcorn, 1985; Smith, 1968, pp. 83-85; Welford, 1987). T h e rate of gain (of information) is t h e slope of t h a t line. R T is g e n e r a l l y an increasing linear f u n c t i o n o f H t (e.g., Crossman, 1953; Gregg, 1954; Warm & Alluisi, 1971). T h a t is, t h e slope constant o f t h e l i n e fitting R T as a f u n c t i o n o f H t is a p o s i t i v e value Some researchers have f o u n d conditions generally, but n o t u n i v e r s a l l y . u n d e r w h i c h R T has failed t o increase w i t h H t (Leonard, 1959; Mowbray, 1960; Mowbray E Rhoades, 1959). O t h e r s have f o u n d conditions u n d e r w h i c h v a r i o u s f a c t o r s a p p a r e n t l y influence t h e degree o f increase (Alluisi, 1965; B r a i n a r d , Irby, Fitts, & Alluisi, 1962; F i t t s & Switzer, 1962; Smith, Warm, & Alluisi, 1966). I n t e r a c t i o n with t h e r a t e of g a i n o f information. H i g h degrees o f experience, familiarity, practice, a n d s k i l l w i t h t h e S-R ensemble have been among t h e conditions f o u n d t o b e associated w i t h b o t h high compatibility a n d low rates o f gain o f information. T h e t y p i c a l p a i r i n g o f number-naming vocal responses with v i s u a l l y presented A r a b i c numerals produces f o r a d u l t humans a highly compatible S-R ensemble, use o f which r e s u l t s i n a measured r a t e o f gain o f information t h a t approaches zero ( B r a i n a r d e t at., 1962). Thus, compatibility a n d information-gain r a t e i n t e r a c t (see Alluisi, Strain, & Thurmond, 1964). T h i s i s shown in t h e data o f F i g u r e 3 . T h e data o f F i g u r e 3 a r e based on t h e vocal responses o f 54 subjects a t t h r e e levels o f stimulus u n c e r t a i n t y (Hs) t o v i s u a l l y presented A r a b i c numerals. T h e stimuli f o r t h e Hs levels o f 1, 2, a n d 3 bits/stimulus were projections o f t h e AND-10400 A r a b i c numerals 1 a n d 2, 1 t h r o u g h 4, a n d 1 t h r o u g h 8, respectively. Ensembles o f low, intermediate, a n d high S - R compatibility were created by p a i r i n g responses w i t h stimuli in t h r e e ways: ( a ) In t h e h i g h - c o m p a t i t i l i t y conditions, t h e numerals w e r e assigned t h e i r usual names, t h a t is, one f o r 1, "two" f o r 2, a n d so f o r t h ; (b) in t h e intermediate condition, subjects were i n s t r u c t e d t o respond vocally w i t h the,,number name o f t h e displayed-numeral-plus-two, t h a t is, "three" f o r 1, f o u r " f o r 2, a n d so f o r t h ; a n d (c) t h e low-compatibility condition was created by randomly p a i r i n g numeral names w i t h numerals d i f f e r e n t l y f o r each subject. T h e t h r e e levels of Hs were combined f a c t o r i a l l y w i t h t h e t h r e e degrees o f S-R compatibility, a n d s i x subjects were assigned a t random t o each o f t h e n i n e conditions. Thus, no subject served u n d e r more t h a n one level o f Hs a n d one level of S - R compatibility, a n d subjects were t e s t e d in random o r d e r d u r i n g 1 - h o u r experimental sessions. The order o f t h e 80 stimulus presentations in each session was random a n d d i f f e r e n t f o r each subject, w i t h equal numbers o f presentations o f t h e assigned stimuli--40 each in t h e two-numeral condition, 20 in t h e f o u r , a n d 10 in t h e eight-numeral condition. Each subject's median R T f o r t h e 80 responses was t h e measure o f performance employed. Performance was essentially f r e e of e r r o r (less t h a n l%), so Hs i s i n t e r p r e t e d as equivalent t o H t .

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1.o

.8

.6

.4

COMPATIBILITY .2

O O

1

2

3

F i g u r e 3. Reaction time ( R T ) as a f u n c t i o n o f stimulus u n c e r t a i n t y (Hs) a t t h r e e levels o f S-R compatibility. Because performance was essentially e r r o r l e s s in t h i s case, Hs i s e q u i v a l e n t t o Ht, t h e amount o f information t r a n s m i t t e d p e r S-R event. (Revised f r o m Alluisi, Strain, & Thurmond, 1964). T h a t t h e t h r e e S-R ensembles r e p r e s e n t d i f f e r e n t levels o f S-R compatibility is e v i d e n t in t h e data o f F i g u r e 3. Means f o r t h e low, intermediate, a n d high c o m p a t i b i l i t y conditions a r e 743.1, 649.8, a n d 477.7 ms, r e s p e c t i v e l y . T h e differences among t h e means a r e s t a t i s t i c a l l y reliable, F(2, 45) = 54.40, p .001. T h a t R T increases w i t h Hs (= Ht, in t h i s case) is also e v i d e n t in t h e data o f F i g u r e 3. Means f o r t h e Hs levels o f 1, 2, a n d 3 bits/stimulus a r e 521 . I , 657.3, a n d 692.2 ms, respectively. The differences among t h e means a r e s t a t i s t i c a l l y reliable, F(2, 45) = 26.78, p < .001. T h a t t h e r a t e o f gain of information is i n v e r s e l y related t o t h e degree o f S-R compatibility is a p p a r e n t in t h e i n t e r a c t i o n o f R T w i t h Hs

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o v e r t h e t h r e e S-R compatibility conditions shown in F i g u r e 3; t h e interaction is s t a t i s t i c a l l y reliable, F(4, 45) = 4.91 p < .005. T h e r a t e o f gain o f information (slope constant) i s 148, 83, a n d 23 ms. w i t h t h e low, intermediate, a n d h i g h - c o m p a t i b i l i t y S-R ensembles, respectively. Summary: S-R c o m p a t i b i l i t y e f f e c t s a n d t h e r a t e o f g a i n of information. R T is an increasing linear f u n c t i o n o f Ht, except in t h e case o f highly compatible S-R ensembles. In general, t h e degree t o w h i c h R T is influenced by H t is an i n v e r s e f u n c t i o n o f t h e degree o f S-R compatibility. T h a t is, t h e r a t e o f gain o f information (i.e., t h e slope o f t h e l i n e r e l a t i n g R T t o H t ) v a r i e s i n v e r s e l y w i t h S-R compatibility. As suggested elsewhere ( A l l u i s i e t al., 19641, it may even b e used as a measure o f S-R compatibility in much t h e same way t h a t "resistance t o e x t i n c t i o n " is used as a measure o f l e a r n i n g . Generality, Limitations, a n d Possible Mechanisms of S - R Compatibility E f f e c t s T h e e a r l i e r studies o f S - R compatibility effects have been emphasized in t h e p r e v i o u s sections of t h i s c h a p t e r . The f i r s t o f two purposes f o r t h i s emphasis is t o c a p t u r e r e l e v a n t studies t h a t a r e in d a n g e r o f b e i n g missed o r i g n o r e d . T h a t is t o say, t h e "corpus" o f t h e PsyclNFO System, t h e electronic databases consisting o f summaries o f t h e "world's l i t e r a t u r e in psychology a n d related fields," begins w i t h summaries p u b l i s h e d in Psychological Abstracts during 1967. Generally, papers p u b l i s h e d e a r l i e r t h a n 1965 a r e n o t included. Some t e x t s a r e i n c l u d e d but o n l y w i t h b r i e f annotations. Chapters o f edited books w i l l n o t appear until 1989 o r 1990. Thus, unless i n c l u d e d i n reviews such as this, papers p u b l i s h e d p r i o r t o t h e rnid-l960s, as well as p a s t t e x t s a n d t h e i r chapters, w i l l soon b e lost t o those g r o w i n g numbers of researchers w h o depend solely o r mostly on computerized searches of those excellent a n d otherwise comprehensive b i b l i o g r a p h i c databases o f t h e PsyclNFO System. T h e second p u r p o s e is t o lay a foundation f o r t h e remainder of t h i s t e x t by documenting, a n d i l l u s t r a t i n g t h r o u g h t h e citations a n d data o f these e a r l i e r studies, t h e major experimental paradigms, performance characteristics, a n d f i n d i n g s t h a t define t h e phenomenon known as "S-R compatibility effects." T h e l a t e r w o r k s o f t h e 1970s a n d 1980s r e p r e s e n t exploitations o f t h e phenomenon. T h e y e x p a n d i t s horizons, seek t o establish j t s limits a n d r e s t r i c t i o n s , a n d p r o v i d e a deeper u n d e r s t a n d i n g o f t h e phenomenon. I n so doing, t h e y c l a r i f y t h e c o n t e x t a n d e x t e n d t h e g e n e r a l i t y o f t h e phenomenon, a n d t h e y b e g i n t o establish i t s mechanisms. T h e f i n a l sections o f t h i s c h a p t e r consist o f b r i e f reviews of these l a t e r studies. Rate of Gain o f Information In h i s review o f choice RT, Smith (1968, pp. 83-85) c i t e d numerous studies t h a t showed t h e r a t e o f gain o f information t o b e i n v e r s e l y related t o S-R compatibility, but h e f o u n d l i t t l e o r no evidence o f t h e slopes e v e r reaching zero. One s t u d y did show b o t h zero a n d negative slopes, but t h e a u t h o r dismissed them as p r o b a b l y a r t i f a c t s o f p r a c t i c e a n d o f p r o c e d u r e - t h e t e s t i n g of all subjects i n an ascending o r d e r o f Hs levels (Alluisi, 1965).

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More recently, L o n g s t r e t h (1988) a r g u e d t h a t t h e relation between R T a n d Hs is n o t linear a t all, b u t r a t h e r curvilinear, c e r t a i n l y beyond u n c e r t a i n t y levels g r e a t e r t h a n 3 b i t s p e r stimulus. He a n d h i s colleagues ( L o n g s t r e t h e t al., 1985) suggested t h e data b e t t e r fit a power law, b u t t h i s view is n o t u n i v e r s a l l y accepted (see L o n g s t r e t h & Alcorn, 1987; Welford, 1987).

It should b e noted t h a t "Hick's Law" relates d i s j u n c t i v e R T to Ht, It says t h a t t h e t h e amount o f information transmitted (see Hick, 1952). relation i s l i n e a r - - t h a t R T is a s t r a i g h t - l i n e f u n c t i o n o f H t . O n l y u n d e r v e r y special conditions w i l l stimulus u n c e r t a i n t y (Hs, o r t h e amount o f information p e r stimulus event) equal H t . For example, w i t h equiprobable stimuli a n d e r r o r l e s s performance, Hs is equal t o H t . I n o t h e r cases, Hs is n o t equal t o Ht, a n d use o f Hs in t h e c o n t e x t o f Hick's Law would b e incorrect. Instead, in such cases, H t would have t o b e computed d i r e c t l y f r o m t h e m a t r i x t h a t records t h e responses made t o t h e stimuli presented. Even so, it would b e unreasonable t o expect t h e linear "Hickks Law" relation t o hold w i t h o u t limit. T h e position t h a t is l i k e l y t o p r o v e valid in t h e l o n g r u n i s essentially t h e same as t h a t stated in a p r i o r section based o n t h e f i n d i n g s of t h e earlier studies; namely, t h a t a t least w i t h i n t h e r a n g e o f H t f r o m 1 t o 3 o r 4 b i t s p e r S-R event,

. .

.

t h e relation between d i s j u n c t i v e R T a n d H t i s linear, t h e slope o f t h e l i n e interacts w i t h S-R compatibility, t h e r e is an i n v e r s e relation between t h e slope a n d t h e degree o f S-R compatibility, and t h e slope approaches zero as S-R compatibility approaches a maximum.

Scope o f S-R Compatibility Effects As noted p r e v i o u s l y (see Maximizing Compatibility), t h e earlier studies o f S-R compatibility focused mostly, b u t n o t exclusively, on perceptual-motor performances, f o r example, on l i g h t - p a t t e r n e d stimulus alphabets p a i r e d w i t h motor-patterned response alphabets. However, vocal-response a n d symbolic-visual alphabets were also widely used, a n d S-R compatibility effects were even demonstrated in t h e paired-associate learning o f nonsense syllables (Baddeley, 1961). Later studies have expanded t h e scope o f t h e phenomenon by exploring, measuring, a n d examining it w i t h d i f f e r e n t S-R ensembles a n d t a s k situations. For example, S-R compatibility effects have been found w i t h S-R ensembles t h a t d o o r d o n o t demonstrate color correspondence (responding w i t h a r e d k e y t o a r e d light, and a green k e y t o a green light; or, w i t h a r e d k e y t o a green light, etc.; Hedge & Marsh, 1975; also see Simon, Sly, & Vilapakkam, 1981). A u d i t o r y stimulus alphabets have also been used in numerous studies, many o f them by Simon a n d h i s colleagues (see B e r t e r a e t al., 1975; Callan e t a l . , 1974; Knowles e t al., 1953; a n d most o f t h e c i t e d chapter references by Simon a n d h i s colleagues). Stimulus o r response alphabets i n o t h e r modalities have also been used, i n c l u d i n g t h e tactual (Leonard, 1959), t h e kinesthetic (Wallace, 1972), a n d t h e oculomotor ( B e r t e r a e t al., 1975). Essentially

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identical S-R compatibility effects as a r e t y p i c a l l y f o u n d in d i s j u n c t i v e R T t a s k s have also been demonstrated in a watchkeeping t a s k ( K u l p & Alluisi, 1967) a n d in a memory scanning t a s k (Ogden & Alluisi, 1980). Shulman a n d McConkie (1973), c i t i n g F i t t s a n d B r i g g s (1959), contended t h a t t h e notions o f S - S compatibility a n d R-R compatibility a r e c o r o l l a r y t o t h e concept o f S-R compatibility. T h e y suggested t h a t a basis f o r S - S compatibility lies in t h e p a s t research o n d i v i d e d a n d selective a t t e n t i o n a n d in Garner's (1970) w o r k o n i n t e g r a l v s . separable etimulus dimensions. It seems reasonable t o agree t h a t S-R compatibility may b e considered along such dimensions as d i s c r i m i n a b i l i t y a n d separableness" (p. 375), since information processing a n d choice R T a r e known t o b e affected by stimulus d i s c r i m i n a b i l i t y a n d d i s s i m i l a r i t y (see A l l u i s i E Sidorsky, 1958; Morgan & Alluisi, 1967; T h u r m o n d & Alluisi, 1963). What m i g h t now b e called "S-S compatibility effects" a r e shown in a s t u d y t h a t predates t h e f i r s t publication o f t h e term, "S-R compatibility." Specifically, F i t t s a n d Simon (1952) used various v i s u a l stimulus p a t t e r n s formed by d i f f e r e n t i n s t r u m e n t a n d p o i n t e r aivangements p a i r e d w i t h a continuous d u a l - p u r s u i t t a s k . T h e y f o u n d t h a t performance i s b e t t e r when i n s t r u m e n t s a r e close t o g e t h e r a n d aligned h o r i z o n t a l l y . Also, performance i s g e n e r a l l y b e t t e r when p o i n t e r s a r e aligned in t h e 9 a n d 12 o'clock d i a l positions, but t h i s f a c t o r i n t e r a c t s w i t h i n s t r u m e n t alignment. T h e 9 o'clock position i s b e t t e r w i t h h o r i z o n t a l l y aligned instruments, but t h e 12 o'clock position is b e t t e r w i t h v e r t i c a l l y aligned instruments. Also, R-R compatibility effects a r e demonstrated by Shulman a n d McConkie (1973), who v a r i e d S-R combinations by c h a n g i n g t h e spatial mapping r u l e i.n a two-choice t a s k a n d v a r i e d R-R combinations by c h a n g i n g t h e specific set o f k e y - p r e s s i n g responses used. T h e i r r e s u l t s showed t h a t t h e effects of response d i s c r i m i n a b i l i t y a n d S-R compatibility are additive. T h e y i n t e r p r e t e d t h a t a d d i t i v i t y along t h e lines o f t h e S t e r n b e r g (1969) paradigm a n d concluded t h a t t h e t w o effects r e p r e s e n t independent c e n t r a l processes. In a more-recent demonstration o f possible response ( R - R ) compatibility effects, M i l l e r (1988) seeks t o resolve an issue raised by t h e r e s u l t s o f c u i n g experiments t h a t show a hand-preparation advantage i.e., R T w i t h f i n g e r s on t h e same h a n d t o b e f a s t e r t h a n w i t h f i n g e r s on d i f f e r e n t hands (Miller, 1982, 1985, 1987). He i n t e r p r e t s this t o b e an R - R compatibility effect, but Reeve a n d Proctor (1984, 1985) p o i n t o u t t h a t t h e h a n d advantage in such c u i n g experiments could b e i n t e r p r e t e d w i t h equal v a l i d i t y as an S-R compatibility effect. Miller's (1988) r e s u l t s show t h e usual advantage o f same v e r s u s d i f f e r e n t hands, but t h e evidence does n o t r u l e o u t t h e p o s s i b i l i t y t h a t it is an S-R compatibility effect. H e discusses t h e issue, but f a i l s t o resolve it. T h i s is but one o f many unresolved issues r e g a r d i n g possible mechanisms o f S-R compatibility effects. Possible Mechanisms o f S -R Compatibility E f f e c t s

As discussed p r e v i o u s l y (see Maximizing Compatibility), t h e e a r l i e r studies established t h a t S-R compatibility effects would t e n d t o b e maximized (a) when t h e elements in t h e stimulus a n d response alphabets a r e p a i r e d t o c o r r e s p o n d in a d i r e c t physical sense a n d (b) where t h e p a i r i n g s a r e consonant w i t h e x i s t i n g population o r i n d i v i d u a l stereotypes.

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(Recall, however, t h a t t h e notion o f correspondence in a d i r e c t physical sense was shown, in t h e c i t e d section, t o b e too c o n f i n i n g a concept, a n d t h e w o r d physical was replaced w i t h Conceptual.) Thus, in an i n i t i a l study, F i t t s a n d Seeger (1953) showed t h a t R T i s f a s t e r (S-R compatibility h i g h e r ) when t h e response k e y s in a motor-response alphabet a r e placed t o correspond p h y s i c a l l y w i t h t h e placement o f t h e spatially located l i g h t s in t h e stimulus alphabet. The underlying mechanism proposed by Simon a n d his colleagues (e.g., C r a f t & Simon, 1970; Simon, 1968; Simon & Rudell, 1967) t o account f o r t h i s finding a n d others, assumed t h a t human subjects have "a p o t e n t n a t u r a l t e n d e n c y t o react t o w a r d t h e major source o f stimulation" (Simon, C r a f t , & Small, 1970, p. 63). However, Wallace (1971) a r g u e d t h a t t h e mechanism i s one o f conceptual correspondence in l a t e r a l i t y . He based t h i s o n f i n d i n g s t h a t t h e effects ( w h i c h h e views as S-S compatibility, r a t h e r t h a n S-R) o c c u r when t h e spatial position o f t h e stimulus ( l e f t o r right) i s matched by t h a t o f t h e response key, regardless o f t h e subject's hands b e i n g crossed o r not. Cotton, Tzeng, a n d H a r d y c k (1977, 1980) disagreed. They a r g u e d f o r a mechanism based on f u n c t i o n a l differences in t h e t w o cerebral hemispheres--an explanation in terms o f c e r e b r a l l a t e r a l i t y f a c t o r s (but see Katz, 1981; Nicoletti e t al., 1982; Whitaker, 1980, 1982). T h e y c i t e d as s u p p o r t i n g evidence f i n d i n g s t h a t show oculomotor R T t o b e f a s t e r when tones signaling l e f t o r r i g h t - l o o k i n g a r e presented in t h e ear c o r r e s p o n d i n g t o t h e d i r e c t i o n o f t h e commanded eye movement, r a t h e r t h a n t h e opposite ear (see B e r t e r a e t al., 1975). Lupker and Katz (1982) reviewed these studies a n d concluded t h a t "unless more compelling data can b e produced, it seems much more reasonable t o r e t a i n t h e s t a n d a r d S-R compatibility explanation t h a n t o i n v o k e one based o n questionable hemisphere assumptions and supported by logically inconsistent data" (p. 98). Thus, it would appear t h a t conceptual correspondence, whatever i t s mechanisms, may y e t p r o v e t o b e t h e more v a l i d a n d parsimonious explanation o f S-R compatibility!

With more t h a n a dozen studies p u b l i s h e d o v e r t h e p a s t t w o a n d a half decades, Simon is easily one o f t h e most p r o l i f i c researchers o n t h e possible mechanisms o f S-R compatibility effects (see C h a p t e r 2, by Simon). He is senior a u t h o r o f 14 studies c i t e d i n t h i s c h a p t e r s reference list, a n d j u n i o r a u t h o r o f y e t another t w o ( C r a f t & Simon, 1970; LeMay & Simon, 1969). Also, these papers have stimulated many o t h e r studies, some s u p p o r t i n g , some opposing t h e i n t e r p r e t a t i o n s a n d mechanisms hypothesized f o r what has come t o b e called ( a f t e r Hedge & Marsh, 1975) t h e "Simon effect" i n S-R compatibility. For example, Simon a n d his associates d i v i d e d S-R compatibility effects i n t o spatial, symbolic, a n d "Simon-effect" compatibilities (Simon e t al., 1981). Spatial compatibility, they said, results from a correspondence between t h e spatial arrangement o f p a i r e d stimulus a n d response alphabets. T h e f i n d i n g s o f e a r l y studies t h a t employed motorp a t t e r n e d responses t o l i g h t - p a t t e r n e d stimuli a r e examples o f t h i s kind o f correspondence (e.g., Deininger & Fitts, 1955; F i t t s & Deininger, 1954; F i t t s & Seeger, 1953). Similarly, Simon e t al. (1981) said t h a t symbolic compatibility r e s u l t s f r o m a nonspatial (or conceptual?) correspondence between stimulus a n d response alphabets. Examples o f t h i s kind o f correspondence can b e f o u n d i n t h e e a r l y studies o f vocal responses t o numerical stimuli (see A l l u i s i F, Martin, 1957, 1958; A l l u i s i & Muller, 1956, 1958; A l l u i s i e t a l . , 1955, 1957).

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The "Simon-effect'' t y p e of compatibility results from conjoint correspondence among t h e responses and t h e relevant and irrelevant characteristics of :he stimuli. Generally, i n t h e experiments t h a t show t h e "Simon effect, both the responses and t h e irrelevant stimulus cues are spatial, whereas t h e relevant stimulus cues are symbolic o r nonspatial. For e.xample, a s t u d y might require a l e f t - o r right-handed response depending on t h e relevant color of a visual stimulus t h a t may appear (irrelevantly) on the l e f t o r r i g h t of t h e display. The findings of such studies typically show t h a t RT is faster when t h e spatial (irrelevant) and color (relevant) cues o r stimulus characteristics both signal t h e same response--a response t h a t corresponds spatially t o t h e irrelevant cue (see C r a f t & Simon, 1970; Simon, 1969, Simon & Craft, Such 1970; Simon, Hinrichs, & Craft, 1970; Simon & Rudell, 1967). demonstrations of t h e "Simon effect" have been interpreted "as reflecting a stereotypic tendency t o respond toward the source of simulation (Simon, 1969)" (Simon e t al., 1981, p. 64). T h e y can also be interpreted i n terms of expectancy (Whitaker, 1980, 1982) o r interference among conjoint o r conflicting conceptual correspondences in t h e p a i r i n g of stimulus and response alphabets t o form the S-R ensemble, o r of the cognitive coding o r recoding of the information t o produce t h e correct response (Brebner, 1979; Brebner, There i s other Shephard, & Cairney, 1972; Hedge & Marsh, 1975). evidence f o r t h i s view. For example, Nicoletti and his associates presented data f o r conceptual correspondence as an explanation of spatial Compatibility. Specifically, t h e y a t t r i b u t e d spatial compatibility "to t h e correspondence, o r lack of it, of t h e locational codes associated w i t h stimuli and responses" (Nicoletti & Umilti, 1984, p. 341; Nicoletti e t al., 1982). T h e y related t h e i r s t u d y t o t h e Teichner and Krebs (1974) view t h a t S-R compatibility effects represent t h e proportion of choice RT "attributable t o stimulus-response translation time" (p. 91), and concluded, "it is not s u r p r i s i n g t h a t even i n t h e adult S-R pairings on t h e same side of t h e body midline are predominant over S-R pairings t h a t cross t h e b o d y midline" (Nicoletti & UmiltB, p. 342). Spatial compatibility effects, especially those related to lateral correspondence, are apparently q u i t e robust. They occur with children as well as adults (Alluisi, 1965; Lidavas, 1983). and w i t h aging adults They occur even when t h e subject's (Simon & Pouraghabagher, 1978). head is rotated and t h e stimulus and response alphabet are arranged perpendicularly (Lidavas & Moscovitch, 1984). They occur w i t h f i n g e r responses, whether t h e hands are held palms up o r down (Heister, Ehrenstein, & Schroeder-Heister, 1986, 1987). They are similar in t h i s regard t o t h e inclusive general class of S-R compatibility effects, as Indeed, S - R demonstrated i n t h e earlier sections of t h i s chapter. compatibility effects may even be found with non-human animals, if Michaels (1988) 'is correct i n t h e i r being describable by Gibson's theory of affordances . Summary Things that go together--S-R ensembles based on stereotyped, corresponding, and S-R compatible alphabets--contribute t o optimum codes and human performances. They are, o r should be, central not only t o t h e repertoire of expertise from which t h e human-factors specialist cont r i b u t e s t o t h e design and use of human-machine systems, b u t also t o the

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i n t e r e s t s o f c o g n i t i v e psychologists who aim t o e x p a n d o u r u n d e r s t a n d i n g o f human c o g n i t i v e behavior. When a set o f discriminable stimuli has been a r r a n g e d i n t o an alphabet, a n d t h e elements o f t h i s stimulus alphabet p a i r e d w i t h those o f a response alphabet, a code o r S - R ensemble w i l l have been formed. T h e ease a n d e x t e n t t o w h i c h t h e ensemble can b e learned a n d used consistently, c o r r e c t l y , a n d r a p i d l y is a f u n c t i o n o f t h e specific p a i r i n g s o f t h e stimuli a n d responses employed. T h e o b s e r v e d variations in human performances t h a t a r e based on t h e specific p a i r i n g s o f elements o f stimulus a n d response alphabets a r e known as S-R compatibility effects ( a f t e r F i t t s & Seeger, 1953, - b a s e d o n a 1951 suggestion o f A r n o l d Small's). It has been l o n g established t h a t S-R compatibility effects a r e r e l a t i v e l y l a r g e in magnitude. They are consistent, a n d independent o f t h e performance measure employed. They a r e r e l a t i v e l y stable but may b e accentuated u n d e r conditions o f workload stress. T h e y approach a maximum when t h e p a i r i n g s o f stimulus a n d response alphabets c o r r e s p o n d t o one another in a d i r e c t conceptual sense. In t h e applicable cases, such as those t h a t employ motor responses t o stimuli w i t h d i f f e r e n t spatial locations, it i s l i k e l y t h a t t h i s w i l l o c c u r when t h e alphabets correspond t o one another in a d i r e c t physical sense. And, in selecting modalities, it i s p r u d e n t t o match t h e stimulus a n d response alphabets t o t h e central-processing, s h o r t - t e r m ( w o r k i n g ) memory requirements o f t h e task, as well.

S - R compatibility effects also t e n d t o b e maximized when p a i r i n g s o f t h e elements in t h e stimulus a n d response alphabets consistent w i t h a n y s t r o n g i n d i v i d u a l o r population stereotypes t h a t found. Examples o f recognized population stereotypes r e l e v a n t t o design a n d use o f human-machine systems a r e l i s t e d by Woodson Conover (1970, p. 1.30), a n d Wickens (1987, pp. 89-90).

the are are the and

If s t r o n g i n d i v i d u a l o r population stereotypes exist, a n d if t h e y a r e violated in t h e c o n s t r u c t i o n o f t h e S-R ensemble, p o o r e r performances w i l l result. Thus, t h e human-factors specialist should follow t h e advice of V a n C o t t a n d Kinkade (1972) t o use population stereotypes in d e s i g n i n g c o n t r o l a n d d i s p l a y panels a n d t o v e r i f y w i t h operational personnel t h a t such stereotypes have n o t been violated, but r a t h e r have been incorporated in t h e system's design. T h e r e is an i n t e r a c t i v e relation between S-R compatibility effects a n d t h e r a t e o f gain o f information. Specifically, w i t h i n t h e r a n g e o f an amount o f information t r a n s m i t t e d ( H t ) between 1 a n d 3 o r 4 b i t s p e r S-R event, (a) t h e relation between d i s j u n c t i v e R T a n d H t is linear, (b) t h e r e is an i n v e r s e relation between t h e slope of t h e l i n e a n d t h e degree o f S - R compatibility, a n d (c) t h e slope approaches zero as S-R compatibility approaches a maximum. Of the possible mechanisms of S-R compatibility effects, correspondence a n d s t e r e o t y p y were t h e earliest o b s e r v e d a n d identified. T h e y a r e well-established as mechanisms. In recent years, many o t h e r possibilities have been investigated. A l t h o u g h none o f t h e o t h e r newer mechanisms postulated has been universally accepted, each has i t s adherents a n d s u p p o r t i n g data.

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For example, a major extension o f t h e phenomenon known as S - R compatibility i s called t h e "Simon effect." T h i s is a compatibility e f f e c t t h a t r e s u l t s f r o m t h e conjoint correspondence o f t h e elements o f t h e response alphabet with those of both relevant and irrelevant characteristics o f t h e S/R alphabet elements. T h e Simon e f f e c t has o c c u r r e d most f r e q u e n t l y ( b u t n o t exclusively) w i t h a spatial response (e.g., l e f t o r right k e y press), a p a i r e d non-spatial relevant stimulus (e.g., r e d o r green light signaling l e f t o r right response, respectively), a n d an associated, but i r r e l e v a n t spatial stimulus characteristic (e.g., t h e r e d light sometimes on t h e left, sometimes o n t h e right). In such cases, R T for t h e l e f t keypress t o t h e r e d - l i g h t - o n - t h e - l e f t i s f a s t e r t h a n f o r t h e same keypress t o t h e r e d - l i g h t - o n - t h e - r i g h t , a n d so f o r t h . U n t i l these o r o t h e r mechanisms are so well established as t o b e u n i v e r s a l l y accepted ( o r n e a r l y so), t h e human-factors specialist should a t t e n d t o t h e mechanisms t h a t a r e known t o e x i s t . A t present, these are t h e empirically d e r i v a b l e stereotypes a n d correspondences, optimization o f which also tends t o optimize t h e beneficial effects o f S - R compatibility in human performances. Thus, in p a i r i n g stimulus and response alphabets t o f o r m S - R ensembles, t h e human-factors specialist should a t t e n d t o t h e stereotypes a n d correspondences, even t o t h e seemingly " i r r e l e v a n t " characteristics o f t h e stimulus a n d response elements a n d pairings, t h a t c o g n i t i v e psycholo g y has demonstrated t o b e important determiners o f human cognitive behavior. O n l y by doing so in t h e design o f t h e codes, displays and controls, human-machine interfaces, a n d w o r k stations t h a t a r e p a r t s o f any human-machine system, w i l l t h e human-performance aspects o f t h e system's performance t e n d t o b e optimized. And, o n l y by such use and t h e r e s u l t i n g optimization w i l l c o g n i t i v e psychology's u n d e r s t a n d i n g and explanation o f human performance b e validated. References T o w a r d optimizing man's tactile communication. Alluisi, E. A . (1961). Perceptual and Motor Skills, 12, 235-245. (1963). Frequencies, preferences, and choice reactions t o Alluisi, E. A . l e t t e r s . Perceptual and Motor Skills, 16, 109-110. Alluisi, E. A . (1965). g a i n of information.

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STIMULUS-RESPONSECOMPATIBILITY R. W. Proctor and T.G. Reeve (Editors 0 Elsevier Science Publishers B. V. (Aorth-Holland). 1990

31

T H E EFFECTS OF A N IRRELEVANT DIRECTIONAL CUE ON HUMAN INFORMATION PROCESSING

J . RICHARD SIMON Department o f Psychology Department o f I n d u s t r i a l a n d Management Engineering U n i v e r s i t y of Iowa

A fundamental concern in c o g n i t i v e psychology a n d in human factors engineering is t o u n d e r s t a n d t h e f a c t o r s t h a t a f f e c t t h e speed o f t r a n s l a t i n g information f r o m a d i s p l a y i n t o an a p p r o p r i a t e c o n t r o l action. T h e t e r m S-R compatibility ( F i t t s & Seeger, 1953) i s commonly used t o describe t h e e x t e n t t o which t h e ensemble o f stimulus a n d response combinations comprising a t a s k results in a high r a t e o f information t r a n s f e r . Many t y p e s o f compatibility have been identified, e.g., symbolic, temporal, 'and spatial compatibility, t o name a few. Symbolic compatibility results f r o m a correspondence between stimulus a n d response codes t h a t simplifies t h e translation process; f o r example, translation i s simplified when a r e d l i g h t signals a r e d - k e y response a n d a green light signals a g r e e n - k e y response r a t h e r than v i c e versa (e.g., LeMay & Simon, 1969; Simon & Sudalaimuthu, 1979). Temporal compatibility effects may occur when processing is affected by t h e o r d e r in which b i t s o f information a r e presented t o t h e operator (LeMay & Simon, 1969). Spatial compatibility is, perhaps, t h e most common a n d results f r o m a correspondence between t h e spatial arrangements o f displays a n d controls; f o r example, compatibility is high when l e f t a n d right l i g h t s a r e associated, respectively, w i t h l e f t a n d right keys ( F i t t s & Seeger, 1953; G a r v e y & Mitnick, 1955; Simon E Craft, 1970b; Simon & Wolf, 1963). T h i s c h a p t e r deals w i t h another t y p e o f compatibility, which m i g h t b e classified as a v a r i e t y o f spatial compatibility. We w i l l describe a series o f related experiments t h a t demonstrate t h a t t h e location o f a stimulus p r o v i d e s a n i r r e l e v a n t directional cue t h a t affects t h e time r e q u i r e d t o process t h e meaning of t h e stimulus. In o t h e r words, t h e r e seems t o b e a s t r o n g stereotypic tendency t o respond i n i t i a l l y t o t h e directional component o f a stimulus r a t h e r than t o i t s symbolic content. Hedge a n d Marsh (1975) used t h e label "Simon effect" t o r e f e r t o t h i s phenomenon. We, too, w i l l use t h i s label as a convenient s h o r t h a n d designation for t h e stereotype. A n Accidental D i s c o v e r y O u r research on t h e Simon effect began q u i t e by accident (Simon & Rudell, 1967). We were o r i g i n a l l y interested in t h e phenomenon o f hemispheric dominance f o r speech a n d designed an experiment t o i n v e s t i gate t h e possible interaction between ear stimulated a n d handedness in an a u d i t o r y reaction time ( R T ) t a s k . Without g o i n g i n t o a detailed rationale, suffice it t o say t h a t we wanted t o t r y t o demonstrate t h a t r i g h t - h a n d e d subjects would respond f a s t e r t o a v e r b a l command in t h e i r right ear t h a n in t h e i r l e f t ear and t h a t left-handed subjects would respond f a s t e r t o a command in t h e i r l e f t ear than i n t h e i r right e a r .

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We selected t w o g r o u p s o f subjects. One g r o u p consisted o f 48 s t r o n g l y r i g h t - h a n d e d students, a n d t h e o t h e r o f 32 s t r o n g l y left-handed students. T h e subjects wore earphones a n d sat a t a table w i t h t h e i r right a n d l e f t i n d e x f i n g e r s each r e s t i n g on a telegraph k e y . T h e y were i n s t r u c t e d t o press t h e right-hand k e y as q u i c k l y as possible a f t e r hearing t h e w o r d right" a n d t o press t h e l e f t - h a n d k e y as q u i c k l y as possib l e a f t e r h e a r i n g t h e w o r d "left." A timer s t a r t e d when a command was presented a n d stopped when t h e subject pressed t h e k e y . Each subject responded t o t h e same series o f 132 tape-recorded,, commands, each comT h e commands mand consisting of t h e w o r d "right" or t h e w o r d l e f t . were presented t o e i t h e r t h e right ear o r t h e l e f t ear in a predetermined random sequence, so t h a t t h e subject h a d n o way o f knowing p r i o r t o t h e presentation o f each command which ear would b e stimulated o r what t h e command would be. A ready signal was presented 2 s p r i o r t o each command, a n d t h e r e was a 6-s i n t e r v a l between commands. T h e median R T was calculated f o r each subject f o r each o f t h e f o u r experimental conditions (i.e., "right" a n d " l e f t " commands t o t h e right a n d l e f t e a r ) . These data were subjected t o a n analysis o f variance. We were disappointed t o find that t h e p r e d i c t e d interaction between ear stimulated a n d handedness was n o t significant. T h e r e was, however, another e f f e c t - - t o t a l l y unexpected--but, as it t u r n e d out, much more i n t e r e s t i n g t h a n t h e e f f e c t t h a t was o u r o r i g i n a l concern. T h e l e f t half o f F i g u r e 1 p i c t u r e s t h i s effect, a s i g n i f i c a n t Command Ear Stimulated interaction. Note t h a t R T was markedly f a s t e r when t h e right" command was h e a r d in t h e right ear t h a n when it was h e a r d in t h e l e f t ear, and, similarly, R T t o t h e " l e f t " command was f a s t e r when it was h e a r d in t h e l e f t ear than when it was h e a r d in t h e right ear. Obviously, a n i r r e l e v a n t cue, t h e ear in which t h e subject h e a r d t h e command, was a f f e c t i n g t h e time r e q u i r e d t o process t h e symbolic content o f t h e command. A t t h i s point, we were s t i l l interested in trying t o demonstrate t h e hypothesized interaction between ear stimulated a n d handedness. We conjectured that, if t h e subject's u n c e r t a i n t y as t o t h e source o f t h e command were removed, t h e n t h e variance c o n t r i b u t e d by t h i s i r r e l e v a n t cue w o u l d b e eliminated, a n d we m i g h t b e able t o detect a n Ear Stimulated x Handedness interaction. We, therefore, conducted another experiment t h a t was essentially t h e same as t h e f i r s t except that, t h i s time, t r i a l s were presented i n blocks r a t h e r t h a n in t h e random-ear o r d e r employed p r e v i o u s l y . R i g h t - a n d left-handed subjects performed on t w o blocks o f t r i a l s . In one block, t h e commands were presented t o t h e right ear, and, in t h e o t h e r block, t h e commands were presented t o t h e l e f t ear. i n s t r u c t i o n s p r i o r t o each t r i a l block stressed t h e f a c t that t h e commands w o u l d , b e h e a r d in t h e right ear o n l y o r in t h e l e f t ear only, as t h e case may be. Again, t h e analysis p r o v i d e d no evidence f o r t h e p r e d i c t e d Ear Stimulated x Handedness interaction. B u t again, as in t h e f i r s t e x p e r i ment, t h e major source o f variance was t h e Command x Ear Stimulated Results o f these t w o interaction p i c t u r e d in t h e right h a l f o f F i g u r e 1. studies, then, clearly indicated t h a t t h e speed o f processing v e r b a l commands was affected by a cue i r r e l e v a n t t o t h e t a s k itself, t h a t is, t h e ear i n which t h e command was heard. R T was s i g n i f i c a n t l y f a s t e r when t h e

Effects of an Irrelevant Directional Cue

33

content of t h e command corresponded t o t h e ear stimulated t h a n when it did not.

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An Isomorphic Association Between Ear Stimulated a n d lpsilateral Hand? A t t h i s point, t h e focus o f o u r research shifted. We decided that, r a t h e r t h a n p u r s u i n g o u r o r i g i n a l hypothesis, we would t r y t o determine t h e reason f o r t h e unexpected Command x Ear Stimulated interaction. O u r f i r s t t h o u g h t was t h a t t h e r e m i g h t b e a s t r o n g tendency t o associate r i g h t - e a r stimulation w i t h a r i g h t - h a n d response a n d left-ear stimulation w i t h a l e f t - h a n d response. If t h i s were t r u e , we reasoned, t h e n t h e interaction should not occur on a unimanual t a s k . If t h e interaction did occur on a unimanual task, t h i s would indicate t h a t it did n o t d e r i v e f r o m a simple isomorphic association between ear stimulated a n d ipsilateral hand.

In o u r n e x t experiment (Simon, 1968a), r i g h t - h a n d e d subjects moved a c o n t r o l handle t o t h e right o r l e f t f r o m a center position in response t o t h e same recorded series o f v e r b a l commands used in t h e f i r s t t w o experiments. Half o f t h e subjects performed w i t h t h e i r right hand, while t h e o t h e r half performed w i t h t h e i r l e f t hand. A timer, which measured RT, s t a r t e d when a command was presented a n d stopped when t h e subject moved t h e handle away f r o m t h e c e n t e r position. A n o t h e r timer, which measured movement time, s t a r t e d when t h e handle

34

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was moved f r o m t h e c e n t e r position a n d stopped when t h e 10 in. lateral movement was completed. Subjects were i n s t r u c t e d t o react a n d move as f a s t as t h e y c o u l d on each t r i a l . T h e R T data revealed t h e same response i n t e r f e r e n c e o b s e r v e d in p r e v i o u s experiments, demonstrating t h a t t h e e f f e c t was n o t d u e t o a n y simple isomorphic association between ear stimulated a n d ipsilateral hand. I n t e r e s t i n g l y , t h e same Command x Ear Stimulated interaction also o c c u r r e d f o r movement time. T h a t is, movements t o t h e right were f a s t e r when t h e "right" command was h e a r d i n t h e right ear t h a n when it was h e a r d in t h e l e f t ear, and, similarly, movements t o t h e l e f t were f a s t e r when t h e " l e f t " command was h e a r d i n t h e l e f t ear t h a n when it was h e a r d in t h e right e a r . These effects on movement time indicate t h a t d i s p l a y c h a r a c t e r i s t i c s can a f f e c t not o n l y R T but also t h e r a t e o f a s u b sequent b a l l i s t i c movement. T e n d e n c y to React T o w a r d t h e Source of Stimulation

It appeared a t t h i s p o i n t t h a t t h e i r r e l e v a n t c u e a f f e c t i n g R T m i g h t b e a n a t u r a l t e n d e n c y t o react t o w a r d t h e source o f stimulation. T o test t h i s notion, we designed a v e r y simple experiment (Simon, 1969). Subjects were i n s t r u c t e d t o move a c o n t r o l handle t o e i t h e r t h e right o r t h e l e f t f r o m i t s c e n t e r position, depending upon t h e ear in w h i c h t h e y h e a r d a tone. Subjects were told, " T h i s is a t e s t t o see how q u i c k l y y o u can react a n d move in response t o a tone t h a t y o u w i l l hear in e i t h e r y o u r right ear o r in y o u r l e f t e a r . " In one block o f t r i a l s , t h e y were i n s t r u c t e d to "...move t h e c o n t r o l handle m a y f r o m t h e side of t h e ear stimulated. In o t h e r words, when you hear t h e tone i n y o u r l e f t ear, move t h e c o n t r o l handle t o t h e right as q u i c k l y as possible, a n d when y o u hear t h e t o n e in y:ur right ear, move t h e c o n t r o l handle t o t h e l e f t In another b l o c k o f t r i a l s , subjects h e a r d t h e as quickly as possible. same sequence o f stimulus tones, but, t h i s time, t h e y were i n s t r u c t e d t o move t h e c o n t r o l handle toward t h e side o f t h e ear stimulated. Subjects reacted s i g n i f i c a n t l y f a s t e r when i n s t r u c t e d t o move t h e c o n t r o l handle toward instead o f away f r o m t h e side o f t h e ear stimulated (292 v s . 351 ms). These r e s u l t s can b e explained by p o s t u l a t i n g a n a t u r a l t e n d e n c y t o react t o w a r d t h e source o f stimulation. T h e necessity f o r o v e r r i d i n g t h i s s t e r e o t y p e b e f o r e r e s p o n d i n g t o t h e tones presented i n t h e "away" b l o c k o f t r i a l s would account f o r t h e slower information processing during t h a t block. Facilitation and/or Interference? L e t me now b a c k t r a c k t o o u r studies i n v o l v i n g R T t o v e r b a l d i r e c tional commands. O u r r e s u l t s t h e r e suggested t h a t t h e a u d i t o r y d i s p l a y p r o v i d e d t w o cues, one r e l e v a n t ( i . e . , t h e c o n t e n t o f t h e command) a n d t h e o t h e r i r r e l e v a n t (i.e., t h e ear stimulated), a n d t h a t t h e time r e q u i r e d t o process t h e f o r m e r c u e was somehow affected by t h e presence o f t h e l a t t e r cue. A t t h i s point, we f e l t t h a t t h e Command x Ear Stimulated i n t e r a c t i o n was d u e t o t h e i r r e l e v a n t c u e interfering w i t h information p r o cessing on t r i a l s in w h i c h it did n o t correspond w i t h t h e symbolic c o n t e n t o f t h e command. It was possible, however, t h a t t h e interaction was d u e t o t h e i r r e l e v a n t cue facilitating information processing on t r i a l s in w h i c h it corresponded w i t h t h e symbolic c o n t e n t of t h e command. It was also possible t h a t t h e i r r e l e v a n t c u e operated both t o f a c i l i t a t e r e s p o n d i n g o n t h e c o r r e s p o n d i n g t r i a l s a n d t o i n t e r f e r e w i t h r e s p o n d i n g on t h e noncorresponding trials.

Effects of an Irrelevant Directional Cue

35

O u r n e x t experiment (Simon & Small, 1969) was designed t o d e t e r mine how t h e Command x Ear Stimulated interaction was produced. It was also designed t o determine t h e g e n e r a l i t y o f t h e interaction; t h a t is, was t h e phenomenon limited t o situations i n v o l v i n g v e r b a l commands, o r did it also occur when simple stimuli such as p u r e tones were used t o p r o v i d e t h e relevant directional information? O u r subjects’ task, t h i s time, was t o press t h e c o r r e c t one o f t w o f i n g e r keys as q u i c k l y as possible a f t e r h e a r i n g e i t h e r a h i g h - p i t c h e d tone (loo0 Hz) o r a low-pitched tone (400 Hz) presented t o one ear o r the other. T h e subjects did n o t know p r i o r t o h e a r i n g t h e tone which ear would b e stimulated o r what t h e tone would be. Half o f t h e subjects were i n s t r u c t e d t o press t h e right k e y when t h e y h e a r d t h e h i g h - p i t c h e d tone a n d t o press t h e l e f t k e y when t h e y h e a r d t h e low-pitched tone. T h e o t h e r h a l f o f t h e subjects were g i v e n t h e opposite t o n e - k e y r u l e . In addition t o t h i s monaural block, each subject also performed o n a block o f t r i a l s i n v o l v i n g b i n a u r a l stimulation. T h e o u t p u t SPL of t h e b i n a u r a l t r i a l s was reduced by 6 dB, so as t o y i e l d approximately t h e same loudness as o n t h e monaural t r i a l s . F i g u r e 2 shows R T t o t h e right a n d l e f t tonal commands presented t o t h e right ear, t h e l e f t ear, o r t o b o t h ears simultaneously. It can b e seen t h a t R T was markedly f a s t e r when t h e right command was h e a r d in

TONAL DIRECTIONAL COMMAND

F i g u r e 2. Reaction time t o tonal directional commands as a f u n c t i o n o f ear(s) stimulated.

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J.R. Simon

t h e right ear t h a n when it was h e a r d in t h e l e f t ear, and, similarly, R T t o t h e l e f t command was f a s t e r when it was h e a r d in t h e l e f t ear t h a n Clearly then, subjects responded s i g when it was h e a r d in t h e right. n i f i c a n t l y f a s t e r o n t r i a l s in which t h e symbolic content o f t h e command corresponded w i t h t h e ear stimulated t h a n o n t r i a l s in which it did not. T h i s r e s u l t demonstrated t h a t t h e Command x Ear Stimulated interaction, heretofore o n l y observed w i t h v e r b a l directional commands, also o c c u r r e d It when p u r e tones were used to signal t h e appropriate response. appears, then, t h a t t h e interaction reflects a basic and general phenomenon t h a t exists independently o f whether t h e command i s comIt also appears t h a t t h e interaction is municated v e r b a l l y o r nonverbally. unrelated t o p r i o r symbolic associations since, in contrast t o v e r b a l directional commands, t h e tones h a d no implicit directional significance. T h e major p u r p o s e o f t h i s experiment, however, was t o determine whether t h e Command x Ear Stimulated interaction was d u e t o a facilitation of information processing o n t h e corresponding t r i a l s o r t o an i n t e r ference w i t h information processing on t h e noncorresponding t r i a l s . On t h e monaural trials, t h e ear in which t h e tone was h e a r d p r o v i d e d an i r r e l e v a n t cue which, o f course, was absent on t h e b i n a u r a l t r i a l s . Therefore, b i n a u r a l R T was used as a baseline f o r evaluating t h e effect o f t h i s i r r e l e v a n t cue. Statistical tests revealed t h a t b i n a u r a l R T was s i g n i f i c a n t l y faster t h a n R T o n noncorresponding as well as corresponding monaural t r i a l s . On t h e noncorresponding monaural t r i a l s , t h e i r r e l e v a n t location c u e apparently conflicted w i t h processing t h e relevant symbolic On t h e corresponding monaural trials, t h e cue, r e s u l t i n g in slower R T . i r r e l e v a n t location cue coincided w i t h t h e relevant symbolic cue, b u t t h i s Results of t h i s correspondence did n o t facilitate information processing. experiment, then, suggested t h a t t h e Command x Ear Stimulated interact i o n was a r e s u l t o f i n t e r f e r e n c e w i t h information processing o n t h e noncorresponding t r i a l s r a t h e r t h a n a facilitation on t h e corresponding t r i a l s . (In l a t e r experiments, u s i n g a d i f f e r e n t experimental design, o u r results indicated t h a t t h e i r r e l e v a n t location c u e may p r o d u c e facilitation as well as i n t e r f e r e n c e [e.g., Acosta & Simon, 1976; Simon 8 Acosta, 1982; Simon & Craft, 1970a; Simon & Pouraghabagher, 19781). T h e Simon E f f e c t w i t h Visual Displays O u r n e x t experiment was concerned w i t h determining whether t h e same phenomena t h a t h a d been observed w i t h a u d i t o r y displays would also A stereoscope was occur w i t h visual displays ( C r a f t & Simon, 1970). modified by removing t h e lenses and i n s e r t i n g an opaque p a r t i t i o n t o d i v i d e t h e visual f i e l d i n t o t w o halves (see F i g u r e 3). Red a n d green stimulus l i g h t s were mounted in t h e center of each h a l f of t h e f i e l d . Subjects operated f i n g e r keys w i t h t h e i r right o r l e f t index fingers. Half of, t h e subjects were told, "If y o u see a r e d light, p u s h t h e r i g h t - h a n d key, a n d if y o u see a green light, p u s h t h e l e f t - h a n d key." T h e o t h e r half o f t h e subjects were g i v e n t h e opposite l i g h t - k e y associations. Subjects performed on t w o blocks o f t r i a l s , a monocular In t h e monocular block, a r e d light o r a block a n d a binocular block. green light was presented t o e i t h e r t h e right eye o r t h e l e f t eye in a

Effects of an Irrelevant Directional Cue

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F i g u r e 3. Modified viewer i n d i c a t i n g location o f stimulus l i g h t s when v i s u a l f i e l d is p a r t i t i o n e d a n d lenses a r e removed. random sequence. I n t h e b i n o c u l a r block, t h e r e d l i g h t s o r t h e g r e e n l i g h t s w e r e p r e s e n t e d t o b o t h eyes simultaneously. F i g u r e 4 shows t h e r e s u l t s o f t h i s experiment. Reactions t o t h e right command were f a s t e r when it was p r e s e n t e d t o t h e right e y e t h a n when it was presented t o t h e l e f t eye, a n d reactions t o t h e l e f t command were f a s t e r when it was p r e s e n t e d t o t h e l e f t eye t h a n when it was p r e sented t o t h e right eye. T h i s Command x Eye Stimulated i n t e r a c t i o n i s a d i r e c t parallel t o t h e Command x Ear Stimulated interaction o b s e r v e d in p r e v i o u s studies. Note, too, t h a t reactions on t h e b i n o c u l a r t r i a l s were f a s t e r t h a n on e i t h e r t h e c o r r e s p o n d i n g o r noncorresponding monocular t r i a l s , s u g g e s t i n g t h a t t h e i n t e r a c t i o n o b s e r v e d on t h e monocular t r i a l s was d u e t o i n t e r f e r e n c e o n noncorresponding t r i a l s r a t h e r t h a n t o f a c i l i tation on corresponding trials. Perceptual R a t h e r t h a n Sensory Explanation for t h e Simon E f f e c t B u t w h a t is t h e source o f t h i s interference? It c o u l d b e t h e eye stimulated p e r se o r it c o u l d b e an i r r e l e v a n t cue, such as t h e spatial locations of t h e stimulus lights, associated with t h e eye stimulated. Our

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n e x t experiment was designed t o isolate t h e c r i t i c a l f a c t o r . T h e proced u r e i n v o l v e d m o d i f y i n g t h e d i s p l a y so as t o eliminate t h e directional cue p r o v i d e d by t h e locus o f t h e stimulus. If t h e directional c u e was responsible f o r t h e interference, t h e n t h e Command x Eye Stimulated interaction should b e eliminated. If, on t h e o t h e r hand, t h e eye stimulated per se was t h e source o f t h e interference, t h e n t h e i n t e r a c t i o n should s t i l l occur. T h e d i s p l a y was t h e same as t h a t used in t h e p r e v i o u s e x p e r i ment except t h a t t h e opaque p a r t i t i o n dividing t h e v i s u a l f i e l d s was removed, a n d t h e lenses were reinstalled. Now, instead o f seeing a r e d o r g r e e n light in e i t h e r t h e l e f t o r right v i s u a l field, t h e subject saw t h e light in t h e c e n t e r o f t h e v i s u a l f i e l d . J u s t as in t h e p r e v i o u s e x p e r i ment, a n y one o f t h e f o u r l i g h t s could b e presented, t h e r e b y stimulating t h e right o r l e f t eye separately. However, since n o spatial o r directional c u e was now associated w i t h t h i s stimulation, subjects c o u l d n o t t e l l w h i c h eye was b e i n g stimulated. Subjects responded t o t h e same random sequence o f monocular t r i a l s used p r e v i o u s l y ; t h a t is, e i t h e r a r e d l i g h t o r a g r e e n light was p r e sented t o e i t h e r t h e right eye o r t h e l e f t eye. Results indicated a comp l e t e absence o f t h e Command x Eye Stimulated interaction, demonstrating conclusively t h a t t h e i n t e r a c t i o n t h a t was so prominent in t h e p r e v i o u s

Effects of an Irrelevant Directional Cue

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experiment was due, n o t t o t h e eye stimulated per se, but t o t h e d i r e c tional c u e associated w i t h t h e eye stimulated. O u r n e x t experiment (Simon, Small, Ziglar, & Craft, 1970) was designed t o determine w h e t h e r t h e Command x Ear Stimulated i n t e r a c t i o n observed in o u r p r e v i o u s studies was d u e t o t h e ear stimulated per se o r t o a directional c u e associated w i t h ear stimulated. Obviously, these t w o factors h a d been confounded in o u r p r e v i o u s experiments. T h a t is, a signal t o t h e right ear appeared t o come f r o m t h e right side, a n d a signal t o t h e l e f t ear appeared t o come f r o m t h e l e f t side. T h e question was how t o unconfound sensory (ear stimulated) a n d perceptual (directional cue) f a c t o r s . Because we c o u l d think o f no w a y o f p r e s e n t i n g monaural stimulation w i t h o u t an associated directional cue, we decided t o eliminate ear stimulated as a f a c t o r . B y employing a p h a s e - s h i f t i n g device, it was possible t o stimulate b o t h ears simultaneously a n d y e t have it appear t o If t h e subjects as if t h e signal were coming f r o m one side o r t h e o t h e r . i n t r o d u c t i o n o f an i r r e l e v a n t directional cue on these b i n a u r a l t r i a l s p r o duced i n t e r f e r e n c e similar t o t h a t on t h e monaural t r i a l s , one m i g h t safely presume t h a t t h e directional cue was responsible f o r t h e i n t e r f e r e n c e on t h e monaural t r i a l s . Subjects performed on t w o blocks o f t r i a l s , one b l o c k i n v o l v i n g monaural stimulation a n d t h e o t h e r i n v o l v i n g b i n a u r a l On t h e monaural trials, subjects pressed a right- 0; l e f t stimulation. right" h a n d k e y in response t o e i t h e r a high- o r low-pitched tone (i.e., o r " l e f t " command) presented t o e i t h e r t h e i r right o r l e f t ear. On a b i n a u r a l b l o c k o f trials, subjects h e a r d t h e same random sequence o f tonal commands, but t h i s time t h e tone was p r e s e n t e d t o b o t h ears w i t h a p h a s e - s h i f t s e t t i n g o f e i t h e r 90" o r 270O. T h e l e f t half o f F i g u r e 5 p i c t u r e s t h e r e s u l t s f r o m t h e monaural block. T h e Command x Ear Stimulated interaction is, by now, a familiar one. T h e right h a l f o f F i g u r e 5 shows t h e r e s u l t s f r o m t h e b i n a u r a l block. With a 270" p h a s e - s h i f t setting, t h e tonal command appeared t o come f r o m t h e right, and, w i t h a 90' p h a s e - s h i f t setting, t h e tone appeared t o come f r o m t h e l e f t . Note t h a t t h e subjects responded f a s t e r t o t h e "right" command when it appeared t o come f r o m t h e right t h a n when it appeared t o come f r o m t h e l e f t a n d responded f a s t e r t o t h e " l e f t " command when it appeared t o come f r o m t h e l e f t t h a n when it appeared t o come f r o m t h e right. These r e s u l t s s t r o n g l y suggested t h a t t h e i n t e r a c t i o n on t h e monaural t r i a l s was n o t d u e t o ear stimulated per se, but t o t h e directional cue associated w i t h t h e ear stimulated. It should b e noted t h a t t h e interaction on t h e b i n a u r a l block was n o t as m a r k e d as o n t h e monaural block. Presumably, t h i s is because t h e i r r e l e v a n t directional cue p r o d u c e d by manipulating i n t e r a u r a l phase was n o t as p o t e n t as t h a t p r o d u c e d by monaural stimulation. Manipulating Potency o f t h e Directional C u e O u r n e x t experiments (Simon, C r a f t , & Small, 1971) used i n t e r a u r a l phase s h i f t as a means o f manipulating n o t o n l y t h e spatial locus o f a stimulus but also t h e potency o f t h e directional cue. If a directional c u e was, in fact, responsible f o r t h e i n t e r f e r e n c e w i t h information processing observed in p r e v i o u s studies, t h e n it should b e possible, by v a r y i n g t h e s t r e n g t h o f t h i s i r r e l e v a n t cue, t o a l t e r t h e amount o f interference. F i g u r e 6 shows that, w i t h a p h a s e - s h i f t s e t t i n g o f 90°, a b i n a u r a l tone appears t o come f r o m t h e l e f t . With a s e t t i n g o f 45", a b i n a u r a l tone

J.R. Simon

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-

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0

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I N T E R A U R A L PHASE DIFFERENCE

F i g u r e 5. Reaction time t o monaural a n d binaural tonal directional commands as a f u n c t i o n of t h e i r actual a n d apparent source. (On monaural trials, t h e tone was presented t o e i t h e r t h e right o r l e f t ear, while on b i n a u r a l t r i a l s t h e tone was presented t o b o t h ears w i t h t h e i n t e r a u r a l phase d i f f e r e n c e adjusted so t h a t i t s apparent source was a t e i t h e r t h e right o r l e f t ear.) s t i l l appears t o come f r o m t h e left, b u t t h e directional cue is, presumSimilarly, w i t h a phase-shift s e t t i n g of ably, n o t as s t r o n g as a t 90'. 270°, a b i n a u r a l tone appears t o come f r o m t h e right. A s e t t i n g o f 315' also produces a tone t h a t appears t o come f r o m t h e right but, presumably, t h e directional cue i s n o t as s t r o n g as a t 270'. T h e subjects' t a s k was t o press a right- o r a l e f t - h a n d k e y as q u i c k l y as possible a f t e r h e a r i n g a high- o r a low-pitched binaural tone. T h e tones were presented in a random sequence a t phase-shift settings o f e i t h e r 90°, 270°, 45O, o r 315', all w i t h i n a single block o f t r i a l s . It should b e emphasized that, a t each o f these phase-shift settings, t h e tones a t each ear were identical w i t h respect t o frequency, sound p r e s sure, a n d moment of onset. T h e o n l y difference was t h a t o f r e l a t i v e phase, a n d it was t h i s cue t h a t led t o t h e apparent position o f t h e sound o n t h e l e f t o r right side. We p r e d i c t e d t h a t a Command x A p p a r e n t Source interaction would o c c u r f o r t h e 270'-90' maximum deviation condit i o n a n d also f o r t h e 315'-45' reduced deviation condition b u t t h a t t h e size o f t h e interaction would n o t b e as marked in t h e l a t t e r case as in t h e former. I n o t h e r words, we p r e d i c t e d a t r i p l e - o r d e r interaction o f Command x A p p a r e n t Source x S t r e n g t h o f Directional Cue.

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INTERAURAL PHASE DIFFERENCE

F i g u r e 6. tings.

A p p a r e n t source o f tonal commands a t various p h a s e - s h i f t set-

T h e r e s u l t s f o r t h e maximum deviation condition ( t h e 270°-90' p h a s e - s h i f t settings) duplicated those f r o m t h e p r e v i o u s s t u d y . However, t h e reduced deviation condition ( t h e 315O-45O p h a s e - s h i f t settings) p r o d u c e d v i r t u a l l y identical r e s u l t s . In o t h e r words, t h e phase-shift settings t h a t we h a d selected f o r t h e reduced deviation condition did n o t p r o d u c e t h e p r e d i c t e d t r i p l e - o r d e r interaction. We recognized t h a t t h e relation between s t r e n g t h o f t h e directional c u e a n d i n t e r a u r a l phase d i f f e r e n c e may n o t b e linear, a n d so we decided t o t r y again, t h i s time f u r t h e r r e d u c i n g t h e i n t e r a u r a l phase difference, a n d therefore, presumably, also f u r t h e r r e d u c i n g t h e s t r e n g t h o f t h e i r r e l e v a n t directional cue. F o r t h i s experiment, p h a s e - s h i f t s e t t i n g s of 345O a n d 15" were selected f o r t h e reduced deviation condition (see F i g u r e 61, a n d these were again contrasted w i t h t h e 270°-90° maximum deviation condition. T h e l e f t h a l f o f F i g u r e 7 shows t h e data f r o m t h e maximum deviat i o n condition. Note t h e exact replication o f p r e v i o u s f i n d i n g s . The right half of F i g u r e 7 shows a similar s i g n i f i c a n t i n t e r a c t i o n f o r t h e reduced deviation condition, but t h i s time, j u s t as predicted, t h e i n t e r action was s i g n i f i c a n t l y smaller t h a n u n d e r t h e maximum deviation condit i o n . Thus, t h e r e s u l t s demonstrated t h a t we c o u l d a l t e r t h e size o f t h e Command x A p p a r e n t Source i n t e r a c t i o n by manipulating t h e s t r e n g t h of t h e i r r e l e v a n t directional cue. T h i s finding p r o v i d e s additional evidence

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t h a t t h e i r r e l e v a n t cue o p e r a t i n g in t h i s a n d in p r e v i o u s studies was, in fact, a directional cue. T h e s t r o n g e r t h a t directional cue, t h e s t r o n g e r was t h e tendency t o react t o w a r d t h e source of stimulation. D E V I A T I O N OF A P P A R E N T

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F i g u r e 7. T h e p r e d i c t e d t r i p l e - o r d e r interaction between command, apparent source, a n d deviation o f apparent source f r o m t h e median plane. Reaction T o w a r d t h e Major Source of Stimulation T h e n e x t question we asked was, is t h i s stereotype a specific tendency t o react t o w a r d t h e relevant stimulus ( t h e command) o r i s it a more general tendency t o react t o w a r d t h e major source o f stimulation? I f it is a tendency t o react t o w a r d t h e major source o f stimulation, it should b e possible t o eliminate t h e Command x Ear Stimulated interaction by p r e s e n t i n g an i r r e l e v a n t stimulus t o t h e opposite ear simultaneously w i t h t h e relevant tonal command. It should also b e possible t o a l t e r t h e size o f t h e interaction by manipulating t h e d B level o f t h e i r r e l e v a n t stimulus t o t h e opposite ear. It m i g h t even b e possible t o reverse t h e d i r e c t i o n o f t h e interaction by increasing t h e d B level o f t h e ' i r r e l e v a n t stimulus t o exceed t h a t o f t h e relevant tonal command. To investigate some o f these notions, we conducted t h r e e e x p e r i ments similar t o those r e p o r t e d p r e v i o u s l y (Simon, Craft, & Small, 1970). In each experiment, subjects were t o l d t o press a right- o r a l e f t - h a n d k e y in response t o high- o r low-pitched tonal commands. In t h e f i r s t experiment, h a l f o f t h e t r i a l s in a block were exactly t h e same as those in p r e v i o u s experiments; that is, a tonal command was presented t o one ear o r t h e other, a n d t h e r e was no stimulus p r o v i d e d t o t h e opposite ear. On t h e o t h e r h a l f o f t h e t r i a l s , an 89 d B broad-band noise was presented

Effects of an Irrelevant Directional Cue

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t o t h e opposite ear simultaneously w i t h t h e i n t r o d u c t i o n o f t h e mand. U n d e r t h e no-noise condition, we obtained t h e same observed so many times before. However, when t h e noise was t o t h e opposite ear, t h e Command x Ear Stimulated interaction nated.

tonal cominteraction introduced was elimi-

T h e n e x t experiment was designed t o determine whether we could a l t e r t h e size o f t h e Command x Ear Stimulated interaction by manipulating t h e i n t e n s i t y o f t h e stimulation t o t h e opposite ear. We reasoned t h a t if an 89 d B noise eliminated t h e interaction, t h e n a noise o f lesser i n t e n s i t y m i g h t reduce b u t n o t eliminate t h e interaction, a n d a noise o f g r e a t e r i n t e n s i t y m i g h t actually reverse t h e d i r e c t i o n o f t h e interaction. F i g u r e 8 shows t h e results o f t h i s experiment. U n d e r t h e no-noise With condition ( l e f t panel), we again observed t h e familiar interaction. 74 d B noise t o t h e opposite ear (center panel), t h e interaction was s t i l l significant b u t was s i g n i f i c a n t l y smaller t h a n u n d e r t h e no-noise condit i o n . W i t h t h e 106 d B noise t o t h e opposite ear (right panel), t h e i n t e r In o t h e r action was statistically significant in t h e opposite direction. words, now R T was faster t o t h e l e f t command when it was h e a r d in t h e right ear t h a n when it was h e a r d in t h e left, a n d R T t o t h e right command was faster when it was h e a r d in t h e l e f t ear t h a n when it was heard in t h e right ear.

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F i g u r e 8. Tonal Command x Ear Stimulated interactions. T h e interaction on t h e no-noise t r i a l s [ l e f t ] was e i t h e r s i g n i f i c a n t l y reduced [center] o r reversed [right], depending on t h e i n t e n s i t y o f noise presented t o t h e ear which did n o t receive t h e tonal command.

J.R. Simon

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Because we wanted t o establish t h a t the crucial factor i n these last two experiments was not t h e presence of the noise per se, but, rather, t h e locus of t h e noise, we performed an additional experiment. On a third of t h e trials, t h e commands were presented alone (i.e., no noise t o opposite ear); on a t h i r d of t h e trials, an 89 d B broad-band noise was presented t o t h e opposite ear simultaneously w i t h t h e command; and on t h e remaining third of t h e trials, t h e 89 d B noise was presented t o t h e sum eur as t h e tonal command. Figure 9 pictures t h e results. The data f o r t h e no-noise condition ( l e f t panel) duplicated previous findings. This time, however, t h e 89 d B noise t o t h e opposite ear (center panel) did more than eliminate t h e i n t e r (This represents t h e action; it actually produced a significant reversal. o n l y instance in t h e e n t i r e series of studies f o r which we failed t o replicate previous results.) Presentation of t h e noise t o t h e same ear as t h e command (right panel) accentuated t h e interaction, demonstrating t h a t t h e locus of t h e irrelevant noise was the crucial factor. In summary, t h e main results of these last t h r e e experiments can all be explained by postulating a strong natural tendency t o react toward t h e major source of stimulation. Changing t h e stimulus-intensity difference at t h e two ears altered t h e strength of an irrelevant directional cue and produced t h e observed shifts in t h e direction and magnitude of the Command x Ear Stimulated interaction.

89 dB Noise in Opposite Ear

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Left

--

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EAR STIMULATED BY TONAL COMMAND Figure 9. Tonal Command x Ear Stimulated interactions. The marked interaction on t h e no-noise trials [left] was either reversed [center] o r accentuated [right], depending on whether the noise was presented t o t h e opposite ear o r t o t h e same ear which received t h e tonal command.

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Mechanisms U n d e r l y i n g A u d i t o r y S-R Compatibility We conducted t w o additional experiments t o i d e n t i f y t h e f a c t o r s c o n t r i b u t i n g t o S-R compatibility in a u d i t o r y information-processing tasks (Simon, Hinrichs, & Craft, 1970). O u r e a r l i e r studies h a d shown t h a t reactions were s i g n i f i c a n t l y f a s t e r when t h e content o f t h e command c o r responded t o t h e ear stimulated t h a n when it did not. In almost all of these studies, however, subjects used t h e i r right h a n d to operate a right k e y a n d t h e i r l e f t hand t o operate a l e f t k e y . It i s n o t clear, therefore, whether t h e f a s t e r responding o n t h e corresponding t h a n on t h e noncorresponding t r i a l s ( t h e compatibility effect) was d u e t o ear/hand correspondence o r ear/response-location correspondence. I n o t h e r words, t h e r e m i g h t b e a tendency to associate r i g h t - e a r stimulation w i t h a righthand response a n d l e f t - e a r stimulation w i t h a l e f t - h a n d response; o r alternatively, r i g h t - e a r stimulation may t e n d t o e l i c i t a right- k e y response a n d left-ear stimulation may t e n d t o e l i c i t a l e f t - k e y response. The reader w i l l recall that, e a r l y i n o u r research program, we were concerned with ear/hand correspondence as a possible explanation f o r o u r f i n d i n g s a n d conducted a s t u d y t h a t demonstrated t h a t t h e compatibility e f f e c t s t i l l o c c u r r e d on a one-hand t a s k (Simon, 1968a). T h i s finding, however, did n o t eliminate t h e p o s s i b i l i t y t h a t ear/hand correspondence m i g h t c o n t r i b u t e t o t h e effect in a two-hand t a s k . We, therefore, designed an experiment t o unconfound these t w o possible sources o f a u d i t o r y S-R compati biIity

.

T h e experiment was similar in many respects t o those r e p o r t e d earlier. Subjects performed on t w o blocks o f trials, responding on one b l o c k w i t h t h e i r arms uncrossed a n d on t h e o t h e r block w i t h t h e i r arms crossed. Thus, o n t h e uncrossed block, subjects operated t h e right k e y with their right-index finger and the left key with their left-index finger, while, on t h e crossed block, t h e y operated t h e right k e y w i t h t h e i r l e f t - i n d e x f i n g e r a n d t h e l e f t k e y w i t h t h e i r r i g h t - i n d e x f i n g e r . On each trial, a high- o r low-pitched tone was presented t o one ear. Half o f t h e subjects were told, “If y o u hear t h e h i g h - p i t c h e d tone, press down t h e key w i t h y o u r l e f t - i n d e x f i n g e r as f a s t as y o u can. If y o u hear t h e low-pitched tone; press down t h e k e y w i t h y o u r r i g h t - i n d e x f i n g e r as fast as y o u can. T h e o t h e r h a l f o f t h e subjects were g i v e n t h e opposite tone-hand rule. A n analysis o f variance o f t h e uncrossed-arms t r i a l s revealed a s i g n i f i c a n t Command x Ear Stimulated interaction; t h a t is, R T was f a s t e r when t h e right-hand command was h e a r d in t h e right ear t h a n when it was h e a r d i n t h e l e f t ear (385 v s . 445 ms), a n d R T t o t h e l e f t - h a n d command was f a s t e r when it was h e a r d in t h e l e f t ear t h a n when it was T h e crossed-arms t r i a l s also heard in t h e right ear (386 vs. 457 ms). revealed a s i g n i f i c a n t interaction, b u t , t h i s time, t h e direction was reversed f r o m t h a t observed on t h e uncrossed trials; t h a t is, R T was f a s t e r when t h e r i g h t - h a n d command was h e a r d in t h e l e f t ear t h a n when it was h e a r d i n t h e right (405 v s . 465 ms), and R T t o t h e l e f t - h a n d command was f a s t e r when it was h e a r d i n t h e r i g h t ear t h a n when it was h e a r d in t h e l e f t (432 v s . 482 ms). T h e f a c t t h a t crossing t h e arms produced a reversal in t h e d i r e c t i o n o f t h e Command x Ear Stimulated interaction s t r o n g l y suggests t h a t ear/response-location correspondence r a t h e r t h a n ear/hand correspondence was t h e crucial f a c t o r accounting f o r t h e interaction. To clarify t h i s point, we combined a n d relabeled t h e means f o r t h e various treatment

46

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combinations t o r e f l e c t t h e presence o r absence o f ear/response-location correspondence a n d ear/hand correspondence. T h i s analysis demons t r a t e d a marked e f f e c t o f ear/response-location correspondence (402 v s . 462 ms), a n d a minimal e f f e c t o f ear/hand correspondence (430 v s . 435 ms). It appears, then, t h a t t h e monaural tonal command p r o v i d e s an i r r e l e v a n t directional cue that tends t o e l i c i t a response on t h e same side as t h e ear stimulated (Simon, 1969). T h i s potent response tendency occurs regardless o f w h e t h e r t h e response is executed by t h e hand i p s i lateral o r contralateral t o t h e ear stimulated.

In t h e above experiment, t h e a u d i t o r y display p r o v i d e d t w o cues: one relevant ( t h e meaning o f t h e tonal command) a n d t h e o t h e r i r r e l e v a n t (the ear stimulated). Results indicated t h a t t h e time r e q u i r e d t o process t h e relevant c u e was affected by t h e presence o f t h e i r r e l e v a n t cue. In o t h e r words, t h e S-R compatibility phenomenon hinged on a stimulus dimension t h a t was i r r e l e v a n t to t h e t a s k itself. B u t what a r e t h e mechanisms u n d e r l y i n g a u d i t o r y S-R compatibility in a simpler t a s k i n v o l v i n g j u s t one relevant cue, t h e ear in which a tone is presented? In an earlier experiment (Simon, 19671, we had subjects respond t o a tone presented t o e i t h e r t h e l e f t o r right ear by p r e s s i n g a l e f t k e y

w i t h t h e i r l e f t h a n d o r a right k e y w i t h t h e i r right hand. On one b l o c k of t r i a l s (corresponding trials), t h e y were i n s t r u c t e d t o press t h e l e f t hand k e y when t h e y h e a r d a tone i n t h e i r l e f t ear and t h e right-hand k e y when t h e y h e a r d a tone in t h e i r right ear. On another block (noncorresponding t r i a l s ) , i n s t r u c t i o n s were t o press t h e l e f t - h a n d k e y when t h e y h e a r d a tone in t h e i r right ear a n d t o press t h e r i g h t - h a n d k e y when t h e y h e a r d a tone in t h e i r l e f t ear. Results indicated s i g n i f i c a n t l y slower responding on t h e noncorresponding trials, b u t since these t r i a l s i n v o l v e d a reversal o f b o t h ear-hand correspondence and earresponse-location correspondence, it was impossible t o determine t h e cont r i b u t i o n of each f a c t o r separately. We, therefore, designed a f a i r l y complex experiment t o achieve t h i s separation (Simon, Hinrichs, & C r a f t , 1970, Experiment II).

In t h a t experiment, subjects pressed one of t w o f i n g e r keys as q u i c k l y as possible depending on t h e ear in which t h e y heard a tone. Each subject performed on t w o blocks o f trials, responding on one block w i t h arms uncrossed and on t h e o t h e r block w i t h arms crossed. On one o f these t w o blocks, t h e subject followed i n s t r u c t i o n s designed t o produce ear/hand correspondence; that is, "If you hear t h e tone in y o u r right ear, p r e s s down t h e k e y w i t h y o u r r i g h t - i n d e x f i n g e r and if y o u hear t h e tone in y o u r l e f t ear, press down t h e k e y w i t h y o u r l e f t - i n d e x finger." On t h e o t h e r block, t h e subject followed i n s t r u c t i o n s designed t o p r o d u c e ear/hand noncorrespondence; t h a t is, "If you hear t h e tone i; y o u r right ear, press down t h e k e y w i t h y o u r l e f t index f i n g e r ...etc. Depending on which i n s t r u c t i o n s (corresponding o r noncorresponding) were p a i r e d w i t h which block (uncrossed o r crossed arms), t h e r e s u l t i n g treatment was either ear/response-location correspondence or ear/response-location noncorrespondence. Subjects were assigned a t random t o one o f t w o groups, a n ear/response-location correspondence g r o u p o r an ear/response-location noncorrespondence g r o u p . T h e first-mentioned g r o u p performed t h e i r uncrossed block w i t h corresponding ear/hand i n s t r u c t i o n s a n d t h e i r crossed b l o c k w i t h noncorresponding ear/hand instructions, whereas t h e l a t t e r g r o u p performed t h e i r uncrossed block w i t h noncorresponding

Effects of ear/hand instructions ear/hand i n s t r u c t i o n s .

and

an Irrelevant Directional Cue their

crossed

block

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corresponding

Reactions were s i g n i f i c a n t l y f a s t e r when b o t h ear stimulated a n d response k e y were o n t h e same side o f t h e b o d y midline (333 vs. 390 ms). In addition, reactions w e r e s i g n i f i c a n t l y f a s t e r when t h e h a n d c o r responded t o t h e ear stimulated (330 v s . 394 ms). Overall, then, r e s u l t s o f t h e Simon, Hinrichs, a n d C r a f t experiments (1970) suggested that, when t h e stimulus was simple a n d p r o v i d e d no information o t h e r t h a n i t s locus, t h e r e were a t least t w o components t o a u d i t o r y S-R compatibility: ear/hand correspondence a n d ear/response-location correspondence. However, when t h e stimulus p r o v i d e d b o t h a r e l e v a n t symbolic cue a n d an i r r e l e v a n t location cue, t h e n ear/response-location correspondence alone accounted f o r S-R compatibility. E f f e c t Across Sensory Modalities I n all o f t h e studies d e s c r i b e d t h u s f a r , b o t h t h e r e l e v a n t a n d t h e i r r e l e v a n t cues i n v o l v e d t h e same sensory modality; f o r example, a u d i t o r y information processing was influenced by an i r r e l e v a n t a u d i t o r y location cue, whereas v i s u a l information processing was a f f e c t e d by an i r r e l e v a n t v i s u a l location cue. It seemed reasonable t o ask w h e t h e r similar effects would o c c u r when t h e r e l e v a n t a n d i r r e l e v a n t cues i n v o l v e d d i f f e r e n t sensory modalities. Conceivably, subjects m i g h t find it easier t o separate relevant f r o m i r r e l e v a n t cues if t h e i r respective i n p u t s i n v o l v e d d i f f e r e n t sensory channels.

T h e experiment (Simon & C r a f t , 1970a) r e q u i r e d subjects t o operate a combination d i s p l a y - c o n t r o l panel consisting o f t w o small w h i t e t r a n s l u c e n t buttons, one on t h e l e f t a n d one on t h e right, t h a t c o u l d b e b a c k l i g h t e d by m i n i a t u r e b u l b s . Below a n d e q u i d i s t a n t f r o m these stimulusresponse bu)tons was a small b u t t o n t h a t p r o v i d e d t h e s t a r t i n g p o i n t f o r t h e subjects response. O n each t r i a l , subjects moved t h e i r r i g h t - i n d e x f i n g e r f r o m t h e s t a r t b u t t o n a n d pressed t h e b u t t o n t h a t l i g h t e d up. O n some t r i a l s , t h e light was presented alone while, on o t h e r trials, t h e light onset was accompanied by a 500-Hz tone p r e s e n t e d e i t h e r monaurally o r b i n a u r a l l y t h r o u g h earphones. T h e r e were, then, e i g h t treatment combinations, t h a t is, l e f t o r right light presented alone o r accompanied by a tone t o t h e l e f t ear, t h e right ear, o r t o b o t h ears. Each t r e a t m e n t comb i n a t i o n was p r e s e n t e d 21 times in a predetermined random sequence w i t h i n a single b l o c k o f t r i a l s . A n analysis o f variance o f t h e monaural t r i a l s revealed a s i g n i f i c a n t Response x Ear Stimulated interaction. Responses t o t h e l e f t light were f a s t e r when t h e tone was presented t o t h e l e f t ear t h a n t o t h e right ear (530 v s . 577 ms), a n d responses t o t h e right l i g h t were f a s t e r when t h e tone was presented t o t h e right ear t h a n t o t h e l e f t (485 v s . 511 ms). Clearly, t h e speed o f r e s p o n d i n g t o t h i s visual d i s p l a y was affected by t h e presentation o f c o n c u r r e n t a u d i t o r y stimulation t h a t h a d n o relevance t o t h e t a s k i t s e l f . Results, then, indicated t h a t t h e i r r e l e v a n t directional c u e p r o d u c e d an e f f e c t across sensory modalities. A second p u r p o s e o f t h i s s t u d y was t o determine w h e t h e r t h e monaural t o n e h a d a f a c i l i t a t i v e o r i n h i b i t o r y e f f e c t on information p r o cessing. B i n a u r a l t r i a l s p r o v i d e d a baseline f o r comparison since t h e y i n v o l v e d t h e presence o f an a u d i t o r y stimulus w i t h o u t t h e associated

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directional cue. A series of planned comparisons led t o the conclusion t h a t t h e monaural tone had both a facilitative and i n h i b i t o r y effect on visual RT. The left response was facilitated by t h e tone t o t h e l e f t ear but inhibited by. t h e tone t o t h e right ear and, similarly, t h e r i g h t response was facilitated by t h e tone t o the. r i g h t ear and inhibited by t h e tone t o t h e l e f t ear. These results d i f f e r e d from o u r previous findings ( C r a f t E Simon, 1970; Simon & Small, 1969), which had suggested t h a t the irrelevant directional cue interfered w i t h processing on t h e noncorresponding t r i a l s b u t did not facilitate responding on t h e corresponding trials. We speculated t h a t t h e d i f f e r e n t pattern of results may be due t o t h e f a c t that, i n t h e present study, the baseline trials were presented in t h e same block as the corresponding and noncorresponding trials, whereas, i n previous studies, t h e baseline trials were presented i n a separate block. Indeed, i n a later study (Simon & Acosta, 1982), we demonstrated t h a t indications of facilitation and/or interference are dependent on t h e context i n which t h e baseline trials are administered. When baseline trials were presented in t h e same block as the correspondi n g and noncorresponding trials, t h e directional cue produced both facilitation and interference. However, when baseline t r i a l s were presented in one block and corresponding and noncorresponding trials were mixed together i n another block, data suggested an interference effect only. The d i f f e r e n t pattern of results from t h e two experimental designs was attributed t o t h e degree of stimulus uncertainty within a t r i a l block (Garner, 1962). When t h e baseline trials were i n a separate block, uncertainty was less (i.e., there was uncertainty about t h e content of t h e command b u t not i t s source), and t h i s resulted i n faster RT, which would bias t h e p i c t u r e against concluding t h a t facilitation occurred on t h e corresponding trials. Finally, mention should be made about t h e no-tone trials, which were included t o provide a baseline f o r evaluating the effect of concurr e n t auditory stimulation on visual RT. Reactions on t h e no-tone trials were significantly slower than on t h e binaural trials. The reason f o r t h i s difference is not clear. It may be t h a t t h e auditory stimulus had an energizing effect on visual RT. Alternatively, it may be t h a t RT on t h e no-tone t r i a l s was slowed because of the absence of t h e tone; t h a t is, because a tone accompanied t h e l i g h t on three f o u r t h s of the trials, subjects may have momentarily delayed t h e i r response t o the l i g h t on t r i a l s i n which the tone was absent. Additional studies would be necessary t o decide between these alternative explanations. Reactions t o Onset and Offset of Lights and Tones Results reported thus f a r support t h e notion t h a t there exists i n humans a stereotypic tendency t o react toward t h e source of stimulation. However, w i t h one exception (Simon, 1969), the evidence concerning t h i s stereotype has been somewhat indirect. We have drawn inferences from situations where t h e processing of a relevant symbolic cue was affected by the presence of an irrelevant directional cue. The objective of o u r n e x t s t u d y (Simon, Craft, & Webster, 1971) was t o construct an information-processing task in which t h e stereotype could be observed more d i r e c t l y and from which differential predictions concerning i t s operation could be generated and tested. The task involved responding to the onset o r offset of one l i g h t on t h e simple two-light display-control panel described previously (Simon &

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Craft, 1970a). If "stimulus" is defined as t h e element o f t h e d i s p l a y t h a t changes, a n d if t h e r e is, in fact, a stereotypic tendency t o react t o w a r d t h e stimulus, t h e n one would p r e d i c t t h a t when t h e stimulus i s a light onset, reactions t o w a r d t h e light t h a t goes on w i l l b e f a s t e r t h a n reactions t o w a r d t h e light t h a t remains o f f . Conversely, when t h e stimulus i s t h e o f f s e t o f one o f t h e t w o lights, one would p r e d i c t t h a t reactions t o w a r d t h e light t h a t goes o f f w i l l b e f a s t e r t h a n reactions t o w a r d t h e light t h a t remains on. Subjects were assigned a t random t o e i t h e r a stimulus-onset g r o u p o r a stimulus-offset g r o u p . Each o f these g r o u p s performed on t w o blocks o f t e s t t r i a l s . O n one block, t h e y pressed t h e stimulus-response b u t t o n t h a t changed and, o n t h e o t h e r block, t h e y pressed t h e stimulusresponse b u t t o n that did n o t change. Specifically, t h e onset g r o u p was told, " A t t h e b e g i n n i n g o f each trial, b o t h o f t h e b u t t o n s w i l l b e o f f . You w i l l hear a w a r n i n g tone and then, one second later, one o f t h e t w o b u t t o n s w i l l light up." O n one b l o c k (response t o changed element), t h e y were told, "Respond as f a s t as y o u can by p r e s s i n g t h e b u t t o n t h a t lights up." On t h e o t h e r block (response t o unchanged element), t h e y were told, "Respond as f a s t as y o u can by p r e s s i n g t h e b u t t o n t h a t doesn't light up." T h e o f f s e t g r o u p was g i v e n parallel i n s t r u c t i o n s . T h e y were told, " A t t h e b e g i n n i n g o f each t r i a l , b o t h o f t h e b u t t o n s w i l l b e l i g h t e d up. You will hear a w a r n i n g ton; a n d then, one second later, one o f t h e t w o l i g h t e d b u t t o n s w i l l g o o f f . On one b l o c k (response t o changed element) i n s t r u c t i o n s we';: t o "respond as f a s t as y o u can by p r e s s i n g t h e b u t t o n t h a t goes off, while on t h e o t h e r b l o c k (response t o unchanged element), i n s t r u c t i o n s were t o "respond as f a s t as y o u can by p r e s s i n g t h e b u t t o n t h a t remains lit." Results were j u s t as p r e d i c t e d . T h e onset g r o u p responded s i g nificantly faster t o t h e b u t t o n t h a t went on (changed element) than t o t h e b u t t o n t h a t remained o f f (unchanged element). T h e o f f s e t group responded s i g n i f i c a n t l y f a s t e r t o t h e b u t t o n t h a t went o f f (changed element) t h a n t o t h e b u t t o n t h a t remained on (unchanged element). Results, then, s u p p o r t e d t h e conclusion t h a t reactions a r e f a s t e r t o w a r d t h e source o f stimulation, t h a t is, t o w a r d t h e d i s p l a y element t h a t changes. A companion experiment (Simon, Craft, 5 Webster, 1971, E x p e r i ment II) investigated a u d i t o r y information processing f r o m an analogous t y p e o f display. Subjects wore earphones and pressed a right- o r l e f t hand k e y i n response t o t h e onset o r t h e o f f s e t o f a tone i n one ear. T h e experimental design was identical t o t h e p r e v i o u s study; t h a t is, subjects were assigned a t random t o e i t h e r a stimulus onset g r o u p o r a stimulus o f f s e t group, a n d each g r o u p performed on t w o blocks o f t r i a l s . On one block, subjects pressed t h e k e y on t h e side o f t h e ear in which t h e stimulus changed, and, on t h e o t h e r block, t h e y pressed t h e k e y o n t h e side o f t h e ear in which t h e stimulus did n o t change. T h e onset g r o u p was told, " A tone w i l l come on in one o f y o u r ears." On one block (response t o changed element), i n s t r u c t i o n s were t o "press t h e k e y o n t h e side where y o u hear t h e tone come o n . " On t h e o t h e r block (response t o unchanged element), i n s t r u c t i o n s were t o "press t h e k e y on t h e side where y o u d o n ' t hear t h e tone." T h e o f f s e t g r o u p was told, " T h e tone in one of y o u r ears w i l l g o o f f . " On one block (response t o changed element), t h e y were t o l d t o "press t h e k e y o n t h e side where you hear t h e tone g o o f f . " On t h e o t h e r block (response t o unchanged

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element), they,, were t o l d t o "press t h e k e y o n t h e side where y o u s t i l l hear t h e tone.

A c u r s o r y examination o f t h i s a u d i t o r y t a s k m i g h t suggest t h a t it is e x a c t l y comparable t o t h e v i s u a l t a s k in t h e companion experiment a n d that, therefore, t h e r e s u l t s o f t h e t w o experiments should b e comparable. In fact, we made d i f f e r e n t p r e d i c t i o n s f o r t h i s experiment. Specifically, we p r e d i c t e d that, f o r t h e tone-onset group, reactions would b e f a s t e r o n t h e side where t h e t o n e came o n (changed element) t h a n o n t h e side w h e r e t h e t o n e was absent (unchanged element). F o r t h e tone-offset group, however, we p r e d i c t e d t h a t reactions would b e f a s t e r on t h e side where t h e t o n e remained o n (unchanged element) t h a n on t h e side where t h e t o n e w e n t off (changed element). Results confirmed these p r e d i c t i o n s . F o r t h e onset group, react i o n s w e r e f a s t e r t o t h e changed element (320 v s . 386 ms), whereas, f o r t h e o f f s e t group, reactions were f a s t e r t o t h e unchanged element (488 v s . 578 ms). Why t h i s a p p a r e n t discrepancy between r e s u l t s o f t h i s p a i r o f experiments? Specifically, w h y , f o r t h e tone-offset group, were reactions f a s t e r o n t h e side o f t h e unchanged element? T h e f i n d i n g s a r e r e a d i l y i n t e r p r e t a b l e if one considers b o t h v i s u a l a n d a u d i t o r y displays in terms o f t h e subjects' perception r a t h e r t h a n in terms o f t h e p h y s i c a l stimulus per se. I n t h e case o f t h e v i s u a l display, t h e stimulus events a n d t h e subjects' p e r c e p t i o n o f them corresponded; t h a t is, a light e i t h e r went o n o r o f f , a n d t h i s i s w h a t subjects perceived. Similarly, on t h e tone-onset t r i a l s , subjects p e r c e i v e d t h e onset o f a monaural t o n e as t h e stimulus t o respond. O n t h e tone-offset t r i a l s , however, t h e r e was a discrepancy between t h e stimulus events a n d subjects' perception o f them. T h e s u b jects r e p o r t e d that, when a b i n a u r a l t o n e changed a b r u p t l y t o a monaural tone, t h e y did n o t p e r c e i v e t h e e v e n t as a tone o f f s e t in one ear. Rather, t h e y h e a r d t h e "onset" o f a tone in t h e o t h e r ear. When viewed in t h i s light, r e s u l t s o f t h e tone o f f s e t t r i a l s d o n o t r e p r e s e n t a d i s c r e p a n t finding. To t h e c o n t r a r y , t h e y b u t t r e s s o t h e r f i n d i n g s ( C r a f t G Simon, 1970; Simon, Small, Ziglar, & C r a f t , 1970) a n d s u p p o r t t h e notion o f a s t e r e o t y p i c t e n d e n c y t o r e a c t t o w a r d t h e apparent source o f stimulation. I t should b e p o i n t e d o u t e x p l i c i t l y t h a t t h e r e s u l t s o f these e x p e r i ments concerned w i t h onset a n d o f f s e t o f l i g h t s a n d tones c o u l d n o t have been p r e d i c t e d f r o m p r e v i o u s data concerned w i t h spatial S-R compatibility (e.g., Biederman 8 Kaplan, 1970; F i t t s C Seeger, 1953). It is well established and, in fact, it seems by now i n t u i t i v e l y obvious t h a t i n f o r mation processing w i l l b e slowed when subjects a r e t o l d t o r e v e r s e " n a t u r a l " S-R relations; f o r example, p r e s s t h e k e y on t h e opposite side o f t h e light" r a t h e r t h a n "press t h e k e y on t h e same side as t h e light. O u r i n s t r u c t i o n s , however, did not, on t h e surface a t least, r e q u i r e subjects t o r e v e r s e spatial S-R correspondence. Therefore, one m i g h t n o t p r e d i c t t h a t p r e s s i n g t h e b u t t o n t h a t l i g h t s - u p would b e f a s t e r t h a n p r e s s i n g t h e b u t t o n t h a t doesn't light-up o r t h a t p r e s s i n g t h e b u t t o n t h a t goes o f f would b e f a s t e r t h a n p r e s s i n g t h e b u t t o n t h a t remains lit-unless, o f course, one conceptualized t h e problem in terms o f a tendency t o react t o w a r d t h e stimulus source.

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Performance E f f e c t s in an O d d i t y T a s k

In another experiment (Simon & Craft, 19721, subjects were r e q u i r e d t o i d e n t i f y a n d respond t o t h e one element o f a three-element display t h a t was d i f f e r e n t f r o m t h e o t h e r t w o elements. Specifically, t h e d i s p l a y consisted of t h r e e stimulus-response b u t t o n s t h a t c o u l d b e l i g h t e d individually. T h e subjects performed o n t w o blocks o f t r i a l s , one b l o c k i n v o l v i n g l i g h t ( s ) onset a n d t h e o t h e r b l o c k i n v o l v i n g l i g h t ( s ) o f f s e t . F o r onset t r i a l s , t h e o r i g i n a l state o f t h e b u t t o n s was off, a n d t h e stimulus was t h e onset o f one o r t w o o f t h e b u t t o n s . Thus, t h e c o r r e c t response, t h a t is, p r e s s i n g t h e o d d b u t t o n , sometimes i n v o l v e d p r e s s i n g t h e l i g h t e d b u t t o n ( t h e element t h a t h a d j u s t changed) a n d sometimes t h e u n l i g h t e d b u t t o n ( t h e element t h a t was unchanged). Similarly, f o r o f f s e t t r i a l s , t h e o r i g i n a l s t a t e o f t h e b u t t o n s was on, a n d t h e stimulus was t h e o f f s e t o f one o r t w o o f t h e t h r e e b u t t o n s . Thus, t h e c o r r e c t response sometimes i n v o l v e d p r e s s i n g t h e u n l i g h t e d b u t t o n ( t h e changed element) a n d sometimes t h e l i g h t e d b u t t o n ( t h e unchanged element). If one conceives o f t h e stimulus as t h e changed element(s) of t h e display, a n d if one hypothesizes a s t e r e o t y p i c t e n d e n c y i n humans t o react t o w a r d t h e stimulus source (Simon, 1969; Simon, C r a f t , & Webster, 19711, t h e n t w o p r e d i c t i o n s follow. I n t h e onset b l o c k o f t h i s o d d i t y task, reactions t o a l i g h t e d b u t t o n should b e f a s t e r t h a n t o an u n l i g h t e d b u t t o n . In t h e o f f set block, t h e r e v e r s e finding should occur; t h a t is, reactions t o t h e u n l i g h t e d b u t t o n should b e f a s t e r t h a n t o t h e l i g h t e d b u t t o n . In o t h e r words, t h e c r u c i a l f a c t o r is n o t w h e t h e r t h e element t o w h i c h a subject responds i s o n o r off, but, r a t h e r , w h e t h e r t h e state o f t h a t element i s changed o r unchanged. Reactions should b e f a s t e r t o w a r d a changed t h a n t o w a r d an unchanged element because t h e r e is no necessity f o r o v e r r i d i n g an i n i t i a l i n c o r r e c t response t e n d e n c y b e f o r e e x e c u t i n g t h e c o r r e c t response. Results, again, were as predicted; t h a t is, subjects reacted s i g n i f i c a n t l y f a s t e r t o a d i s p l a y element when t h e state o f t h a t element h a d j u s t changed t h a n when it was unchanged (594 v s . 636 ms). T h i s effect, a f a s t e r reaction t o w a r d t h e source o f stimulation, o c c u r r e d o n t h e onset block (615 v s . 644 ms) as well as t h e o f f s e t b l o c k (573 vs. 628 ms). These f i n d i n g s i l l u s t r a t e t h a t i r r e l e v a n t cues i n a c h a n g i n g d i s p l a y can e l i c i t i n t e r f e r i n g response tendencies, w h i c h r e s u l t in slower information processing on a problem-solving t a s k .

A P e r s i s t e n t Phenomenon A l l o f t h e studies d e s c r i b e d t h u s f a r have i n v o l v e d observations g a t h e r e d during a single experimental session. While t h e e f f e c t o f t h e stereotype in t h a t single session has been extremely potent, one m i g h t ask w h e t h e r t h e phenomenon is s h o r t - l i v e d o r e n d u r i n g . Clearly, if t h e e f f e c t is a t r a n s i e n t one t h a t disappears q u i c k l y w i t h practice, it is n o t as important a behavioral phenomenon as if it p e r s i s t s i n d e f i n i t e l y . We decided, therefore, t o investigate t h e e f f e c t o f e x t e n d e d p r a c t i c e on t h e tendency t o react t o w a r d t h e stimulus source (Simon, Craft, & Webster, 1973). We also wanted t o conduct a formal analysis o f e r r o r s in a t a s k i n v o l v i n g c o n f l i c t i n g directional a n d symbolic cues. Such an analysis h a d been p r e c l u d e d i n p r e v i o u s studies by a combination o f a low e r r o r r a t e a n d a limited number o f observations f o r each subject. We f e l t t h a t an analysis o f t h e r e l a t i v e latency a n d f r e q u e n c y o f t h e d i f f e r e n t k i n d s o f e r r o r s would p r o v i d e valuable i n s i g h t s i n t o t h e mechanism u n d e r l y i n g t h e

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effect previously observed on correct response t r i a l s (i.e., slower i n f o r mation processing when the source and content of t h e stimulus did not correspond.) We had suggested (Simon, Small, Ziglar, & Craft, 1970) t h a t t h i s slower information processing on noncorresponding trials was due t o an initial tendency t o react t o t h e source of t h e stimulus rather than t o i t s meaning, and t h e necessity f o r o v e r r i d i n g t h i s initial response tendency produced t h e observed delay i n information processing. It would follow, then, t h a t e r r o r s on noncorresponding trials represent failures t o i n h i b i t t h e initial incorrect response tendency. It was, therefore, hypothesized that t h e latency of e r r o r responses on noncorresponding t r i a l s should be less than t h e latency of correct responses on these t r i a l s . Subjects performed 24 practice trials and 192 t e s t trials on each of 5 consecutive days. On each trial, subjects pressed a l e f t o r r i g h t key in response t o a high- o r low-pitched monaural tone. Figure 10 shows that, on each of t h e 5 days, RT was significantly faster when t h e r i g h t command was heard i n t h e r i g h t ear than when it was heard i n t h e l e f t ear, and R T t o t h e l e f t command was faster when it was heard i n t h e left ear than when it was heard i n t h e r i g h t ear. Thus, although overall performance improved w i t h practice, t h e Simon effect showed no sign of disappearing.

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The Tonal Command x Ear Stimulated interaction over a 5-day

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Analysis of t h e e r r o r data b u t t r e s s e d o u r i n t e r p r e t a t i o n o f t h e Simon e f f e c t as r e p r e s e n t i n g an i n i t i a l t e n d e n c y t o react t o t h e source o f a stimulus r a t h e r t h a n t o i t s content. E r r o r s on noncorresponding t r i a l s were s i g n i f i c a n t l y more f r e q u e n t t h a n e r r o r s on c o r r e s p o n d i n g t r i a l s , which i s w h a t one m i g h t expect if e r r o r s r e p r e s e n t f a i l u r e s t o inhibit an i n i t i a l i n c o r r e c t response tendency. C o r r e c t responses on noncorresponding t r i a l s were also s i g n i f i c a n t l y slower t h a n e r r o r responses (368 v s . 321 ms), w h i c h reflected t h e additional time r e q u i r e d t o o v e r r i d e an i n i t i a l i n c o r r e c t response tendency. I r r e l e v a n t C u e Presented P r i o r to Relevant Stimulus In all o f t h e research d e s c r i b e d t h u s far, t h e directional c u e o c c u r r e d simultaneously w i t h t h e r e l e v a n t stimulus. T h a t is, t h e d i r e c tional cue was e i t h e r an i r r e l e v a n t dimension o f t h e stimulus itself, o r it was p r o v i d e d by a simultaneous but i r r e l e v a n t stimulus t o another sensory modality. T h e p u r p o s e o f o u r n e x t s t u d y (Simon, Acosta, & Mewaldt, 1975) was t o i n v e s t i g a t e t h e e f f e c t o f an i r r e l e v a n t directional c u e presented p r i o r t o t h e onset o f t h e r e l e v a n t stimulus.

Subjects pressed a l e f t - o r r i g h t - h a n d k e y depending on t h e ear in Half were i n s t r u c t e d t o p r e s s t h e k e y w h i c h t h e y h e a r d a stimulus tone. on t h e same side as t h e ear stimulated ( c o r r e s p o n d i n g S-R mapping), whereas t h e o t h e r half p r e s s e d t h e k e y o n t h e opposite side (noncorresponding S-R mapping). P r i o r t o t h e stimulus, subjects h e a r d a w a r n i n g tone in e i t h e r t h e l e f t ear, t h e r i g h t ear, o r b o t h ears. We p r e d i c t e d t h a t reactions would b e f a s t e r when w a r n i n g t o n e a n d response were on t h e same side o f t h e b o d y t h a n when t h e y were o n opposite sides. Assuming t h a t t h e locus o f t h e w a r n i n g tone h a d t h e p r e d i c t e d effect, we were also i n t e r e s t e d in d e t e r m i n i n g t h e persistence o f t h e effect. Presumably, t h e potency o f an i r r e l e v a n t c u e dissipates o v e r time. Hence, t h e i n t e r v a l between w a r n i n g a n d stimulus tones was manipulated w i t h t h e expectation that, as w a r n i n g i n t e r v a l increased, t h e r e m i g h t b e a r e d u c t i o n in t h e e f f e c t o f w a r n i n g t o n e locus o n R T t o t h e e n s u i n g stimulus. F i g u r e 11 p i c t u r e s t h e r e s u l t s . Note t h a t t h e p r e d i c t e d i n t e r a c t i o n o c c u r r e d in t h e noncorresponding S-R mapping t a s k but n o t in t h e c o r r e sponding t a s k . Note, too, t h a t t h e interaction was as p o t e n t a t 400 ms following t h e w a r n i n g tone as it was a t 200 ms.

Why did t h e expected i n t e r a c t i o n o c c u r in t h e noncorresponding t a s k but n o t in t h e c o r r e s p o n d i n g task? We believe t h a t t h e explanation lies in t h e r e l a t i v e s t r e n g t h o f competing response tendencies. In t h e c o r r e s p o n d i n g S-R mapping task, t h e i n s t r u c t i o n s corresponded with a s t r o n g n a t u r a l t e n d e n c y t o respond t o a stimulus in one ear by making an ipsilateral response. T h e r e was l i t t l e o r no t e n d e n c y t o make t h e comp e t i n g contralateral response. In t h i s situation, then, a n y additional response t e n d e n c y elicited by t h e w a r n i n g t o n e was n o t p o t e n t enough t o a f f e c t t h e balance between already unequal response tendencies associated w i t h t h e stimulus. In t h e noncorresponding task, however, t h e i n s t r u c t i o n s t o p r e s s t h e k e y on t h e opposite side o f t h e stimulated e a r conf l i c t e d w i t h t h e s t r o n g n a t u r a l tendency t o make an ipsilateral response. T h e slower R T o n t h e noncorresponding t a s k reflects t h e competition between t h e t e n d e n c y t o make t h e response as i n s t r u c t e d a n d t h e tendency t o make t h e response t h a t was n a t u r a l . T h e p o i n t is, however,

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Locus of Warning Tone F i g u r e 11. E f f e c t o f locus o f a w a r n i n g tone a n d w a r n i n g i n t e r v a l on R T f o r right a n d l e f t responses in noncorresponding a n d c o r r e s p o n d i n g S-R mapping tasks. Note t h a t t h e speed o f information processing on t h e noncorresponding t a s k is affected by t h e r e l a t i v e location o f t h e w a r n i n g t o n e in relation t o t h a t o f t h e response. t h a t in t h i s situation i n v o l v i n g r e l a t i v e l y equal but competing responses t o t h e stimulus, t h e added response t e n d e n c y elicited by t h e w a r n i n g tone was p o t e n t enough t o have a s i g n i f i c a n t e f f e c t . I n summary, then, these f i n d i n g s e x t e n d e d o u r u n d e r s t a n d i n g o f t h e Simon e f f e c t by specifying t h e r a n g e o f situations in w h i c h it operated. Whether o r n o t i n t e r f e r e n c e o c c u r r e d depended o n t h e n a t u r e o f t h e response t h a t was r e q u i r e d t o t h e r e l e v a n t stimulus. Furthermore, since t h e d i f f e r e n c e between t h e c o r r e s p o n d i n g a n d noncorresponding t a s k s was p u r e l y a difference in response assignment, t h e r e s u l t s suggested t h a t response selection was t h e stage affected by t h e i r r e l e v a n t directional cue. Stages of Processing T h e Simon, Acosta, a n d Mewaldt (1975) data did n o t establish conc l u s i v e l y t h a t t h e i r r e l e v a n t directional c u e affects o n l y t h e responseIn fact, some o f o u r e a r l i e r f i n d i n g s m i g h t b e i n t e r selection stage. p r e t e d t o suggest t h a t t h e i r r e l e v a n t cue affects both t h e stimulusencoding stage a n d t h e response-selection stage. Specifically, Simon a n d Small (1969) a n d C r a f t a n d Simon (1970) f o u n d t h a t b o t h c o r r e s p o n d i n g a n d noncorresponding t r i a l s were s i g n i f i c a n t l y slower t h a n baseline t r i a l s

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presented i n a b l o c k in w h i c h t h e directional c u e was absent. This f i n d i n g m i g h t suggest t h a t t h e i r r e l e v a n t directional c u e n o t o n l y t e n d e d t o e l i c i t a response t o w a r d t h e source o f stimulation but t h a t it also increased t h e time r e q u i r e d t o encode t h e r e l e v a n t symbolic information o n t h e c o r r e s p o n d i n g a n d noncorresponding t r i a l s . O u r n e x t experiment, then, employed t h e S t e r n b e r g (1969) a d d i t i v e f a c t o r s method t o t e s t t h e hypothesis t h a t t h e i r r e l e v a n t directional c u e affected t h e stimulus encoding stage. T h e method involves c o n d u c t i n g f a c t o r i a l experiments t o determine w h i c h variables p r o d u c e s i g n i f i c a n t interactions a n d w h i c h d o not. T h e variables t h a t i n t e r a c t a r e assumed t o a f f e c t a stage in common, whereas t h e variables t h a t d o n o t i n t e r a c t a r e assumed t o a f f e c t d i f f e r e n t stages. Furthermore, f r o m t h e n a t u r e o f t h e variables t h a t interact, one can d e f i n e t h e processing t h a t i s accomplished by a c e r t a i n stage. T h o u g h t h e a d d i t i v e f a c t o r s method involves some t h o r n y methodological problems (Pachella, 1974; Taylor, 19761, it has p r o v i d e d an extremely u s e f u l tool in information-processing research. O u r s t u d y (Acosta & Simon, 1976) i n v o l v e d h a v i n g some subjects p r e s s a l e f t - o r r i g h t - h a n d k e y i n response t o t h e c h a r a c t e r X o r 0. O t h e r subjects performed a unimanual t a s k t h a t i n v o l v e d m o v i n g a t o g g l e switch t o w a r d o r away f r o m t h e i r b o d y in response t o t h e same stimuli used in t h e bimanual t a s k . O n half o f t h e t r i a l s , a t r a n s l u c e n t Plexiglas p l a t e was placed in f r o n t o f t h e v i s u a l d i s p l a y t o r e d u c e t h e d i s c r i m i n a b i l i t y o r q u a l i t y o f t h e alphanumeric characters. Previous research indicates t h a t stimulus d i s c r i m i n a b i l i t y affects t h e stimulusencoding stage a n d does n o t a f f e c t a n y o t h e r processing stage (Biederman & Kaplan, 1970; Sternberg, 1967, 1969). Simultaneous w i t h t h e onset o f t h e alphanumeric character, subjects h e a r d a t o n e in e i t h e r t h e l e f t ear, t h e right ear, o r b o t h ears. If t h i s i r r e l e v a n t a u d i t o r y stimulus (i.e., t h e directional cue) affected t h e stimulus-encoding stage, i t s effects should i n t e r a c t w i t h discriminability, whereas, if t h e i r r e l e v a n t directional c u e affected another separate stage, t h e r e should b e no i n t e r a c t i o n w i t h discriminability. Results indicated t h a t when response- k e y locations were isomorphic t o t h e source o f t h e i r r e l e v a n t tone ( i . e . , bimanual task), t h e directional c u e h a d a p o t e n t e f f e c t on performance. However, when response-select i o n operations were altered by employing responses t h a t were n e u t r a l w i t h respect t o t h e source o f t h e i r r e l e v a n t tone (i.e., unimanual task), t h e directional c u e h a d no e f f e c t . In addition, d e g r a d i n g t h e q u a l i t y o f t h e v i s u a l stimulus r e s u l t e d in slower R T but did n o t a l t e r t h e e f f e c t o f t h e i r r e l e v a n t directional cue. A c c o r d i n g t o a d d i t i v e f a c t o r s logic, t h i s p a t t e r n o f f i n d i n g s suggested t h a t t h e i r r e l e v a n t directional c u e did n o t a f f e c t t h e stimulus-encoding stage but, instead, affected t h e responseselection stage. I n t e r a c t i o n Between Stereotypes T h u s f a r , most o f o u r investigations of t h e stereotype have i n v o l v e d situations in which t h e a l t e r n a t i v e commands, stimulus locations, a n d response locations were e i t h e r l e f t o r right. In retrospect, t h i s manipulation along t h e horizontal dimension may have been unfortunate, f o r it has necessitated elaborate experimental p r o c e d u r e s t o establish t h a t t h e Simon e f f e c t was n o t related t o factors such as hemispheric dominance, r e s p o n d i n g b o d y member, o r sense o r g a n stimulated. T h e major

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purpose o f o u r n e x t s t u d y (Simon, Mewaldt, Acosta, & Hu, 1976) was t o determine whether t h e stereotypic reaction t o w a r d t h e stimulus source o c c u r r e d in t h e v e r t i c a l dimension, ,y well. Evidence that t h e stereotype o c c u r r e d in a situation i n v o l v i n g u p " a n d "down" commands, stimulus locations, a n d responses would n o t o n l y establish t h e g e n e r a l i t y o f t h e phenomenon, but would p r o v i d e convincing evidence t h a t l a t e r a l i t y was n o t a f a c t o r in i t s production. A second p u r p o s e was t o investigate t h e e f f e c t of t h e frequency-space stereotype on a u d i t o r y R T . Mudd (1963) investigated spatial stereotypes associated w i t h various dimensions o f sound by h a v i n g subjects move a p e g in a panel f r o m one position t o another, in a manner suggested by t h e relation between t w o tones. He f o u n d t h a t subjects h a d a v e r y s t r o n g tendency t o associate h i g h - p i t c h e d tones w i t h an u p response a n d low-pitched tones w,ith a down response. We hypothesized t h a t if p i t c h , in fact, provides a n a t u r a l " indication o f t h e c o r r e c t response, t h e n subjects should react f a s t e r when t h e toneresponse r u l e (i.e., stimulus-response, o r S - R mapping) corresponded w i t h t h e frequency-space stereotype (i.e., high tone = u p response and low tone = down response) t h a n when it did n o t (high tone = down response a n d low tone = u p response). In o t h e r words, t h e c o r responding S - R mapping should b e "compatible" ( F i t t s & Seeger, 19531, in t h a t t h e ensemble o f stimulus a n d response combinations comprising t h e t a s k should r e s u l t in a h i g h e r r a t e o f information t r a n s f e r t h a n w i t h t h e noncorresponding mapping. A third p u r p o s e o f t h e s t u d y was t o i n v e s t i gate t h e possible interaction between t h e t w o stereotypes; t h a t is, does t h e s t r e n g t h o f t h e tendency t o react t o w a r d t h e stimulus source depend o n w h e t h e r t h e tone-response r u l e corresponds w i t h t h e "natural" f r e quency-space stereotype? Subjects were seated in a soundproof booth a t a table o n which a display-control panel was placed. T h e panel contained t w o small speakers mounted one above t h e other, 45 cm a p a r t . Midway between t h e speakers was a three-position toggle switch, which flormally rested in t h e center position, b u t could b e toggled up o r down. Subjects r e s t e d t h e i r elbow on t h e t a b l e a n d grasped t h e switch between t h e t h u m b a n d f o r e f i n g e r o f t h e i r p r e f e r r e d hand. T h e y were i n s t r u c t e d that, on each trial, t h e y would hear e i t h e r a h i g h - p i t c h e d tone (500 Hz) o r a low-pitched tone (200 Hz) emanating f r o m e i t h e r t h e t o p o r bottom speaker a n d t h a t t h e i r t a s k was t o make t h e c o r r e c t response as q u i c k l y a n d accurately as possible. Subjects performed on t w o blocks o f t r i a l s . On one block (i.e., c o r r e sponding block), t h e y were i n s t r u c t e d t o move t h e switch u p in response t o t h e h i g h - p i t c h e d tone a n d down in response t o t h e low-pitched tone. On t h e o t h e r b l o c k (i.e., noncorresponding block), t h e y moved t h e switch u p in response t o t h e low-pitched tone a n d down in response t o t h e h i g h - p i t c h e d tone. F i g u r e 12 p i c t u r e s t h e r e s u l t s . Note, f i r s t , that R T was faster in t h e corresponding S-R mapping t a s k ( l e f t panel) t h a n in t h e noncorresponding S-R mapping t a s k (right panel). Note, too, t h e significant Command x Command Location interaction in t h e corresponding t a s k ( l e f t panel). T h a t is, u p responses were f a s t e r when t h e h i g h - p i t c h e d tone emanated from t h e t o p speaker t h a n when it emanated f r o m t h e bottom speaker, a n d down responses were f a s t e r when t h e low tone came f r o m t h e bottom speaker t h a n when it came f r o m t h e t o p speaker. However, in t h e noncorresponding S - R mapping t a s k ( r i g h t panel), t h i s Command x Command Location interaction was n o t significant. T h e o n l y significant

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F i g u r e 12. Effect o f location o f tonal command on reaction time f o r up and down responses in corresponding and noncorresponding S-R mapping tasks. effect i n t h e noncorresponding t a s k was a main e f f e c t o f command, indicating t h a t u p responses were f a s t e r t h a n down responses (412 ms v s . 454 ms). T h e finding t h a t R T was f a s t e r when t h e S-R mapping c o r r e sponded w i t h t h e frequency-space stereotype is n o t p a r t i c u l a r l y s u r p r i s ing in view of Mudd's (1963) finding t h a t tone f r e q u e n c y has a s t r o n g n a t u r a l association w i t h t h e v e r t i c a l dimension o f space; t h a t is, high tones suggest a n u p response a n d low tones suggest a down response. In previous studies, we had employed h i g h - and low-pitched tones t o signal l e f t o r right responses, w i t h half o f t h e subjects i n s t r u c t e d t h a t a h i g h tone means right a n d a low tone means left, while t h e o t h e r h a l f were g i v e n t h e reverse S-R mapping (e.g., Simon, C r a f t , & Small, 1970; Simon, Craft, & Webster, 1973, Simon & Small, 1969). We h a d n e v e r detected a n y d i f f e r e n c e in R T between t h e t w o S-R mapping conditions. In t h e p r e s e n t study, however, where responses of u p o r down r a t h e r than right o r l e f t were involved, one S-R mapping condition was c l e a r l y "compatible" in t h e sense t h a t it resulted i n a h i g h e r r a t e of information

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t r a n s f e r ( F i t t s & Seeger, 1953). T h i s r e s u l t may have p r a c t i c a l applicat i o n s in t h e design o f a u d i t o r y displays f o r man-machine systems where, f o r example, a p p r o p r i a t e l y chosen tonal commands m i g h t facilitate t h e u p o r down movements o f a c o n t r o l o r a vehicle. T h e second finding, t h a t s t e r e o t y p i c reactions t o w a r d t h e stimulus source o c c u r r e d in t h e v e r t i c a l plane, i s of i n t e r e s t because it establishes t h e g e n e r a l i t y of a phenomenon t h a t has been demonstrated p r e v i o u s l y along t h e horizontal dimension. More importantly, it establishes c o n v i n c i n g l y t h a t t h e Simon e f f e c t is n o t related t o f a c t o r s such as l a t e r a l i t y o r hemispheric dominance. T h e interaction between t h e stereotypes is, theoretically a t least, t h e most i n t e r e s t i n g and, a t t h e same time, t h e most p u z z l i n g o f o u r results. Subjects reacted f a s t e r t o t h e up command when it emanated f r o m t h e t o p speaker t h a n when it emanated f r o m t h e bottom speaker, a n d t h e y reacted f a s t e r t o t h e down command when it came f r o m t h e bottom speaker t h a n when it came f r o m t h e t o p s p e a k e r - - b u t o n l y o n t h e b l o c k o f t r i a l s where t h e S-R mapping corresponded w i t h t h e frequency-space stereotype. T h i s reaction t o w a r d t h e source o f stimulation did n o t o c c u r when subjects h a d t o process commands t h a t c o n f l i c t e d w i t h t h e frequency-space stereotype (i.e., low-pitched t o n e = u p a n d h i g h - p i t c h e d t o n e = down). It appears t h a t processing t h e symbolic c o n t e n t o f a tonal command t h a t c o n f l i c t s w i t h one p o w e r f u l stereotype may have so t h o r o u g h l y engaged t h e information-processing mechanisms as t o eliminate t h e o t h e r stereotype. T h e reader w i l l recall t h a t t h i s s o r t o f complex i n t e r action between n a t u r a l response tendencies had been o b s e r v e d i n an e a r l i e r s t u d y (Simon, Acosta, E Mewaldt, 1975). I n t h a t study, subjects pressed a l e f t - o r r i g h t - h a n d k e y depending on t h e ear in w h i c h t h e y h e a r d a tone. Some subjects pressed t h e k e y on t h e same side as t h e e a r stimulated ( c o r r e s p o n d i n g condition), while o t h e r s pressed t h e k e y on t h e opposite side (noncorresponding c o n d i t i o n ) . T h e locus o f a w a r n i n g tone, w h i c h preceded t h e stimulus tone, affected R T o n noncorresponding t r i a l s , but n o t o n c o r r e s p o n d i n g t r i a l s . Those e a r l i e r f i n d i n g s , then, a r e similar to, b u t in a sense, opposite f r o m those obtained in t h e p r e s e n t s t u d y w h e r e t h e i r r e l e v a n t directional cue affected o n l y t h e c o r r e s p o n d i n g trials. How does one rationalize t h i s a p p a r e n t discrepancy? F i r s t , i n t h e e a r l i e r s t u d y , t h e directional c u e was relevant, whereas i n t h e p r e s e n t s t u d y , t h e symbolic c u e was r e l e v a n t . We know t h a t t h e r e a r e i m p o r t a n t differences in information processing, depending o n w h e t h e r t h e cue is directional o r symbolic (Simon & C r a f t , 1971). Second, in t h e e a r l i e r study, t h e i r r e l e v a n t directional cue was i n t h e w a r n i n g tone, whereas i n t h e p r e s e n t s t u d y , t h e directional c u e was an i r r e l e v a n t dimension o f t h e r e l e v a n t symbolic stimulus. We know t h a t t h e basic mechanisms u n d e r l y ing a u d i t o r y S-R compatibility a r e d i f f e r e n t when stimulus location is t h e r e l e v a n t c u e t h a n when location i s an i r r e l e v a n t dimension o f t h e r e l e v a n t symbolic cue (Simon, H i n r i c h s , & C r a f t , 1970). In retrospect, then, it seems reasonable t o a r r a n g e t h e treatment conditions o f t h e p r e s e n t experiment a n d o u r e a r l i e r s t u d y (Simon, Acosta, & Mewaldt, 1975) along a single compatibility dimension. At the high e n d o f t h i s continuum would b e t h e c o r r e s p o n d i n g condition o f t h e e a r l i e r s t u d y , w h i c h r e q u i r e d r e s p o n d i n g t o a directional cue by p r e s s i n g a k e y o n t h e same side as t h e e a r stimulated. Subjects could p e r f o r m t h i s t a s k in a r e f l e x - l i k e way, almost w i t h o u t t h i n k i n g , a n d it is n o t s u r p r i s i n g t h a t a directional c u e f r o m a w a r n i n g tone presented p r i o r t o t h e

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stimulus did n o t have a d i s c e r n i b l e e f f e c t on R T . A t t h e low e n d o f t h i s same compatibility continuum w o u l d b e t h e noncorresponding condition of t h e p r e s e n t s t u d y , in w h i c h subjects h a d t o process a symbolic c u e by employing a tone-response r u l e t h a t r a n c o u n t e r t o t h e p o t e n t f r e q u e n c y space stereotype. T h e r e q u i r e d symbolic t r a n s l a t i o n may have been so complex as t o overload t h e information-processing apparatus, t h u s p r e v e n t i n g t h e directional c u e f r o m h a v i n g a n y impact. Between these extremes would b e t h e noncorresponding condition o f t h e e a r l i e r s t u d y a n d t h e c o r r e s p o n d i n g condition o f t h e p r e s e n t study; in both, t h e stereotypic reaction t o w a r d t h e stimulus source o c c u r r e d . O u r results, then, p r o v i d e general s u p p o r t f o r t h e Sheridan a n d F e r r e l l (1974) s u g gestion t h a t t h e conception o f t h e o p e r a t o r as a limited channel o f i n f o r mation transmission is least accurate u n d e r conditions where each stimulus has, in effect, i t s own channel (high compatibility) a n d w h e r e t h e S - R mapping is a r b i t r a r y o r r u n s c o u n t e r t o a common stereotype. It is clear, however, t h a t much remains t o b e learned about information-processing mechanisms in situations t h a t i n v o l v e i n t e r a c t i n g s t e r e o t y p i c response tendencies. Time C o u r s e o f t h e Phenomenon O u r n e x t s t u d y (Simon, Acosta, Mewaldt, & Speidel, 1976) was conc e r n e d w i t h c h a r t i n g t h e time course o f t h e s t e r e o t y p i c t e n d e n c y t o respond t o w a r d t h e stimulus source and, also, t o localize i t s e f f e c t in relation t o t h e stages o f information processing. T w o d i f f e r e n t paradigms were used. I n one paradigm, subjects responded t o a r e d o r g r e e n light presented on t h e right o r l e f t side by p r e s s i n g a right o r l e f t k e y . T h e location o f t h e light p r o v i d e d t h e i r r e l e v a n t cue. Subjects were i n s t r u c t e d t o delay t h e i r r e s p o n d i n g t o t h e l i g h t until t h e y h e a r d a g o signal presented 0, 150, 250, o r 350 ms a f t e r t h e light. F i g u r e 13 shows t h a t t h e Command x Command Location interaction o c c u r r e d up t o a delay i n t e r v a l of about 250 ms. When t h e delay i n t e r v a l was increased t o 350 ms, t h e source o f t h e stimulus no longer h a d a n y detectable e f f e c t on R T . These r e s u l t s suggested t h a t t h e e f f e c t of t h e i r r e l e v a n t directional cue was localized in t h e l a t e r stages o f processing, perhaps in t h e response-selection stage. Since t h e t y p i c a l magnitude o f t h e Command x Command Location interaction is o n t h e o r d e r o f 60 ms, it is d i f f i c u l t t o conceive how t h e e f f e c t could p e r s i s t undiminished f o r 250 ms and, yet, have i t s onset e a r l y in t h e decision process. In another paradigm, subjects pressed e i t h e r a l e f t - o r r i g h t - h a n d k e y labeled " r e d " o r "green" in response t o t h e pitch o f a stimulus tone; t h a t is, low tone signaled r e d k e y a n d h i g h tone signaled g r e e n k e y . T h e location o f t h e tone ( l e f t o r right ear) p r o v i d e d t h e i r r e l e v a n t d i r e c tional cue. A l i g h t - e m i t t i n g diode (LED) capable o f p r o d u c i n g e i t h e r a r e d o r a g r e e n light was located b e h i n d each k e y . B y employing t h e LEDs, it was possible t o v a r y t h e labeling o f t h e keys f r o m t r i a l t o t r i a l , t o delay labeling t h e keys until a f t e r t h e stimulus tone was presented, a n d t o manipulate t h e i n t e r v a l between stimulus presentation a n d labeling o f t h e k e y s . F o r some groups, t h e r e d a n d g r e e n k e y s were designated (i.e., labeled) p r i o r t o t h e onset of t h e tone; t h a t is, t h e right k e y m i g h t b e r e d a n d t h e l e f t k e y green. Thus, upon onset o f t h e tone, subjects h a d all t h e information t h e y needed t o i n i t i a t e t h e i r response. For o t h e r groups, t h e keys were n o t labeled until some time a f t e r t h e tone had been presented, so subjects h a d t o delay t h e i r response until t h e y c o u l d determine w h i c h k e y was r e d a n d w h i c h k e y was g r e e n on t h a t

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Location of Command (Eye Stimulated) Figure 13. E f f e c t o f an i r r e l e v a n t directional c u e i n a choice reaction time t a s k i n v o l v i n g a g o signal. T h e basic t a s k i n v o l v e d p r e s s i n g e i t h e r a l e f t - o r a r i g h t - h a n d key, depending on t h e color o f a r e d o r a g r e e n command light. T h e light appeared in e i t h e r t h e l e f t o r t h e right half of t h e v i s u a l field, t h u s p r o v i d i n g an i r r e l e v a n t directional cue. Subjects w e r e i n s t r u c t e d t o delay e x e c u t i n g t h e i r response until t h e y h e a r d an a u d i t o r y g o signal, w h i c h was presented e i t h e r simultaneously w i t h t h e command light (Panel 1) o r following t h e light by 150, 250, o r 350 ms (Panels 2 t h r o u g h 4). Reaction time was measured f r o m t h e onset of t h e g o signal. particular trial. In t h i s way, subjects were p r e v e n t e d f r o m b e g i n n i n g t h e response-selection stage until t h e i n s t a n t specified by t h e e x p e r i menter. O u r idea was t h a t a n y i n i t i a l tendency t o react t o t h e source o f t h e tone w o u l d dissipate d u r i n g t h e enforced delay b e f o r e responding. We f e l t that, by i d e n t i f y i n g t h e c r i t i c a l i n t e r v a l a t w h i c h t h e stereotype no l o n g e r occurred, we c o u l d estimate t h e d u r a t i o n o f t h e e f f e c t a n d also localize it in t h e information-processing sequence. F i g u r e 14 shows t h a t t h e source o f t h e tonal command affected information processing only when t h e response keys were labeled p r i o r to, o r simultaneously with, t h e onset o f t h e tone. When t h e response k e y s were labeled a f t e r onset o f t h e tone, t h e i r r e l e v a n t directional cue

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h a d n o s i g n i f i c a n t effect. T h i s finding has i m p o r t a n t implications because it i s f u r t h e r evidence t h a t t h e i r r e l e v a n t c u e affects t h e response-select i o n stage; t h a t is, t h e directional c u e h a d an e f f e c t o n l y when subjects could proceed d i r e c t l y t o t h e response-selection stage. When t h e labeling o f t h e response k e y s was delayed, subjects w e r e p r e v e n t e d f r o m p r o ceeding t o t h e response-selection stage, a n d t h e directional c u e in t h e stimulus t o n e h a d n o e f f e c t .

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Location of Command (Ear Stimulated) Figure 14. E f f e c t of an i r r e l e v a n t d i r e c t i o n a l cue on t h e time r e q u i r e d to t r a n s l a t e p i t c h information i n t o a k e y p r e s s response. T h e basic t a s k i n v o l v e d p r e s s i n g e i t h e r a l e f t - o r a r i g h t - h a n d key, labeled "red" o r "green," in response t o t h e pitch o f a monaural tonal command. Each o f s i x g r o u p s (see Panels 1-6) p e r f o r m e d a d i f f e r e n t v a r i a t i o n o f t h i s basic task. F o r G r o u p 1, t h e r e d k e y a n d t h e g r e e n k e y were labeled b e f o r e t h e s t a r t o f t h e experiment, a n d these designations remained f i x e d throughout. F o r G r o u p 2, t h e keys were labeled 1 s p r i o r t o t h e onset o f each tonal command, a n d these labels v a r i e d f r o m t r i a l t o t r i a l ; i.e., on one t r i a l , t h e right k e y m i g h t b e r e d a n d t h e l e f t k e y green, whereas o n t h e n e x t t r i a l t h e r e v e r s e labeling m i g h t occur. Reaction times f o r Groups 1 a n d 2 were measured f r o m t h e onset o f t h e tonal command. For Groups 3, 4, 5, a n d 6, t h e k e y labels also v a r i e d f r o m t r i a l t o t r i a l . T h e labeling, however, o c c u r r e d e i t h e r simultaneously w i t h presentation o f t h e tonal command o r 150, 250, o r 350 ms a f t e r t h e onset o f t h e command. Reaction times f o r Groups 3 - 6 were measured f r o m t h e i n s t a n t t h e keys were labeled.

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J.R. Simon A B u f f e r Model of Response Selection

We have developed a model t h a t employs a scanning notion as a means o f d e s c r i b i n g t h e response-selection process. T h e model proposes t h a t a response " b u f f e r " is established f o r each acceptable response in a choice R T t a s k . - T h e r e f o r e , in a t a s k t h a t r e q u i r e s p r e s s i n g e i t h e r a l e f t - o r r i g h t - h a n d key, t w o b u f f e r s a r e created, one f o r t h e l e f t - a n d one f o r t h e r i g h t - h a n d response. Each b u f f e r contains a representational l i s t i n g o f t h e stimuli associated w i t h t h a t p a r t i c u l a r response. F o r example, if t h e stimulus "X" indicates a l e f t - k e y response a n d t h e stimulus 0" indicates a r i g h t - k e y response, a representation o f "X" is contained w i t h i n t h e l e f t b u f f e r a n d a representation of "0" i s contained w i t h i n t h e right b u f f e r . We assume that, once a presented stimulus has b-een identified, each o f t h e response b u f f e r s is searched i n a serial, self-termin a t i n g scan until t h e matching representation is located a n d t h e associated response i s selected. We f u r t h e r assume t h a t an i r r e l e v a n t d i r e c tional c u e associated w i t h t h e stimulus affects R T by biasing t h e subject t o search f i r s t t h e b u f f e r t h a t corresponds t o t h e i r r e l e v a n t cue. Thus, when t h e r e l e v a n t a n d i r r e l e v a n t cue correspond, R T w i l l b e r e l a t i v e l y f a s t because t h e c o r r e c t b u f f e r w i l l b e searched f i r s t . However, when t h e r e l e v a n t a n d i r r e l e v a n t c u e d o n o t correspond, R T w i l l b e r e l a t i v e l y slow because t h e i n c o r r e c t b u f f e r w i l l b e searched f i r s t . T h e above model accounts f o r t h e r e s u l t s o f t h e p r e v i o u s t w o experiments. F o r example, i n t h e f i r s t experiment, u n d e r t h e 150-ms delay, t h e g o signal o c c u r r e d w h i l e t h e subjects were s t i l l i d e n t i f y i n g t h e stimulus, a n d so response selection proceeded in t h e same fashion as in t h e no-delay condition; t h a t is, RTs reflected a p o t e n t Simon e f f e c t . However, when t h e g o signal was delayed f o r more t h a n 250 ms, t h e subject h a d time t o scan both response b u f f e r s a n d select t h e a p p r o p r i a t e response before t h e signal to execute was presented. Thus, under t h e long-delay conditions, R T no longer reflected t h e beneficial o r detrimental effects p r o d u c e d by scanning a p a r t i c u l a r response b u f f e r f i r s t . In t h e second experiment, t h e t y p i c a l Simon e f f e c t o c c u r r e d when subjects were able t o p r o g r a m t h e stimulus-response associations p r i o r t o stimulus presentation, t h a t is, when t h e response k e y s were labeled p r i o r to the trial. T h e s t e r e o t y p e also o c c u r r e d when t h e response keys were labeled simultaneously w i t h t h e tone. U n d e r t h i s l a t t e r condition, subjects a p p a r e n t l y f i r s t i d e n t i f i e d t h e k e y labels so t h e y could load t h e response b u f f e r s . T h i s additional step slowed R T but, once completed, subjects c o u l d proceed w i t h o u t delay t h r o u g h t h e stimulus-identification a n d response-selection stages. When t h e labeling of response k e y s was delayed, t h e sequence of operations was altered. Now, presumably, subjects f i r s t processed t h e tone, d u e t o i t s temporal p r i o r i t y , a n d t h e n proceeded t o i d e n t i f y t h e k e y lab:ls a n d load t h e b u f f e r s . Apparently t h e engagement of t h e subjects' c e n t r a l processing u n i t " in i d e n t i f y i n g t h e k e y labels a n d loading t h e b u f f e r s eliminated t h e i n i t i a l scanning bias p r o d u c e d by t h e locus o f t h e tone a n d t h e r e b y eliminated t h e Simon effect.

To elaborate a n d t e s t t h i s b u f f e r model o f response selection, we designed a t a s k t h a t i n v o l v e d t h e assignment o f unequal numbers o f stimuli t o k e y - p r e s s responses by t h e l e f t o r right h a n d (Mewaldt, Connelly, & Simon, 1980). Specifically, t w o visual stimuli were assigned t o a response by one h a n d (two-item set) a n d f o u r v i s u a l stimuli were assigned t o a response by t h e o t h e r h a n d ( f o u r - i t e m set). Each time a

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stimulus was presented, it was accompanied by a monaural t o n e t h a t p r o v i d e d t h e i r r e l e v a n t directional cue. Sometimes t h e t o n e was presented t o t h e ear c o r r e s p o n d i n g t o t h e response t o b e made on t h a t t r i a l (henceforth, called c o r r e s p o n d i n g trials), a n d sometimes t h e tone was presented t o t h e ear t h a t did n o t correspond t o t h e response (noncorresponding t r i a l s ) . T h e model makes p r e d i c t i o n s c o n c e r n i n g t h e relations among t h e R T s o b s e r v e d f o r t h e f o u r d i f f e r e n t combinations o f r e l e v a n t a n d i r r e l e v a n t information. These p r e d i c t i o n s a r e based o n t h e assumptions t h a t t h e locus o f t h e tone determines t h e b u f f e r searched f i r s t a n d t h a t R T is a d i r e c t f u n c t i o n o f t h e number o f items scanned d u r i n g response selection. Table 1 shows t h e presumed average number o f items t h a t m u s t b e scanned in o r d e r t o i d e n t i f y a stimulus f r o m a two- o r f o u r - i t e m set u n d e r conditions in which t h e i r r e l e v a n t directional c u e corresponds with t h e response a n d conditions i n w h i c h it does n o t . F o r example, consider t h e case in w h i c h a stimulus f r o m a two-item set is accompanied by a noncorresponding tone. T h e t o n e biases t h e subject t o search f i r s t t h r o u g h t h e f o u r - i t e m b u f f e r , w h i c h in t h i s instance is t h e i n c o r r e c t b u f f e r . We assume t h a t all f o u r items in t h i s b u f f e r a r e scanned b e f o r e t h e two-item b u f f e r is searched. T h e stimulus is f i n a l l y located as e i t h e r t h e f i r s t o r second item in t h e two-item b u f f e r . In t h i s example, then, t h e subject has t o scan an average o f 5.5 items (4 1.5) b e f o r e t h e c o r r e c t response can b e selected a n d executed. If a stimulus f r o m a t w o item set is accompanied by a c o r r e s p o n d i n g tone, t h e c o r r e c t b u f f e r (i.e., t h e two-item b u f f e r ) is searched f i r s t , a n d t h e stimulus is located a f t e r scanning an average o f 1.5 items. T h e average number o f items t h a t must b e scanned t o locate a stimulus f r o m t h e f o u r - i t e m set is calculated in t h e same manner, a n d these f i g u r e s a r e presented in t h e right column o f Table 1. +

Table 1 Predicted Average Number o f Items Scanned to Locate a Stimulus From a Two- o r F o u r - I t e m Set

Correspondence o f Tone a n d Response Noncor res pondi ng ( N ) Corresponding ( C ) N - C Difference

T w o - Itern Set 5.5 1.5 4.0

Four-Item Set 4.5 2.5 2.0

Note. Model assumes t h a t t h e location o f t h e i r r e l e v a n t tone determines t h e b u f f e r searched f i r s t a n d t h a t t h e search is self-terminating. T h e major p r e d i c t i o n d e r i v e d f r o m t h i s model i s t h a t t h e d i f f e r e n c e i n R T between c o r r e s p o n d i n g a n d noncorresponding t r i a l s should b e g r e a t e r f o r stimuli f r o m t h e two-item set t h a n f o r stimuli f r o m t h e f o u r item set. We base t h i s p r e d i c t i o n on t w o assumptions: (a) t h e location o f t h e i r r e l e v a n t t o n e determines t h e b u f f e r searched f i r s t , a n d (b) R T

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is a f u n c t i o n o f t h e number o f items scanned. As Table 1 indicates, t h e d i f f e r e n c e in items scanned between c o r r e s p o n d i n g a n d noncorresponding t r i a l s should b e g r e a t e r f o r stimuli f r o m t h e two-item set t h a n f o r stimuli f r o m t h e f o u r - i t e m set. O u r r e s u l t s confirmed t h i s p r e d i c t i o n . Furthermore, a f t e r we c o r r e c t e d f o r a 31 ms l o n g e r identification time f o r items f r o m t h e f o u r - i t e m set, o u r data (see Table 2) showed a n e a r l y p e r f e c t relation between t h e RTs f o r t h e v a r i o u s treatment conditions a n d t h e number o f items p r e sumably scanned in each condition. O u r f i n d i n g s , then, p r o v i d e f i r m s u p p o r t f o r o u r b u f f e r model o f response selection. Table 2 Reaction Time (in Milliseconds) A d j u s t e d t o Remove t h e Set-Size E f f e c t

Correspondence o f Tone a n d Response Noncorresponding (N) C o r r e s p o n d i n g (C) Mean N - C Difference

Note.

Two-Item Set

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540 504 522 36

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530

535

513 522 17

508

Adjustment i n v o l v e d s u b t r a c t i n g 31 msec f r o m RTs t o t h e f o u r - i t e m

set. T h e Logical Recoding Notion Hedge a n d Marsh (1975) used a paradigm similar t o o u r s t o i n v e s t i gate t h e e f f e c t of manipulating symbolic compatibility on reactions t o w a r d t h e stimulus source. I n one v a r i a n t o f t h e t a s k , t h e y p r e s e n t e d a g r e e n o r a r e d stimulus o n e i t h e r t h e l e f t o r t h e right side o f a v e r t i c a l panel. Subjects responded by moving t h e i r r i g h t h a n d f r o m a c e n t r a l s t a r t b u t t o n t o one o f a p a i r o f response keys, g r e e n on t h e l e f t a n d r e d o n t h e right. Subjects p e r f o r m e d on t w o blocks o f t r i a l s . In one b l o c k ( t h e compatible o r same-color condition), subjects were i n s t r u c t e d t o p r e s s t h e g r e e n k e y when t h e g r e e n stimulus appeared a n d t o p r e s s t h e r e d k e y when t h e r e d stimulus appeared. In another block ( t h e incompatible o r alternate-color condition), subjects were i n s t r u c t e d t o p r e s s t h e r e d k e y when t h e g r e e n stimulus appeared a n d t o p r e s s t h e g r e e n k e y when t h e r e d stimulus appeared. F i g u r e 15 shows in diagrammatic f o r m t h e S-R relations t h a t c h a r acterized t h e various k i n d s o f t r i a l s contained in t h e compatible a n d incompatible blocks. Diagrams (a), (b), (c), a n d (d) r e p r e s e n t t h e f o u r k i n d s o f compatible t r i a l s , whereas diagrams ( e l , (f), (g), a n d (h) r e p r e s e n t t h e f o u r k i n d s o f incompatible t r i a l s . In each diagram, t h e small squares r e p r e s e n t t h e l e f t a n d right windows that, when illuminated, displayed e i t h e r t h e g r e e n ( G ) o r t h e r e d (R) stimulus. The

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MAPPING (SAME-COLOR CONDITION 1

(C)

INCOMPATIBLE S-R

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--

F i g u r e 15. Diagrammatic representation o f t h e various S-R relations t h a t characterized t h e compatible a n d incompatible S-R mapping t r i a l s o f Hedge a n d Marsh (1975) and Simon e t a l . (1981). In each diagram, t h e small squares represent t h e l e f t and right windows that, when illuminated, displayed e i t h e r a green ( G ) o r r e d ( R ) stimulus. T h e circles represent t h e green ( G ) a n d r e d (R) response k e y s . T h e a r r o w in each diagram indicates t h e c o r r e c t response k e y f o r t h e p a r t i c u l a r t r i a l . small circles represent t h e t w o keys, green (G) on t h e l e f t a n d r e d (R) on t h e right. T h e a r r o w in each diagram indicates t h e c o r r e c t response key f o r the particular trial. Hedge a n d Marsh f o u n d t h a t , in t h e compatible block, t h e l e f t response was f a s t e r when t h e stimulus appeared o n t h e l e f t side t h a n when it appeared on t h e right ( a < c), a n d t h e right response was f a s t e r when t h e stimulus appeared on t h e right side t h a n when it appeared o n t h e l e f t (b < d ) . In o t h e r words, reactions were f a s t e r on t r i a l s in which t h e location o f t h e stimulus a n d response corresponded t h a n o n t r i a l s in which t h e y did n o t correspond. Hedge a n d Marsh used t h e label "Simon effect" t o r e f e r t o t h i s t y p i c a l finding. T h e i n t e r e s t i n g aspect o f t h e Hedge a n d Marsh findings, however, was t h e reversal o f t h e Simon e f f e c t i n t h e incompatible block, f o r which reactions were now f a s t e r on t r i a l s in which t h e location o f stimulus and response did n o t correspond than on t r i a l s in which t h e y corresponded. T h a t is, t h e l e f t response was f a s t e r when t h e stimulus appeared on t h e right side t h a n when it appeared on t h e l e f t (e < g), a n d t h e right response was f a s t e r when t h e

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stimulus appeared o n t h e l e f t side t h a n when it appeared on t h e right (f < h). Hedge a n d Marsh explained t h i s a p p a r e n t r e v e r s a l o f t h e Simon e f f e c t in terms o f a notion t h a t t h e y called logical recoding. "For a g i v e n logical r e c o d i n g ( i d e n t i t y o r reversal) o f t h e r e l e v a n t a t t r i b u t e (colour), r e s p o n d i n g was f a s t e r f o r t r i a l s i n w h i c h t h e recoding o f t h e i r r e l e v a n t a t t r i b u t e (position) was o f t h e same logical t y p e as t h a t o f t h e r e l e v a n t a t t r i b u t e , t h a n f o r t r i a l s in w h i c h t h e logical recoding o f t h e i r r e l e v a n t a t t r i b u t e was opposite in t y p e " (p. 435). I n o t h e r words, Hedge a n d Marsh suggested t h a t when subjects h a d t o t r a n s l a t e a r e d stimulus i n t o a g r e e n k e y press, t h e y also recoded o r r e v e r s e d t h e i r r e l e v a n t position c u e so t h a t t h e y now responded f a s t e r on t h e side opposite t h e location o f t h e stimulus. D i s p l a y - C o n t r o l A r r a n g e m e n t Correspondence We f e l t t h a t t h e concept o f logical recoding was n o t necessary t o explain t h e a p p a r e n t r e v e r s a l o f t h e Simon e f f e c t u n d e r incompatible S-R mapping i n s t r u c t i o n s . Rather, we hypothesized t h a t t h e Hedge a n d Marsh r e s u l t s c o u l d b e explained in terms o f a c o n f o u n d i n g w i t h a v a r i e t y o f spatial cfmpatibility, w h i c h we call "display-control arrangement c o r r e spondence. B y this, we mean a correspondence between t h e spatial arrangement ( l e f t a n d right) o f t h e g r e e n a n d r e d l i g h t s o n t h e d i s p l a y panel a n d t h e g r e e n a n d r e d response k e y s . We suggest t h a t , i n t h e Hedge a n d Marsh experiment, half o f t h e t r i a l s in each b l o c k were characterized by d i s p l a y - c o n t r o l arrangement correspondence, a n d h a l f were characterized by d i s p l a y - c o n t r o l a r r a n g e ment noncorrespondence. T h e diagrams in F i g u r e 15 show t h e d i s p l a y c o n t r o l c o n f i g u r a t i o n t h a t characterized t h e various t y p e s o f t r i a l s . Of t h e compatible t r i a l s , types, (a) a n d (b) i n v o l v e d a c o r r e s p o n d i n g arrangement; t h a t is, t h e stimulus light a n d response k e y o f t h e same color were o n t h e same side o f t h e console. T y p e s (c) a n d (d) i n v o l v e d a noncorresponding arrangement. Similarly, o f t h e incompatible trials, t y p e s (e) a n d (f) i n v o l v e d a c o r r e s p o n d i n g arrangement, whereas t y p e s (9) a n d (h) i n v o l v e d a noncorresponding arrangement. (Note that, in F i g u r e 15(a), f o r example, t h e g r e e n light appeared in t h e l e f t window w h i l e t h e right window was "empty." We characterize t h i s d i s p l a y - c o n t r o l c o n f i g u r a t i o n as c o r r e s p o n d i n g because t h e g r e e n light a n d g r e e n k e y a r e o n t h e same side, even t h o u g h t h e r e is n o t a v i s i b l e r e d stimulus in t h e right window t o c o r r e s p o n d t o t h e r e d k e y on t h e right.) Because one m i g h t expect f a s t e r reactions when t h e d i s p l a y - c o n t r o l arrangement i s c o r r e s p o n d i n g t h a n when it is noncorresponding, one would p r e d i c t t h a t t r i a l t y p e s (a) a n d (b) would b e f a s t e r t h a n (c) a n d (d) a n d t h a t t r i a l t y p e s (e) a n d (f) would b e f a s t e r t h a n (9) a n d (h). T h i s is e x a c t l y w h a t Hedge a n d Marsh f o u n d . Thus, we a r e s u g g e s t i n g t h a t a v a r i a t i o n i n d i s p l a y - c o n t r o l arrangement correspondence p r o d u c e d t h e Hedge a n d Marsh r e s u l t s a n d t h a t t h e r e was no need f o r them t o r e s o r t t o t h e more i n v o l v e d explanation in terms o f t h e Simon e f f e c t a n d i t s r e v e r s a l u n d e r incompatible S-R mapping i n s t r u c t i o n s (i.e., t h e notion o f logical recoding). O u r experiment, then, represented an attempt t o replicate t h e Hedge a n d Marsh r e s u l t s by deliberately manipulating d i s p l a y - c o n t r o l arrangement correspondence (Simon, Sly, & Vilapakkam, 1981). Subjects were assigned a t incompatible S - R mapping g r o u p .

random t o e i t h e r a compatible o r an The compatible g r o u p was told, "If a

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green l i g h t appears, press t h e green k e y as f a s t as y o u y:n, while if a r e d light appears, press t h e r e d k e y as f a s t as y o u can. T h e incompatible g r o u p was told, "If a green light appears, press t h e r e d k e y as f a s t as y o u can, while if a r e d light appears, press t h e green k e y as f a s t as y o u can." Both t h e compatible and incompatible g r o u p s performed on t w o blocks o f trials, one b l o c k characterized by display-control arrangement correspondence a n d t h e o t h e r block characterized by display-control arrangement noncorrespondence. Specifically, r e f e r r i n g again t o F i g u r e 15, t h e compatible g r o u p received t r i a l t y p e s (a) a n d (b) in t h e i r c o r r e sponding block a n d t r i a l t y p e s (c) a n d ( d ) i n t h e i r noncorresponding block. T h e incompatible g r o u p received t r i a l t y p e s (e) a n d ( f ) in t h e i r corresponding block a n d t r i a l t y p e s ( 9 ) a n d ( h ) in t h e i r noncorresponding block. T h e reader w i l l note that, in t h e blocks characterized by d i s play-control arrangement correspondence, when t h e green stimulus was presented, it always appeared in t h e l e f t window, a n d when t h e r e d stimulus was presented, it always appeared in t h e right window. In o t h e r words, t h e spatial arrangement o f t h e green ( l e f t ) a n d r e d (right) l i g h t s o f t h e display corresponded w i t h t h e spatial arrangement o f t h e green ( l e f t ) and r e d ( r i g h t ) keys. I n t h e t r i a l blocks characterized by display-control arrangement noncorrespondence, when t h e green stimulus was presented, it always appeared in t h e right window, a n d when t h e r e d stimulus was presented, it always appeared i n t h e l e f t window. In o t h e r words, t h e spatial arrangement o f t h e green (right) a n d r e d ( l e f t ) d i s p l a y l i g h t s did n o t correspond w i t h t h e spatial arrangement o f t h e green ( l e f t ) and r e d (right) keys. The verbal instructions f o r the corresponding and noncorresponding blocks o f t r i a l s were identical. The experimenter ,,demonstrated by p o i n t i n g and, thus, p u r p o s e l y avoided r e f e r r i n g t o l e f t " o r "right" windows, t o ensure t h a t t h e subjects would respond in terms o f t h e relevant stimulus (color) r a t h e r t h a n in terms o f t h e redundant i r r e l e v a n t stimulus (stimulus location). In t h e Hedge a n d Marsh procedure, w i t h e i t h e r color appearing i n e i t h e r window, stimulus location was i r r e l e v a n t b u t n o t redundant. A n overall analysis o f variance confirmed o u r hypothesis; t h a t is, t h e corresponding block was s i g n i f i c a n t l y f a s t e r t h a n t h e noncorrespondi n g block (719 v s . 807 ms). T h i s correspondence e f f e c t o c c u r r e d in t h e compatible g r o u p (633 v s . 746 ms), as well as t h e incompatible g r o u p (804 v s . 868 ms), a n d t h e r e was no interaction between correspondence a n d compatibility. We t h e n relabeled o u r d a t a a n d analyzed them as Hedge a n d Marsh had done. F i g u r e 16 shows that, f o r t h e compatible S-R mapp i n g group, t h e r e was t h e t y p i c a l Simon effect. In o t h e r words, reactions were f a s t e r when t h e location o f t h e stimulus a n d t h e response c o r responded than when it did n o t . For t h e incompatible group, t h a t effect was reversed. T o summarize, we have shown t h a t a manipulation o f display-control arrangement correspondence produced t h e same p a t t e r n o f results as t h a t observed by Hedge a n d Marsh. It seems t o us more parsimonious t o explain t h e Hedge a n d Marsh f i n d i n g s i n terms o f t h i s main e f f e c t t h a n it is t o assert t h a t subjects recode (reverse) t h e i r r e l e v a n t position cue u n d e r conditions o f incompatible S - R mappings ( i . e . , t h e notion o f logical recoding). O u r experiment, however, did not eliminate logical recoding as a viable alternative explanation f o r o u r f i n d i n g s . That is, one m i g h t s t i l l

J.R. Simon

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a r g u e t h a t t h e d i f f e r e n c e between corresponding a n d noncorresponding blocks in t h e compatible g r o u p was d u e t o t h e Simon effect, whereas t h e difference between corresponding a n d noncorresponding blocks in t h e incompatible g r o u p was d u e t o logical recoding. T h e problem is t h a t t h e r e a r e t h r e e k i n d s o f compatibility which a r e i n e x t r i c a b l y confounded in t h e Hedge a n d Marsh t a s k . T h e y a r e display-control arrangement correspondence, t h e Simon effect, and symbolic compatibility. Because o f t h i s confounding, t h e r e is n o way t h a t t h e Hedge a n d Marsh paradigm can b e used t o p r o v i d e a c r u c i a l t e s t o f whether t h e Simon e f f e c t reverses u n d e r incompatible mapping i n s t r u c t i o n s . We, therefore, employed a d i f f e r e n t paradigm which eliminated t h e factor o f display-control arrangement correspondence by p r e s e n t i n g t h e relevant visual stimulus f r o m a single window in t h e center o f t h e d i s p l a y panel. T h e i r r e l e v a n t directional cue was p r o v i d e d by a monaural tone presented simultaneously w i t h t h e relevant visual cue. O u r previous research had demonstrated t h a t responses t o a visual stimulus a r e f a s t e r

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when t h e location o f t h e i r r e l e v a n t tone a n d t h e response c o r r e s p o n d t h a n when t h e y d o n o t (e.g., Acosta & Simon, 1976; Mewaldt e t al., 1980; Simon & Craft, 1 9 7 0 ~ ; Simon & Pouraghabagher, 1978). With t h i s paradigm, then, it was possible t o independently manipulate t h e i r r e l e v a n t directional cue a n d compatibility o f S-R mapping w i t h o u t encountering t h e troublesome confounding o f these f a c t o r s w i t h display-control arrangement correspondence t h a t characterized o u r f i r s t experiment. T h e subjects' response in t h i s second experiment was t h e same as in t h e f i r s t ; t h a t is, t h e y pressed e i t h e r a r e d o r green k e y in response t o t h e onset o f a r e d o r green stimulus t h a t appeared in one c e n t r a l l y located d i s p l a y window. On each trial, t h e stimulus was accompanied by t h e simultaneous onset o f a tone presented t h r o u g h earphones t o t h e A compatible g r o u p was i n s t r u c t e d t o press subjects' right o r l e f t ear. t h e green k e y if t h e green light appeared and t o press t h e r e d k e y if t h e r e d light appeared, whereas an incompatible g r o u p was g i v e n t h e opposite S-R mapping i n s t r u c t i o n s . If a reversal o f t h e Simon e f f e c t were t o occur on t h i s t a s k u n d e r incompatible S-R mapping instructions, we would have clear evidence t o s u p p o r t t h e logical recoding notion. O u r results did n o t s u p p o r t t h e notion o f logical recoding. With compatible SR mapping, we obtained t h e t y p i c a l Simon effect; t h a t is, reactions were f a s t e r when t h e location o f t h e i r r e l e v a n t cue a n d t h e response However, w i t h incompatible S-R corresponded than when t h e y did not. mapping, t h e r e was no evidence o f a reversal o f t h e Simon effect; rather, t h e location of t h e i r r e l e v a n t c u e simply h a d n o effect. It appears t h a t t h e incompatible S-R mapping i n s t r u c t i o n s may have so loaded t h e information-processing apparatus as t o negate t h e e f f e c t o f t h e i r r e l e v a n t directional cue. Thus, evidence f r o m t h i s s t u d y s u p p o r t s e a r l i e r indications (Simon e t al., 1976) t h a t a complex interaction may o c c u r between d i f f e r e n t sources o f compatibility.

A Research Tool for S t u d y i n g A g i n g We have used t h e Simon e f f e c t as a tool, in combination w i t h S t e r n b e r g ' s a d d i t i v e f a c t o r s method, t o determine t h e e f f e c t o f a g i n g on t h e encoding a n d response-selection stages o f a choice R T t a s k . Young (average age 20 years) a n d o l d (average age 73 years) subjects pressed The a l e f t - o r r i g h t - h a n d k e y in response t o t h e onset o f an X o r 0. q u a l i t y o r d i s c r i m i n a b i l i t y o f these characters was manipulated so as t o affect t h e stimulus-encoding stage (Beiderman & Kaplan, 1970; Sternberg, 1967, 1969). A monaural tone presented simultaneously w i t h t h e characters p r o v i d e d an i r r e l e v a n t directional cue, which o u r p r e v i o u s research had shown t o a f f e c t t h e response-selection stage b u t n o t t h e stimulus-encoding stage (e.g., Acosta & Simon, 1976).

If aging affects t h e stimulus-encoding stage, t h e r e should b e a s i g n i f i c a n t Stimulus Quality x Age interaction. If a g i n g affects t h e response-selection stage, t h e r e should b e a s i g n i f i c a n t Directional Cue x Age interaction. If older subjects t e n d t o encode i r r e l e v a n t stimuli t h a t younger subjects ignore, t h i s should manifest i t s e l f as a t r i p l e - o r d e r interaction o f Age x Stimulus Q u a l i t y x Directional Cue. F i g u r e 17 shows t h a t each o f t h e t h r e e independent variables h a d a significant e f f e c t on RT; reactions t o t h e degraded stimulus were slower t h a n t o t h e i n t a c t stimulus; t h e i r r e l e v a n t directional c u e p r o d u c e d t h e t y p i c a l cross e f f e c t (Simon effect) i n each f i g u r e panel; a n d o l d subjects responded slower t h a n y o u n g subjects.

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Of p r i m a r y importance was t h e p a t t e r n of interactions between t h e independent variables. F i r s t , t h e e f f e c t o f t h e i r r e l e v a n t directional c u e did n o t v a r y as a f u n c t i o n o f stimulus q u a l i t y . T h i s f i n d i n g suggests t h a t t h e manipulation of t h e i r r e l e v a n t directional c u e a n d t h e manipulat i o n o f t h e stimulus q u a l i t y affected separate stages. Second, t h e r e was a s i g n i f i c a n t Stimulus Q u a l i t y x A g e interaction; t h a t is, t h e d i f f e r e n c e between i n t a c t a n d degraded R T was more t h a n t w i c e as l a r g e f o r o l d subjects as f o r y o u n g . T h i s interaction c l e a r l y implicates t h e stimulusencoding stage as t h e p r i m a r y locus o f t h e slowing i n information p r o Third, t h e i r r e l e v a n t directional c u e did cessing t h a t accompanies aging. n o t i n t e r a c t w i t h age. Because t h e directional cue affects response selection, t h i s f i n d i n g suggests t h a t t h e response-selection stage was n o t Finally, t h e r e was n o t h i n g in t h e o v e r a l l p a t t e r n o f affected by aging. r e s u l t s t o suggest t h a t o l d e r people process information a n y d i f f e r e n t l y

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t h a n y o u n g people. T h e y simply do it more slowly, a n d t h e main locus o f t h e slowing seems t o b e t h e stimulus-encoding stage. We w i l l n o t detail t h e additional analyses conducted t o e x p l o r e possible a l t e r n a t i v e explanat i o n s f o r o u r f i n d i n g s , because o u r main p o i n t i s simply t o i l l u s t r a t e how t h e Simon e f f e c t can b e used as a tool f o r i n v e s t i g a t i n g o t h e r s u b s t a n t i v e areas. Processing of Single- a n d D u a l - T a s k s In a related s t u d y , we employed t h e Simon effect, along w i t h a d d i t i v e f a c t o r s logic, t o determine w h e t h e r t h e flow o f information u n d e r a dual-tasks condition was d i f f e r e n t f r o m t h e f l o w t h a t p r e v i o u s research h a d suggested o c c u r r e d u n d e r t h e single-task condition (Simon, 1982). Subjects pressed a l e f t - o r r i g h t - h a n d k e y i n response t o t h e onset o f an X o r 0 ( i n t a c t o r degraded), w h i c h was accompanied by a monaural o r b i n a u r a l tone. I n t h e single-task condition, t h e tone was i r r e l e v a n t t o t h e t a s k assigned t h e subjects, whereas, in t h e dual-tasks condition, t h e subject h a d t o respond v e r b a l l y (i.e., left, right, o r b o t h ) t o t h e locat i o n o f t h e tone while also making t h e k e y - p r e s s response. Thus, singlea n d dual-tasks were identical in terms o f t h e stimuli i n v o l v e d but d i f f e r e n t in terms o f t h e response requirements. We were especially interested i n t h e e f f e c t o f t h e tone location on processing t h e v i s u a l stimulus in t h e dual-tasks condition. Would making t h e p r e v i o u s l y i r r e l e v a n t t o n e relev a n t s e r v e t o uncouple i t s e f f e c t on v i s u a l R T o r would t h e t o n e location s t i l l p r o d u c e interactions? If interactions existed, we c o u l d i n f e r a s h a r i n g o f processing capacity a t some stage(s). The pattern o f interact i o n s m i g h t also h e l p t o i d e n t i f y those mechanisms t h a t may b e simultaneo u s l y i n v o l v e d i n processing v i s u a l a n d a u d i t o r y stimuli.

O u r r e s u l t s enabled us t o c o n s t r u c t a model o f t h e flow o f informat i o n t h r o u g h t h e v a r i o u s stages o f processing i n t h e single-task condition, as well as in t h e dual-tasks condition. F i g u r e 18 represents a model o f information processing in t h e single-task condition. T h e diagram indicates t h a t t h e i r r e l e v a n t tone did n o t a f f e c t encoding o f t h e v i s u a l stimulus b u t did a f f e c t response selection. In d r a w i n g t h i s diagram, we were n o t s u r e w h e t h e r t o show t h e a u d i t o r y location cue as a f f e c t i n g t h e response-selection stage d i r e c t l y , o r alternatively, as passing t h r o u g h an In o t h e r words, encoding stage p r i o r t o i t s e f f e c t on response selection. since t h e a u d i t o r y stimulus was i r r e l e v a n t t o t h e subjects' task, it is n o t clear t h a t t h e information was encoded in t h e " o r d i n a r y " sense. What is clear f r o m o u r r e s u l t s is t h a t a n y encoding of t h e a u d i t o r y stimulus t h a t may have o c c u r r e d was separate f r o m t h e encoding of t h e v i s u a l stimulus. O u r major focus, however, was on information processing u n d e r t h e dual-tasks condition. F i g u r e 19 presents f o u r possible models. Model A shows t h e visual a n d a u d i t o r y stimuli b e i n g processed separately, e i t h e r sequentially o r in parallel. Model B p i c t u r e s t h e visual a n d a u d i t o r y stimuli s h a r i n g capacity a t t h e encoding stage. Model C p i c t u r e s t h e v i s u a l a n d a u d i t o r y stimuli s h a r i n g capacity a t b o t h t h e encoding a n d response-selection stages. Model D shows t h e v i s u a l a n d a u d i t o r y stimuli b e i n g encoded separately but s h a r i n g capacity a t t h e response-selection stage.

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STIMULUS

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Figure 19. Four possible models representing information processing in the dual-tasks condition.

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O u r f i n d i n g s indicated t h a t b o t h correspondence (i.e., location o f t h e tone in relation t o t h e c o r r e c t key-press response) a n d stimulus q u a l i t y affected visual RT, b u t these t w o effects did n o t i n t e r a c t . In addition, t h e correspondence e f f e c t was more pronounced in t h e dualtasks condition t h a n in t h e single-task condition. Finally, visual R T was slower in t h e dual- t h a n in t h e single-task condition. These r e s u l t s a r e all consistent w i t h Model D and inconsistent, in one way o r t h e other, w i t h t h e a l t e r n a t i v e models. We w i l l n o t spell o u t t h e complete reasoning here. Suffice it t o say t h a t t h e p a t t e r n o f main effects a n d interactions observed in t h i s s t u d y suggested t h a t t h e same model o f information flow described b o t h single- a n d dual-tasks conditions; t h a t is, visual a n d a u d i t o r y stimuli were encoded separately b u t shared capacity a t t h e response-selection stage. Resemblance t o O t h e r Phenomena T h e Simon effect bears a c e r t a i n resemblance t o o t h e r phenomena. For example, we speculated e a r l y o n that it m i g h t b e a manifestation of a basic o r i e n t i n g r e f l e x (OR) which, in i t s f u l l y developed form, can i n v o l v e a specific molar reaction o f t u r n i n g t o w a r d t h e source o f stimulat i o n (Razran, 1961). O r d i n a r i l y , however, t h e OR e v o k i n g capacity of a stimulus decreases w i t h repeated presentations, whereas t h e Simon e f f e c t is c e r t a i n l y n o t a t r a n s i e n t phenomenon (e.g., Simon e t al., 1973). Similarly, t h e Simon e f f e c t bears some resemblance t o t h e well-known Stroop phenomenon (Jensen & Rohwer, 1966). I n a t y p i c a l Stroop task, t h e stimuli consist o f names o f colors p r i n t e d i n o t h e r colors. Interference occurs when subjects are asked t o name t h e colors a n d i g n o r e t h e words. A t t h e superficial level, t h i s interference, produced by t h e i n c o n g r u i t y o f ink color a n d t h e v e r b a l content o f t h e color word, is similar t o t h e interference we have observed when t h e source o f a command does n o t correspond t o i t s symbolic content. In o u r research, however, t h e interference i s n o t related t o a c o n f l i c t between t w o symbolic cues b u t seems instead t o r e f l e c t a p r i m i t i v e innate tendency t o react t o w a r d t h e source o f stimulation. We believe t h a t t h i s p o t e n t behavioral p a t t e r n may b e analogous t o t h e t a x i s o r d i r e c t e d orientation reactions o f lower animals (Fletcher, 1966; Fraenkel & Gunn, 1961; Lorenz & T i n b e r gen, 1957). T h e necessity o f o v e r r i d i n g t h i s i n i t i a l response tendency before responding t o a relevant symbolic cue would account f o r t h e slower information processing we have observed. Some o f t h e tasks that we have employed bear n o resemblance t o t h e S t r o o p - t y p e t a s k and, yet, we have been able t o generate, test, a n d confirm predictions based o n t h e existence o f t h i s u n i q u e a n d p o t e n t response tendency (e.g., Simon & C r a f t , 1972; Simon, C r a f t , & Webster, 1971). Possible Practical Implications What about t h e practical implications o f o u r research t o engineers who a r e concerned w i t h t h e design a n d arrangement o f displays a n d controls? Clearly, designers should b e m i n d f u l t h a t t h e i r decisions conc e r n i n g t h e location o f a display on a console may, inadvertently, i n t r o duce i r r e l e v a n t directional cues t h a t i n t e r f e r e w i t h a n operator's a b i l i t y t o process relevant symbolic information f r o m t h e display. B u t can we d o more t h a n simply caution designers t o avoid interference f r o m i r r e l e v a n t directional cues? For example, can t h e directional cue actually b e utilized t o facilitate information processing? T w o studies were conducted t o explore t h i s p o s s i b i l i t y . In one s t u d y (Simon, 1968b), we attempted t o facilitate processing o f verbal directional commands by u s i n g t h e ear

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stimulated t o p r o v i d e a r e l e v a n t ( r e d u n d a n t ) directional cue. We asked w h e t h e r subjects would respond f a s t e r t o a d i s p l a y t h a t combined d i r e c t i o n a l a n d symbolic cues (i.e., monaural v e r b a l commands of "right" a n d " l e f t " in t h e ear c o r r e s p o n d i n g t o t h e c o n t e n t o f t h e command) t h a n t h e y would to displays tha! i n v o l v e d ei)Fer a symbolic c u e alone (i.e., b i n a u r a l v e r b a l commands o f right" a n d l e f t " ) o r a directional c u e alone (i.e., monaural p u r e tones). Results showed that, w h i l e responses t o t h e comb i n e d cues were f a s t e r t h a n t o t h e symbolic c u e alone (322 v s . 420 ms), t h e y were n o t f a s t e r t h a n t o t h e directional c u e alone (307 ms). Findings f r o m t h i s study, however, cannot b e used t o s u p p o r t generalizations conc e r n i n g t h e r e l a t i v e effectiveness of symbolic v e r s u s directional cues o r t h e general e f f e c t of combining these cues because q u a l i t a t i v e l y d i f f e r e n t stimuli (tones a n d v e r b a l commands) were used f o r t h e d i f f e r e n t conditions. A n o t h e r s t u d y (Simon & Craft, 1971) o b v i a t e d t h e above-mentioned difficulty by employing p u r e tones t o p r e s e n t b o t h directional a n d symIn separate blocks o f trials, subjects pressed a right- o r bolic cues. (a) high- o r l o w - p i t c h e d b i n a u r a l tonal l e f t - h a n d k e y in response to: commands (symbolic cue), (b) t h e e a r stimulated by a single p u r e tone (directional cue), a n d (c) high- o r low-pitched monaural tonal commands in t h e e a r c o r r e s p o n d i n g t o t h e symbolic c o n t e n t o f t h e command (combined directional a n d symbolic cues). T h e combined ( r e d u n d a n t ) cue condition p r o d u c e d s i g n i f i c a n t l y f a s t e r R T (295 ms) t h a n t h e directional c u e alone (304 ms) o r t h e symbolic c u e alone (371 ms). T h i s finding f i t s i n t o a small b o d y o f l i t e r a t u r e w h i c h suggests that, u n d e r c e r t a i n conditions, p r o v i d i n g r e d u n d a n t information in a d i s p l a y may f a c i l i t a t e i n f o r However, mation processing (e.g., Woodworth & Schlosberg, 1954). because t h e o b s e r v e d d i f f e r e n c e was so small, t h i s finding i s of p e r h a p s g r e a t e r theoretical t h a n p r a c t i c a l significance. Methodological Implications O u r research o n t h e stereotype has important methodological implications f o r studies o f hemispheric dominance a n d interhemispheric t r a n s f e r time, because it i l l u s t r a t e s how i r r e l e v a n t directional cues in t h e d i s p l a y i n t e r a c t w i t h t h e response location t o a f f e c t information processing (Bashore, 1981; Simon & Agens, 1980; Swanson, Ledlow, & Kinsbourne, 1978; Young, 1982). Researchers must b e especially a l e r t t o these d i r e c tional cues because t h e cues may b e confounded w i t h t h e v a r i a b l e t h a t is manipulated (Babkoff, Genser, & Hegge, 1985; Bradshaw & Umilt5, 1984; Cohen, 1973; Elias & Elias, 1976; G a r d n e r e t al., 1977; G a r d n e r & Ward, 1979; Genesee e t al., 1978; l z d e b s k i & Shipp, 1978; Klatzky, 1972). Consider, f o r example, a t a s k t h a t involves presentation o f a stimulus t o t h e right o r l e f t v i s u a l f i e l d in o r d e r t o stimulate t h e l e f t o r right hemisphere. T h e subject is asked t o respond by p r e s s i n g keys labeled Sup"same" o r "different," located o n t h e right a n d left, respectively. pose t h a t same judgments a r e s i g n i f i c a n t l y f a s t e r when t h e stimulus is presented t o t h e right v i s u a l f i e l d t h a n when it is presented t o t h e l e f t . A n i n v e s t i g a t o r m i g h t a t t r i b u t e t h i s finding t o t h e s u p e r i o r i t y o f t h e l e f t hemisphere o v e r t h e r i g h t when, actually, t h e d i f f e r e n c e c o u l d b e d u e t o f a s t e r reactions t o w a r d t h e source o f stimulation (e.g., Simon, 1970). In a similar fashion, r e s u l t s o f c e r t a i n a u d i t o r y asymmetry studies may b e affected by i r r e l e v a n t directional cues associated w i t h t h e ear stimulated Suppose, f o r example, t h a t speech sounds (e.g., Elias & Elias, 1976). a r e p r e s e n t e d monaurally a n d subjects p r e s s a l e f t - o r r i g h t - h a n d k e y t o r e g i s t e r a "yes-no" o r a "same-different" judgment. We would c l e a r l y

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expect t o find f a s t e r reactions w i t h t h e k e y on t h e same side as t h e ear stimulated (Simon & Rudell, 1967). Related Research T h e p u r p o s e o f t h i s c h a p t e r has been t o review o u r own research on a t y p e o f spatial S-R compatibility t h a t Hedge a n d Marsh (1975) have labeled t h e "Simon e f f e c t . " We have indicated how a serendipitous findi n g launched us on t h i s p r o g r a m o f research. We have t h e n d e s c r i b e d a series o f related experiments t h a t demonstrate t h a t t h e location of a stimulus p r o v i d e s an i r r e l e v a n t directional c u e t h a t affects t h e time r e q u i r e d t o process t h e meaning o f t h e stimulus. O u r r e s u l t s suggest t h a t t h e r e may b e a s t r o n g n a t u r a l t e n d e n c y t o respond i n i t i a l l y t o t h e directional component o f a stimulus r a t h e r t h a n t o i t s symbolic content. T h i s i r r e l e v a n t location c u e biases t h e subject t o search a p a r t i c u l a r response b u f f e r f i r s t , which, on c e r t a i n trials, produces a delay in R T . While o u r main aim h e r e has been t o summarize o u r own research, it seems a p p r o p r i a t e t o a t least mention related studies. T h e r e have been a number o f papers concerned w i t h r e p l i c a t i n g a n d / o r e x t e n d i n g o u r findi n g s (Anzola, Bertoloni, Buchtel, & Rizzolatti, 1977; Bauer & Miller, 1982; B e r n s t e i n & Edelstein, 1971; Bertera, Callan, Parsons, & Pishkin, 1975; Brebner, 1973, 1979; Brebner, Shephard, & Cairney, 1972; Callan, Klisz, & Parsons, 1974; Cotton, Tzeng, & H a r d y c k , 1977, 1980; Faber, v a n d e r Molen, Keuss, & Stoffels, 1986; Guiard, 1983; Hammond & Barber, 1978; Hasbroucq, 1987; Hedge & Marsh, 1975; Katz, 1981; Klisz & Parsons, 1975; Ladavas, 1987; Nicoletti & Umilta, 1984; Nicoletti, Umilta, & Ladavas, 1984; Nicoletti, Anzola, Luppino, Rizzolatti, & Umilta, 1982; Pieters, 1981; Puleo & Sheldon, 1977; Ragot, 1984; Ragot & Lesevre, 1986; Ragot & Renault, 1985; Ragot, Renault, & Remond, 1980; 1982; Riggio, Gawryszewski, & Umilti, 1986; Rastatter & Gallaher, Schaefer, Gunn, & Dubowski, 1977; Shulman & McConkie, 1973; Stoffels, v a n d e r Molen, & Keuss, 1985, 1986; Umilta & Nicoletti, 1985; v a n d e r Molen & Keuss, 1981; Wallace, 1971, 1972; Whitaker, 1982). O t h e r papers have suggested a l t e r n a t i v e explanations f o r t h e Simon e f f e c t (Heister & Schroeder-Heister, 1985; Kahneman, 1973; Kinsbourne, 1970; L i d a v a s & Moscovitch, 1984; L u p k e r & Katz, 1982; Nickerson, 1973; Proctor & Reeve, 1985; Smith & Brebner, 1983; Stoffels, 1988; Swanson e t al., 1978). O u r own explanation o f t h e phenomenon has been i n t e r p r e t e d too b r o a d l y on some occasions (e.g., Nicoletti e t al., 1982) a n d too n a r r o w l y on o t h e r s (e.g., U m i l t i & Nicoletti, 1985). We have n e v e r meant t o suggest t h a t all spatial S-R compatibility effects can b e explained in terms o f an innate tendency t o react t o w a r d t h e source o f stimulation. In fact, we have been c a r e f u l t o d i s t i n g u i s h experiments in which t h e stimulus location p r o v i d e s an i r r e l e v a n t c u e f r o m those in w h i c h t h e stimulus location i s the r e l e v a n t cue ( e . g . , Simon, Hinrichs, & C r a f t , 1970; Simon e t al., 1981). N o r d o we d e n y t h e role o f coding a n d Indeed, translation processes in e x p l a i n i n g some compatibility effects. o u r own research p r o v i d e s examples of t h e Simon e f f e c t i n t e r a c t i n g w i t h symbolic compatibility, t h a t is, w i t h t r a n s l a t i o n processes (e.g., Simon e t al., 1976). Umilta a n d Nicoletti (1985) conducted seyeral experiments designed t o t e s t t h e i r "coding hypothesis" against o u r attentional" explanation o f t h e Simon e f f e c t . T h e y conclude t h a t t h e i r r e s u l t s s u p p o r t t h e coding

J.R. Simon hypothesis. In fact, we would have p r e d i c t e d t h e same results. Specifically, we would expect t h e Simon e f f e c t t o occur when b o t h stimuli ( l e f t a n d right) as well as b o t h responses ( l e f t a n d right) a r e located o n t h e In o u r view, t h e Simon e f f e c t does n o t same side o f t h e b o d y midline. necessitate t h a t an actual movement b e made t o w a r d t h e source o f stimulation (see L u p k e r & Katz, 1982). Umilta and Nicoletti's finding o f a significant, t h o u g h reduced, Simon e f f e c t when t h e i r r e l e v a n t location cue did n o t v a r y w i t h i n a block o f t r i a l s was also n o t s u r p r i s i n g because we had obtained t h e same r e s u l t e a r l y in o u r research (Simon & Rudell, 1967). Finally, we have no problem w i t h t h e i r finding t h a t t h e Simon e f f e c t depends, in c e r t a i n cases, o n r e l a t i v e r a t h e r t h a n a b s o l u p location cues. We suggest that, r a t h e r t h a n thinking o f "coding" a n d attention" as t w o mutual!y exclusive explanations o f t h e phenomenon a n d t h e n trying t o design a crucial" experiment t o r e f u t e one explanation and s u p p o r t t h e other, t h e explanations should b e viewed as r e f l e c t i n g a difference in emphasis. Nicoletti a n d Umilta (19851, Wallace (19711, a n d others seem t o view t h e e f f e c t in terms o f a high-level c o g n i t i v e process t h a t involves coding t h e i r r e l e v a n t location cue a n d comparing it t o t h e code f o r t h e relevant cue. In contrast, we t e n d t o minimize t h e c o g n i t i v e component and view t h e e f f e c t in terms o f a more p r i m i t i v e reaction t o t h e location o f t h e stimulus t h a t biases t h e subject t o search a p a r t i c u l a r response buffer first. O u r view has enabled u s t o p r e d i c t results on tasks where t h e r e is no l e f t - r i g h t coding o f stimulus and/or response positions (e.g., Simon & Craft, 1972; Simon, Craft, & Webster, 1971). The continuing i n t e r e s t in t h e Simon e f f e c t by numerous investigators and t h e i r d i f f e r i n g explanations f o r it indicate t h a t t h e r e is s t i l l much t o b e learned about t h i s p o t e n t phenomenon. Summary T h i s c h a p t e r describes almost 40 related experiments concerned w i t h what we believe t o b e a basic and important human response mechanism, a tendency t o react t o w a r d t h e source o f stimulation. T h e following i s a b r i e f summary o f o u r s t e p - b y - s t e p attempt t o u n d e r s t a n d t h e basic n a t u r e o f t h i s phenomenon a n d t o explore some o f i t s practical, methodological, a n d theoretical implications. 1. We discovered q u i t e by accident t h a t R T t o v e r b a l "right" a n d "left" commands was s i g n i f i c a n t l y f a s t e r when t h e content o f t h e command corresponded t o t h e ear stimulated t h a n when it did not. In o t h e r words, t h e location o f t h e stimulus p r o v i d e d a n i r r e l e v a n t cue t h a t affected t h e time r e q u i r e d t o process t h e meaning o f t h e stimulus. This e f f e c t has been labeled t h e Simon e f f e c t .

2. Removing subjects' u n c e r t a i n t y as t o which ear would b e stimulated did n o t eliminate t h i s effect. 3. T h e Simon e f f e c t was n o t d u e t o a n y simple isomorphic associat i o n between ear stimulated a n d ipsilateral hand, because t h e same response i n t e r f e r e n c e also o c c u r r e d in a one-hand t a s k .

4. Subjects reacted f a s t e r when t h e y were t o l d t o move toward r a t h e r t h a n away f r o m t h e source o f a n a u d i t o r y signal, suggesting t h a t t h e r e was a n a t u r a l tendency t o react t o w a r d t h e source o f stimulation. T h e necessity f o r o v e r r i d i n g t h i s n a t u r a l response tendency would account f o r t h e slower information processing on t h e "away" block o f trials.

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5. Whether t h e i r r e l e v a n t directional c u e h a d a f a c i l i t a t i v e and/or i n h i b i t o r y e f f e c t on information processing seemed t o depend o n t h e e x perimental design; specifically, t h e e f f e c t depended o n t h e c o n t e x t in w h i c h baseline t r i a l s were presented.

6. T h e Simon e f f e c t o c c u r r e d when p u r e tones were used t o signal l e f t a n d right responses, i n d i c a t i n g t h a t t h e phenomenon did n o t depend on p r i o r symbolic associations attached t o v e r b a l directional commands.

7. T h e Simon e f f e c t was also o b s e r v e d in a v i s u a l R T t a s k t h a t paralleled t h e o r i g i n a l a u d i t o r y t a s k . T h i s e f f e c t was n o t d u e t o eye stimulated per se but, r a t h e r , t o t h e directional c u e associated w i t h eye stimulated. 8. Additional studies showed t h a t t h e i r r e l e v a n t c u e p r o d u c i n g t h e e f f e c t in t h e a u d i t o r y experiments was n o t t h e ear stimulated per se but r a t h e r t h e directional cue associated w i t h ear stimulated. In o t h e r words, perceptual f a c t o r s r a t h e r t h a n sensory f a c t o r s were responsible f o r t h e Simon e f f e c t . 9. B y manipulating i n t e r a u r a l phase s h i f t , we v a r i e d n o t o n l y t h e a p p a r e n t spatial locus o f a stimulus but also t h e p o t e n c y o f t h e d i r e c tional cue. T h e s t r o n g e r t h e directional cue, t h e s t r o n g e r was t h e t e n d e n c y t o react t o w a r d t h e source o f stimulation.

10. B y manipulating t h e i n t e n s i t y a n d locus o f b r o a d - b a n d noise presented c o n c u r r e n t l y w i t h t h e command, we eliminated, reduced, accentuated, a n d even r e v e r s e d t h e Simon effect. In o t h e r words, t h e stereotype involves a t e n d e n c y t o react t o w a r d t h e major source o f stimulation a n d n o t simply a t e n d e n c y t o react t o w a r d t h e r e l e v a n t stimul u s , t h a t is, t h e command. 11. T h e mechanisms u n d e r l y i n g a u d i t o r y S-R compatibility d i f f e r e d f o r d i f f e r e n t tasks. When stimulus location was t h e r e l e v a n t cue, t h e r e ear/hand corresponwere t w o components t o a u d i t o r y S-R compatibility: dence a n d ear/response-location correspondence. However, when stimulus c o n t e n t was t h e r e l e v a n t cue a n d location t h e i r r e l e v a n t cue, t h e n ear/response-location correspondence alone accounted f o r S-R compatibility effects. 12. T h e i r r e l e v a n t directional cue p r o d u c e d an e f f e c t across sens o r y modalities; t h a t is, speed o f r e s p o n d i n g t o a v i s u a l d i s p l a y was affected by t h e location o f a c o n c u r r e n t but i r r e l e v a n t a u d i t o r y stimulus.

13. Experiments i n v o l v i n g responses t o t h e changed element o f a two-element d i s p l a y (i.e., onset a n d o f f s e t o f l i g h t s a n d tones) p r o d u c e d r e s u l t s consistent w i t h t h e notion o f a s t e r e o t y p i c t e n d e n c y t o react t o w a r d t h e a p p a r e n t source o f stimulation. 14. I n an experiment i n v o l v i n g an o d d i t y task, subjects h a d t o i d e n t i f y a n d respond t o t h e one element of a three-element d i s p l a y t h a t was d i f f e r e n t f r o m t h e o t h e r t w o elements. I r r e l e v a n t cues f r o m t h i s c h a n g i n g d i s p l a y a p p a r e n t l y elicited a s t e r e o t y p i c response t h a t i n t e r f e r e d w i t h information processing. 15. T h e stereotype p e r s i s t e d r e l a t i v e l y undiminished f o r more t h a n 1,000 t r i a l s o v e r a 5 - d a y p e r i o d . Furthermore, analysis o f t h e latency

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a n d f r e q u e n c y o f d i f f e r e n t k i n d s o f e r r o r s s u p p o r t e d t h e conclusion t h a t t h e e f f e c t represented an i n i t i a l tendency t o react t o w a r d t h e source o f a stimulus r a t h e r t h a n t o i t s meaning.

16. A n i r r e l e v a n t d i r e c t i o n a l c u e presented p r i o r t o t h e onset o f a r e l e v a n t stimulus i n t e r f e r e d w i t h t h e processing o f t h a t stimulus. 17. Studies employing a d d i t i v e f a c t o r s logic suggested t h a t t h e directional c u e affected t h e response-selection stage a n d did n o t a f f e c t t h e stimulus-encoding stage. 18. A s t e r e o t y p i c reaction t o w a r d t h e stimulus source o c c u r r e d in a t a s k i n v o l v i n g high- a n d low-pitched "up" a n d "down" commands emanating f r o m a t o p o r bottom speaker. T h i s f i n d i n g p r o v i d e d c o n v i n c i n g e v i dence t h a t l a t e r a l i t y was n o t a f a c t o r in p r o d u c i n g t h e Simon effect. Results also revealed an interaction between t w o p o t e n t population stereotypes, a frequency-space stereotype, a n d a t e n d e n c y t o respond t o w a r d t h e source of stimulation.

19. T w o d i f f e r e n t paradigms were used t o c h a r t t h e time course o f t h e phenomenon and, also, t o localize i t s e f f e c t in relation t o t h e stages o f information processing. Results p r o v i d e d additional c o n v e r g i n g e v i dence t h a t t h e directional c u e affected response selection. 20. We developed a n d t e s t e d a b u f f e r model o f t h e response-select i o n process. Results s u p p o r t e d t h e notion t h a t response selection i n v o l v e d a serial s e l f - t e r m i n a t i n g search o f response b u f f e r s a n d t h a t t h e i r r e l e v a n t directional c u e determined t h e b u f f e r searched f i r s t . 21. We e x p l o r e d a n d rejected t h e suggestion that, u n d e r incomp a t i b l e S-R mapping i n s t r u c t i o n s , subjects m i g h t recode (i.e., reverse) t h e i r r e l e v a n t location cue a n d respond f a s t e r o n t h e side opposite t h e location o f t h e stimulus. O u r r e s u l t s indicated t h a t a confounded factor, d i s p l a y - c o n t r o l arrangement correspondence, p r o v i d e d a more parsimonious explanation for t h e a p p a r e n t r e v e r s a l o f t h e Simon e f f e c t . 2 2 . We have used t h e stereotype as a tool i n combination w i t h S t e r n b e r g ' s a d d i t i v e f a c t o r s logic to s t u d y performance changes w i t h a g i n g a n d t o examine information processing u n d e r dual-tasks conditions.

23. We suggested t h a t d i s p l a y designers attempt t o avoid i n t r o d u c i n g i r r e l e v a n t directional cues t h a t i n t e r f e r e w i t h information processing. B u t can designers f a c i l i t a t e information processing by making t h e directional cue relevant? We f o u n d t h a t p r o v i d i n g r e d u n d a n t information in a d i s p l a y may, u n d e r c e r t a i n conditions, f a c i l i t a t e information processing, but t h e e f f e c t is so small as t o b e o f d o u b t f u l p r a c t i c a l significance. 24. O u r f i n d i n g s have important methodological implications f o r researchers ex p l o r i ng hemispheric domi na nce effects, because ir r e l e v a nt directional cues may b e confounded w i t h t h e variable b e i n g manipulated. 25. We suggest t h a t t h e t e n d e n c y f o r humans t o react t o t h e source o f stimulation may b e analogous t o t h e t a x i s o r d i r e c t e d orientation react i o n s o f lower animals.

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(1982). E f f e c t o f an a u d i t o r y stimulus on t h e processing Simon, J. R. o f a v i s u a l stimulus u n d e r single- a n d d u a l - t a s k s conditions. Acta Psychologica, 51, 61 -73. Simon, J. R., & Acosta, E., J r . (1982). E f f e c t o f i r r e l e v a n t information on the processing of relevant information. Perception & Psychophysics, 31, 383-388. Simon, J. R., Acosta, E., Jr., & Mewaldt, S . P. (1975). E f f e c t o f locus of w a r n i n g tone on a u d i t o r y choice reaction time. Memory & Cognition, 3 , 167-170. Simon,

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to

stages

of

Retrieval time in a monaural

Simon, J. R., & Craft, J. L. (1970a). Effects of an i r r e l e v a n t a u d i t o r y Journal of Experimental stimulus on v i s u a l choice reaction time. Psychology, 86, 272-274.

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Simon, J. R., & C r a f t , J. L. (1970b). Effects of altered d i s p l a y - c o n t r o l relationships o n information processing f r o m a visual display. Journal of Applied Psychology, 54, 253-257. Simon, J. R., & C r a f t , J. L. (1971). Communicating directional i n f o r mation w i t h an a u d i t o r y d i s p l a y . Journal of Applied Psychology, 55, 241 -243. Simon, J . R., & C r a f t , J. L. (1972). Reaction time in an o d d i t y t a s k : Responding t o t h e " d i f f e r e n t " element o f a t h r e e - l i g h t d i s p l a y . Journal of Experimental Psychology, 92, 405-411. Simon, J. R., Craft, J. L., & Small, A . M., Jr. (1970). Manipulating t h e s t r e n g t h o f a stereotype: I n t e r f e r e n c e effects in an a u d i t o r y information-processing t a s k . Journal of Experimental Psychology, 8 6 , 63-68. Simon, J. R., Craft, J. L., & Small, A . M., Jr. (1971). t o w a r d t h e a p p a r e n t source o f an a u d i t o r y stimulus. Experimental Psychology, 8 9 , 203-206.

Reactions Journal of

Reaction time t o Simon, J. R., C r a f t , J. L., & Webster, J . B. (1971). onset a n d o f f s e t o f l i g h t s a n d tones: Reactions t o w a r d t h e changed element in a two-element display. Journal of Experimental Psychology, 89, 197-202. Simon, J. R., C r a f t , J. L., & Webster, J. B. (1973). Reactions t o w a r d t h e stimulus source: Analysis o f c o r r e c t responses a n d e r r o r s o v e r a f i v e - d a y p e r i o d . Journal of Experimental Psychology, 101, 175-178. Simon, J. R . , Hinrichs, J. V., & C r a f t , J . L. (1970). A u d i t o r y S-R compatibility: Reaction time as a f u n c t i o n o f e a r - h a n d correspondence Journal of Experimental a n d ear-response-location correspondence. Psychology, 86, 97-102. Simon, J. R., Mewaldt, S . P . , Acosta, E., Jr., & Hu, J. (1976). Processing a u d i t o r y information: I n t e r a c t i o n o f t w o population stereot y p e s . Journal of Applied Psychology, 6 1 , 354-358. Simon, J. R., & Pouraghabagher, A. R . (1978). T h e e f f e c t o f aging on Journal of t h e stages o f processing in a choice reaction time t a s k . Gerontology, 3 3 , 553-561. Simon, J. R . , & Rudell, A . P. (1967). A u d i t o r y S - R compatibility: T h e Journal of e f f e c t o f an i r r e l e v a n t c u e on information processing. Applied Psychology, 5 1 , 300-304. E f f e c t of compatiSimon, J. R., Sly, P . E., & Vilapakkam, S . (1981). bility o f S - R mapping on reactions t o w a r d t h e stimulus source. Acta Psychologica, 47, 63-81. Simon, J . R., E Small, A . M., J r . (1969). Processing a u d i t o r y i n f o r Journal of Applied mation: I n t e r f e r e n c e f r o m an i r r e l e v a n t cue. Psychology, 5 3 , 433-435. Simon, J. R., Small, A . M., Jr., Ziglar, R . A., & C r a f t , J. L. Response i n t e r f e r e n c e in an information processing t a s k :

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Simon, J. R., & Sudalaimuthu, P. (1979). Effects o f S-R mapping a n d Journal of response modality on performance on a Stroop t a s k . Experimental Psychology: Human Perception and Performance, 5 , 176187. Simon, J. R., & Wolf, J. D. (1963). Choice reaction time as a f u n c t i o n o f angular stimulus-response correspondence a n d age. Ergonomics, 6 , 99- 105. (1983). S-R compatibility: T h e r e l a t i v e Smith, P., & Brebner, J. Australian effects o f "relevant" spatial a n d non-spatial variables. Journal of Psychology, 35, 1-10. Sternberg, S . (1967). T w o operations in character recognition: Some Perception & evidence from reaction-time measurements. Psychophysics, 2 , 45-52. Sternberg, S. (1969). of Donders' method.

T h e discovery of processing stages: Extensions Acta Psychologica, 30, 276-315.

Stoffels, E. J. (1988). Reactions toward the stimulus source: The locus of the effect. Unpublished doctoral dissertation, Free U n i v e r s i t y , Amsterdam, T h e Netherlands. Stoffels, E. J., van d e r Molen, M. W., & Keuss, P. J. G. (1985). l n t e r s e n s o r y facilitation a n d i n h i b i t i o n : Immediate arousal a n d location Acta effects o f a u d i t o r y noise on visual choice reaction time. Psychologica, 58, 45-62. Stoffels, E. J., van d e r Molen, M. W., & Keuss, P. J. G . (1986). Motoric effects on precued discrete finger responses: A different-hand advantage, not a same-hand advantage. Paper presented a t t h e a n n u a l meeting of the Psychonomic Society, New Orleans, LA. Swanson, J., Ledlow, A . , & Kinsbourne, M. (1978). Lateral asymmet r i e s revealed by simple reaction time. I n M. Kinsbourne (Ed.), Asymmetrical function of the brain ( p p . 274-291). Cambridge: Cambridge U n i v e r s i t y Press. Taylor, D. A . (1976). Bulletin, 83, 161-191.

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Umiltti, C., & Nicoletti, R . (1985). Attention a n d coding effects in S - R compatibility d u e t o i r r e l e v a n t spatial cues. I n M. I. Posner & 0. S. M. Marin (Eds.), Attention and performance XI (pp. 457-471). Hillsdale, NJ: Erlbaum. Response selection van d e r Molen, M. W., & Keuss, P. J. G . (1981). and t h e processing o f a u d i t o r y i n t e n s i t y . Quarterly Journal of Experimental Psychology, 33A, 177- 184. Wallace, R. J. (1971). S-R compatibility a n d t h e idea o f a response code. Journal of Experimental Psychology, 88, 354-360.

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Wallace, R . J. (1972). Spatial S - R compatibility effects involving kinesthetic cues. Journal of Experimental Psychology, 93, 163-168. (1982). Stimulus-response compatibility for left-right Whitaker, L. A . discriminations as a function of stimulus position. Journal of Experimental Psychology: Human Perception and Performance, 8 , 865-874. Woodworth, R . S . , & Schlosberg, H . New York: Holt.

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Young, A. W. (1982). Methodological and theoretical bases of visual hemifield studies. I n J . G . Beaumont ( E d . ) , Divided visual field studies of cerebral organisation (pp. 11 -27). L.ondon: Academic Press.

P A R T II MENTAL REPRESENTATION T h e f o u r chapters o f P a r t II p r o v i d e evidence f o r c o g n i t i v e coding as a major f a c t o r in stimulus-response compatibility. A l l o f t h e chapters address aspects o f spatial-compatibility, in w h i c h spatial-location stimuli a r e assigned t o d i s c r e t e k e y p r e s s responses a t d i s t i n c t locations. C h a p t e r 3, by U m i l t i a n d Nicoletti, is organized a r o u n d a c u r r e n t t a x onomy o f compatibility effects. T h e c h a p t e r evaluates t h e roles o f egocentric a n d r e l a t i v e spatial dimensions f o r stimuli a n d responses, b o t h when stimulus locations a r e r e l e v a n t t o t h e t a s k a n d when t h e y a r e irrelevant. A l t e r n a t i v e hypotheses r e g a r d i n g t h e n a t u r e o f t h e effects a r e contrasted. C h a p t e r 4, by Heister, Schroeder-Heister, and Ehrenstein, proposes a hierarchical model in w h i c h spatial coding a n d spatio-anatomical mapping p l a y roles. C h a p t e r 5, by Lidavas, presents a similar hierarchical model a n d p r o v i d e s evidence t h a t p r e - v e r b a l c h i l d r e n show spatial-compatibility effects. C h a p t e r 6, by Reeve a n d Proctor, extends t h e investigation o f spatial c o d i n g t o more complex, four-choice It also demonstrates how compatibility effects can arise p r e c u i n g tasks. f r o m symbolic stimuli assigned t o keypress responses.

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STIMULUS-RESPONSE COMPATIBILITY R. W. Proctor and T.G. Reeve (Editors

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S P A T I A L STIMULUS-RESPONSE C O M P A T I B I L I T Y CARLO U M I L T ~ l s t i t u t o di Fisologia Umana U n i v e r s i t a di Parma, I t a l y ROBERTO N l C O L E T T l Dipartimento di Psicologia Generale U n i v e r s i t a di Padova, I t a l y

F i t t s (1951) observed t h a t people, if allowed t o choose one response t o a stimulus among a set o f possible responses, were consist e n t in t h e i r choice. T o describe t h e phenomenon h e used t h e t e r m "population stereotype," meaning t h a t f o r most people t h e r e is an o p t i mal p a i r i n g o f t h e elements o f t h e stimulus set w i t h t h e elements o f t h e response set. T h e tasks t h a t conform t o such population stereotypes a r e easier t o perform, i n terms o f b o t h speed a n d accuracy, t h a n those t h a t d o n o t conform t o them. In subsequent studies ( F i t t s & Deininger, 1954; F i t t s & Seeger, 1953), performance in these t a s k s was shown n o t t o depend o n t h e p a r t i c u l a r sets o f stimuli a n d responses t h a t were used, but on t h e way in which i n d i v i d u a l stimuli a n d responses were p a i r e d w i t h each o t h e r . In o t h e r words, t h e t y p e o f stimulus-response (S-R) p a i r i n g adopted in t h e t a s k determined t h e speed a n d accuracy w i t h which t h e t a s k was executed. T h e t e r m "population stereotype" r e f e r r e d t o t h e f a c t that, g i v e n t h e occurrence o f a c e r t a i n element o f t h e stimulus set, subjects showed a consistent tendency t o select a p a r t i c u l a r element o f t h e response set. I n contrast, t h e t e r m "stimulus-response compatibility," w h i c h l a t e r became popular, r e f e r s t o t h e f a c t t h a t some S-R p a i r i n g s a r e easier t o use t h a n o t h e r s . F o r example, i n choice reaction time ( R T ) tasks, S - R p a i r i n g s t h a t y i e l d s h o r t e r latencies a n d lower e r r o r rates a r e said t o b e more compatible t h a n S-R p a i r i n g s t h a t y i e l d longer latencies a n d h i g h e r e r r o r rates. In s p i t e o f t h e p o p u l a r i t y enjoyed by t h e t e r m S - R compatibility, i t s use does n o t indicate a b e t t e r u n d e r s t a n d i n g o f t h e phenomenon t h a n t h e use o f t h e o l d e r t e r m "population stereotype." In fact, b o t h terms a r e merely d e s c r i p t i v e . Population stereotype describes t h e p r o b a b i l i t y w i t h w h i c h a response is chosen, whereas S-R compatibility describes t h e speed a n d accuracy w i t h w h i c h a response is emitted. Yet, g i v e n a c e r t a i n stimulus, t h e most probable response is also t h e fastest a n d most accurate one (Nicoletti E Umilt6, 1984). A Taxonomy of S-R Compatibility V e r y likely, t h e vagueness of t h e t e r m S-R compatibility, coupled w i t h t h e false impression t h a t t h e t e r m possesses an e x p l a n a t o r y value,

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r e n d e r e d it so p o p u l a r as t o b e employed t o describe a number o f a p p a r e n t l y dissimilar phenomena. Simon, S l y a n d Vilapakkam (1981; see also Hedge & Marsh, 1975) d i s t i n g u i s h e d t h r e e t y p e s o f S-R compatibility. T h e f i r s t was "symbolic compatibility," w h i c h r e s u l t s f r o m t h e v e r b a l labels associated w i t h t h e stimulus a n d t h e response. T h e seco n d was "spatial compatibility," w h i c h occurs when t h e position o f t h e stimulus signals t h e position o f t h e r e q u i r e d response. T h e third was t h e "Simon effect," w h i c h occurs when t h e position o f t h e stimulus, t h o u g h i r r e l e v a n t , biases t h e selection o f t h e response. In our previous w o r k (see, e.g., Nicoletti & U m i l t i , 1984; Umilta & Nicoletti, 1985) we made use o f t h i s taxonomy. However, v e r y recently, Kornblum, Hasbroucq, a n d Osman (in press) succeeded in p r o v i d i n g a much more comprehensive taxonomy, w h i c h w i l l t h u s b e adopted i n t h i s c h a p t e r . Pivotal t o K o r n b l u m e t a l . ' s (in press) taxonomy i s t h e notion of dimensional o v e r l a p o f t h e S - R ensemble, according t o w h i c h t h e stimul u s set may o r may n o t share categorical a t t r i b u t e s w i t h t h e response set. T o t h e e x t e n t t h a t it does, it generates v a r y i n g degrees o f dimensional o v e r l a p in t h e S-R ensemble. Note t h a t t h e notion o f dimensional o v e r l a p i s n o t r e s t r i c t e d t o t h e p h y s i c a l a t t r i b u t e s o f stimuli a n d responses but extends also t o t h e i r mental representations. A n o t h e r important aspect o f t h e taxonomy is t h e d i s t i n c t i o n between r e l e v a n t a n d i r r e l e v a n t dimensions o f t h e stimulus set. A stimulus dimension is r e l e v a n t when t h e r e q u i r e d response depends on t h e value o f t h e stimulus in t h a t dimension, whereas a stimulus dimension is i r r e l e v a n t if values on it a r e uncorrelated w i t h t h e r e q u i r e d response. B y combining dimensional o v e r l a p w i t h dimensional relevancy, K o r n b l u m e t a l . (in press) o b t a i n f o u r t y p e s o f S-R ensembles. Most of t h e following examples a r e taken f r o m t h e i r p a p e r . Examples of spatial S - R ensembles, more p e r t i n e n t t o t h e purposes o f t h e p r e s e n t chapter, w i l l b e p r o v i d e d in subsequent sections.

Type I Ensembles T h i s t y p e o f ensemble occurs when t h e r e is no dimensional o v e r l a p in e i t h e r t h e r e l e v a n t o r t h e i r r e l e v a n t dimension. T h a t is t o say, i n it t h e stimulus set does n o t share a n y categorical a t t r i b u t e s w i t h t h e response set. Because o f this, a n y S - R p a i r i n g is as easy as a n y o t h e r p a i r i n g , a n d t h e v e r y basis of S-R compatibility effects is l a c k i n g in t h e t a s k . A good example o f T y p e I ensemble is t h a t o f a condition i n F i t t s a n d Deininger's (1954) s t u d y i n w h i c h movements (response set) were p a i r e d w i t h p r o p e r names (stimulus set). Not s u r p r i s i n g l y , all S - R p a i r i n g s p r o v e d e q u a l l y good.

It is perhaps w o r t h n o t i n g t h a t T y p e I ensembles can b e more d i f f i c u l t t o find t h a n one would t h i n k . For instance, L i d a v a s (1987) f o u n d an e f f e c t o f S - R p a i r i n g in a t a s k i n which t h e subjects were r e q u i r e d to p r e s s t h e l e f t o r right b u t t o n in response t o a light shown above or below t h e f i x a t i o n p o i n t . H e r r i g h t - h a n d e d subjects did b e t t e r when t h e l e f t b u t t o n was p a i r e d w i t h t h e stimulus located below t h e f i x a t i o n point, a n d t h e right b u t t o n was p a i r e d w i t h t h e stimulus located above it, t h a n when t h e p a i r i n g s were reversed. T h a t means t h a t t h i s S - R ensemble showed an unexpected dimensional overlap.

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T y p e I I Ensembles T h i s t y p e o f ensemble is characterized by t h e presence o f dimensional o v e r l a p in t h e r e l e v a n t dimension. Therefore, it satisfies t h e condition f o r o b t a i n i n g S-R compatibility effects. Most o f t h e studies w i t h T y p e I I ensembles use spatial stimulus a n d response dimensions: A light is p r e s e n t e d in t h e l e f t o r i n t h e right visual f i e l d a n d t h e subject is r e q u i r e d t o p r e s s e i t h e r t h e l e f t o r t h e right k e y in response. R T is f a s t e r when t h e imperative stimulus a n d t h e response a r e on t h e same side t h a n when t h e y a r e on opposite sides (see, e.g., Nicoletti, Anzola, Luppino, Rizzolatti, & UmiltS, 1982; Simon, 1969). Similar spatial S-R compatibility effects a r e observed when t h e light stimuli a r e p r e s e n t e d in t h e u p p e r o r i n t h e lower v i s u a l f i e l d a n d t h e response keys a r e located one above t h e o t h e r (see, e.g., Nicoletti & Umilta, 1984, 1985). A c c o r d i n g t o o u r p r e v i o u s terminology, these effects i n v o l v e d "spatial compatibility. "

Also, nonspatial T y p e 1 1 ensembles p r o d u c e S-R compatibility effects. F o r instance, S t e r n b e r g (1969) a n d Blackman (1975) i n s t r u c t e d t h e i r subjects t o respond t o d i g i t s w i t h t h e name o f t h e digit o r w i t h Similarly, Sanders (1970) used vowels as t h e name of t h e n e x t digit. stimuli a n d t h e name o f t h e vowel o r t h a t o f t h e n e x t vowel in t h e response. I n all cases, t h e r e were c l e a r - c u t S-R compatibility effects, in t h e sense t h a t t h e name o f t h e stimulus gave f a s t e r R T s t h a n t h e In t h e o l d taxonomy, these effects were called "symbolic n e x t name. compatibility . " T y p e I l l Ensembles

In t h i s t y p e o f ensemble, t h e relevant stimulus dimension has no o v e r l a p w i t h a n y o f t h e response dimensions, whereas t h e i r r e l e v a n t dimension does. T y p i c a l l y , subjects a r e presented w i t h t w o geometric f i g u r e s o r t w o color l i g h t s a n d a r e i n s t r u c t e d t o p r e s s t h e l e f t k e y i n response t o one f i g u r e o r color a n d t h e r i g h t k e y i n response t o t h e o t h e r f i g u r e o r color. T h e stimuli a r e randomly shown in t h e l e f t o r right side o f t h e d i s p l a y . Despite t h e f a c t t h a t side is i r r e l e v a n t , R T is f a s t e r when t h e side o f t h e response k e y corresponds t o t h e side where t h e stimulus appears t h a n when stimulus a n d response o c c u r on opposite sides (see, e . g . , U m i l t i & Nicoletti, 1985; Wallace, 1971). Simon a n d Small (1969) obtained similar r e s u l t s i n t h e a u d i t o r y modality when t h e subjects h a d t o p r e s s t h e l e f t o r right k e y i n response t o h i g h o r low p i t c h e d tones, randomly d e l i v e r e d t o t h e l e f t o r right ear. Side o f stimulated ear was i r r e l e v a n t , b u t R T was f a s t e r when t h e side of stimulation corresponded t o t h e side of t h e response t h a n when it did not. B y following t h e o l d taxonomy, these effects would have been termed "Simon effects. " It is i n t e r e s t i n g t o note t h a t in all studies t h a t showed S-R comp a t i b i l i t y effects w i t h T y p e I I I ensembles, t h e i r r e l e v a n t dimension was spatial. T h a t a p p a r e n t l y means t h a t correspondence i n space is a p a r t i c u l a r l y p o w e r f u l way o f o b t a i n i n g an o v e r l a p between stimulus set a n d response set.

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92 T y p e I V Ensembles

In t h i s t y p e o f ensemble, t h e stimulus a n d response sets overlap' o n b o t h a r e l e v a n t a n d an i r r e l e v a n t dimension. When t h e relevant a n d t h e i r r e l e v a n t dimensions a r e d i f f e r e n t , t h e ensemble is characterized as T y p e I V - A , a n d when t h e t w o dimensions a r e equal, t h e ensemble is characterized as T y p e IV-€3. A n example o f a T y p e I V - A ensemble can b e f o u n d in a s t u d y by Hedge a n d Marsh (1975). I n t h e i r s t u d y t h e subjects performed a choice R T t a s k w i t h r e d o r g r e e n l i g h t s appearing t o t h e l e f t o r right o f a f i x a t i o n p o i n t . T h e responses consisted o f p r e s s i n g r e d a n d g r e e n k e y s positioned t o t h e l e f t o r right o f t h e b o d y midline. Color was t h e r e l e v a n t dimension f o r mapping stimuli o n t o responses, whereas side was t h e i r r e l e v a n t dimension. S - R compatibility effects a t t r i b u t a b l e t o b o t h dimensions were f o u n d . Examples o f T y p e I V - B ensembles a r e those t y p i c a l l y employed i n S t r o o p - t y p e t a s k s (see, e.g., reviews in D y e r , 1973, a n d in Jensen & Rohwer, 1966) f o r w h i c h t h e r e l e v a n t dimension i s t h e color o f t h e stimulus a n d t h e i r r e l e v a n t dimension is t h e meaning o f t h e stimulus (i.e., t h e w o r d "green" w r i t t e n in green o r r e d ink). A somewhat d i f f e r e n t example o f a T y p e IV-8 ensemble can b e f o u n d i n a s t u d y by Simon, Mewaldt, Acosta, a n d H u (1976b), i n w h i c h subjects were i n s t r u c t e d t o move a t o g g l e s w i t c h up i n response t o a h i g h - p i t c h e d tone a n d down i n response t o a low-pitched tone. T h e tones were emitted randomly by t w o loudspeakers positioned one above t h e o t h e r . Frequency o f t h e sound p r o v i d e d t h e r e l e v a n t dimension a n d position o f t h e loudspeaker p r o v i d e d t h e i r r e l e v a n t dimension. Simon e t al. showed t h a t b o t h dimensions p r o d u c e d S-R compatibility effects. Summary T h e taxonomy i l l u s t r a t e d above is meant t o encompass all t y p e s of S - R compatibility effects a n d w i l l b e applied h e r e t o t h e special case o f spatial S-R compatibility. T h e general p l a n o f t h e c h a p t e r is t h e following. F i r s t , t w o sections a r e devoted t o discussing spatial stimulus a n d response dimensions t h a t o v e r l a p t o p r o d u c e S - R compatibility effects. Then, in t h e subsequent section, we i l l u s t r a t e a phenomenon, namely, t h e dominance o f t h e l e f t - r i g h t o v e r t h e above-below dimension, which can shed light on how t h e stimulus dimensions a r e processed. Finally, in t h e last section, e x p l a n a t o r y hypotheses o f spatial S-R comp a t i b i l i t y a r e t a k e n i n t o consideration. Egocentric a n d Relative Stimulus Dimensions T h e v i s u a l choice R T studies t h a t w i l l b e considered in t h i s sect i o n employed ensembles o f T y p e I I , T y p e I l l , a n d T y p e IV. In all cases t h e o v e r l a p p i n g dimension f o r t h e stimulus a n d response sets was spatial, a n d i t s values were specified in terms of l e f t - r i g h t positions. In o t h e r words, t h e response set a n d t h e stimulus set always were comp r i s e d o f t w o elements, namely, a l e f t a n d a r i g h t position.

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When t h e stimulus set comprises a l e f t and a r i g h t element, l e f t and r i g h t values can be specified along two dimensions, one egocentric and t h e other relative. I n t h e case of t h e egocentric dimension, t h e stimulus can be on either t h e l e f t o r r i g h t side i n relation t o an egocentric reference axis, such as t h e body midline, t h e head midline, o r t h e vertical retinal meridian. (Note t h a t if t h e observer fixates a point without t u r n i n g t h e head o r t h e body, t h e three axes are aligned and t h e stimulus is on t h e l e f t o r r i g h t side in relation t o each axis). In t h e case of t h e relative dimension, t h e stimulus can be on either t h e l e f t o r right position i n relation t o an external reference point, such as t h e other stimulus. From now on we will use t h e term side" t o mean t h e position in relation t o one o r more of t h e egocentric axes (which are assumed t o be aligned) and t h e term "relative position" t o mean t h e position i n relation t o an external reference point (usually, t h e other stimulus). The two dimensions cannot be distinguished if t h e two stimuli are shown t o t h e l e f t o r r i g h t of t h e egocentric axes: The stimulus on t h e l e f t side occupies t h e l e f t relative position, and t h e stimulus on t h e Consider, however, a r i g h t side occupies t h e right relative position. display such as t h a t shown in Figure 1 . The stimulus can appear in two positions only (i.e., those marked by the two boxes), and both positions are on t h e r i g h t side. Side and relative position are

LVF

RVF

r Figure 1. Schematic diagram of stimulus display and response keys. I n t h e delay condition t h e two boxes preceded the stimulus by 500 ms. (RVF, r i g h t visual field o r r i g h t side; LVF, l e f t visual f i e l d o r l e f t side).

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unconfounded a t t h e box near t h e f i x a t i o n point, which is on t h e right side but occupies t h e l e f t r e l a t i v e position. Side a n d r e l a t i v e position a r e unconfounded also a t t h e box closer t o f i x a t i o n o n t h e l e f t side ( n o t shown i n F i g u r e 1). O f course, t h e d i s t i n c t i o n between t h e egocentric a n d t h e r e l a t i v e dimensions holds f o r t h e response set as well. T h a t is, f o r t h e positions o f t h e responses, t h e r e is also a l e f t o r right side a n d a l e f t o r right r e l a t i v e position. Side a n d position can b e p a r t i a l l y unconfounded by p o s i t i o n i n g t h e t w o responses on t h e same side in relation t o t h e egocentric axes. T h e issue o f response dimensions w i l l Here we w i l l confine b e dealt w i t h separately in t h e n e x t section. ourselves t o discussing stimulus dimensions, a n d t h e experiments t a k e n i n t o consideration were aimed a t d e t e r m i n i n g t h e role o f t h e t w o t y p e s o f l e f t - r i g h t dimension in p r o d u c i n g spatial S-R compatibility effects.

Left-Right Stimulus Dimensions for T y p e II Ensembles In t h e case o f T y p e I I ensembles, t h e dimensional o v e r l a p occurs in t h e r e l e v a n t dimension, which must b e processed intentionally because it is t h e value o f t h e stimulus i n t h a t dimension t h a t allows t h e selection o f t h e c o r r e c t response, ( i . e . , t h e c o r r e c t value in t h e response set). F o r example, if t h e stimulus is on t h e right side, t h e n t h e right response (compatible S-R p a i r i n g ) o r t h e l e f t response (incompatible S-R p a i r i n g ) must b e selected. I n terms o f t h e controlled-automatic dichotomy o f information-processing modes (see, e . g . , Schneider, Dumais, & S h i f f r i n , 1984; S h i f f r i n & Schneider, 19771, it can b e said t h a t in T y p e II ensembles t h e l e f t - r i g h t dimension u n d e r goes c o n t r o l l e d processing, w h i c h is comparatively slow, serial, e f f o r t ful, capacity-limited, subject t o interference, a n d can b e stopped. Note t h a t by s a y i n g t h a t in T y p e II ensembles t h e relevant dimension undergoes controlled processing, we d o n o t mean t o imply t h a t t h e position o f t h e stimulus a n d t h e position o f t h e response a r e encoded t h r o u g h a controlled process. We simply want t o stress t h e f a c t t h a t S-R p a i r i n g s a r e formed i n t e n t i o n a l l y . In most studies t h a t employed spatial T y p e I 1 ensembles, side a n d r e l a t i v e position were confounded, because t h e t w o stimuli were shown t o t h e l e f t o r r i g h t o f t h e f i x a t i o n p o i n t (see, e.g., Anzola, Bertoloni, Buchtel, & Rizzolatti, 1977; Brebner, Shepard, & Cairney, 1972; Simon, 1969). Therefore, these studies a r e uninformative about which dimension caused t h e S-R compatibility effects. Nicoletti e t al. (1982) a n d Umilt6 a n d L i o t t i (1987, Experiment 1) attempted t o show S - R compatibility effects p u r e l y a t t r i b u t a b l e t o t h e processing o f r e l a t i v e position. In these studies, t h e subjects made unimanual, discriminative, k e y p r e s s i n g responses t o t w o v i s u a l stimuli, b o t h shown t o e i t h e r t h e l e f t o r right o f t h e f i x a t i o n p o i n t (see F i g u r e 1 f o r an example o f such d i s plays). More .precisely, in t h e compatible condition t h e subject was i n s t r u c t e d t o p r e s s t h e l e f t k e y if t h e stimulus was shown i n t h e l e f t r e l a t i v e position (i.e., t h e position closer t o f i x a t i o n on t h e right side a n d t h e more p e r i p h e r a l position in t h e l e f t side) a n d t o p r e s s t h e right k e y if t h e stimulus was shown in t h e right r e l a t i v e position (i.e., t h e position closer t o f i x a t i o n on t h e l e f t side a n d t h e more p e r i p h e r a l posit i o n on t h e right side). In t h e incompatible condition, t h e assignment

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was reversed, so t h a t t h e l e f t k e y was used i n response t o t h e stimulus in t h e right r e l a t i v e position a n d t h e right k e y was used in response t o t h e stimulus in t h e l e f t r e l a t i v e position. In b r i e f , r e l a t i v e position o f t h e stimulus signaled t h e position o f t h e c o r r e c t response, whereas side was i r r e l e v a n t . T h e r e were S - R compatibility effects d u e t o t h e relat i v e dimension, whereas no effects a t t r i b u t a b l e t o t h e egocentric dimension c o u l d b e f o u n d . T h e r e s u l t s showed, therefore, t h a t S-R compatibility effects o c c u r o n l y f o r t h e dimension t h a t is submitted t o cont r o l l e d processing ( i . e . , t h e r e l a t i v e one in t h i s case). T o t h e b e s t o f o u r knowledge, t h e o n l y s t u d y t h a t attempted t o demonstrate S - R compatibility effects p u r e l y a t t r i b u t a b l e t o t h e processing o f t h e egocentric dimension, by u n c o n f o u n d i n g it f r o m t h e r e l a t i v e In dimension, was done by Umilt5 a n d L i o t t i (1987, Experiment 2 ) . t h e i r stimulus d i s p l a y (see F i g u r e 21, f o u r positions were m a r k e d by empty boxes, t w o on each side o f f i x a t i o n . T h e t w o boxes w i t h b r o k e n contours h a d t h e o n l y p u r p o s e o f m a r k i n g t h e r e l a t i v e position o f t h e stimulus, while t h e stimulus i t s e l f was always shown w i t h i n one o f t w o boxes w i t h continuous contours. In t h e compatible condition t h e subject pressed t h e l e f t k e y if t h e stimulus appeared on t h e l e f t side (i.e., w i t h i n t h e box w i t h continuous contours t o t h e l e f t o f f i x a t i o n ) a n d t h e right k e y if t h e stimulus appeared on t h e r i g h t side (i.e., within the

LVF I I I

RVF

I I

r

B

F i g u r e 2. Schematic diagram o f stimulus d i s p l a y a n d response k e y s . I n t h e delay condition t h e f o u r boxes preceded t h e stimulus by 500 ms. T h e stimulus was always shown in one o f t h e solid boxes. T h e dashed boxes marked t h e r e l a t i v e positions.

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box w i t h continuous contours t o t h e l e f t o f f i x a t i o n ) . T h e assignment was r e v e r s e d in t h e incompatible condition, so t h a t t h e response was t o b e emitted w i t h t h e l e f t k e y t o stimuli shown on t h e r i g h t side a n d w i t h t h e right k e y t o stimuli shown o n t h e l e f t side. Because t h e i n s t r u c t i o n s were g i v e n in terms o f l e f t a n d right sides, t h e egocentric dimension was relevant, whereas t h e r e l a t i v e dimension was i r r e l e v a n t . T h e r e were S-R compatibility effects d u e t o t h e egocentric dimension, whereas effects a t t r i b u t a b l e t o t h e r e l a t i v e dimension were absent. I n o t h e r words, it was confirmed t h a t S - R compatibility effects o c c u r r e d now t h e egocentric one, whose elements u n d e r o n l y in t h e dimension, went c o n t r o l l e d processing. It c o u l d b e a r g u e d t h a t t h e experimental conditions j u s t described come close t o making a T y p e I V - A o r a T y p e IV-B ensemble. T h i s is because, by unconfounding side a n d r e l a t i v e position, t w o stimulus dimensions a r e made available t h a t both can o v e r l a p w i t h t h e response dimension because t h e y b o t h comprise l e f t - r i g h t elements. Which dimension is r e l e v a n t a n d w h i c h is i r r e l e v a n t depends on w h e t h e r t h e subject i s i n s t r u c t e d t o process side o r r e l a t i v e position. It remains t o b e seen w h e t h e r these t w o dimensions a r e equal ( T y p e IV-B) o r different (Type IV-A). However, it is apparent t h a t e i t h e r dimension ceases t o h a v e any effects when it becomes i r r e l e v a n t a n d does n o t und e r g o controlled processing. Thus, these studies approximate but d o n o t q u i t e make T y p e IV ensembles in which, by definition, b o t h dimensions must p r o d u c e S-R compatibility effects.

L e f t - R i g h t Stimulus Dimensions f o r T y p e II 1 Ensembles B y u s i n g t h i s t y p e o f ensemble it can b e demonstrated t h a t spat i a l S-R compatibility effects a r e n o t r e s t r i c t e d t o t h e r e l e v a n t stimulus dimension but a r e also obtained w i t h an i r r e l e v a n t stimulus dimension, p r o v i d e d t h a t t h e i r r e l e v a n t dimension overlaps w i t h t h e response dimension. T h e r e l e v a n t stimulus dimension can b e a nonspatial f e a t u r e o f t h e stimulus, such as color, w h i c h signals t h e l e f t - r i g h t value o f t h e c o r r e c t element in t h e response set. For example, if t h e light is red, t h e n t h e l e f t response must b e selected; if t h e light is green, t h e n t h e right response must b e selected. T h e lights, however, appear randomly on e i t h e r t h e l e f t o r right side, a n d t h u s an i r r e l e v a n t stimulus dimension overlaps w i t h t h e response dimension. T h a t t h i s dimensional o v e r l a p is e f f e c t i v e is shown by t h e f a c t t h a t t h e r e d light is responded t o f a s t e r when it is presented on t h e l e f t t h a n t h e r i g h t side (compatible a n d incompatible condition, respectively) a n d v i c e versa f o r t h e g r e e n light. T o explain t h e e f f e c t o f t h e i r r e l e v a n t dimension, it seems necess a r y t o assume t h a t t h i s dimension is processed automatically, t h a t is, t h a t i t s processing takes place u n i n t e n t i o n a l l y a n d cannot b e bypassed. B y definition, automatic processing i s fast, parallel, r e l a t i v e l y immune t o interference, a n d f a i r l y effortless; it i s n o t limited by s h o r t - t e r m memory capacity and, perhaps, cannot b e stopped (see, e.g., Schneider e t al., 1984; S h i f f r i n & Schneider, 1977).

Also, with Type

as is t h e case f o r T y p e I I ensembles, in most of t h e studies I l l ensembles t h e t w o stimuli were shown t o t h e l e f t a n d

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right o f fixation, a n d t h u s side a n d r e l a t i v e position w e r e confounded (see, e.g., Simon, 1969; Wallace, 1971, 1972). T h e r e are, however, a few studies t h a t allow independent determination o f t h e r o l e o f t h e egoc e n t r i c a n d t h e r e l a t i v e dimensions w i t h T y p e I l l ensembles. I n a s t u d y by Umilts a n d Nicoletti (1985, Experiments 2 a n d 4), t h e r e l e v a n t stimulus dimension was color, a n d processing a stimulus position was n o t necessary f o r choosing t h e c o r r e c t response. In fact, subjects were i n s t r u c t e d t o p r e s s one b u t t o n f o r t h e g r e e n light a n d t h e o t h e r b u t t o n f o r t h e r e d light, regardless o f stimulus position. T h e t w o stimuli could appear i n t w o d i f f e r e n t positions but always w i t h i n t h e same visual f i e l d (as in F i g u r e I), t h u s u n c o n f o u n d i n g side a n d r e l a t i v e position. Umilt.5 a n d L i o t t i (1987, Experiment 3) did a similar experiment in w h i c h t h e relevant stimulus dimension was shape a n d t h e subject was r e q u i r e d t o make l e f t - r i g h t discriminative responses t o a square a n d a rectangle, i r r e s p e c t i v e of where t h e stimulus was shown. In t h e condition u n d e r consideration here, t w o boxes were randomly presented t o t h e l e f t o r right o f t h e f i x a t i o n m a r k (see F i g u r e 11, a n d then, a f t e r an i n t e r v a l o f 500 ms, t h e imperative stimulus ( t h e square o r t h e rectangle) appeared w i t h i n one b o x . Therefore, in t h e case of t h e d i s p l a y shown in F i g u r e 1, f o r instance, t h e subject knew 500 ms in advance t h a t t h e imperative stimulus was g o i n g t o appear o n t h e right side. What was u n c e r t a i n u n t i l stimulus onset was, besides i t s shape, w h e t h e r t h e stimulus was t o b e shown in t h e l e f t o r right r e l a t i v e position. Therefore, side a n d r e l a t i v e position were unconfounded. Note t h a t in b o t h studies t h e r e l e v a n t stimulus dimension (color o r shape) did n o t o v e r l a p w i t h t h e response dimension, whereas t h e i r r e l e v a n t stimulus dimension ( r e l a t i v e position) did. In b o t h studies t h e r e were S - R compatibility effects d u e t o t h e r e l a t i v e dimension a n d no effects a t t r i b u t a b l e t o t h e egocentric dimension. T h a t can mean t h a t o n l y r e l a t i v e position was automatically processed a n d p r o duced S - R compatibility effects.

Umilt6 a n d L i o t t i (1987, Experiment 4) also t r i e d t o show S-R compatibility effects a t t r i b u t a b l e t o t h e automatic processing o f t h e egoc e n t r i c dimension, n o t confounded w i t h t h e r e l a t i v e dimension. In one condition, t h e d i s p l a y o f F i g u r e 2 was shown and, a f t e r an i n t e r v a l of 500 ms, t h e imperative stimulus (again, a square o r a rectangle) appeared w i t h i n one o f t h e t w o boxes w i t h continuous contours. Therefore, i n t h e case o f t h e d i s p l a y shown i n F i g u r e 2, f o r example, t h e subject knew 500 ms i n advance t h a t t h e stimulus was g o i n g t o appear in t h e right r e l a t i v e position. What was u n c e r t a i n was w h e t h e r it was t o b e shown t o t h e l e f t o r right side o f f i x a t i o n . In t h i s experiment, too, t h e r e l e v a n t stimulus dimension (shape) did n o t o v e r lap w i t h t h e response dimension, whereas t h e i r r e l e v a n t dimension (side) did. T h e r e s u l t s showed S - R compatibility effects d u e t o t h e egocentric dimension a n d no effects d u e t o t h e r e l a t i v e dimension. It seems, thus, t h a t o n l y t h e egocentric dimension was automatically processed a n d p r o d u c e d S - R compatibility effects. It is w o r t h n o t i n g t h a t also these experiments could b e c o n s t r u e d as instances o f T y p e IV-A o r T y p e I V - B ensembles, depending on whether one considers t h e t w o dimensions t o b e d i f f e r e n t o r equal.

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Actually, t h e y would make r a t h e r special instances o f T y p e IV ensembles, because it i s t w o i r r e l e v a n t stimulus dimensions (egocentric a n d relative) that can o v e r l a p w i t h t h e response dimension, instead o f a r e l evant a n d an i r r e l e v a n t stimulus dimension, as is t y p i c a l o f T y p e IV ensembles. However, since one o f t h e t w o stimulus dimensions (i.e., t h e one t h a t becomes available p r i o r t o stimulus presentation), does n o t p r o d u c e S-R compatibility effects, we p r e f e r t o consider these as instances o f T y p e I l l ensembles. L e f t - R i g h t Stimulus Dimensions for T y p e I V Ensembles T h i s t y p e o f ensemble is characterized by t h e f a c t t h a t t h e r e is overlap between t h e stimulus a n d t h e response sets in b o t h t h e relevant and t h e i r r e l e v a n t dimensions. As already said, t h e experimental conditions described above, u n d e r t h e heading T y p e I I ensembles, could b e construed as belonging t o T y p e IV, in which one stimulus dimension was egocentric a n d t h e o t h e r relative. Depending on t h e instructions, t h e egocentric dimension was relevant a n d t h e r e l a t i v e dimension was irrelevant, o r v i c e versa. However, because t h e i r r e l e v a n t dimension did n o t produce S-R compatibility effects, we considered those t o b e Similarly, in t h e experimental conditions described T y p e II ensembles. u n d e r t h e heading T y p e I l l ensemble, t h e r e were t w o i r r e l e v a n t stimulus dimensions (again, one egocentric a n d t h e o t h e r relative) t h a t could overlap w i t h t h e response dimension. However, because o n l y one dimension, depending on t h e experiment, produced S-R compatibility effects, we considered those t o b e T y p e I l l ensembles. In t h e s t u d y by U m i l t i and L i o t t i (1987, Experiments 3 a n d 4) t h e r e were conditions t h a t were closer approximations o f T y p e IV ensembles. T h a t happened when t h e r e was n o delay between presentat i o n o f t h e boxes a n d presentation o f t h e imperative stimulus (see F i g ures 1 a n d 2 ) . Note that t h e o n l y d i f f e r e n c e w i t h respect t o t h e experimental conditions already discussed concerned t h e t i m i n g o f t h e display elements. More specifically, in these new experimental conditions, t h e information needed t o determine stimulus position i n t h e egoc e n t r i c dimension a n d in t h e r e l a t i v e dimension became available a t t h e same time. T h e results showed t h a t n e i t h e r t h e egocentric dimension n o r t h e r e l a t i v e dimension p r o d u c e d S - R compatibility effects. Rememb e r t h a t w i t h t h e delay o f 500 ms t h e r e were, instead, effects d u e t o e i t h e r t h e r e l a t i v e dimension (Experiment 3) o r t h e egocentric dimension (Experiment 4). U m i l t i a n d L i o t t i argued that, in t h e absence o f t h e delay, b o t h stimulus dimensions overlapped w i t h t h e response dimension, b u t t h e S-R compatibility effects t h e y produced cancelled each other out.

I t must b e r e i t e r a t e d t h a t t h e conditions w i t h o u t delay o f E x p e r i ments 3 a n d 4 o f U m i l t i and L i o t t i d o n o t fit exactly a T y p e IV ensemble. T h i s i s because t h e t w o stimulus dimensions t h a t overlapped w i t h t h e response dimension were b o t h i r r e l e v a n t (in fact, t h e relevant stimulus dimension was shape), r a t h e r than one relevant a n d t h e o t h e r irrelevant, as r e q u i r e d by a t r u e T y p e IV ensemble. It is also n o t clear w h e t h e r those conditions approximated a T y p e IV-B o r a T y p e IVA ensemble because it is d i f f i c u l t t o establish if t h e t w o stimulus dimensions were equal o r d i f f e r e n t . T h e important p o i n t here, however, is

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t h a t those experiments confirmed t h a t t h e subject can process automatically b o t h stimulus dimensions. T h i s p o i n t depends, o f course, on accepting t h e explanation o f f e r e d f o r t h e disappearance o f t h e S-R comp a t i b i l i t y effects in t h e absence o f t h e delay, namely, t h a t b o t h dimensions p r o d u c e d S-R compatibility effects t h a t cancelled each o t h e r o u t . T h i s p o i n t w i l l b e discussed i n t h e last section of t h e chapter, w h e r e i n t e r p r e t i v e models a r e t a k e n i n t o consideration. Egocentric a n d Relative Response Dimensions Anatomical Factors Before discussing t h e role o f egocentric a n d r e l a t i v e response dimensions, a p r e l i m i n a r y issue concerning possible anatomical f a c t o r s in spatial S-R compatibility must b e c l a r i f i e d . T h e studies considered so f a r have a l l employed choice R T tasks. However, t h e time t a k e n t o react manually t o a lateralized v i s u a l stimulus depends on t h e relation between t h e stimulus a n d t h e responding h a n d in simple R T tasks, as well. T h e r e can b e l i t t l e d o u b t t h a t i n simple R T t a s k s t h e directness o f t h e anatomical connections between t h e stimulated hemiretinae a n d t h e responding h a n d plays an important role (Anzola e t al., 1977; Berlucchi, Crea, D i Stefano, & Tassinari. 1977; Berlucchi, Heron, Hyman, Rizzolatti, & UmiltB, 1971; see C h a p t e r 7, by Bashore). Responses made w i t h t h e h a n d ipsilateral t o a stimulus p r e s e n t e d t o t h e l e f t o r right v i s u a l f i e l d a r e f a s t e r t h a n those made w i t h t h e contralatera1 hand, because n e u r a l connections a r e s h o r t e r in t h e f o r m e r t h a n in t h e l a t t e r case: lpsilateral responses a r e mediated by n e u r a l c i r c u i t s located w i t h i n a single hemisphere, whereas contralateral responses r e q u i r e an e x t r a time f o r t r a n s f e r o f information f r o m one hemisphere t o t h e other. T h i s e x t r a time is on t h e o r d e r o f a few milliseconds, w i t h ipsilateral responses b e i n g about 2-3 ms f a s t e r t h a n contralateral responses (also see review in Bashore, 1981). B y contrast, in choice R T tasks t h e influence o f t h e anatomical c o n n e c t i v i t y becomes almost immaterial a n d spatial S-R compatibility emerges as t h e most important determinant o f speed o f response. Note also t h a t t h e d i f f e r e n c e in speed o f response between compatible a n d incompatible S-R p a i r i n g s i s on t h e o r d e r o f several t e n s o f milliseconds instead o f j u s t 2 o r 3 ms (Anzola e t al., 1977; Bradshaw & UmiltB, 1984). T h e d i f f e r e n t i a l r o l e played by anatomical connections a n d by S-R compatibility i n simple a n d choice R T t a s k s was c l a r i f i e d by those experiments in which t h e subjects e i t h e r k e p t t h e i r hands in anatomical position o r crossed them so t h a t t h e l e f t h a n d was o n t h e right side o f t h e b o d y a n d t h e right h a n d was on t h e l e f t side. I n simple R T tasks, regardless o f h a n d position, t h e r i g h t h a n d was a few milliseconds f a s t e r i n r e s p o n d i n g t o stimuli shown in t h e right visual f i e l d t h a n in t h e l e f t v i s u a l f i e l d a n d v i c e versa f o r t h e l e f t h a n d (Anzola e t al., 1977; B e r l u c c h i e t al., 1977; also see reviews i n Bashore, 1981, a n d in Bradshaw & Umilt5, 1984).

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A q u i t e d i f f e r e n t situation arose in choice R T tasks. When t h e subjects crossed t h e i r hands, t h e right h a n d responded f a s t e r t o t h e stimuli p r e s e n t e d t o t h e l e f t v i s u a l f i e l d a n d t h e l e f t h a n d responded f a s t e r t o t h e stimuli p r e s e n t e d t o t h e right v i s u a l f i e l d . T h e d i f f e r e n c e was in t h e o r d e r o f t e n s o f milliseconds (Anzola e t al., 1977; B r e b n e r e t al., 1972; Nicoletti, UmiltB, & LBdavas, 1984; Wallace, 1971, 1972; also see reviews in Bashore, 1981, a n d i n Bradshaw & Umiltti, 1984). T h i s finding cannot b e explained in terms o f f i x e d anatomical connect i o n s a n d shows t h a t in choice R T t a s k s what matters is t h e relation between position o f t h e stimulus a n d position o f t h e e f f e c t o r . Crossed-Hand Effects Once established that, in choice R T tasks, o n l y t h e position o f t h e e f f e c t o r p l a y s a r o l e in S-R compatibility, it becomes i n t e r e s t i n g t o ask w h e t h e r t h e response dimension t h a t produces those compatibility effects i s egocentric o r relative. O f course, t h i s is t h e same question we asked b e f o r e f o r t h e stimulus dimension. Also in t h e case o f t h e effectors, egocentric dimension is d e f i n e d w i t h reference t o an egocent r i c axis, such as t h e b o d y a n d / o r t h e head midlines, whereas r e l a t i v e dimension is d e f i n e d in relation t o an e x t e r n a l reference point, such as t h e other effector. T h e question o f t h e r e l a t i v e importance o f these t w o response dimensions c o u l d n o t b e asked in most o f t h e studies about spatial S-R compatibility because t h e t w o e f f e c t o r s were positioned on opposite sides in relation t o t h e b o d y midline, a n d t h u s side a n d r e l a t i v e position were confounded. As a l r e a d y p o i n t e d out, t h e t w o dimensions can b e unconfounded, a t least p a r t i a l l y , by p o s i t i o n i n g t h e t w o effectors on t h e same side o f t h e b o d y midline. T h i s was done by Nicoletti e t al. (1982, Experiment 2; 1984, Experiments 2 a n d 31, w h o showed S - R compatibility effects a t t r i b u t a b l e t o t h e r e l a t i v e position o f t h e effectors. When subjects cross t h e i r hands so t h a t t h e right h a n d is located t o t h e l e f t o f t h e l e f t h a n d a n d t h e l e f t h a n d is located t o t h e right o f t h e right hand, spatial S-R compatibility a p p a r e n t l y reverses i t s d i r e c tion. R T becomes f a s t e r when t h e l e f t stimulus commands a response w i t h t h e e f f e c t o r located t o t h e left, even t h o u g h t h i s is t h e r i g h t hand, a n d v i c e versa f o r t h e right stimulus. As a l r e a d y p o i n t e d out, such a p p a r e n t reversal is i m p o r t a n t because it disproves a n y i n t e r p r e t a t i o n of spatial S - R compatibility effects i n terms o f anatomical connect i o n s a n d u n d e r l i n e s t h e importance o f t h e spatial relation between stimulus and effector. T h e f a c t t h a t such apparent reversal in t h e d i r e c t i o n o f S - R compatibility occurs also if t h e t w o hands a r e positioned on t h e same side o f t h e b o d y midline, namely, b o t h on t h e l e f t o r right side egocentrically (Nicoletti e t al., 1984, Experiments 2 a n d 3), can b e t a k e n as f u r t h e r evidence o f t h e r o l e o f t h e r e l a t i v e response dimension in p r o d u c i n g S - R compatibility effects. A n o t h e r i n t e r e s t i n g consequence caused by t h e c r o s s i n g hands i s t h a t R T becomes o v e r a l l longer t h a n when t h e hands anatomical (i.e., uncrossed) position (Anzola e t al., 1977; Greim, Mendicino, & Koening, 1979; Nicoletti e t al., 1982, Nicoletti e t al. (1984) showed t h a t t h e l e n g t h e n i n g o f R T w i t h

of the a r e in Klapp, 1984). hands

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crossed cannot b e a t t r i b u t e d t o bio-mechanical f a c t o r s (i.e., an a w k w a r d posture; see B e r l u c c h i e t al., 1977; Klapp e t al., 1979) but i s d u e t o a mismatch between t h e position o f t h e e f f e c t o r a n d i t s anatomical status. In o t h e r words, when, f o r example, t h e right h a n d becomes t h e l e f t effector, a c o n f l i c t arises between t w o representations, one positional a n d t h e o t h e r anatomical, o f t h e same e f f e c t o r . Because o f t h i s conflict, responses t o t h e l e f t stimulus a r e slower w i t h t h e crossed right h a n d t h a n w i t h t h e uncrossed l e f t h a n d (i.e., t h e crossed-hand effect). Remember, however, t h a t responses t o t h e l e f t stimulus a r e f a s t e r w i t h t h e crossed right h a n d t h a n w i t h t h e crossed l e f t h a n d (i.e., t h e a p p a r e n t r e v e r s a l o f S-R c o m p a t i b i l i t y ) . T h e explanation o f t h e crossed-hand e f f e c t in terms o f a c o n f l i c t between positional a n d anatomical representations o f t h e e f f e c t o r w i l l b e discussed in t h e last section. What is o f i n t e r e s t h e r e is that, also f o r t h e crossed-hand effect, one can ask w h e t h e r it is d u e t o t h e egocent r i c o r t h e r e l a t i v e dimension. T h a t is t o say, one can ask w h e t h e r t h e position o f t h e e f f e c t o r is determined w i t h reference t o t h e b o d y m i d l i n e o r t o t h e other effector. If w h a t matters is t h e egocentric dimension, t h e n t h e crossed-hand e f f e c t should o c c u r o n l y when t h e t w o hands a r e located on t h e contralateral sides across t h e b o d y midline. If w h a t matters is t h e r e l a t i v e dimension, t h e n t h e crossed-hand e f f e c t should o c c u r also when t h e t w o hands a r e crossed on t h e same side, t h a t is, when o n l y one o f them lies across t h e b o d y midline. In most o f t h e experiments t h a t employed a crossed-hand condition, t h e t w o dimensions were confounded because t h e subjects placed t h e i r hands by c r o s s i n g t h e b o d y midline, a n d t h u s each h a n d occupied t h e "wrong" position, b o t h egocentrically a n d r e l a t i v e l y . In t h e s t u d y by Nicoletti e t at. (1984), t h e t w o dimensions were unconfounded because t h e subjects k e p t t h e i r hands e i t h e r uncrossed o r crossed o n t h e same side o f t h e b o d y midline. It was f o u n d t h a t t h e crossed-hand e f f e c t depended on t h e r e l a t i v e position o f t h e effector. In conclusion, t h e r e s u l t s o f t h e f o r e g o i n g studies c o n v e r g e in showing that, also in t h e case o f t h e response set, t h e values o f t h e r e l e v a n t spatial dimension a r e specified w i t h reference t o t h e r e l a t i v e position o f t h e effectors. B y contrast, t h e role o f t h e egocentric response dimension in d e t e r m i n i n g t h e o v e r l a p w i t h t h e stimulus dimension has y e t t o b e demonstrated, i n t h e absence o f a c o n f o u n d i n g w i t h t h e r e l a t i v e dimension. Remember t h a t i n t h e case o f t h e stimulus set b o t h dimensions have, instead, p r o v e d e f f e c t i v e in p r o d u c i n g S-R comp a t i b i l i t y effects. Position o f t h e Response Goal So far, we have considered t h e response set by making reference t o t h e positions o f t h e effectors in general. However, t w o positional features of t h e response can b e distinguished, as suggested, f o r example, by G u i a r d (1984) a n d by Klapp e t al. (1979). One is t h e position of t h e e f f e c t o r (i.e., t h e h a n d i n t h e case o f a k e y - p r e s s i n g response) a n d t h e o t h e r is t h e position o f t h e response goal (i.e., t h e k e y ) . In spatial S-R compatibility studies, b o t h positional features have been t a k e n i n t o consideration. F o r instance, Wallace (1971, 1972) stressed

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t h e importance o f e f f e c t o r emphasized k e y position.

position,

whereas

Brebner et

al.

(1972)

A s t u d y by Riggio, Gawryszewski, a n d Umilta (1986) was aimed a t d i s e n t a n g l i n g t h e role o f t h e position of t h e e f f e c t o r a n d t h a t o f t h e response goal. In t h e i r Experiment 2, t h e hands were uncrossed but operated t w o sticks, which c o u l d b e e i t h e r uncrossed o r crossed a t t h e b o d y midline. In t h e uncrossed condition, t h e effectors ( i . e . , t h e t w o hands) a n d t h e response goals (i.e., t h e t w o keys) occupied t h e same position in b o t h egocentric (i.e., in relation t o t h e b o d y midline) a n d r e l a t i v e (i.e., in relation t o t h e o t h e r e f f e c t o r o r response goal) terms. I n t h e crossed condition, w h i c h was t h e c r i t i c a l one, t h e effectors a n d t h e response goals occupied opposite positions, again e i t h e r egocentrically o r r e l a t i v e l y . It was reasoned t h a t if t h e determining f a c t o r f o r S-R compatibility was t h e position o f t h e effector, responses t o stimuli presented on t h e side w h e r e t h e h a n d was located, but opposite t o t h e key, should b e f a s t e r t h a n those to stimuli presented on t h e o t h e r side Of w i t h respect t o t h e hand, but on t h e t h e same side o f t h e k e y . course, if t h e determining f a c t o r was t h e position o f t h e response goal, t h e n t h e p r e d i c t i o n would b e reversed. T h e r e s u l t s were unequivocal a n d demonstrated t h e importance o f t h e position o f t h e response goal. T h e position o f t h e e f f e c t o r h a d no influence. I n t h e crossed condition, t h e right hand, which was on t h e right side but operated t h e l e f t key, was f a s t e r f o r t h e l e f t t h a n t h e right stimulus, whereas t h e l e f t hand, which was on t h e l e f t side b u t operated t h e right key, was f a s t e r f o r t h e right t h a n t h e l e f t stimulus. T h i s r e s u l t is s u r p r i s i n g if one considers t h a t t h e subject c o u l d r e n d e r compatible (i.e., easier) t h e incompatible condition by coding t h e posit i o n o f t h e e f f e c t o r instead of t h a t o f t h e response goal. F o r example, in t h e incompatible condition, when t h e l e f t stimulus was shown, t h e subject h a d t o p r e s s t h e right k e y w i t h a movement performed by t h e l e f t hand, w h i c h was located o n t h e same side as t h e stimulus. Had t h e subject coded t h e position o f t h e hand, t h e S-R p a i r i n g would have become compatible because t h e l e f t stimulus would have been p a i r e d t o t h e left effector. However, t h e r e s u l t s clearly show t h a t S-R compatibility depended on t h e position of t h e response goal, which dominated o v e r t h e position o f t h e e f f e c t o r . I n t e r e s t i n g l y enough, also in t h i s experiment t h e responses w i t h t h e s t i c k s crossed were slower t h a n those w i t h t h e s t i c k s uncrossed, despite t h e f a c t t h a t t h e hands were always i n t h e anatomical uncrossed position a n d o n l y t h e s t i c k s t h e y h e l d crossed t h e midline t o reach t h e response k e y s . O f course, t h i s f i n d i n g rules o u t any possible explanat i o n i n terms o f bio-mechanical f a c t o r s .

A l l t h e studies reviewed i n t h i s section used T y p e I I ensembles. Thus, it can b e concluded that, f o r t h e response set, t h e relevant dimension o n .which t h e dimensional o v e r l a p o c c u r r e d was r e l a t i v e position of t h e response goal.

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T h e Dominance o f t h e L e f t - R i g h t Dimension O v e r t h e Above-Below Dimension T h e Phenomenon of L e f t - R i g h t Dominance

A series o f experiments conducted in o u r l a b o r a t o r y (Nicoletti & Umilta, 1984, 1985; Nicoletti, Umilta, Tressoldi, & Marzi, 1988) has e v i T h i s phenomenon i s t h e dominance denced an intriguing phenomenon. o f t h e l e f t - r i g h t dimension o v e r t h e above-below dimension in spatial S R compatibility. These experiments employed r a t h e r unusual instances o f T y p e I I ensembles, i n w h i c h t h e stimulus a n d response sets o v e r lapped in t w o r e l e v a n t spatial dimensions ( i . e . , above-below a n d l e f t right), instead o f one only, as happens w i t h r e g u l a r T y p e I I ensembles. T h a t is t o say, each element o f t h e stimulus set as well as each element o f t h e response set c o u l d unambiguously b e specified in e i t h e r t h e above-below o r t h e l e f t - r i g h t dimension. E i t h e r dimension was p e r f e c t l y p r e d i c t i v e o f t h e c o r r e c t S-R p a i r i n g . F o r example, t h e r e c o u l d b e t w o possible stimulus positions, f o r instance, t o p / r i g h t a n d bottom/left, and two response positions, again, top/right and bottom/left. T h e subject could, therefore, b e i n s t r u c t e d t o make use o f e i t h e r dimension f o r f o r m i n g t w o e x h a u s t i v e sets o f compatible a n d incompatible S-R p a i r i n g s w i t h t w o independent T y p e I I ensembles. When t h e i n s t r u c t i o n s were couched in above-below terms, t h e p a i r i n g s were t o p stimulus/top response, t o p stimulus/bottom response, bottom stimulus/bottom response, bottom stimulus/top response. When t h e i n s t r u c t i o n s were couched in l e f t - r i g h t terms, t h e p a i r i n g s were right s t i m u l u s / r i g h t response, r i g h t stimulus/left response, l e f t stimulus/left response, l e f t s t i m u l u s / r i g h t response. Note that, a l t h o u g h o n l y one o f t h e t w o dimensions was mentioned in t h e i n s t r u c t i o n s , these cannot b e considered t o b e T y p e IV-A ensembles ( n o doubt, t h e t w o dimensions were d i f f e r e n t ) , because b o t h dimensions were relevant, instead o f one relevant a n d t h e o t h e r i r r e l e v a n t , as r e q u i r e d by T y p e IV ensembles. Regardless o f t h e t y p e o f S-R ensemble, t h e spatial dimension most f r e q u e n t l y employed i n S-R compatibility paradigms i s t h e l e f t right one. However, S-R compatibility effects can b e easily shown also when t h e stimulus a n d response sets o v e r l a p on t h e above-below dimension ( L i d a v a s & Moscovitch, 1984; Nicoletti G Umilt5, 1984, 1985; Nicoletti e t al., 1988). For example, in t h e case o f a r e g u l a r T y p e I I ensemble, R T is s h o r t e r when t h e t o p stimulus commands a response w i t h t h e t o p t h a n w i t h t h e bottom key, a n d v i c e versa f o r t h e bottom stimulus. Furthermore, above-below S - R compatibility effects a r e o f more o r less t h e same magnitude as l e f t - r i g h t effects. Therefore, t h e r e seems t o b e no i n t r i n s i c d i f f i c u l t y i n u s i n g t h e above-below spatial dimension f o r mapping stimuli o n t o responses. T h e situation changes radically when stimulus a n d response sets overlap, n o t o n l y on t h e above-below dimension, b u t also on t h e l e f t right dimension, which too happens t o b e r e l e v a n t . If b o t h l e f t - r i g h t a n d above-below values a r e available i n t h e stimulus set a n d in t h e response set, t h e n t h e use o f t h e above-below dimension f o r mapping stimuli o n t o responses becomes more d i f f i c u l t , o r altogether impossible,

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as demonstrated by t h e weakening (Nicoletti & Umilti, appearance (Nicoletti & Umilti, 1985; Nicoletti e t al., compatibility effects in t h a t dimension.

1984) o r t h e d i s 1988) o f t h e S-R

In a paradigmatic experiment, subjects a r e e x p l i c i t l y t o l d t o use t h e above-below dimension o n l y f o r mapping stimuli onto responses. T h e t w o dimensions a r e combined orthogonally, so t h a t f o u r experiment a l conditions originate. In t h e f i r s t , t h e S-R p a i r i n g s a r e compatible f o r both t h e above-below a n d t h e l e f t - r i g h t dimensions. For example, when t h e t o p right light is shown, t h e subject responds by p r e s s i n g t h e t o p right k e y . In t h e second condition, t h e S-R p a i r i n g s are comp a t i b l e f o r t h e above/below dimension o n l y . F o r example, t h e t o p right light commands a response with t h e t o p l e f t k e y . T h e third condition comprises S-R p a i r i n g s t h a t a r e compatible f o r t h e l e f t - r i g h t dimension only. In t h i s case, t h e t o p right light commands a response w i t h t h e bottom right k e y . T h e f o u r t h condition is, instead, compatible f o r n e i t h e r dimension. For example, t h e t o p right light commands a response w i t h t h e bottom l e f t k e y . These were t h e experimental conditions t h a t demonstrated t h e dominance o f t h e l e f t - r i g h t o v e r t h e abovebelow dimension. T h a t is, despite t h e f a c t t h a t t h e i n s t r u c t i o n s were g i v e n exclusively in terms o f above-below S - R p a i r i n g s (i.e., in t h e f i r s t t w o examples above, t h e subject was t o l d t o use t h e t o p key, whereas in t h e second t w o examples h e was t o l d t o use t h e bottom key), S-R compatibility effects were p r e s e n t f o r t h e l e f t - r i g h t dimension o n l y o r were much s t r o n g e r f o r t h a t dimension. I n t e r p r e t a t i o n s of t h e Phenomenon Even t h o u g h t h e phenomenon o f t h e l e f t - r i g h t dominance is s t r o n g a n d reliable, i t s i n t e r p r e t a t i o n has p r o v e d t o b e r a t h e r elusive. The f i r s t possible explanation i s that, when t h e subject uses t h e hands f o r responding, t h e values o f t h e response set a r e more n a t u r a l l y specified in terms o f l e f t - r i g h t t h a n above-below positions. However, Nicoletti and U m i l t i (1985) could n o t s u p p o r t t h i s explanation because t h e l e f t right dominance was p r e s e n t also in a n experiment i n which t h e responses were emitted w i t h one hand a n d one foot instead o f t h e t w o It would seem t h a t t h e positions o f hand and foot a r e more hands. n a t u r a l l y specified along t h e above-below dimension t h a n t h e l e f t - r i g h t dimension.

A second explanation is based on t h e consideration t h a t i n t h e experiments t h a t showed t h e l e f t - r i g h t dominance, t h i s dimension extended across t h e b o d y midline ( i . e . , t h e egocentric a n d relative dimensions were confounded). I n contrast, above-below values were n o t specified in relation t o any egocentric reference axes. It could b e argued t h a t it was t h e availability o f unambiguous reference axes t h a t rendered l e f t - r i g h t values more salient and easier t o process t h a n above-below values. T h i s explanation is scarcely tenable, however, since it has been demonstrated t h a t S - R compatibility effects a r e o f t h e same magnitude i r r e s p e c t i v e o f whether t h e l e f t - r i g h t values a r e specif i e d across t h e b o d y midline (Nicoletti et a l . , 1982; U m i l t i & Nicoletti, 1985) .

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A third explanation o f t h e l e f t - r i g h t dominance was r e c e n t l y t e s t e d a n d rejected by Nicoletti e t al. (1988). We reasoned that, because d i s c r i m i n a t i n g between l e f t a n d right is d i f f i c u l t f o r a horizont a l l y symmetrical organism, it c o u l d b e t h a t a t t e n t i o n is selectively allocated t o t h e l e f t - r i g h t dimension, t h u s r e n d e r i n g values in t h a t dimension comparatively more salient. If t h i s hypothesis w e r e correct, s t r o n g e r S-R compatibility effects should b e f o u n d f o r t h e above-below dimension t h a n t h e l e f t - r i g h t dimension in t h e a u d i t o r y modality. This p r e d i c t i o n arises because sound localization is known t o b e more difficult, and, presumably more a t t e n t i o n demanding in t h e v e r t i c a l t h a n in t h e horizontal plane. I n s p i t e o f that, i n t h e experiment by Nicoletti e t al., t h e l e f t - r i g h t dominance was f o u n d also when acoustical stimuli were used in place o f v i s u a l stimuli. O f course, these r e s u l t s r u l e o u t t h e p o s s i b i l i t y t h a t t h e reason w h y a t t e n t i o n is selectively allocated t o t h e l e f t - r i g h t dimension is t h e d i f f i c u l t y human o b s e r v e r s experience in d i s c r i m i n a t i n g l e f t f r o m right. It may well be, however, t h a t a t t e n t i o n is selectively allocated t o t h e l e f t - r i g h t dimension f o r another, as y e t unknown, reason a n d t h u s l e f t - r i g h t cues a r e always more salient t h a n above-below ones, i r r e s p e c t i v e o f t h e modality tested. Relevant a n d I r r e l e v a n t Dimensions

It is w o r t h n o t i n g a c e r t a i n s i m i l a r i t y between t h e r e s u l t s o f t h e experiments t h a t demonstrate t h e l e f t - r i g h t dominance a n d t h e r e s u l t s of some o f t h e experiments discussed u n d e r t h e heading l e f t - r i g h t stimulus dimensions f o r T y p e I I ensembles, (i.e., U m i l t i E Liotti, 1987, E x p e r i ments 1 a n d 2 ) . In t h e s t u d y by U m i l t i a n d Liotti, t h e values o f t h e stimuius set were specified o n e i t h e r an egocentric o r a r e l a t i v e l e f t right dimension. Depending on t h e instructions, one dimension was r e l e v a n t a n d t h e o t h e r was i r r e l e v a n t . Despite t h e f a c t t h a t stimulus a n d response sets overlapped on b o t h dimensions, o n l y t h e r e l e v a n t dimension, regardless o f w h e t h e r it happened t o b e t h e egocentric o r t h e r e l a t i v e one, p r o d u c e d S-R compatibility effects. Actually, t h i s was t h e reason w h y those were assumed t o b e T y p e I I ensembles. Taken together, these r e s u l t s suggest t h a t a subject, if engaged in processing one spatial dimension in a c o n t r o l l e d way, i s unable t o process simultaneously another spatial dimension, n o t even automatically. I n t h e case o f t h e l e f t - r i g h t dominance, it can b e a r g u e d t h a t , f o r u n k n o w n reasons, t h e l e f t - r i g h t dimension p r e f e r e n t i a l l y a t t r a c t s attention and, because o f this, undergoes c o n t r o l l e d processing a t t h e expense o f t h e above-below dimension. Note t h a t spatial dimensions, regardless o f t h e i r n a t u r e ( i . e . , l e f t - r i g h t egocentric, l e f t - r i g h t relat i v e o r above-below), a r e processed automatically when t h e competing dimension t h a t undergoes c o n t r o l l e d processing is nonspatial. T h i s is t h e cause o f t h e spatial S-R compatibility effects obtained w i t h T y p e I l l a n d T y p e IV ensembles. A l t e r n a t i v e l y , a n d perhaps more likely, it could b e t h a t t h e automatic processing o f t h e i r r e l e v a n t ( o r nondominant) spatial dimension does occur, but i t s effects a r e cancelled o u t by t h e effects o f t h e c o n t r o l l e d processing o f t h e other, r e l e v a n t ( o r dominant) spatial dimension. A d i f f e r e n t situation arises when t h e t w o spatial stimulus dimensions t h a t can o v e r l a p w i t h t h e spatial response dimension(s) a r e b o t h

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irrelevant. In t h i s case, t h e outcome depends on t h e r e l a t i v e t i m i n g in t h e a v a i l a b i l i t y o f t h e t w o i r r e l e v a n t spatial dimensions (see Umilt5 G Liotti, 1987, Experiments 3, 4, a n d 5). If t h e t w o spatial dimensions become available one a f t e r t h e other, t h e n S-R compatibility effects can b e o b s e r v e d o n l y f o r t h e dimension t h a t is processed second. If t h e t w o spatial dimensions become available a t t h e same time, t h e n n e i t h e r Because i r r e l e v a n t spatial dimension produces S-R compatibility effects. dimensions can no d o u b t be, processed automatically, if n o competing spatial dimension i s simultaneously p r e s e n t (note, again, t h a t t h i s is t h e cause o f spatial S-R compatibility effects obtained w i t h T y p e I l l a n d T y p e IV ensembles), it must b e concluded t h a t t h e r e s u l t s o f t h e automatic processing o f t w o spatial dimensions cancel each o t h e r o u t . This p o s s i b i l i t y was already mentioned in t h e section about stimulus dimensions w i t h T y p e IV ensembles a n d w i l l b e discussed f u r t h e r in t h e n e x t section. E x p l a n a t o r y Hypotheses of Spatial S-R Compatibility Spatial C o d i n g Hypothesis f o r T y p e II Ensembles Wallace (1971, 1972; also, see Nicoletti e t al., 1982) proposed an explanation o f spatial S-R compatibility effects in terms of t h e correspondence, o r lack o f correspondence, between t h e spatial code t h a t specifies t h e position o f t h e stimulus a n d t h e spatial code t h a t specifies t h e position o f t h e e f f e c t o r . Response latencies a r e f a s t e r when t h e t w o positions s h a r e t h e same code t h a n when t h e y a r e coded differently. In o t h e r words, Wallace suggested t h a t t h e positions of stimulus a n d e f f e c t o r a r e related t o spatial codes a n d t h a t it is t h e o u t come o f a comparison between t h e i r representations in these codes t h a t y i e l d s t h e d i f f e r e n c e in R T between compatible a n d incompatible S-R pairings. T e i c h n e r a n d K r e b s (1974) extended t h i s hypothesis t o nonspatial t y p e s o f S-R compatibility a n d r e n d e r e d it more e x p l i c i t by i n d i c a t i n g a stage in processing a t w h i c h S-R compatibility effects emerge. These a u t h o r s d i s t i n g u i s h e d f o u r temporal components in choice R T tasks: (a) A constant r e p r e s e n t i n g t h e sum of v a r i o u s n e u r a l transmission lags, (b) t h e time r e q u i r e d f o r stimulus categorization, (c) t h e time r e q u i r e d t o p e r f o r m some t y p e of t r a n s l a t i o n between stimulus a n d response, a n d (d) t h e time r e q u i r e d f o r response selection. In t h e i r view, S - R compatibility effects o c c u r a t t h e level o f t h e third compo:ent a n d t h e degree o f S-R compatibility i n h e r e n t in a t a s k depends on . . . t h e p r o p o r t i o n o f choice reaction time t h a t is a t t r i b u t a b l e t o stimulusresponse t r a n s l a t i o n time" (p. 91). T h a t is t o say, S - R compatibility would depend on t h e number o f S-R c o d i n g transformations: T h e f e w e r t h e t r a n s l a t i o n s required, t h e h i g h e r t h e degree o f compatibility o f t h e t a s k . T h e h i g h e s t level o f compatibility is obtained when t h e coda t h a t specifies t h e value o f t h e imperative stimulus w i t h i n t h e stimulus set is t h e same as t h e code t h a t specifies t h e value o f t h e c o r r e s p o n d i n g response w i t h i n t h e response set. Welford (1976), too, has a similar view a n d speaks e x p l i c i t l y o f t r a n s l a t i o n mechanisms t h a t u n d e r l i e S - R compatibility effects. Note t h a t , according t o t h e spatial coding hypothesis, S-R compatibility effects a r e d u e t o a l e n g t h e n i n g o f R T f o r

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incompatible S-R pairings, r a t h e r t h a n t o a facilitation o f R T f o r comp a t i b l e ones. T h i s i m p o r t a n t point, however, is f a r f r o m b e i n g s e t t l e d (see Simon & Acosta, 1982; Simon, Acosta, Mewaldt, & Speidel, 1976a). T h e c o d i n g hypothesis is n o d o u b t more c o n v i n c i n g in t h e case o f T y p e I I ensembles (Nicoletti e t al., 1982; Nicoletti 6 U m i l t i , 1984). d e s p i t e t h e f a c t t h a t it was proposed by Wallace (1971, 1972) t o e x p l a i n spatial S-R compatibility effects obtained w i t h ensembles t h a t can b e classified as T y p e I l l . In t h e case o f T y p e I I ensembles, codes must b e assigned t o t h e values o f stimuli a n d responses in t h e o v e r l a p p i n g dimension. T h e n some s o r t of t r a n s l a t i o n f r o m t h e stimulus code t o t h e response code m u s t t a k e place because it is t h e v a l u e o f t h e imperative stimulus in t h e o v e r l a p p i n g dimension t h a t indicates t h e v a l u e o f t h e r e q u i r e d response. Therefore, it i s reasonable t o assume t h a t response latency i s f a s t e r when t h e t r a n s l a t i o n is simple t h a n when it is complex. A n o t h e r important consideration concerns t h e f a c t t h a t t h e t r a n s l a t i o n process performed on t h e r e l e v a n t dimension is v e r y l i k e l y t o o c c u r in a controlled mode. T h i s is because t h e subject is required, In p a r by t h e t a s k instructions, t o allocate a t t e n t i o n t o t h a t process. t i c u l a r , t h e subject i s e x p l i c i t l y t o l d t o use t h e stimulus code f o r selecting t h e c o r r e c t response. T h e coding hypothesis can e x p l a i n t h e lack o f S-R compatibility effects f o r a spatial dimension o f t h e stimulus, t h a t also o v e r l a p s w i t h t h e response dimension, but is d i f f e r e n t f r o m t h e one t o w h i c h t h e subject's a t t e n t i o n i s d i r e c t e d . (Remember t h a t t h i s second stimulus dimension p r o v e s i n e f f e c t i v e regardless o f w h e t h e r it i s r e l e v a n t o r irrelevant.) One has simply t o make t h e additional assumption t h a t t r a n s l a t i o n processes t h a t o c c u r in a c o n t r o l l e d mode (i.e., that are attended t o ) a n n u l t h e effects o f similar, a n d p o t e n t i a l l y competing, processes t h a t a t t h e same time o c c u r automatically, (i.e., t h a t a r e n o t attended t o ) . T y p e I l l a n d T y p e I V Ensembles A hypothesis t h a t a t t r i b u t e s t h e l e n g t h e n i n g o f incompatible response latencies t o a stage o f t r a n s l a t i o n f r o m t h e stimulus t o t h e response code is, however, much less c o n v i n c i n g f o r T y p e Ill a n d T y p e IV ensembles. In fact, t h e stimulus a n d response sets o v e r l a p o n an i r r e l e v a n t dimension, w h i c h need n o t b e processed. Therefore, no such t r a n s l a t i o n is necessary. I n t h e case o f S-R compatibility effects d u e t o an i r r e l e v a n t stimu l u s dimension that, however, overlaps w i t h t h e response dimension, t h r e e e x p l a n a t o r y hypotheses have been proposed. Actually, t h e f i r s t t w o t o b e considered below were proposed t o e x p l a i n spatial S-R comp a t i b i l i t y in general, n o t j u s t t h e effects d u e t o an i r r e l e v a n t stimulus dimension. However, it seems t o us t h a t spatial S-R compatibility effects obtained w i t h T y p e I I ensembles a r e s a t i s f a c t o r i l y accounted f o r by t h e coding hypothesis a n d a r e n o t i n need o f f u r t h e r explanations. A n i m p o r t a n t p o i n t t h a t should n o t b e overlooked is t h a t spatial S - R compatibility effects p r o d u c e d on a r e l e v a n t dimension (i.e., w i t h

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T y p e I I ensembles) a r e c o n s i s t e n t l y g r e a t e r t h a n those p r o d u c e d on an i r r e l e v a n t dimension (i.e., w i t h T y p e I l l a n d T y p e IV ensembles). The f o r m e r a r e o f about 50-70 ms, whereas t h e l a t t e r a r e n o t g r e a t e r t h a n approximately 25 ms ( f o r estimates o f t h e t w o t y p e s o f effects o b s e r v e d in comparable conditions, see Nicoletti e t al., 1982; Nicoletti & UmiltO, 1984, 1985; UmiltO & Liotti, 1987; UmiltO E Nicoletti, 1985). It seems, therefore, t h a t t h e d i s t i n c t i o n between spatial S-R compatibility effects in a r e l e v a n t o r in an i r r e l e v a n t dimension is n o t merely semantic.

All t h r e e hypotheses c l e a r l y p o i n t t o response selection as t h e stage a t w h i c h S-R compatibility effects emerge. T h i s i s no d o u b t a l i k e l y possibility, because it has been shown (see UmiltO 8 Liotti, 1987; Umilta & Nicoletti, 1985) t h a t these effects o c c u r o n l y f o r t h a t dimension t h a t becomes available w h i l e t h e response is b e i n g selected. The stimulus dimension t h a t can b e processed 500 ms b e f o r e response select i o n does n o t p r o d u c e a n y effects. F o r example, if information about side is p r e s e n t e d in advance, whereas information about r e l a t i v e posit i o n i s p r e s e n t e d simultaneously w i t h t h e imperative stimulus, t h e n S-R compatibility effects o c c u r f o r r e l a t i v e position o n l y . Conversely, if information a b o u t r e l a t i v e position i s p r e s e n t e d in advance, whereas information about side is p r e s e n t e d a t t h e same time as t h e imperative stimulus, t h e n S-R compatibility effects o c c u r f o r side. Similarly, Simon e t al. (1976a) f o u n d t h a t spatial S-R compatibility effects disapp e a r if t h e response is delayed f o r approximately 300 ms a f t e r stimulus onset. T h e t h r e e hypotheses also share, e x p l i c i t l y o r implicitly, t h e assumption t h a t t h e processes t h a t cause spatial S-R compatibility effects w i t h T y p e I l l a n d T y p e IV ensembles a r e automatic, t h a t is, d o n o t r e q u i r e attention. T h i s t o o seems t o b e a l i k e l y p o s s i b i l i t y because, as repeatedly said, t h e stimulus dimension t h a t overlaps w i t h t h e response dimension, a n d t h u s produces S-R compatibility effects, is n o t t a s k r e l e v a n t and, presumably, does n o t u n d e r g o processing in a controlled mode.

The Attentional Hypothesis. T o explain spatial S-R compatibility effects, Simon (1968; Simon & Rudell, 1967) i n v o k e d a basic n a t u r a l t e n d e n c y t o respond t o w a r d t h e source o f t h e imperative stimulus. When t h e side o f t h e stimulus does n o t match t h e side o f t h e response, comparatively l o n g e r R T s a r e obtained because t h e i n a p p r o p r i a t e response t e n d e n c y must b e i n h i b i t e d . When t h e side o f t h e stimulus a n d t h e side o f t h e response correspond, RTs a r e s h o r t e r because t h e basic response t e n d e n c y is consistent w i t h t h e r e q u i r e d response. T h i s is an attentional explanation, as attested by t h e f a c t t h a t Simon (1969) l i k e n e d t h e n a t u r a l t e n d e n c y t o respond t o w a r d t h e stimulus source t o an o r i e n t i n g response. T h e imperative stimulus would p r o v i d e a d i r e c tional cue, w h i c h t e n d s t o e l i c i t automatically a response on i t s same side (Simon, Hinrichs, E C r a f t , 1970; f o r a modified v e r s i o n o f t h e attentional hypothesis, see Verfaellie, Bowers, & Heilman, 1988). T h e attentional hypothesis becomes scarcely tenable if one considers t h a t spatial S-R compatibility effects can b e obtained w i t h T y p e I l l ensembles also when t h e i r r e l e v a n t dimension is relative, t h a t is, i n a condition in w h i c h b o t h t h e t w o stimuli a n d t h e t w o responses o c c u r

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o n t h e same side (see UmiIta & Liotti, 1987; Umilth & Nicoletti, 1985). If t h e d e t e r m i n i n g f a c t o r were t h e t e n d e n c y t o react in t h e d i r e c t i o n o f t h e imperative stimulus, n o effects should instead b e f o u n d because e i t h e r stimulus should cause a response t e n d e n c y t h a t i s identical f o r e i t h e r response. In o t h e r words, if t h e t w o stimuli appear in t h e right v i s u a l field, t h e n b o t h should e l i c i t a t e n d e n c y t o r e s p o n d t o w a r d t h e right side. Also t h e f a c t t h a t S-R compatibility effects vanish when t w o i r r e l e v a n t spatial stimulus dimensions a r e simultaneously available (see U m i l t i & Liotti, 1987, Experiments 3 a n d 4) creates problems f o r t h e attentional hypothesis. I n fact, t h e hypothesis cannot e x p l a i n why t h e stimulus t h a t appears o n t h e right side (see F i g u r e s 1 a n d 2 ) should n o t f a v o r t h e response located o n t h e same side, simply because t h a t stimulus occupies also t h e l e f t r e l a t i v e position.

A f u r t h e r difficulty encountered by t h e hypothesis concerns t h e fact t h a t S-R compatibility effects a r e absent in simple R T tasks. (Remember t h a t in simple R T tasks w h a t matters is anatomical connectivity, r a t h e r t h a n spatial S-R Compatibility.) It i s unclear w h y a stimulus shown in t h e right v i s u a l f i e l d should n o t p r o d u c e a response t e n d e n c y t h a t f a v o r s t h e response o n t h e right side o v e r t h e response on t h e l e f t side also w i t h simple R T s . T h e Dimensional O v e r l a p Hypothesis. According t o Kornblum e t al. (in press), S-R compatibility effects a r e p r o d u c e d w i t h an i r r e l e v a n t dimension of t h e stimulus because, d u e t o t h e o v e r l a p w i t h t h e response dimension, presentation o f a p a r t i c u l a r element o f t h e stimulus set causes t h e automatic activation o f a p a r t i c u l a r element o f t h e response set. If t h e response t h a t is activated by t h e stimulus i s t h e one assigned t o it on t h e basis o f t h e r e l e v a n t dimension, t h e n response latency w i l l show a b e n e f i t . If t h e activated response d i f f e r s f r o m t h e r e q u i r e d one, t h e n response latency w i l l show a cost. O f course, in t h e absence of o v e r l a p between t h e i r r e ! e v a n t stimulus dimension a n d t h e response dimension, n e i t h e r costs n o r b e n e f i t s w i l l b e f o u n d . S-R compatibility effects a r e due, in t h i s view, t o t h e automatic p r i m i n g , by an i r r e l e v a n t stimulus element, o f a response element. T h e response element t h a t is p r i m e d may o r may n o t coincide w i t h t h e response element t h a t must b e selected on t h e basis o f t h e r e l e v a n t stimulus element. K o r n b l u m e t al.'s ( i n p r e s s ) hypothesis a p p a r e n t l y f a r e s b e t t e r t h a n t h e p r e v i o u s one in t h e face of t h e empirical f i n d i n g s . It has no d i f f i c u l t i e s i n accounting f o r t h e S-R compatibility effects d u e t o t h e r e l a t i v e position o f t h e stimuli a n d / o r t h e responses. In fact, t h e r e i s no reason w h y t h e stimulus a n d response sets should n o t o v e r l a p o n t h e r e l a t i v e dimension. Thus, in t h a t dimension too, a p a r t i c u l a r stimulus element can activate a p a r t i c u l a r response element. T h e absence o f spatial S-R compatibility effects in simple R T t a s k s can b e easily explained, based on t h e f a c t t h a t i n those t a s k s t h e stimulus a n d response sets comprise one element o n l y . Therefore, t h e r e cannot b e a d i f f e r e n t i a l a c t i v a t i o n of response elements by a stimulus element.

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T h e absence o f S-R compatibility effects if t w o spatial dimensions a r e p r e s e n t a t t h e time when t h e response i s b e i n g selected seems t o b e more problematic f o r t h e hypothesis. One can argue, o f course, t h a t t w o d i f f e r e n t responses a r e p r i m e d a n d t h e n e t e f f e c t is null. However, t h a t implies t h a t one single stimulus can y i e l d t w o independent stimulus elements. T h i s is an a d hoc assumption, which is in need o f empirical s u p p o r t . T h e Coding Hypothesis. A d i f f e r e n t t a c k was t a k e n by Umilta a n d L i o t t i (1987; also see UmiltB & Nicoletti, 19851, who attempted t o account f o r spatial S-R compatibility effects p r o d u c e d by an i r r e l e v a n t stimulus dimension by e x t e n d i n g t o them t h e c o d i n g hypothesis. (Remember t h a t t h i s was also t h e o r i g i n a l proposal o f Wallace, 1971, 1972.) T h e basic idea is t h a t S-R compatibility effects a r i s e because of t h e presence, a t t h e moment when t h e response i s selected, o f t w o spat i a l codes. One i s t h e response code, w h i c h must b e formed in o r d e r t o p e r f o r m t h e t a s k . T h e o t h e r is a stimulus code, w h i c h is o f no r e l evance f o r p e r f o r m i n g t h e task, but is formed automatically. T h e stimulus code can influence t h e speed w i t h w h i c h t h e response code is formed. If t h e t w o codes a r e equal, t h e selection o f t h e response is facilitated. If t h e t w o codes a r e d i f f e r e n t , t h e selection o f t h e response is delayed. Note t h a t t h i s v e r s i o n o f t h e c o d i n g hypothesis ascribes t h e S-R compatibility effects p r o d u c e d by an i r r e l e v a n t stimulus dimension t o t h e stage in w h i c h response selection occurs, r a t h e r t h a n t o t h e stage in w h i c h a t r a n s l a t i o n occurs f r o m t h e stimulus code t o t h e response code. B y contrast, S - R compatibility effects p r o d u c e d by a r e l e v a n t dimension a r e t h o u g h t t o o r i g i n a t e a t t h e stage in which such t r a n s l a t i o n takes place. T h e d i s t i n c t i o n between t h e stage in w h i c h t h e response i s selected a n d t h e stage i n w h i c h t h e stimulus code is t r a n s l a t e d i n t o t h e response code is based on t h e consideration t h a t a t t h e t r a n s l a t i o n stage t h e spatial codes o f t h e stimulus set a r e mapped i n t o t h e spatial codes o f t h e response set, whereas a t t h e selection stage t h e a p p r o p r i a t e element o f t h e response set is chosen. T h e coding hypothesis can easily account f o r t h e S - R compatibility effects d u e t o r e l a t i v e stimulus position. Actually, t h i s is t h e reason w h y it was i n i t i a l l y put f o r w a r d (see Umilta & Nicoletti, 1985). In t h i s view, t h e r e l a t i v e position o f t h e stimulus is automatically p r o cessed, a n d t h i s processing g i v e s r i s e t o a spatial code, w h i c h can speed u p o r delay t h e p r o d u c t i o n o f t h e spatial response code. The same happens, o f course, when t h e stimulus dimension is side, instead of r e l a t i v e position, w h i c h is processed while t h e response code i s b e i n g formed (see Umilta & Liotti, 1987, Experiments 3 a n d 4).

If r e l a t i v e position a n d side a r e b o t h coded a t t h e moment when t h e response code is formed, a n d t h e outcome o f t h e t w o independent c o d i n g processes is d i f f e r e n t , t h e n t h e facilitation a n d i n h i b i t i o n effects balance each o t h e r o u t . T h e n e t r e s u l t is t h e absence o f spatial S - R compatibility effects (see, again, Umilt.5 & Liotti, 1987, Experiments 3 a n d 4). If r e l a t i v e position a n d side a r e b o t h processed b e f o r e p r e sentation o f t h e imperative stimulus, t h e n no spatial code is a c t i v e a t t h e moment of response selection. Also, in t h i s case, S - R compatibility effects vanish (see Umilta & L i o t t i , 1987, Experiment 5).

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As for t h e absence o f S-R compatibility e f f e c t s i n simple R T tasks, t h e explanation o f f e r e d by t h e c o d i n g hypothesis i s t h a t no spat i a l response code needs t o b e formed because t h e r e i s one response only. Therefore, even if a spatial stimulus code i s automatically formed, t h e v e r y basis f o r a n i n t e r f e r e n c e between t h i s code a n d t h e response code is lacking.

The Crossed-Hand Effect In a p r e v i o u s section, we d e s c r i b e d t h e consequences t h a t r e s u l t when t h e subject crosses h i d h e r hands. T h e t y p e o f ensemble ( T y p e I I o r T y p e I l l ) is immaterial because t h e spatial response dimension i s always r e l e v a n t . One consequence is t h a t spatial S-R compatibility a p p a r e n t l y reverses i t s d i r e c t i o n . T h e o t h e r is t h a t R T becomes l o n g e r o v e r a l l (i.e., t h e crossed-hand e f f e c t ) . T h e a p p a r e n t r e v e r s a l o f S-R compatibility poses n o problems f o r a n y o f t h e t h r e e hypotheses. For t h e f i r s t hypothesis, t h e t e n d e n c y t o r e s p o n d t o w a r d t h e stimulus influences t h e response t h a t is emitted in a p a r t i c u l a r position, n o t t h e h a n d t h a t emits t h e response. F o r t h e second hypothesis, a stimulus element activates automatically a response element t h a t is located in t h e c o n g r u e n t position, n o t t h e h a n d t h a t emits t h e response. F o r t h e third hypothesis, w h a t i s automatically coded i s t h e position o f t h e response, n o t t h e h a n d t h a t emits t h e response. B y contrast, t h e crossed-hand e f f e c t o f slowing o v e r a l l RTs can b e explained o n l y w i t h i n t h e f r a m e w o r k o f t h e c o d i n g hypothesis (see Nicoletti e t al., 1984; Riggio e t al., 1986). T h i s hypothesis maintains t h a t t h e anatomical s t a t u s of t h e e f f e c t o r i s coded along with t h e position o f the effector. If t h e t w o codes a r e d i f f e r e n t , as happens in t h e crossed-hand condition, a c o n f l i c t arises a t t h e stage o f response select i o n which, therefore, occurs w i t h a c e r t a i n delay. Summary In t h i s chapter, K o r n b l u m e t al.'s (in press) taxonomy was T h e most imporadopted t o c l a s s i f y spatial S-R compatibility effects. t a n t f e a t u r e o f t h e taxonomy is t h e d i s t i n c t i o n between r e l e v a n t a n d i r r e l e v a n t dimensions o f t h e stimulus set. A stimulus dimension is said t o b e r e l e v a n t when t h e r e q u i r e d response is signaled by t h e v a l u e of t h e stimulus in t h a t dimension. A stimulus dimension i s said t o b e i r r e l e v a n t if values on it d o n o t signal t h e r e q u i r e d response. In t h e f i r s t section, we discussed choice R T studies in w h i c h t h e l e f t - r i g h t dimension of t h e stimulus c o u l d b e e i t h e r r e l e v a n t (i.e., T y p e I I ensembles, according t o K o r n b l u m e t a l . ' s taxonomy) o r i r r e l e v a n t (i.e., T y p e I l l or T y p e IV ensembles). When t h e stimulus set comp r i s e s a l e f t a n d a right element, l e f t a n d right values can b e specified along t w o dimensions: T h e stimulus can b e on e i t h e r t h e l e f t o r right side i n relation t o an egocentric reference axis (i.e., t h e egocentric dimension); a l t e r n a t i v e l y , it can b e e i t h e r t h e l e f t o r right position in relation t o an e x t e r n a l reference p o i n t ( i . e . , t h e r e l a t i v e dimension). We showed t h a t these t w o dimensions can b e unconfounded a n d

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independently p r o d u c e S-R compatibility effects w i t h T y p e II, T y p e I I I , a n d T y p e IV ensembles.

I n t h e second section o f t h e c h a p t e r it was a r g u e d t h a t in simple R T tasks, spatial S-R compatibility i s immaterial a n d w h a t matters i s t h e directness o f t h e anatomical connections between t h e stimulated hemiretinae a n d t h e r e s p o n d i n g hand. B y contrast, in choice R T tasks, speed o f response depends p r i m a r i l y on t h e spatial relations between t h e position o f t h e response and t h e position of t h e imperative stimulus, t h a t is, o n spatial S-R compatibility. Also in t h e case of t h e response position, egocentric a n d r e l a t i v e l e f t - r i g h t dimensions can b e unconfounded. T h e studies discussed in t h i s section show S-R compatibility effects a t t r i b u t a b l e to t h e r e l a t i v e position o f t h e response. More p r e cisely, if one distinguishes between t h e r e l a t i v e position o f t h e e f f e c t o r a n d t h e r e l a t i v e position o f t h e response goal, it appears t h a t S-R comp a t i b i l i t y effects depend on t h e l a t t e r . T h e r o l e o f t h e egocentric dimension o f t h e e f f e c t o r o r response goal in p r o d u c i n g compatibility effects has y e t t o b e demonstrated. In t h i s same section, t w o phenomena t h a t a r e caused by t h e position o f t h e effectors were discussed: When subjects cross t h e i r hands, S-R compatibility a p p a r e n t l y reverses i t s direction, a n d R T becomes o v e r a l l longer. In t h e third section, t h e dominance o f t h e l e f t - r i g h t dimension o v e r t h e above-below dimension in spatial S-R compatibility was d i s cussed. T h i s phenomenon manifests i t s e l f w i t h special instances o f T y p e I 1 ensembles, in w h i c h t h e stimulus a n d response sets o v e r l a p in t w o spatial dimensions (i.e., above-below a n d l e f t - r i g h t ) , instead o f one, as happens w i t h r e g u l a r T y p e I I ensembles. A l t h o u g h S-R comp a t i b i l i t y effects a r e easily f o u n d when t h e stimulus a n d t h e response sets o v e r l a p o n t h e above-below dimension only, t h i s dimension i s n o t used f o r mapping stimuli o n t o responses if also l e f t - r i g h t values a r e available in b o t h t h e stimulus a n d t h e response set. Finally, in t h e f o u r t h section, several e x p l a n a t o r y hypotheses o f spatial S-R compatibility effects were t a k e n i n t o consideration. It was concluded t h a t S-R compatibility effects p r o d u c e d by e i t h e r a r e l e v a n t o r an i r r e l e v a n t stimulus dimension can b e s t b e explained w i t h i n t h e framework o f t h e c o d i n g hypothesis. In t h e case o f T y p e I I ensembles, spatial codes a r e assigned t o t h e values o f stimuli a n d responses in t h e o v e r l a p p i n g dimension. T h e n a t r a n s l a t i o n f r o m t h e stimulus code t o t h e response code takes place. T h i s is because it i s t h e value o f t h e imperative stimulus in t h e o v e r l a p p i n g dimension t h a t signals t h e value of t h e r e q u i r e d response. Therefore, response latency is s h o r t e r when t h e t r a n s l a t i o n i s simple ( i . e . , compatible S-R p a i r i n g s ) t h a n when it is complex (i.e., incompatible S-R p a i r i n g s ) .

In t h e case o f T y p e I l l a n d T y p e IV ensembles, spatial S-R comp a t i b i l i t y effects o c c u r because o f t h e presence, a t t h e moment when t h e response is selected, of t w o spatial codes. One is t h e response code, w h i c h must b e formed t o p e r f o r m t h e t a s k . T h e o t h e r is a stimu l u s code, w h i c h i s o f no relevance f o r p e r f o r m i n g t h e task, but is formed automatically. If t h e t w o codes a r e equal ( i . e . , compatible S - R p a i r i n g s ) , t h e selection o f t h e response is facilitated; if t h e t w o codes

Spatial S-R Compatibility are different (i.e., response is delayed.

incompatible S-R

pairings),

113 t h e selection o f t h e

In conclusion, t h e coding hypothesis a t t r i b u t e s t h e spatial S-R compatibility effects p r o d u c e d by an i r r e l e v a n t stimulus dimension t o t h e stage in w h i c h t h e response selection occurs, whereas it a t t r i b u t e s t h e effects p r o d u c e d by a r e l e v a n t dimension t o t h e stage in w h i c h t h e t r a n s l a t i o n f r o m t h e stimulus code t o t h e response code o c c u r s .

Acknowledgements Preparation o f t h e p a p e r was s u p p o r t e d by g r a n t s f r o m t h e Consiglio Nazionale delle Ricerche a n d t h e M i n i s t e r 0 della Pubblica l s t r u z i o n e t o C.U. T h e a u t h o r s a r e g r a t e f u l t o S. Kornblum, T. Hasbroucq, a n d A . O m a n f o r allowing t h e use o f t h e i r taxonomy.

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SPATIAL CODING AND SPATIO-ANATOMICAL MAPPING: EVIDENCE FOR A HIERARCHICAL MODEL OF SPATIAL STl MULUS- RESPONSE COMPAT I B I L IT Y GABRIELE HEISTER A b t e i l u n g Neuropsychologie Universitaetsspital Zuerich, Switzerland PETER SCHROEDER-HEISTER Zentrum Philosophie und Wissenschaftstheorie U n i v e r s i t a e t Konstanz, FRG WALTER H. EHRENSTEIN l n s t i t u t f u e r Arbeitsphysiologie an d e r U n i v e r s i t a e t Dortmund, FRG

Spatial stimulus-response (S-R) compatibility u s u a l l y denotes t h e reaction-time ( R T ) advantage f o r spatially compatible S-R p a i r i n g s o v e r incompatible p a i r i n g s in choice-reaction tasks. T h i s notion goes b a c k t o F i t t s a n d Seeger (1953), who c r e d i t e d t h e t e r m "compatibility" t o a suggestion by A. M. Small. It i s difficult t o g i v e a d e f i n i t i o n t h a t captures e v e r y t h i n g t h a t has been associated w i t h t h i s term. Quite generally, one may call S-R p a i r i n g s spatially compatible if, w i t h respect t o a c e r t a i n spatial characteristic, t h e stimuli a n d responses c o r r e s p o n d t o each o t h e r and incompatible if they have opposite spatial characteristics. A t y p i c a l example o f spatial compatibility is t h e situation in w h i c h subjects react t o a light stimulus in t h e right o r l e f t visual f i e l d by p r e s s i n g a r i g h t o r l e f t response b u t t o n w i t h t h e i r right or l e f t hand, respectively. One normally f i n d s a spatial S-R compatibility effect, in t h e sense t h a t r i g h t - h a n d reactions t o right l i g h t s a n d l e f t - h a n d reactions t o l e f t l i g h t s (compatible condition) a r e f a s t e r t h a n r i g h t - h a n d reactions t o l e f t l i g h t s a n d l e f t - h a n d reactions t o right l i g h t s (incompatible condition; Brebner, Shephard, & Cairney, 1972; C r a f t & Simon, 1970; Simon & Wolf, 1963; Wallace,1971; upright head conditions o f Lddavas & Moscovitch, 1984, Experim n t s 1 a n d 3, a n d o f Schroeder-Heister, Heister, & Ehrenstein, 1988).? Usually it is n o t specified w h e t h e r t h i s e f f e c t l T h e r e is a c e r t a i n ambiguity i n t h e notion o f S-R compatibility. B y S-R compatibility, one may designate (1) a f e a t u r e o f t h e experimental stimulus-response arrangement whose e f f e c t on which performance is measured (i.e., an independent variable, which is experimentally manipulated); (2) t h e e f f e c t t h a t compatible responses a r e f a s t e r t h a n incompati b l e responses (i.e., an observed r e s u l t ) ; (3) a c e r t a i n i n t e r p r e t a t i o n o f t h i s compatibility e f f e c t emphasizing i t s c o g n i t i v e c h a r a c t e r as opposed, e.g., t o neuroanatomic explanations (see Heister E Schroeder-Heister, 1985). I n t h e following, it w i l l always b e clear f r o m t h e c o n t e x t w h a t is meant.

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means (a) t h a t t h e r i g h t hand reacts faster t o t h e r i g h t l i g h t than t o the l e f t light, and t h e l e f t hand faster t o t h e l e f t light than t o t h e r i g h t light, (b) t h a t t h e right hand reacts faster than the l e f t hand t o t h e r i g h t light, and t h e l e f t hand faster than t h e r i g h t hand t o t h e l e f t light, o r (c) both of these possibilities. Similar examples could be given f o r stimuli i n the upper and lower visual fields (LBdavas & Moscovitch, 1984, u p r i g h t head conditions of Umilt6, Experiments 1 and 4; Nicoletti & Umilt6, 1984; Nicoletti, Tressoldi, & Marzi, 1988) and f o r auditory stimulation (Simon 1969; Simon & Rudell, 1967; see also Chapter 2, by Simon). However, because the pattern of results is similar f o r these d i f f e r e n t experimental conditions, i n the following section, i n which some basic results of compatibility research are t o be discussed, we will mainly deal with the paradigm of bimanual choice reactions t o r i g h t and l e f t lights. O u r considerations can be extended t o most other S-R arrangements i n a straightforward way. For t h e sake of simplicity, we will also r e f r a i n from distinguishing between spatial S-R compatibility effects f o r relevant stimulus location, f o r which t h e subject's task concerns the spatial distinction t h a t is i n question f o r t h e compatibility effect, and irrelevant stimulus location, f o r which t h e task concerns responding t o some other feature of the stimulus (such as i t s color o r i t s semantic content). What is Compatible or Incompatible in Spatial S-R Compatibility? With bimanual choice reactions t o r i g h t and l e f t stimuli, there are at least t h r e e d i f f e r e n t r i g h t h e f t distinctions on t h e response side, leading t o three d i f f e r e n t relations between stimuli and responses. First, we have a r i g h t h e f t distinction between t h e positions of the response keys. Then, we can distinguish between t h e r i g h t o r l e f t position of the responding hand. And, finally, there is t h e anatomical distinction between the right and the left hand. Correspondingly, the compatibility effect observed may be due t o the compatibility o r incompatibility of either (a) stimulus position and response key position, (b) stimulus position and response effector position, o r (c) stimulus position and responding hand. Obviously, all three possibilities are confounded i n t h e normal paradigm, in which the right key i s operated w i t h t h e right hand held i n right position. T h e confounding between responding hand and i t s position was resolved by Simon, Hinrichs and C r a f t (197Ob--auditory stimulation) and I n these studies, independently by Wallace (1971 --visual stimulation). subjects r e a c t e d ' w i t h t h e i r arms crossed, so t h a t the r i g h t hand was placed on t h e l e f t side and t h e l e f t hand was placed on t h e r i g h t side. T h e compatibility between stimulus position and t h e position of responding hand (or response key), and not t h e compatibility between stimulus position and right o r l e f t hand, was shown t o be crucial f o r t h e observed effect. (For applications of t h i s technique, see Anzola, Bertoloni, 1972; Callan, Klisz, & Buchtel, & Rizzolatti, 1977; Brebner et al., Parsons, 1974; Nicoletti, Anzola, Luppino, Rizzolatti, & Umilta, 1982.) Riggio, Gawryszewski, and Umilta (1986) resolved t h e confounding between position of response keys and position of responding hand. They studied responses w i t h hands held i n r i g h t o r l e f t positions, w i t h

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o n l y t h e r e s p o n d i n g index f i n g e r s crossed, so t h a t subjects pressed t h e l e f t k e y w i t h t h e i r r i g h t - i n d e x f i n g e r a n d t h e right k e y w i t h t h e i r l e f t index f i n g e r . T h e compatibility between stimulus position a n d position o f response keys, a n d n o t between stimulus position a n d position o f r e s p o n d i n g hand, caused t h e o b s e r v e d compatibility e f f e c t . This result was confirmed by an experiment i n w h i c h subjects used crossed s t i c k s t o p r e s s t h e response buttons, so t h a t even t h e index f i n g e r s o f t h e r e s p o n d i n g hands were on t h e i r normal right o r l e f t sides. Because Riggio e t al.'s (1986) p a p e r was t h e f i r s t s t u d y t h a t e x p l i c i t l y d e a l t w i t h t h e hand-position/key-position d i s t i n c t i o n (Klapp, Greim, Mendicino, & Koenig, 1979, Experiment 1, dealt w i t h it implicitly, see later), t h e empirical basis f o r t h i s d i s t i n c t i o n is s t i l l r e l a t i v e l y small. In particular, n o investigation has been c a r r i e d o u t f o r i r r e l e v a n t stimulus location. O n t h e stimulus side, too, t h e r e is a t least one confounding, namely between a p p a r e n t location o f t h e stimulus a n d t h e sensory o r g a n stimulated. C r a f t a n d Simon (1970, Experiment 2) used stereoscopic p r e sentation o f stimuli t o show t h a t t h e compatibility e f f e c t disappears if t h e a p p a r e n t position o f t h e stimulus is in t h e middle o f t h e v i s u a l field, alt h o u g h actually e i t h e r t h e right or t h e l e f t eye is stimulated. Therefore, o n t h e stimulus side, t h e apparent position o f t h e stimulus is essential f o r S-R compatibility. Analogous r e s u l t s were also obtained f o r a u d i t o r y stimulation, u s i n g phase transformation t o p r o d u c e an a p p a r e n t r i g h t h e f t location w i t h b i n a u r a l stimulus presentation (Simon, C r a f t , & Small, 1971; Simon, Small, Ziglar, & C r a f t , 1970a). T h e r e s u l t s f r o m t h e above-mentioned studies show t h a t spatial S-R compatibility is related t o t h e compatibility between p e r c e i v e d stimulus position a n d t h e position o f t h e response k e y . However, in most designs, t h e apparent a n d actual stimulus positions, as well as t h e positions o f t h e r e s p o n d i n g hands a n d response keys, a r e t h e same. Thus, we w i l l t r e a t t h e relation between stimulus position a n d response position as essential f o r spatial S-R compatibility. In o t h e r words, t h e c e n t r a l d i s t i n c t i o n on t h e response side t h a t we a r e addressing in t h i s p a p e r i s t h a t between response position (which may b e k e y position o r h a n d posit i o n ) a n d response effector (e.g., anatomically d e f i n e d right o r l e f t hand). T h e d i s t i n c t i o n between h a n d position a n d k e y position w i l l nevertheless b e i n c l u d e d in t h e hierarchical model proposed subsequently. Several explanations have been proposed t o account f o r t h e importance o f spatial positions. A c c o r d i n g t o Simon, t h e stimulus perceived is a command t o react in a c e r t a i n way, namely, t o p r e s s t h e right o r left button. Because, as a "population stereotype" (Simon & Rudell, 1967, p. 3001, a " ' n a t u r a l ' tendency t o respond t o w a r d thf source o f stimulation" (Simon, 1969, p. 174) i s assumed, t h e r e is t h e necessity o f o v e r r i d i n g t h i s i n i t i a l response tendency" (Simon e t al., 1970a, p. 314) when t h e content o f t h e command (i.e., t h e locus o f t h e c o r r e c t response) does n o t agree w i t h t h e locus o f t h e stimulus source, y i e l d i n g l o n g e r RTs. In e a r l i e r papers, Simon h a d considered t h i s stereotype t o express an o r i e n t i n g r e f l e x (Simon, 1968, 19691, but h e l a t e r abandoned t h i s i n t e r p r e t a t i o n because such an e f f e c t should v e r y q u i c k l y disappear w i t h practice, which is n o t t r u e f o r t h e spatial S - R compatibility e f f e c t (see Simon, 1970; Faber, v a n d e r Molen, Keuss, & Stoffels, 1986). Furthermore, Simon did n o t claim t h a t t h i s n a t u r a l tendency is "natural" in t h e s t r i c t sense: "It m i g h t v e r y well b e learned" (Simon, 1970, p. 51).

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A c c o r d i n g t o Wallace (1971), spatial S-R compatibility is d u e t o a comparison o f spatial codes f o r stimulus a n d response positions, leading t o l o n g e r RTs when t h e representations o f stimulus a n d response posit i o n s d o n o t coincide. T h i s c o d i n g i n t e r p r e t a t i o n resembles F i t t s a n d Seeger's (1953) t h e o r y t h a t S-R compatibility is related t o t h e r a t e o f L a t e r researchers have f u r t h e r information t r a n s f e r in reaction tasks. advanced Wallace's c o d i n g i n t e r p r e t a t i o n o f spatial S-R compatibility (in p a r t i c u l a r , Nicoletti e t al., 1982; Nicoletti & Umilt6, 1984, 1985; Umilt6 & Nicoletti, 1985; Umilt6 & L i o t t i 1987; see C h a p t e r 3 by UmiltA & Nicoletti) . However, whereas these l a t t e r researchers saw a fundamental d i s t i n c t i o n between Simon's approach, w h i c h t h e y called "attentional" (Nicoletti e t al., 1982; we followed t h i s perhaps u n f o r t u n a t e terminology in Heister, Ehrenstein, & Schroeder-Heister, 1986, 1987), a n d t h e coding hypothesis, Wallace did not. Rather, h e r e g a r d e d h i s coding t h e o r y as a more a b s t r a c t basis f o r Simon's "tendency t o respond t o w a r d t h e source o f stimulation" (see Wallace, 1971, p. 360). With respect t o t h e stage o f c o g n i t i v e processing in which S-R comp a t i b i l i t y effects a r e generated, t h e major position is t h a t t h e responseselection stage, r a t h e r t h a n t h e stimulus-encoding stage, is r e l e v a n t (cf. Sanders, 1980). T h i s hypothesis is s u p p o r t e d by r e s u l t s u s i n g Sternberg's (1969) a d d i t i v e f a c t o r s paradigm (see Acosta & Simon, 1976; Callan e t al., 1974; Simon, 1982; Simon, Acosta, & Mewaldt, 1975; Simon, Acosta, Mewaldt, & Speidel, 1976). However, t h e r e is also some evidence c o n c e r n i n g t h e importance o f t h e stimulus side, in p a r t i c u l a r t h e dependence o f t h e size o f t h e o b s e r v e d e f f e c t on t h e distance o f t h e stimuli (Ehrenstein, Heister, & Schroeder-Heister, in preparation; Gunia, 1987; Rabbitt, 1967; Simon e t al., 1971).

In t h e following, we w i l l r e f e r t o t h e coding hypothesis as t h e major theoretical explanation o f spatial S-R compatibility, while b e i n g aware t h a t "coding" has a r a t h e r vague meaning. We u n d e r s t a n d "coding hypothesis" t o b e used mainly as a label f o r theories assuming t h a t spatial S-R compatibility is a genuinely c o g n i t i v e phenomenon t h a t has t o d o w i t h t h e i n t e r n a l representation o f spatial relationships. Unimanual T w o - F i n g e r Choice Reactions S t a n d a r d designs f o r s t u d y i n g spatial S-R compatibility t y p i c a l l y i n v e s t i g a t e bimanual reactions t o lateralized v i s u a l o r a u d i t o r y stimuli. Sometimes unimanual movement reactions have also been employed, such as moving a handle t o t h e right o r l e f t (Simon 1968, 1969) o r moving t h e i n d e x f i n g e r f r o m a s t a r t i n g location t o one o f t w o response b u t t o n s (Bauer & Miller, 1982; Brebner, 1979; Cotton, Tzeng, & Hardyck, 1980; Results f r o m Katz (1981) indicate t h a t spatial S Hedge & Marsh, 1975). R compatibility effects also a r e obtained between stimulus locations a n d t h e t w o f i n g e r s o f one hand. In an experiment w i t h i r r e l e v a n t stimulus location a n d unimanual reactions, w i t h t h e h a n d h e l d i n a middle position, Katz showed t h a t compatible v i s u a l f i e l d / f i n g e r p a i r i n g s (e.g., stimulus in t h e right field, response w i t h t h e right f i n g e r ) were f a s t e r t h a n incomp a t i b l e ones, independent o f w h i c h h a n d was used f o r responding. In t h e following, we w i l l call t h i s k i n d o f spatial S-R compatibility finger Compatibility, as d i s t i n g u i s h e d f r o m hand Compatibility. I n t h e context of unimanual t w o - f i n g e r choice reactions, hand-compatibility effects denote R T advantages f o r compatible field/hand relations t h a t may b e obtained in addition t o f i n g e r - c o m p a t i b i l i t y effects. These l a t t e r effects a r e p r o b a b l y

Spatial Coding and Spatio-Anatomical Mapping p r e s e n t in most data f o r choice reactions w i t h t w o o r more fingers, a r e u s u a l l y n o t evaluated.

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T h e Comparison of Prone a n d Supine H a n d Orientations

In several experiments we have i n v e s t i g a t e d compatibility effects w i t h t w o - f i n g e r choice reactions, placing p a r t i c u l a r emphasis o n t h e d i s t i n c t i o n between spatial a n d anatomical f a c t o r s . T o d i s t i n g u i s h between spatial a n d anatomical influences, an analog t o t h e crossed-arms t e s t o f t h e bimanual case h a d t o b e developed. I n Heister e t al. (1986, 1987), we used t h e comparison between p r o n e h a n d position (response b u t t o n s pressed as usual f r o m above) a n d supine h a n d position (response b u t t o n s pressed f r o m below). When t h e h a n d is t u r n e d , t h e spatial relations between t h e r e s p o n d i n g f i n g e r s a r e reversed: T h e f i n g e r t h a t is spatially r i g h t w i t h t h e h a n d h e l d p r o n e i s spatially l e f t in t h e s u p i n e hand. Comparing t h e f i n g e r - c o m p a t i b i l i t y effects f o r b o t h positions can t h e r e f o r e indicate w h e t h e r t h e o b s e r v e d e f f e c t is d u e t o the position o f t h e responding f i n g e r o r t o some anatomical relation. These possibilities were confounded in Katz's (1981) experiment. Results f o r r e l e v a n t stimulus location. T h e main question o f o u r f i r s t s t u d y (Heister e t al., 1986) was w h e t h e r a spatial S - R compatibility e f f e c t occurs w i t h t w o - f i n g e r choice reactions when t h e palms a r e f a c i n g down a n d w h e t h e r it p e r s i s t s when t h e hands a r e t u r n e d o v e r . Eight female subjects h a d t o react t o w a r d a r i g h t o r a l e f t light w i t h t h e index o r middle f i n g e r of t h e i r right o r l e f t hand, w h e r e b y t h e r e s p o n d i n g h a n d was h e l d in a normal r i g h t o r l e f t position. T h e r e s u l t s showed a c l e a r - c u t f i n g e r - c o m p a t i b i l i t y e f f e c t when t h e palms were f a c i n g down a n d an even more pronounced e f f e c t when t h e palms were f a c i n g up. These r e s u l t s s u p p o r t t h e coding hypothesis of spatial S-R compatibility, which says t h a t t h e r e l a t i v e spatial positions o f stimuli a n d responses a r e encoded a n d compared i r r e s p e c t i v e o f t h e anatomical response organs. A n y explanation o f t h e r e s u l t s r e l y i n g on callosal c r o s s i n g time (pathway hypothesis) is immediately r e f u t e d because t h e movements o f t w o f i n g e r s o f one h a n d a r e i n i t i a t e d w i t h i n t h e same hemisphere. Because b o t h responses a r e g i v e n on t h e same side o f t h e b o d y midline, t h e r e s u l t s cannot b e explained by t h e tendency t o react towards t h e source of stimulation ( a t least n o t w i t h o u t making an a d hoc extension o f t h i s hypothesis). Finally, no r i g h t / l e f t classification o f t h e anatomical f i n g e r s can account f o r these results, because t h e spatial compatibility e f f e c t p e r s i s t e d f o r hands in supine position. T h a t is, such a classificat i o n is r e v e r s e d by turning t h e hands and, therefore, would p r e d i c t t h e converse e f f e c t w i t h t h e palm-up position. No hand-compatibility e f f e c t was observed in addition t o t h e s t r o n g f i n g e r - c o m p a t i b i l i t y effect, a l t h o u g h t h e r e s p o n d i n g h a n d was h e l d in lateral r a t h e r t h a n middle position, opposed t o Katz's (1981) design. T h i s negative r e s u l t may b e explained by t h e f a c t t h a t t h e experimental t a s k did n o t demand a choice between responding hands (which were o n l y changed between blocks o f t r i a l s ) , but between t h e responding f i n g e r s o f one hand.

Results f o r i r r e l e v a n t stimulus location a n d f i x e d v e r s u s a l t e r n a t i n g hands. In a subsequent s t u d y (Heister e t a l . , 1987), spatial S - R comp a t i b i l i t y effects f o r t w o - f i n g e r choice reactions were i n v e s t i g a t e d f o r

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i r r e l e v a n t stimulus location. Tests also were i n c l u d e d f o r determining w h e t h e r additional hand-compatibility effects w o u l d b e obtained w i t h i r r e l e v a n t stimulus location a n d w h e t h e r such effects may depend on h o l d i n g t h e r e s p o n d i n g h a n d f i x e d t h r o u g h o u t an experimental session. Eight new female subjects responded w i t h t h e i r index o r middle f i n g e r t o a bicolor diode t h a t emitted e i t h e r r e d o r g r e e n light i n t h e right o r l e f t visual field. T h e subjects were tested w i t h t h e palms f a c i n g down a n d t h e palms f a c i n g up, w i t h t h e r e s p o n d i n g h a n d b e i n g altered in b o t h cond i t i o n s between blocks o f t r i a l s . T h e y also were tested w i t h t h e palms f a c i n g down, but w i t h t h e same h a n d b e i n g used t h r o u g h o u t an e x p e r i mental session. T h e r e s u l t s showed a s t r o n g f i n g e r - c o m p a t i b i l i t y e f f e c t f o r all t h r e e conditions. T h e tendency f o r t h e f i n g e r - c o m p a t i b i l i t y e f f e c t t o increase f r o m t h e palm-down condition ( w i t h a l t e r n a t i n g hands) t o t h e palm-up condition, w h i c h was already p r e s e n t in t h e p r e v i o u s s t u d y (Heister e t al., 19861, now became s t a t i s t i c a l l y s i g n i f i c a n t . As in t h e p r e v i o u s s t u d y , no h a n d -cornpati bi I ity e f f e c t occu r r e d in t h e conditions w i t h a It e r n a t i ng hands. In t h e condition w i t h nonalternating hands, however, an a d d i tional, small hand-compatibility e f f e c t was obtained. These r e s u l t s again suggest t h e prevalence o f t h e spatial coding f a c t o r in S-R compatibility, because t h e compatibility e f f e c t f o r f i n g e r s depends on t h e r e l a t i v e spatial positions of t h e r e s p o n d i n g f i n g e r s w i t h respect t o each o t h e r a n d n o t on t h e i r anatomical relation. T h e small hand-compatibility e f f e c t f o r t h e condition w i t h t h e responding h a n d b e i n g f i x e d t h r o u g h o u t an experimental session can b e explained by a modified attentional hypothesis. A c c o r d i n g t o t h i s hypothesis, constant use o f one h a n d d i r e c t s more a t t e n t i o n t o w a r d t h e c o r r e s p o n d i n g side o f stimulation. However, t h e empirical basis f o r t h i s hand-compatibility e f f e c t is s t i l l too small t o allow f o r more t h a n a t e n t a t i v e hypothesis. Experimental w o r k t o elucidate t h i s e f f e c t is b e i n g performed p r e s e n t l y (Schroeder-Heister, Ehrenstein, & Heister, in p r e p a r a t i o n ) . Spatial Versus Anatomical F i n g e r Distance A n o t h e r experiment w i t h t w o - f i n g e r choice reactions tested t h e hypothesis t h a t t h e size o f t h e spatial S - R compatibility e f f e c t is d i s tance-dependent w i t h respect t o t h e response. T h a t is, we i n v e s t i g a t e d w h e t h e r a n d in w h a t manner t h e compatibility e f f e c t changes if t h e r e l a t i v e distance o f responses i s altered. A s an a l t e r n a t i v e t o t h e supine hands test, s u c h an i n v e s t i g a t i o n should discriminate between coding of response positions a n d anatomical r i g h t / l e f t classification o f f i n g e r s , if one distinguishes between t h e spatial distance of t h e response b u t t o n s A n example o f a n d t h e anatomical distance o f t h e responding f i n g e r s . anatomical distance is t h a t t h e second a n d f o u r t h f i n g e r s a r e nearer t o each o t h e r t h a n a r e t h e f i r s t a n d fifth, even if t h e y operate t h e same pair of buttons. If t h e size o f t h e observed compatibility e f f e c t o n l y depends on t h e spatial distance a n d n o t on t h e anatomical distance, one may i n f e r t h a t spatial coding is t h e e f f e c t i v e f a c t o r . If t h e size o f t h e o b s e r v e d e f f e c t o n l y depends on t h e anatomical distance a n d n o t on t h e

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spatial distance, t h e n an anatomical r i g h t - l e f t discrimination can b e conc l u d e d t o b e essential. F u r t h e r possibilities a r e t h a t t h e size o f t h e comp a t i b i l i t y e f f e c t depends on b o t h spatial a n d anatomical distance, o r t h a t t h e r e is no distance-dependence a t all. Therefore, t h e question o f t h e following experiment was w h e t h e r t h e f i n g e r compatibility e f f e c t remains stable when t h e response i s g i v e n w i t h d i f f e r e n t spatial a n d d i f f e r e n t anatomical distances between responding fingers. I n o u r p r e v i o u s studies, we used index a n d middle f i n g e r s f o r response, but now we chose t h e second ( i n d e x ) a n d f o u r t h f i n g e r ( r i n g ) o r t h e f i r s t (thumb) a n d fifth ( l i t t l e ) f i n g e r , respectively, so t h a t t h e third (middle) f i n g e r c o u l d b e r e g a r d e d as t h e middle o f t h e r e s p o n d i n g hand. To v a r y spatial a n d anatomical distance between f i n g e r s independently, in one o f t h e experimental conditions subjects placed t h e i r f i r s t a n d fifth f i n g e r on response b u t t o n s a t a n a r r o w separ a t i o n t h a t would normally b e used f o r t h e i r second a n d f o u r t h f i n g e r s . Six r i g h t - h a n d e d female college s t u d e n t s (aged 17 t o 19 years) s e r v e d as subjects. T h e stimuli w e r e t w o r e d l i g h t - e m i t t i n g diodes, positioned a t 5 O o f v i s u a l angle t o t h e l e f t a n d r i g h t o f t h e f i x a t i o n p o i n t a n d Response k e y s were t w o c i r c u l a r microswitches, presented f o r 100 ms. whose centers were separated by 45 mm ( n a r r o w b o x ) o r by 110 mm (wide box); these distances were average r e s u l t s f o r h a n d measurements t a k e n o f t h e subjects i n advance: T h e n a r r o w box was used f o r t h e i n d e x finger/ring-finger condition a n d f o r t h e t h u m b / l i t t l e - f i n g e r , narrow spacing condition; t h e wide box was used f o r t h e t h u m b / l i t t l e - f i n g e r wide ( o r n a t u r a l ) spacing condition. T h e subjects attended t w o sessions on d i f f e r e n t days. In one session t h e y responded w i t h t h e i r r i g h t hands, in t h e o t h e r session w i t h t h e i r l e f t hands. In each session, t h r e e conditions were investigated: (a) index-finger/ring-finger operating the narrow buttons, (b) t h u m b / l i t t l e - f i n g e r o p e r a t i n g t h e wide buttons, a n d (c) t h u m b / l i t t l e - f i n g e r operating the narrow buttons. T h e t h r e e conditions were tested in a sequence o f six blocks o f t r i a l s ( i . e . , t w o blocks f o r each condition), each consisting o f s i x p r a c t i c e t r i a l s followed by 60 t e s t t r i a l s (30 in t h e right field, 30 i n t h e l e f t f i e l d ) . I n t h r e e blocks o f each session, t h e subjects made spatially compatible responses; i n t h e o t h e r t h r e e blocks, t h e y made spatially incompatible responses. So as n o t t o d r a w t h e i r attention t o spatial relations, t h e i n s t r u c t i o n s were in terms o f index-, r i n g - , l i t t l e f i n g e r s , o r thumbs, r a t h e r t h a n l e f t o r right position o f t h e fingers. E r r o r s were few, a n d e r r o r t r i a l s were repeated a t t h e e n d o f each block. Mean RTs were subjected t o an analysis o f variance, w i t h response condition (a, b, c), responding h a n d ( r i g h t A e f t ) , f i e l d o f stimulus p r e sentation ( r i g h t / l e f t ) , a n d r e s p o n d i n g f i n g e r ( r i g h t / l e f t ) as w i t h i n - s u b j e c t factors (see Table 1 f o r cell means a n d c o r r e s p o n d i n g s t a n d a r d d e v i ations). T h e r e were t w o s i g n i f i c a n t main effects, one f o r experimental condition, F(2, 10) = 4.61, p < .05, a n d one f o r f i e l d o f stimulus presentation, F(1, 5 ) = 49.52, p < .001. Responses were f a s t e s t w i t h second a n d f o u r t h f i n g e r (Condition a: 281 ms) a n d slowest w i t h f i r s t a n d fifth f i n g e r p r e s s i n g t h e n a r r o w b u t t o n s (Condition c : 293 ms). Reactions were overall f a s t e r w i t h r i g h t - f i e l d t h a n w i t h l e f t - f i e l d stimulation (283 v s . 293 m s ) , which m i g h t b e i n t e r p r e t e d a s e x p r e s s i n g a left-hemisphere s u p e r i o r i t y f o r spatial decision tasks a n d is i n agreement w i t h o u r

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p r e v i o u s r e s u l t s f o r S-R designs w i t h r e l e v a n t stimulus location (Heister T h e s i g n i f i c a n t interaction between f i e l d a n d r e s p o n d i n g e t al., 1986). f i n g e r , F(1, 5) = 87.47, p < .001, expresses t h e S-R compatibility e f f e c t f o r f i n g e r s . T h e r e was a s i g n i f i c a n t change in t h e size o f t h i s compatibility e f f e c t between t h e d i f f e r e n t response conditions, as indicated in t h e t h r e e - w a y interaction between response condition, f i e l d o f stimulation, a n d f i n g e r , F(2, 10) = 9.06, p < .01 (see F i g u r e 1). Table 1 Mean Reaction Times [in milliseconds) a n d S t a n d a r d Deviations f o r Experimental Condition a (second a n d f o u r t h f i n g e r s o p e r a t i n g n a r r o w buttons), Condition b ( f i r s t a n d f i f t h f i n g e r s o p e r a t i n g wide distance buttons), a n d Condition c ( f i r s t a n d f i f t h f i n g e r s o p e r a t i n g n a r r o w buttons)

L e f t light

R i g h t light Responding f i n g e r

Left

Right

Left

Condition a Right hand

264 (37)

310 (39)

Left hand

259 (25)

314 (22) Condition b

Right hand

32 1 (37)

272 (28)

295 (34)

Left hand

Condition c Right hand

266 (30)

315 (35)

Left hand

282 (41 1

324 (27)

Note.

T h e numbers in parentheses a r e s t a n d a r d deviations.

Right

Spatial Coding and Spatio-Anatomical Mappins

CONDITION a

CONDITION b

125

CONDITION c

RTIms) 340

280

260 240

LEFT LIGHT

RIGHT LlGHT

LEFT LIGHT

RIGHT LIGHT

LEFT LIGET

R IGHT LIGHT

F i g u r e 1. Mean R T s f o r responses t o l i g h t s in t h e right o r l e f t v i s u a l f i e l d w i t h t h e right ( f i l l e d circles) a n d l e f t f i n g e r s ( u n f i l l e d circles), averaged o v e r b o t h hands. Condition a: Second a n d f o u r t h f i n g e r s o p e r a t i n g n a r r o w buttons; Condition b: F i r s t a n d fifth f i n g e r s o p e r a t i n g wide distance buttons; Condition c: F i r s t a n d f i f t h f i n g e r s o p e r a t i n g n a r r o w buttons.

U n d e r Condition b, in w h i c h t h e wide-distance b u t t o n s were pressed w i t h t h e f i r s t a n d f i f t h fingers, t h e compatibility e f f e c t was significantly smaller t h a n in t h e o t h e r conditions ( d i f f e r e n c e between incomp a t i b l e a n d compatible reactions in Condition a: 51 ms, Condition b: 26 ms, Condition c: 46 ms; see F i g u r e 1 ) . Because t h i s was especially t r u e f o r l e f t h a n d responses, t h e i n t e r a c t i o n between response condition, hand, field, a n d f i n g e r was significant, F(2, 10) = 7.57, p = .01. A s i g n i f i c a n t d i f f e r e n c e in t h e compatibility e f f e c t between right a n d l e f t hands took place o n l y in Condition b, in w h i c h t h e f i r s t a n d fifth f i n g e r s operated t h e wide-distance b u t t o n s (interaction between hand, field, a n d f i n g e r f o r t h i s condition: F(1, 5) = 11.21, p < .05). F u r t h e r research t o c l a r i f y w h y t h i s e f f e c t took place o n l y f o r l e f t - h a n d responses i s b e i n g performed. T h e most i n t e r e s t i n g r e s u l t is t h a t t h e r e was no d i f f e r e n c e in t h e size o f t h e S - R compatibility e f f e c t f o r Condition a (index f i n g e r / r i n g f i n g e r - - n a r r o w b u t t o n s ) a n d Condition c ( t h u m b / l i t t l e f i n g e r - - n a r r o w buttons), which d i f f e r w i t h respect t o anatomical f i n g e r distance, but n o t w i t h respect t o spatial f i n g e r distance. T h i s means t h a t t h e anatomical distance o f t h e r e s p o n d i n g f i n g e r s alone is n o t r e l e v a n t for the size o f t h e compatibility effect. B u t t h e r e was a s i g n i f i c a n t d i f f e r e n c e between finger--wide buttons) a n d Condition c Condition b ( t h u m b / l i t t l e ( t h u m b / l i t t l e f i n g e r - - n a r r o w b u t t o n s ) , w h i c h d i f f e r w i t h respect t o spatial but n o t w i t h respect t o anatomical f i n g e r distance f i n g e r distance, (interaction between condition, field, a n d f i n g e r in t h e subanalysis f o r data o f Conditions b a n d c : F(1, 5 ) = 15.49, p = .01). T h e S-R comp a t i b i l i t y e f f e c t was smaller when t h u m b a n d l i t t l e f i n g e r pressed t h e wide

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b u t t o n s than when t h e y pressed t h e n a r r o w buttons, which means t h a t o n l y t h e spatial (environmental) distance o f t h e responding f i n g e r s seems t o b e relevant to t h e size o f t h e effect. Therefore, we can conclude t h a t t h e spatial S - R compatibility e f f e c t f o r t w o - f i n g e r choice reactions i t s e l f essentially depends on t h e p u r e l y spatial r a t h e r t h a n anatomical relations o f t h e responses. Spatial C o d i n g a n d Spatio-Anatomical Mapping T h e Effectiveness of Subordinate Factors When t h e spatial r i g h t h e f t d i s t i n c t i o n on the stimulus side is eliminated by a r r a n g i n g t h e stimulus along t h e spatial top/down dimension, but response b u t t o n s a r e s t i l l o n t h e right o r l e f t side, t h e n t h e compatibility e f f e c t disappears (see Simon & Wolf, 1963; Soetens, Deboeck, Hueting, & Merckx, 1984; Wallace 1971, 1972--nevertheless, t h e r e may b e an association o f dominant h a n d w i t h t o p position, see LBdavas, 1987). However, a compatibility e f f e c t may b e obtained when on the response side t h e r i g h t h e f t d i s t i n c t i o n f o r response keys is eliminated, b u t stimulus l i g h t s a r e s t i l l o n t h e right o r l e f t side, because f o r t h e responses o t h e r r i g h t h e f t distinctions (i.e., position o f response effector or anatomical classification) s t i l l remain in force. Klapp e t a l . (1979, Experiment 1--crossed and uncrossed conditions) had subjects operate a single switch, which was mounted in a middle position, w i t h t h e i r thumbs f r o m t h e right o r l e f t side, a n d observed a spatial-compatibility e f f e c t ( R T advantage f o r compatible handlight relations). B y p e r f o r m i n g t h i s experiment w i t h crossed a n d uncrossed arms, Klapp e t a l . made s u r e t h a t t h e e f f e c t was n o t d u e t o t h e anatomical status o f t h e hands as right o r l e f t b u t t o t h e position of t h e responding hand. Furthermore, when t h e r i g h t / l e f t distinction f o r response keys, as well as f o r positions o f responding hands, is eliminated, a compatibility effect is observed f o r which t h e anatomical status o f t h e responding hand as right o r l e f t is relevant. Again, experiments by Klapp e t al. (1979) can b e used as evidence: T h e y had subjects respond t o l e f t a n d right stimuli, w i t h hands h e l d in t h e up/down dimension, by o p e r a t i n g a single A h i g h l y significant interaction b u t t o n w i t h t h e i r respective thumbs. between ( r i g h t o r l e f t ) f i e l d o f stimulation and (anatomically right o r l e f t ) responding hand was f o u n d b o t h f o r uncrossed and crossed arms (Klapp e t al., 1979, Experiment 1, up/down condition w i t h stimulus location relevant; Experiment 2, experimental condition w i t h stimulus location i r r e l e vant). These f i n d i n g s also were obtained when subjects l a y on t h e i r sides and t h e spatial r i g h t / l e f t positions o f t h e responding arms were These results show t h a t eliminated (Klapp e t al., 1979, Experiment 3 ) . those spatial relations t h a t are subordinate t o t h e basic stimulus position/response- k e y position relation (i.e., t h e stimulus-position/responseeffector position relation and t h e stimulus-position/anatomical-hand relation) can b e effective.

A corresponding r e s u l t f o r f i n g e r compatibility was obtained in o u r s t u d y w i t h two-finger choice reactions and orthogonal S-R relations (Ehrenstein, Schroeder-Heister, & Heister, 1989). T h e main question was whether, u n d e r conditions in which no d i r e c t r i g h t - l e f t association of

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stimulus positions a n d positions o f response effectors is possible, an anatomical r i g h t / l e f t classification o f r e s p o n d i n g f i n g e r s becomes essential. F o r t h i s purpose, an experimental design was chosen in w h i c h stimuli a n d responses w e r e p e r p e n d i c u l a r t o each o t h e r . In f o u r d i f f e r e n t sessions, subjects reacted t o r e d o r g r e e n l i g h t s in t h e right o r l e f t v i s u a l f i e l d ( i r r e l e v a n t stimulus location) by p r e s s i n g response k e y s t h a t w e r e a r r a n g e d in t h e following ways: (a) on t h e right o r l e f t side o f t h e b o d y midline ( c o n t r o l condition), (b) p e r p e n d i c u l a r t o t h e stimuli horizontal in t h e midsaggital plane, (c) same as in Condition b but pressed f r o m below (supine h a n d position), a n d (d) p e r p e n d i c u l a r t o t h e stimuli v e r t i c a l in t h e midsaggital plane, pressed w i t h t h e palms f a c i n g t h e b o d y . T h e r e s u l t s f o r Condition a ( t h e c o n t r o l condition) showed t h e usual spatial S-R compatibility e f f e c t f o r f i e l d o f stimulus presentation and responding finger. F o r Conditions b a n d d, a s t r o n g S-R compatibility effect was f o u n d f o r t h e same stimulus/finger p a i r i n g s . For Condition c, t h a t is, reactions w i t h supine h a n d position, R T advantages f o r t h e opposite s t i m u l u d f i n g e r p a i r i n g s were obtained. T h i s means t h a t those s t i m u l u d f i n g e r relations were f a s t e r t h a n t h e y would b e if one t u r n e d t h e h a n d b a c k i n t o t h e normal right o r l e f t position (while keeping i t s p r o n e o r supine orientation). These f i n d i n g s s t r o n g l y s u p p o r t t h e hypothesis t h a t an anatomical r i g h t / l e f t d i s t i n c t i o n becomes e f f e c t i v e if t h e r i g h t / l e f t d i s t i n c t i o n between response positions i s eliminated. The s h i f t o f t h i s relation u n d e r Condition c can b e i n t e r p r e t e d as showing that, u n l i k e t h e case o f t h e right a n d l e f t hands, t h e r e i s no canonical r i g h t / l e f t classification o f anatomical f i n g e r s . T h a t is, t h i s classification may depend o n h a n d o r i e n t a t i o n ( p r o n e v s . supine). One m i g h t suppose t h a t t h e effectiveness o f t h e anatomical r i g h t / l e f t d i s t i n c t i o n s f o u n d by Klapp e t al. (1979) c o u l d b e explained by neuroanatomical c o n n e c t i v i t y . However, t h e effects o b s e r v e d a r e much h i g h e r t h a n s t a n d a r d estimates o f t h e callosal c r o s s i n g time a n d h a v e t h e size o f a s t a n d a r d compatibility e f f e c t f o r w h i c h t h e positions o f response keys a r e important. Furthermore, t h e y o n l y a r e obtained, a t least in t h e size observed, if t h e r e is a choice reaction between t h e r e s p o n d i n g hands. If t h e r e is n o choice between t h e r e s p o n d i n g hands, a compatibility e f f e c t f o r hands is, if e x i s t e n t a t all, v e r y small (see Klapp e t al., 1979, Experiment 2). And, finally, o u r r e s u l t s w i t h orthogonal S - R relations r e f u t e t h i s hypothesis, because t h e responses o f t w o f i n g e r s o f one h a n d a r e i n i t i a t e d f r o m t h e same hemisphere. T h i s suggests t h a t t h e o b s e r v e d anatomical r i g h t / l e f t effects a r e c o g n i t i v e l y based, as a r e t h e o t h e r S-R compatibility effects mentioned. One may hypothesize t h a t t h e r e is a coding o f t h e right a n d l e f t h a n d as spatially right a n d left, respectively. T h i s code is based on t h e f a c t t h a t t h e anatomically right a n d l e f t hands a r e s p a t i a l l y right a n d left, when t h e hands a r e h e l d i n normal positions on t h e respective sides o f the body. T h i s code is t h e n kept, even when t h e hands a r e h e l d in non-normal positions, t h a t is, in middle positions o r on opposite sides o f t h e body. One may also speak of a " n a t u r a l association" o f t h e anatomically right a n d l e f t hands as spatially right a n d l e f t . Correspondingly, f o r t w o - f i n g e r choice reactions, we may assume t h a t t h e r e a r e codes f o r anatomical f i n g e r s as right o r l e f t (even if these codes a r e n o t u n i q u e a n d canonical as f o r right a n d l e f t hands), a n d t h a t these codes a r e k e p t even when t h e response keys a r e n o t mounted parallel t o t h e stimuli.

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T o a v o i d confusion, we propose t h e t e r m spatio-anatomical mapping (in s h o r t : mapping) f o r t h i s association o f spatial positions w i t h anatomical distinctions, a n d r e s e r v e t h e t e r m spatial coding f o r t h e coding o f response positions. We w i s h t o emphasize h e r e t h e c o g n i t i v e c h a r a c t e r o f spatio-anatomical mapping, w h i c h is n o t e x p l i c i t l y stated in Heister e t al. (19861, w h e r e t h i s t e r m was i n t r o d u c e d . 2 Evidence f o r spatial S-R compatibility effects in situations f o r w h i c h t h e response k e y s a r e n o t o r i e n t e d along t h e r i g h t / l e f t dimension can also b e f o u n d in neuropsychological investigations o f cerebral functioning, w h i c h we i l l u s t r a t e by some data o f o u r own. C o m p a t i b i l i t y E f f e c t s With O r t h o g o n a l S-R Relations in Divided-VisualF i e l d Studies of C e r e b r a l Lateralization Because S-R compatibility effects t a k e place n o t o n l y w i t h relevant stimulus location but also w i t h i r r e l e v a n t location, compatibility m i g h t a f f e c t e v e r y normal d i v i d e d v i s u a l - f i e l d s t u d y o f c e r e b r a l lateralization in w h i c h choice reactions a r e r e q u i r e d t o stimuli in t h e right o r l e f t visual f i e l d . Therefore, t h e R T asymmetry ( f i e l d effect), w h i c h u s u a l l y is used as an i n d i c a t o r o f t h e n a t u r e o f hemispheric processing, is d i f f i c u l t t o i n t e r p r e t because o f potential S-R compatibility effects. We f i r s t obtained evidence f o r such j o i n t effects o f cerebral lateralization a n d S-R compatibility by r e p l i c a t i n g a lexical decision t a s k u n d e r t h r e e d i f f e r e n t response conditions: (a) normal unimanual t w o - f i n g e r choice reactions, w i t h t h e hands positioned on t h e right a n d l e f t sides; (b) unimanual t w o - f i n g e r choice reactions, w i t h t h e response keys positioned one b e h i n d t h e o t h e r along t h e midline o f t h e experimental d e s k (medial position); (c) vocal responses (see Heister & Schroeder-Heister, 1987). T h e experiment w i t h lateral response position (a) showed a handcompatibi ‘ty e f f e c t t h a t s i g n i f i c a n t l y decreased w i t h medial response posit i o n (b).

5

Therefore, in a subsequent s t u d y on hormonal influences on hemis p h e r i c processing during t h e menstrual cycle (Heister, Landis, Regard, 6 Schroeder-Heister, 19891, we tried t o exclude such possible additional S-R compatibility effects by p l a c i n g t h e response b u t t o n s ( f o r bimanual reactions) one b e h i n d t h e o t h e r in medial position (i.e., orthogonal t o t h e stimuli along t h e midline o f t h e experimental d e s k ) . In t h i s way, t h e r i g h t / l e f t d i s t i n c t i o n f o r positions o f response k e y s was eliminated. However, in v i e w o f t h e r e s u l t s r e p o r t e d in t h e p r e v i o u s section, t h i s arrangement m i g h t n o t b e able t o fully eliminate S-R compatibility, because a d i s t i n c t i o n between right a n d l e f t h a n d is s t i l l possible. Therefore, a reanalysis of t h e data f r o m t h i s experiment is presented, w h i c h focuses o n compatibility effects. T h e basic hypothesis is t h a t spatial S-R compatibility effects s t i l l a r e obtained in addition t o c e r e b r a l effects a n d hormonal influences. 2The possible c o g n i t i v e representation o f anatomical d i s t i n c t i o n s is also t a k e n i n t o consideration by G u i a r d (1984, p . 20). 3Because o f t h e i n i t i a l main i n t e r e s t in interhemispheric relations, f i n g e r - c o m p a t i b i l i t y effects were n o t evaluated in Heister a n d SchroederHeister (1987).

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Twelve normally m e n s t r u a t i n g females aged 22 to 39 y e a r s took p a r t i n t w o experiments, one lexical-decision t a s k ( f u n c t i o n w o r d s v s . nonsense syllables) a n d one analogously c o n s t r u c t e d face-decision t a s k (photographically p r o d u c e d composites o f normal v s . scrambled faces). Half o f t h e subjects gave yes-decisions w i t h t h e f a r t h e r b u t t o n a n d nodecisions w i t h t h e n e a r e r one, a n d t h e o t h e r h a l f h a d t h e r e v e r s e d relation. T h e stimuli were presented f o r 130 ms in t h e right o r l e f t v i s u a l field. E v e r y subject took p a r t f o u r times i n e v e r y experiment in f o u r d i f f e r e n t phases o f t h e menstrual cycle. T h r e e subjects s t a r t e d in each o f t h e f o u r c y c l e phases ( f o r a more detailed d e s c r i p t i o n see Heister e t al., 1989). In Heister e t al. (1’989), t h e data were evaluated w i t h respect t o phase o f menstrual c y c l e w i t h o u t c o n s i d e r i n g t h e r e s p o n d i n g h a n d as a separate f a c t o r . T h e following reanalysis p r e s e n t s t h e data g r o u p e d w i t h respect t o o r d e r o f t e s t sessions (session 1 t o session 4) a n d effects o f r e s p o n d i n g hands (right v s . l e f t ) . T h i s reanalysis may also answer t h e question o f w h e t h e r p r a c t i c e a n d experience w i t h t h e t a s k s influence a possible S-R compatibility e f f e c t . F o r each o f t h e t w o experiments ( v e r b a l a n d non-verbal), an analysis o f variance was conducted f o r means o f t h e c o r r e c t RTs w i t h t e s t session (1 t o 41, f i e l d o f stimulus presentation (right/left), a n d r e s p o n d i n g h a n d ( r i g h t h e f t ) as w i t h i n - s u b j e c t f a c t o r s . O n l y r e l e v a n t data a r e presented here.

In t h e lexical-decision experiment, in addition t o an o v e r a l l p r a c t i c e e f f e c t (linear decrease o f RTs f r o m session 1 t o session 4) a n d a right f i e l d ( l e f t hemisphere) s u p e r i o r i t y , a s i g n i f i c a n t interaction was obtained between t e s t session, f i e l d a n d hand, F ( 3 , 33) = 3.43, p < .05. T h e r e was an S-R compatibility e f f e c t in t h e f i r s t t e s t session (compatible reactions [right f i e l d / r i g h t hand, l e f t f i e l d / l e f t hand] b e i n g 57 ms f a s t e r t h a n incompatible ones), w h i c h disappeared during t h e replications a n d even s h i f t e d t o t h e opposite i n t h e last session, in w h i c h incompatible reactions were 14 ms f a s t e r t h a n compatible ones (see F i g u r e 2). T h e analysis f o r t h e face-decision experiment also showed a p r a c t i c e e f f e c t and, as expected, a l e f t f i e l d (right hemisphere) s u p e r i o r i t y . In addition, t h e r e was a s i g n i f i c a n t interaction between f i e l d o f stimulus p r e sentation a n d r e s p o n d i n g hand, F ( 1 , 11) = 6.52, p < .05. T h i s interact i o n expresses an overall S - R compatibility effect, w i t h compatible responses b e i n g 15 ms f a s t e r t h a n incompatible ones (see F i g u r e 3).

W i t h small S-R compatibility effects, one can always object t h a t t h e y were actually pathway effects. However, i n t h e p r e s e n t case t h i s could n o t explain w h y t h e e f f e c t disappeared i n t h e lexical-decision t a s k a f t e r t h e f i r s t session a n d even s h i f t e d in t h e last session, because pathway effects should r e f l e c t h a r d w a r e and, thus, should b e stable. In general, these d i v i d e d v i s u a l - f i e l d studies o f cerebral lateralization showed evidence f o r S-R compatibility effects f o r t h e responding hand, although t h e response b u t t o n s were in t h e midsaggital plane a n d t h u s orthogonal t o t h e stimuli, a n d t h e position o f t h e stimuli was i r r e l e v a n t f o r t h e decision. I n addition, t h e r e were differences between t h e lexical-decision a n d t h e face-decision t a s k t h a t m i g h t have t o d o w i t h

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differences in t a s k d i f f i c u l t y : For t h e more d i f f i c u l t t a s k (face decisions), t h e S - R compatibility e f f e c t did n o t change d u r i n g f o u r r e t e s t sessions, whereas it did f o r t h e easier t a s k (lexical decisions). L E X I C A L DECISIONS RT(as) 900

..

850

.-

800

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750

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700

.-

650

.-

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I

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1

1

2

3

4

EXPERIMENTAL SESSION

F i g u r e 2. Mean R T s f o r responses t o v e r b a l stimuli f o r compatible ( f i l l e d circles) a n d incompatible ( u n f i l l e d circles) f i e l d - h a n d relationships f o r experimental sessions 1 t o 4.

O u r r e s u l t s s u p p o r t t h e hypothesis that spatio-anatomical mapping becomes e f f e c t i v e if spatial coding o f response positions i s excluded by a r r a n g i n g response k e y s p e r p e n d i c u l a r t o t h e stimuli. One m i g h t object t h a t n o t t h e anatomical r i g h t / l e f t d i s t i n c t i o n , but r a t h e r t h e spatial posit i o n o f r e s p o n d i n g hands ( w h i c h were s t i l l on t h e right o r l e f t side, even t h o u g h t h e response b u t t o n s , b e i n g pressed w i t h t h e index fingers, were mounted in middle position) was essential f o r t h e e f f e c t observed. However, even if t h i s objection i s correct, w h i c h cannot b e decided on t h e basis o f t h e data available, t h e r e s u l t s show t h a t h a n d position, as d i s t i n g u i s h e d f r o m k e y position, can b e e f f e c t i v e . Finally, t h e r e s u l t s c o n f i r m t h e claim t h a t spatial S - R compatibility n o t o n l y i s obtained w i t h low-level t a s k s (such as r i g h t / l e f t o r red/green discrimination) but also w i t h h i g h e r - l e v e l tasks (such as lexical o r face decisions)--a t o p i c t h a t has n o t y e t been g i v e n a p p r o p r i a t e attention in compatibility research. T h i s means t h a t in neuropsychological studies o f t h i s kind, spatial S-R compatibility effects cannot b e t o t a l l y r u l e d o u t .

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FACE DECISIONS

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F i g u r e 3. Mean R T s f o r responses t o n o n - v e r b a l stimuli f o r compatible ( f i l l e d circles) a n d incompatible ( u n f i l l e d circles) f i e l d - h a n d relationships f o r experimental sessions 1 t o 4.

A Hierarchical Model of Spatial S-R C o m p a t i b i l i t y T h e T h r e e Factors a n d T h e i r Rank O r d e r We have d i s t i n g u i s h e d t h r e e kinds o f compatibility t h a t d i f f e r w i t h respect t o t h e spatial coding o f t h e response: (a) compatibility d u e t o t h e coding o f positions o f response keys (coding of key position), (b) compatibility d u e t o t h e c o d i n g of positions o f r e s p o n d i n g hands o r f i n g e r s (coding of effector position), a n d (c) compatibility d u e t o t h e coding o f t h e response e f f e c t o r s as right o r l e f t (spatlo-onotomical m p p i n g ) . A c c o r d i n g t o t h e r e s u l t s reported, w h i c h o f these compatibilities is e f f e c t i v e in a g i v e n situation is n o t an u n p r e d i c t i b l e f u n c t i o n o f t h e response conditions t h a t a r e used in an experimental design. Rather, t h e i r appearance follows a clear p a t t e r n : C o d i n g o f k e y position is t h e dominant f a c t o r (see Riggio e t al., 1986). However, if such a coding is impossible (as in K l a p p e t al., 1979, Experiment 1, crossed a n d uncrossed conditions, f o r w h i c h t h e r e was no spatial d i f f e r e n c e between t h e response goals), t h e n spatial coding o f e f f e c t o r position becomes t h e dominant f a c t o r . Furthermore, if an a p p r o p r i a t e coding o f positions of response effectors i s likewise impossible (as in Klapp e t al., 1979, Experiment 1, up/down condition, f o r w h i c h stimuli were in r i g h t / l e f t

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position but r e s p o n d i n g h a n d in up/down position, o r in t h e orthogonal S - R arrangements f o r t w o - f i n g e r choice reactions o f Ehrenstein e t al., 19881, t h e n spatio-anatomical mapping becomes t h e dominant f a c t o r . Thus, we propose a model t h a t assumes a hierarchical o r d e r o f t h e following f a c t o r s : (a) spatial coding o f response keys, (b) spatial coding o f e f f e c t o r positions, (c) spatio-anatomical mapping. These f a c t o r s j o i n t l y determine spatial S-R compatibility. Factor (a) i s dominant o v e r f a c t o r s (b) a n d (c), a n d f a c t o r (b) i s dominant o v e r (c). However, if one f a c t o r is n o t applicable, t h e n t h e f a c t o r o f n e x t lower r a n k becomes t h e dominant one. T h i s approach, w h i c h develops f u r t h e r an idea by Klapp e t al. (1979) on anatomical a n d environmental dimensions o f S-R compatibility, enables us t o t r e a t spatial S-R compatibility as a u n i f o r m phenomenon t h a t i s determined by several f a c t o r s w h i c h become e f f e c t i v e according t o t h e i r r a n k o r d e r . T h e h i e r a r c h i c a l model proposed is c e r t a i n l y n o t fully exhaustive. It p u t s p a r t i c u l a r emphasis on d i s t i n c t i o n s between ways o f coding spatial relations on t h e response side. D i s t i n g u i s h i n g ways o f coding spatial relations on t h e stimulus side may lead t o an extension o f t h e model. What we propose is o n l y a f i r s t attempt to, overcome t h e situation t h a t a f t e r more t h a n t w e n t y [now: thirty] years research t h e r e is n o m e t r i c f o r compatibility t h a t transcends p a r t i c u l a r experimental conditions" (Welford, 1980, p. 99). I n t e r a c t i o n s Between t h e Factors of t h e Hierarchical Model T h e hierarchical model a n d i t s r a n k o r d e r were motivated by t h e r e s u l t s t h a t show t h a t a c e r t a i n t y p e of compatibility becomes r e l e v a n t if c e r t a i n o t h e r s a r e n o t applicable. However, t h e r e a r e also r e s u l t s t h a t suggest t h a t t h e d i f f e r e n t f a c t o r s do i n t e r a c t . T h i s means t h a t t h e f a c t o r s o f t h e model n o t o n l y replace each o t h e r following a c e r t a i n r a n k o r d e r , but can j o i n t l y determine an o b s e r v e d e f f e c t (by p r e s e r v i n g t h i s order). With respect t o spatial coding v e r s u s spatio-anatomical mapping, t h e r e s u l t s a n d theoretical proposals by Nicoletti, U m i l t i , a n d LAdavas (1984) o n t h e slowing down o f reactions w i t h crossed arms can b e i n t e r p r e t e d i n t h a t way. T h e y showed t h a t w i t h simple R T s t h e r e is no slowing down o f reactions if t h e r e s p o n d i n g h a n d is h e l d on t h e opposite side o f t h e b o d y midline. T h e y concluded t h a t t h e longer o v e r a l l R T s u s u a l l y observed u n d e r crossed-arms conditions is d u e t o a mismatch between t h e spatial codes o f h a n d a n d h a n d position and, t h u s , is a c o g n i t i v e phenomenon. T h e same t h e o r i z i n g applies t o t h e slowing down o f reactions o b s e r v e d f o r supine as compared t o p r o n e h a n d o r i e n t a t i o n in o u r e x p e r i ments w i t h t w o - f i n g e r choice reactions. Because turning t h e hands causes a r e v e r s a l o f t h e spatial o r d e r o f t h e anatomical fingers, t h i s slowing down o f reactions can b e a t t r i b u t e d t o a mismatch between spatial codes o f anatomical f i n g e r s a n d f i n g e r positions. However, it was statist i c a l l y s i g n i f i c a n t o n l y w i t h r e l e v a n t stimulus location (Heister e t al., 1986), showing a n o n - s i g n i f i c a n t t r e n d w i t h i r r e l e v a n t stimulus location (Heister e t al., 1987). T h i s may b e d u e t o t h e f a c t t h a t anatomical r i g h t / l e f t codes a r e n o t so obvious a n d u n i q u e l y determined f o r responding f i n g e r s as f o r r e s p o n d i n g hands. A n e a r l y s i g n i f i c a n t increase o f RTs u n d e r t h e supine h a n d condition was also o b s e r v e d in Ehrenstein e t

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al. (1988, Condition c), w h e r e response k e y s were a r r a n g e d perpend i c u l a r t o t h e stimuli, so t h a t no a p p r o p r i a t e spatial code f o r t h e f i n g e r positions c o u l d b e formed. H e r e t h i s increase may b e d u e t o a mismatch between d i f f e r e n t anatomical f i n g e r codes f o r p r o n e a n d supine orientation. However, r e s u l t s a r e n o t y e t available w i t h simple R T s f o r p r o n e v s . supine h a n d orientation, by means of w h i c h one c o u l d c l e a r l y d i s t i n g u i s h between a c o g n i t i v e e f f e c t d u e t o a mismatch between d i f f e r e n t codes a n d a p u r e l y bio-mechanical d i f f e r e n c e between t h e t w o h a n d o r i e n tations. Even t h e generally slower reaction f o r t h e condition in w h i c h t h e f i r s t a n d fifth f i n g e r s pressed t h e n a r r o w b u t t o n s ( t h e f i n g e r - d i s t a n c e experiment r e p o r t e d in t h i s c h a p t e r ) m i g h t b e i n t e r p r e t e d in - a similar way. In t h i s condition, a mismatch may b e seen as o c c u r r i n g between t h e codes o f a n a r r o w spatial distance a n d a wide anatomical distance, leading t o t h e o v e r a l l increase o f R T s . However, t h i s idea m u s t b e cons i d e r e d v e r y t e n t a t i v e l y , because an explanation t h a t simply r e f e r s t o t h e mechanical d i f f i c u l t y o f t h e t a s k has n o t been experimentally excluded. Analogous hypotheses can b e applied t o Riggio e t al.'s (1986) r e s u l t s on t h e slowing down o f reactions w i t h crossed f i n g e r s o r crossed sticks. These f i n d i n g s may b e considered as b e i n g d u e t o a mismatch between t h e spatial codes f o r response goals a n d position o f response effectors. ( R i g g i o e t al. themselves explained t h e f i n d i n g s by a mismatch between t h e spatial codes f o r response goals a n d t h e anatomical r i g h t h e f t distinction. 1 O u r r e f e r r a l t o "interaction" o f f a c t o r s o f t h e hierarchical model is j u s t i f i e d in t h e s t r i c t statistical sense. When we consider t h e paradigm o f bimanual reactions, t h e slowing down o f reactions u n d e r t h e crossed-arms condition can b e i n t e r p r e t e d as a main e f f e c t o f experimental condition, as an i n t e r a c t i o n between r e s p o n d i n g h a n d a n d i t s position, a n d as an i n t e r action between compatibility w i t h respect t o hands a n d compatibility w i t h respect t o t h e i r positions, depending on how f a c t o r s a r e g r o u p e d in t h e analysis o f variance. Suppose t h a t i n a hypothetical experiment, we obtained t h e data displayed in Table 2 as r e s u l t s f o r r i g h t - h a n d a n d l e f t h a n d reactions t o right a n d l e f t l i g h t s w i t h uncrossed a n d crossed arms. Then, we can f i r s t i n t e r p r e t these data as e x p r e s s i n g a main e f f e c t f o r experimental condition (uncrossed v s . crossed arms). Second, we can a r r a n g e them as i n t h e u p p e r panel o f Table 3, i n t e r p r e t i n g them as an i n t e r a c t i o n between h a n d a n d h a n d position (as in t h e studies o f Nicoletti e t al., 1984, a n d B r e b n e r e t al., 1972). Finally, however, we may a r r a n g e them as i n t h e lower panel o f Table 3. With respect t o t h i s arrangement, we can i n t e r p r e t t h e data as e x p r e s s i n g an interaction b e t ween stimulus-hand compatibility a n d stimulus-hand position compatibility (i.e., between f a c t o r s (b) a n d (c) o f o u r hierarchical model.) This means t h a t t h e f i e l d - h a n d position compatibility effect, w h i c h is d u e t o spatial coding, takes place essentially o n l y u n d e r a compatible f i e l d - h a n d relation. T h i s arrangement o f data was proposed by Simon e t al. (1970b) a n d also used by Callan e t al. (1974). In t h e following, we put p a r t i c u l a r emphasis on t h e relations between f a c t o r s (a) a n d (b) on t h e one side a n d f a c t o r (c) o n t h e other, t h a t is, t h e relation between c o d i n g o f response k e y o r e f f e c t o r position a n d spatio-anatomical mapping.

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Table 2 Reaction Times in a Hypothetical Experiment for R i g h t Hand (RH) a n d L e f t Hand (LH) Responses t o R i g h t a n d L e f t L i g h t s f o r t h e Experimental Conditions o f Arms Held Uncrossed a n d Arms Held Crossed

Condition

L e f t Light LH RH

R i g h t Light LH RH

Mean

Uncrossed

300

350

350

300

325

Crossed

400

350

350

400

375

Table 3 Arrangement o f t h e Data o f Table 2 A c c o r d i n g t o t h e Factors Hand a n d Hand Position ( u p p e r panel), a n d A c c o r d i n g t o t h e Factors Stimulus-Hand Compatibility a n d Stimulus-Hand Position Compatibility (bottom panel)

Hand Hand Position

Left

Left Right

325 375

Stimulus-Hand Position Compatibility

Right

375 325

St imuI us - Hand Cornp a t ibi I ity Compatible Incompatible

Compatible Incompatible

300 400

350 350

D i r e c t Effects of 'Spatio-Anatomical Mapping

So f a r we have dealt w i t h t h e case in w h i c h spatial coding is replaced by spatio-anatomical mapping if c o d i n g is undefined, a n d w i t h t h e case where coding is t h e major factor, b u t mapping indirectly i n f l u ences coding by i n t e r a c t i n g w i t h it. T h e l a t t e r produces a slowing down o f reactions u n d e r crossed-arms o r supine-hand conditions. However, t h e r e a r e also situations in w h i c h coding i s possible in p r i n c i p l e ( i . e . , i s n o t undefined), but mapping replaces coding o r is a t least directly effect i v e by n o t o n l y i n f l u e n c i n g o v e r a l l RTs but also q u a l i t a t i v e l y o r q u a n t i t a t i v e l y i m p a i r i n g t h e coding e f f e c t i t s e l f . Ladavas a n d Moscovitch's (1984) r e s u l t s can be t a k e n as an example o f coding b e i n g replaced by mapping, a l t h o u g h a c o d i n g hypothesis makes sense. Schroeder-Heister

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e t al. (1989) showed t h a t mapping can inhibit c o d i n g w i t h o u t fully elimin a t i n g it. I n several experiments, Lddavas a n d Moscovitch (1984) s t u d i e d t h e relation between head tilt a n d frames o f reference by i n v e s t i g a t i n g t h r e e conditions of head. position: head h e l d upright, t i l t e d 9 0' t o t h e right, a n d t i l t e d 90' t o t h e l e f t . In t h e t i l t e d - h e a d conditions, stimuli a n d responses were always p e r p e n d i c u l a r t o each other, w i t h e i t h e r t h e stimuli a r r a n g e d along t h e horizontal dimension a n d t h e (bimanual) responses along t h e v e r t i c a l dimension o r v i c e versa. In h a l f o f t h e experiments, t h e responding hands were exchanged, similar to t h e crossed-arms t e s t . Ladavas a n d Moscovitch f o u n d an association between stimuli a n d r e s p o n d i n g hands ( n o t hand positions) o f t h e - following p a t t e r n : T h e r e was an R T advantage f o r those stimulus-hand p a i r i n g s t h a t correspond t o each o t h e r if t h e stimulus is encoded as right o r l e f t in t h e a p p r o p r i a t e frame o f reference (environmental v s . egocentric) t h a t makes a r i g h t h e f t d i s t i n c t i o n possible. T h i s r e s u l t is explained by t h e idea t h a t u n d e r t h e conditions o f t h e g i v e n experimental design, t h e c o d i n g o f response positions was replaced by t h e c o d i n g o f t h e anatomical r i g h t h e f t c l ssification, t h a t is, by spatio-anatomical mapping in o u r terminology.$ T h i s replacement o f spatial coding by spatio-anatomical mapping took place although a s o r t o f spatial coding is imaginable in principle: One may, f o r example, assume t h a t t h e positions o f stimuli a n d responses a r e coded in d i f f e r e n t frames o f reference, a r r i v i n g e i t h e r a t a top/down o r r i g h t h e f t classification f o r b o t h stimuli a n d respo ses (see Ladavas f, Moscovitch, 1984, Discussion o f Experiments 1 a n d 2 ) .

s

T o resolve t h e confound between head tilt a n d o r t h o g o n a l i t y o f stimuli a n d responses in LBdavas a n d Moscovitch's s t u d y , we i n v e s t i g a t e d t h e condition o f head t i l t e d alone (Schroeder-Heister e t al., 1989). We h a d subjects react u n d e r t h e conditions o f head h e l d upright, head t i l t e d t o t h e right, a n d head t i l t e d t o t h e l e f t (as i n Lddavas E Moscovitch, 1984), but w i t h stimuli always a r r a n g e d h o r i z o n t a l l y a n d responses b e i n g g i v e n always by t w o b u t t o n s mounted o n t h e r i g h t o r l e f t side o f t h e experimental desk. Subjects were tested b o t h w i t h uncrossed a n d w i t h crossed arms. In t h e cpright head condition, t h e usual spatial S - R compatibility effect between stimulus position a n d response position was obtained f o r b o t h uncrossed a n d crossed arms. In t h e t i l t e d - h e a d conditions, t h i s e f f e c t was s t i l l present, but s i g n i f i c a n t l y decreased f r o m t h e uncrossed-arms t o t h e crossed-arms conditions, w i t h o u t r e v e r s i n g t o 4Ladavas a n d Moscovitch (1984) vacillate between t h e i n t e r p r e t a t i o n t h a t it is essentially t h e condition of head tilt u n d e r w h i c h spatial coding is replaced by spatio-anatomical mapping ( p . 213, bottom o f l e f t column a n d bottom o f right column) a n d t h e i n t e r p r e t a t i o n t h a t head tilt in combination w i t h o r t h o g o n a l i t y o f stimuli a n d responses is responsible f o r t h e observed e f f e c t (p. 214, t o p o f right column). 5LBdavas a n d Moscovitch's (1984) r e s u l t s also seem t o express a tendency towards a slowing down o f reactions w i t h exchanged hands, which may indicate an i n d i r e c t influence of spatial coding in addition t o t h e dominant e f f e c t of spatio-anatomical mapping. However, t h i s cannot b e e x a c t l y established on t h e basis o f t h e p u b l i s h e d data.

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t h e opposite. In o t h e r words, w i t h crossed arms, R T s were n o t o n l y o v e r a l l slower, but t h e size o f t h e spatial-compatibility e f f e c t was diminished. B y n o t u s i n g experimental condition (arms uncrossed v s . crossed) but instead h a n d compatibility a n d position compatibility as f a c t o r s in t h e analysis of variance (see t h e hypothetical experiment discussed above a n d Tables 2 a n d 3), t h e r e s u l t s f o r t h e t i l t e d - h e a d conditions can also b e i n t e r p r e t e d as follows: B o t h a s i g n i f i c a n t position-compatibility e f f e c t a n d an (anatomical) hand-compatibility e f f e c t were obtained, as was a s i g n i f i c a n t i n t e r a c t i o n between these effects. T h i s shows t h a t besides c o d i n g o f response positions, w h i c h remains t h e dominant factor, spatio-anatomical mapping can b e d i r e c t l y present, now r e d u c i n g t h e coding e f f e c t u n d e r t h e crossed-arms condition. T h e i n t e r a c t i o n o f coding a n d spatioanatomical mapping expresses again t h e i n d i r e c t influence o f mapping by increasing t h e o v e r a l l R T s in t h e crossed-arms condition. We explained t h i s r e s u l t as i n d i c a t i n g t h a t spatial coding o f response positions is p a r t i c u l a r l y difficult when b o t h t h e head i s t i l t e d a n d t h e arms a r e crossed, leading t o a g r e a t e r u n c e r t a i n t y in coding spatial positions as right o r left. Because o u r r e s u l t s show t h a t head tilt alone does n o t eliminate spatial coding o f response positions as t h e dominant factor, t h e additional difficulty in stimulus encoding (choice o f an a p p r o p r i a t e frame o f r e f e r ence) in L i d a v a s a n d Moscovitch's (1984) experiments may lead subjects t o r e l y o n t h e clear anatomical r i g h t h e f t d i s t i n c t i o n f o r t h e i r responses. T h i s would mean t h a t response encoding may also depend on t h e condit i o n s o f stlmulus encoding, because these a r e t h e o n l y essential d i f f e r ences between o u r design a n d LBdavas a n d Moscovitch's design f o r t h e horizontal response arrangement. T h e f i n d i n g s o f L i d a v a s a n d Moscovitch (1984) a n d SchroederHeister e t al. (1989) fully s u p p o r t o u r hierarchical model. In a situation w h e r e spatial c o d i n g i s made difficult, spatio-anatomical mapping has t h e chance t o i n t e r f e r e o r , u n d e r extremely difficult conditions, even t o dominate o v e r spatial coding. Some Methodological Remarks In t h e p r e s e n t paper, we have placed o u r main emphasis on t h e response side o f spatial S-R compatibility, d i s t i n g u i s h i n g between various ways of c o d i n g t h e response: c o d i n g o f positions o f response keys, coding o f positions o f r e s p o n d i n g hands o r f i n g e r s , a n d coding o f (anatomically right o r l e f t ) hands o r f i n g e r s , t h a t is, spatio-anatomical mapping. Correspondingly, t h e techniques considered t o separate influences o f t h e various f a c t o r s were manipulations o f t h e response a r r a n g e ment. One o f t h e basic experimental methods t o d i s t i n g u i s h between p u r e l y spatial a n d spatio-anatomical aspects o f S - R compatibility is a comp a r i s o n between data f o r arms h e l d in normal position a n d f o r arms crossed. T h e r e is a general difficulty w i t h i n t e r p r e t i n g such data t h a t has implications f a r b e y o n d t h e crossed-arms paradigm.

A c c o r d i n g t o one possible i n t e r p r e t a t i o n , uncrossed-arms v s . crossed-arms a r e experimental conditions whose influence o n t h e manner o f c o g n i t i v e processing is experimentally investigated. If a d i f f e r e n c e in performance i s o b s e r v e d when t h e arms a r e h e l d crossed, t h i s d i f f e r e n c e

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i s i n t e r p r e t e d as e x p r e s s i n g a change in c o g n i t i v e processing. For example, t h e o v e r a l l slowing down o f reactions i n crossed-arms conditions can b e i n t e r p r e t e d as r e f l e c t i n g a c o g n i t i v e c o n f l i c t between anatomical a n d spatial codes t h a t is n o t p r e s e n t when t h e arms a r e h e l d normally, but t h a t arises when t h e arms a r e crossed. A c c o r d i n g t o another i n t e r p r e t a t i o n , t h e crossed-arms condition is j u s t a measurement device o f t h e experimenter t o d i s t i n g u i s h between effects d u e t o h a n d a n d t o h a n d position, w i t h o u t a f f e c t i n g c o g n i t i v e p r o cedures. A change in performance w i t h arms crossed i s n o t i n t e r p r e t e d as e x p r e s s i n g a change in information processing, but as r e v e a l i n g feat u r e s o f i t s permanent s t r u c t u r e t h a t cannot show up, a l t h o u g h present, w i t h hands in normal position. T a k i n g t h e same example as above, t h e slowing down o f reactions w i t h arms crossed is i n t e r p r e t e d as i n d i c a t i n g some general interaction o f anatomical a n d spatial codes t h a t is always present, but manifests i t s e l f in a specific w a y when t h e arms a r e crossed. One may t r y t o d i s t i n g u i s h experimentally,,between these t w o i n t e r between” uncrossed a n d pretations by p r o v i d i n g a baseline condition If t h e crossed-arms t e s t is a condition t h a t changes p r o crossed arms. cessing by causing a c o g n i t i v e c o n f l i c t between spatial a n d anatomical codes, an i n h i b i t i o n o f processing should b e expected w i t h arms crossed. However, if t h e crossed-arms t e s t i s n o t h i n g but a methodological i n s t r u ment demonstrating a permanent c o g n i t i v e i n t e r f e r e n c e of anatomical a n d spatial codes, processing should n o t j u s t b e i n h i b i t e d if b o t h codes a r e i n c o n g r u e n t (i.e., u n d e r t h e crossed-arms condition), but also f a c i l i t a t e d if b o t h codes a r e c o n g r u e n t (i.e., u n d e r t h e uncrossed-arms condition). Without detailed discussion, K l a p p e t al. (1979, Experiment 1) i m p l i c i t l y investigated t h i s question by s t u d y i n g a baseline i n t h e i r up/down condit i o n (hands o p e r a t i n g a b u t t o n f r o m above a n d below), w h i c h t h e y used in addition t o t h e i r crossed/uncrossed conditions (hands o p e r a t i n g a b u t t o n f r o m t h e right o r l e f t side w i t h arms crossed o r arms uncrossed). T h e o v e r a l l RTs i n t h e crossed-arms condition were s i g n i f i c a n t l y l a r g e r (by n e a r l y 100 ms) t h a n in b o t h t h e uncrossed-arms a n d t h e up/down conditions, whereas t h e r e was p r a c t i c a l l y no R T d i f f e r e n c e between t h e uncrossed-arms a n d t h e up/down condition. T h i s indicates t h a t t h e r e is i n h i b i t i o n o f processing u n d e r t h e crossed-arms condition, but no facilitation u n d e r t h e uncrossed-arms condition. T h u s , one may conclude t h a t something a c t u a l l y “happens” i n t e r n a l l y when one crosses arms. T h a t is, t h e crossed-arms t e s t manipulates t h e manner o f processing. However, s t u d y i n g baseline conditions l i k e t h e one j u s t mentioned, which is a r a t h e r neglected t o p i c i n compatibility research, cannot fully eliminate t h e basic difficulty. I n principle, one cannot exclude t h a t t h i s baseline condition i t s e l f represents an experimental manipulation t h a t may change processing a n d t h u s r e q u i r e s another ( “ h i g h e r o r d e r ” ) baseline condition t o establish w h e t h e r it is in f a c t a n e u t r a l condition between t h e uncrossed- a n d crossed-arms conditions, a n d so f o r t h ad infiniturn. Obviously, t h i s problem occurs n o t j u s t w i t h t h e crossed-arms t e s t in compatibility experiments, but w i t h all s o r t s o f changes o f t h e response arrangements t h a t a r e u n d e r t a k e n e i t h e r t o isolate o r t o exclude anatomy-related f a c t o r s as relevant f o r an o b s e r v e d effect. Besides t h e crossed-arms test, we have p a r t i c u l a r l y discussed t h e comparison between p r o n e a n d supine h a n d positions a n d t h e d i s t i n c t i o n between anatomical a n d spatial f i n g e r distance. Related questions can b e posed if one wants

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t o use t h e crossed-arms methodology t o establish effects d u e t o neuroanatomical c o n n e c t i v i t y (see Anzola e t al., 1977; Berlucchi, Crea, D i Stefano, E Tassinari, 1977; Bradshaw & Umilti, 1984; Levy, 1984). It seems t h a t t h e o n l y way t o cope w i t h these difficulties, a t least in p a r t , is t o consider baseline conditions on t h e stimulus side (e.g., by p r e s e n t i n g stimuli in middle position) independently o f manipulations on t h e response side. What we a r e facing h e r e w i t h respect t o S-R compatibility is t h e general problem o f experimental methodology: Certain methods o f observation change what i s observed in a way essential f o r t h e purpose o f t h e investigation. Summary

A consideration o f d i f f e r e n t r i g h t / l e f t relations on t h e response side distinguishes a t least t h r e e t y p e s o f spatial S-R compatibility: (a) compatibility between stimulus positions and positions o f response keys, (b) compatibility between stimulus positions a n d positions o f response effectors (hands o r fingers), a n d (c) compatibility between stimulus posiCompatibilities of tions a n d anatomically right o r l e f t response effectors. t y p e s (a) a n d (b?, may b e a t t r i b u t e d to an i n t e r n a l coding of response positions (called spatial coding") a n d compatibility o f t y p e (c) t o an i n t e r n a l coding o f anatomical r i g h t / l e f t classifications (called "spatioanatomical mapping"). Standard results in compatibility research show t h a t spatial coding i s dominant in determining compatibility effects a n d t h a t anatomy-related factors p l a y a subordinate role. For bimanual choice reactions t h i s has been established by t h e crossed-arms test, whereas it has been established f o r unimanual t w o - f i n g e r choice reactions by comparing p r o n e and supine hand orientations. As a f u r t h e r possibility t o separate spatial f r o m anatomical factors in unimanual t w o - f i n g e r choice reactions, we p r o pose t h e d i s t i n c t i o n between anatomical a n d spatial f i n g e r distance. Preliminary experimental data presented h e r e show t h a t t h e size o f t h e compatibility e f f e c t varies w i t h spatial and n o t w i t h anatomical f i n g e r distance. Notwithstanding t h e p r i o r i t y o f spatial coding, several results show t h a t spatio-anatomical mapping may b e effective if spatial r i g h t / l e f t d i s In particular, mapping is tinctions on t h e response side a r e eliminated. effective if response b u t t o n s a r e arranged in middle position perpend i c u l a r t o t h e stimuli. T h i s conclusion i s also supported by t h e reanalysis o f data o f a ,divided visual-field s t u d y o f cerebral lateralization. T o i n t e g r a t e t h e seemingly d i v e r g e n t findings, we have proposed a hierarchical model, according t o which spatial S-R compatibility is a j o i n t r e s u l t of t h e t h r e e factors mentioned above, which dominate each o t h e r according t o a c e r t a i n rank o r d e r and which can i n t e r a c t in various ways.

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SOME ASPECTS OF SPATIAL STIMULUS-RESPONSE COMPATIBILITY IN ADULTS AND NORMAL CHILDREN E L I S A B E T T A LXDAVAS Dipartimento di Psicologia U n i v e r s i t a di Bologna, I t a l y I n several choice reaction-time ( R T ) t a s k s employing an a r r a y o f stimuli a n d an a r r a y o f responses, c e r t a i n stimulus-response (S-R) p a i r i n g s lead t o f a s t e r RTs t h a n d o o t h e r s . T h e more e f f i c i e n t p a i r i n g s Simon, a r e termed "compatible" a n d t h e less efficient, "incompatible. " Sly, a n d Vilapakkam (1981) d i s t i n g u i s h e d t h r e e t y p e s o f compatibility: symbolic compatibility, spatial compatibility a n d a third t y p e t h a t can b e Symbolic compatibility termed t h e "Simon effect" (Hedge & Marsh, 1975). r e s u l t s f r o m t h e correspondence between t h e v e r b a l labels associated w i t h t h e stimulus a n d with t h e response. F o r example, if t h e r e a r e r e d a n d g r e e n l i g h t s a n d r e d a n d g r e e n keys, responses a r e f a s t e r when t h e t a s k r e q u i r e s t h a t t h e r e d k e y b e pressed in response t o t h e r e d light a n d t h e g r e e n k e y b e pressed in response t o t h e g r e e n light, as compared t o a t a s k i n v o l v i n g t h e opposite S-R p a i r i n g s (Hedge & Marsh, 1975; Simon e t al., 1981; Simon & Sudalaimuthu, 1979).

T h e speed o f response also can b e affected by t h e spatial charact e r i s t i c s o f t h e stimulus a n d response codes, regardless o f w h e t h e r t h e selection o f response is based o n a spatial c u e o r n o t . When t h e selection o f t h e response i s d i r e c t l y based on t h e position o f t h e stimulus, a "spatial compatibility effect" occurs. Thus, if t h e r e a r e t w o light stimuli, one in t h e right visual f i e l d a n d t h e o t h e r in t h e left, a n d t w o response keys, one o n t h e right a n d t h e o t h e r on t h e left, R T i s f a s t e r when t h e t a s k r e q u i r e s t h a t t h e right k e y b e pressed in response t o t h e right light a n d t h e l e f t k e y b e pressed i n response t o t h e l e f t light, as compared t o a t a s k i n v o l v i n g t h e opposite S-R p a i r i n g s (Anzola, Bertoloni, Buchtel, & Rizzolatti, 1977; B r e b n e r , Shephard, & Cairney, 1972). A d i f f e r e n t v a r i e t y o f spatial compatibility e f f e c t is t h e Simon effect, which can b e o b s e r v e d i n R T t a s k s f o r w h i c h t h e position o f t h e stimulus i s i r r e l e v a n t f o r t h e selection o f t h e response a n d y e t has an influence on r e s p o n d i n g speed. T h i s compatibility e f f e c t is apparent when a visual p a t t e r n (e.g., a triangle), p r e s e n t e d sometimes in t h e right a n d sometimes in t h e l e f t v i s u a l field, m u s t b e responded t o by p r e s s i n g t h e right key, a n d another p a t t e r n (e.g., a square), also p r e sented sometimes i n t h e right a n d sometimes in t h e l e f t visual field, must Reactions associated w i t h b e responded t o by p r e s s i n g t h e l e f t k e y . spatially c o r r e s p o n d i n g S-R p a i r i n g s (i.e., stimulus a n d k e y on t h e same side) a r e f a s t e r t h a n reactions associated w i t h non-corresponding S-R p a i r i n g s (i.e., stimulus o n one side a n d k e y o n t h e o t h e r side), even t h o u g h spatial position is i r r e l e v a n t (Simon, 1968; Simon e t al., 1970).

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Spatial compatibility a n d t h e Simon e f f e c t a r e n o t d u e t o t h e basic anatomical c o n n e c t i v i t y o f t h e responding h a n d w i t h t h e hemispheric representation o f t h e t w o v i s u a l f i e l d s (Myers, 1965; S p e r r y , 1964), but t o t h e correspondence between t h e spatial positions o f t h e stimulus a n d o f t h e response. T h e neuroanatomical pathway hypothesis was r u l e d o u t by experiments in w h i c h t h e position o f t h e response h a n d was crossed (Anzola e t al., 1977; Wallace, 1971). When t h e right h a n d was on t h e right side o f t h e b o d y a n d t h e l e f t h a n d on t h e l e f t side ( t h e uncrossed position), t h e right h a n d responded f a s t e r t o stimuli presented t o t h e right v i s u a l f i e l d a n d t h e l e f t h a n d t o stimuli presented t o t h e l e f t visual In contrast, when t h e subjects crossed t h e i r hands so t h a t t h e field. right h a n d was on t h e l e f t side o f t h e b o d y a n d t h e l e f t h a n d on t h e right side ( t h e crossed position), t h e right h a n d was f a s t e r f o r l e f t - s i d e d stimuli a n d t h e l e f t h a n d f o r r i g h t - s i d e d stimuli. T h i s was t r u e f o r b o t h t h e spatial-compatibility e f f e c t (Anzola e t JI., 1977; B r e b n e r e t al., 1972; Nicoletti e t al., 1982) a n d t h e Simon e f f e c t (Simon e t al., 1970; Wallace 1971, 1972). T h e Responding H a n d as a Determining Factor in P r o d u c i n g t h e Spatial C o m p a t i b i l i t y E f f e c t s One i n t e r e s t i n g f i n d i n g o f t h e experiments t h a t used uncrossed a n d crossed positions was t h a t R T s were f a s t e r in t h e uncrossed condition (Anzola e t al., 1977; B e r l u c c h i e t al., 1977; B r e b n e r e t al., 1972; Nicol e t t i e t al., 19821, i r r e s p e c t i v e o f w h e t h e r t h e stimulus-response arrangement was compatible o r incompatible. There are two tentative explanations f o r t h i s observation. T h e f i r s t one, the postural hypothesis, suggests t h a t t h e slower RTs w i t h crossed hands a r e d u e t o an e f f e c t o f arm p o s t u r e on t h e movement speed o f t h e r e s p o n d i n g f i n g e r . T h i s p o s t u r a l e f f e c t c o u l d arise because t h e crossed position is somewhat uncomfortable. A n o t h e r explanation, t h e coding hypothesis (Nicoletti e t al., 1984; Riggio, Gawryszewski, & Umilta, 1986), is in terms o f compatibility between t h e position of t h e response h a n d a n d t h e side o f t h e body w i t h w h i c h t h e h a n d is connected. A c c o r d i n g t o t h e coding hypothesis, t w o spatial codes related t o t h e e f f e c t o r a r e formed, one t h a t describes t h e position in space where t h e h a n d is located (right o r l e f t ) a n d t h e o t h e r t h a t describes t h e side o f t h e b o d y w i t h which t h e h a n d is connected. When a stimulus is presented, i t s spatial code is compared w i t h t h e spatial codes d e s c r i b i n g t h e position o f t h e h a n d a n d t h e side o f t h e b o d y w i t h w h i c h t h e h a n d is connected. T h e overall slowing of R T w i t h t h e hands crossed is a t t r i b u t e d t o a mismatch between t h e codes d e s c r i b i n g t h e positions i n space where t h e h a n d is located (right o r l e f t ) a n d t h e side o f t h e b o d y w i t h w h i c h t h e h a n d is connected. With hands crossed, a conflict would arise between t w o i n t e r n a l representations o f t h e same effector, one locational a n d t h e o t h e r anatomical. Nicoletti, Umilta, a n d Ladavas (1984) rejected t h e p o s t u r a l h y p o t h esis by showing t h a t t h e crossed-hand e f f e c t is absent in a simple R T task, although t h e response h a n d is k e p t in an uncomfortable position, (e.g., on t h e opposite side o f t h e b o d y t o which it is connected). Therefore, t h e y concluded t h a t t h e crossed-hand e f f e c t is b e t t e r explained in terms o f a c o n f l i c t between t h e codes d e s c r i b i n g t h e response h a n d a n d t h e side where t h e response h a n d is located. These

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r e s u l t s can b e considered as i n d i r e c t evidence t h a t t h e anatomical status o f t h e t w o hands is coded. More d i r e c t evidence of S-R compatibility effects based o n a rightl e f t classification of t h e r e s p o n d i n g h a n d comes f r o m a s t u d y by LBdavas a n d Moscovitch (1984). T h i s s t u d y showed t h a t t h e r e s p o n d i n g hand, r a t h e r t h a n t h e response position, determines t h e d i f f e r e n t i a l S-R associations u n d e r l y i n g t h e spatial-compatibility effects. T w o spatial codes can b e used t o describe t h e position o f t h e v i s u a l stimuli: an environmental ( o r physical) frame o f reference a n d an egocentric ( o r retinal) frame o f reference. When t h e head is in t h e normal upright position, t h e t w o f r a n e s o f reference coincide. F o r example, " l e f t " in one frame i s also " l e f t " in t h e o t h e r frame. B u t if t h e subject t i l t s h i s o r h e r head t o t h e r i g h t by 90°, t h e t w o frames o f reference n o l o n g e r coincide because "left" in t h e physical frame o f reference is "down" in t h e egocentric Thus, a subject frame o f reference, a n d so on f o r t h e o t h e r positions. w i t h t h e head t i l t e d can use t w o codes, one environmental a n d t h e o t h e r egocentric, f o r d e s c r i b i n g a n y position in t h e v i s u a l space. LBdavas a n d Moscovitch (1984) demonstrated t h a t when t h e e n v i ronmental a n d egocentric frames o f reference coincide (as when t h e head i s upright) a n d stimuli a n d responses a r e d i s t r i b u t e d along t h e same plane (horizontal o r vertical), t h e n t h e r e l a t i v e positions o f stimuli a n d responses determine S-R compatibility effects. I n contrast, if t h e stimuli a n d responses a r e p e r p e n d i c u l a r t o each o t h e r a n d t h e frames o f reference d o n o t coincide (as when t h e head is tilted), r e s p o n d i n g h a n d replaces response position as t h e r e l e v a n t f a c t o r . This introduces a tendency t o code stimulus position in terms o f l e f t a n d right. As a consequence, subjects adopt t h e frame of reference t h a t enables them t o classify t h e stimuli as l e f t a n d right, regardless o f w h e t h e r t h e y a r e horizontally o r v e r t i c a l l y d i s t r i b u t e d . T h e l e f t - r i g h t stimulus classification is mapped o n t o t h e r e s p o n d i n g hand, t h e r e b y p r o d u c i n g t h e o b s e r v e d compatibility effects. Similar S-R compatibility effects between t h e r e s p o n d i n g hands a n d t h e spatial position o f t h e stimuli were obtained by Lddavas (1987) in a visuomotor choice R T t a s k t h a t i n v o l v e d a p e r p e n d i c u l a r arrangement o f displays a n d controls (e.g., t h e stimuli were displayed v e r t i c a l l y a n d t h e responding hands were placed h o r i z o n t a l l y ) . A l t h o u g h t h i s arrangement does n o t imply a n y o v e r t p r e f e r e n t i a l association between t h e spatial p r o p e r t i e s o f t h e stimuli a n d those of t h e responses, responses o f t h e dominant h a n d (right f o r r i g h t - h a n d e r s a n d l e f t f o r l e f t - h a n d e r s ) were f a s t e r t o t h e u p p e r light a n d those o f t h e non-dominant h a n d were f a s t e r t o t h e lower light. These effects were comparable in magnitude t o t h e t y p i c a l compatibility effects f o u n d w i t h a parallel arrangement o f displays a n d controls. Theories proposed t o explain S-R compatibility effects cannot easily accommodate t h e effects obtained in absence of a n y o v e r t correspondence between t h e spatial p r o p e r t i e s o f t h e stimulus a r r a y a n d those o f t h e response a r r a y . Simon a n d associates (Simon, 1968; Simon e t al., 1970) postulated t h a t spatial S-R compatibility effects a r i s e f r o m a basic t e n dency t o react t o w a r d t h e source o f t h e stimulus, such t h a t a n y reaction o c c u r r i n g in t h a t general d i r e c t i o n i s facilitated, whereas a l t e r n a t i v e reactions in t h e d i r e c t i o n opposite t o t h e p r e f e r r e d one a r e h i n d e r e d . T h e postulated t e n d e n c y t o move t o w a r d t h e source of t h e stimulus m i g h t i n t e r a c t w i t h t h e motor outflow r e q u i r e d f o r k e y - p r e s s i n g . But, g i v e n

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t h e arrangement of displays a n d controls used in Lddavas's experiment (1987), such interaction, if effective, should produce t h e same p a t t e r n o f responses f o r b o t h hands (e.g., it could facilitate t h e response o f b o t h hands to t h e u p p e r light a n d h i n d e r t h e response o f b o t h hands t o t h e lower light). Therefore, t h e p r e f e r e n t i a l associations o f t h e dominant hand w i t h t h e u p p e r stimulus a n d o f t h e non-dominant hand w i t h t h e lower stimulus l i k e l y r e s u l t f r o m o t h e r factors, such as some relation between t h e positional coding o f t h e stimuli a n d t h a t o f t h e hands. Wallace (1971, 1972) proposed a general model o f sensorimotor integ r a t i o n based o n positional coding. According t o Wallace, t h e c o r respondence between t h e position o f the stimuli a n d t h e f e l t (proprioceptive) position o f t h e responding p a r t o f t h e b o d y (e.g., a hand) i s t h e c r u c i a l f a c t o r t h a t facilitates compatible responses. Thus, a stimulus a n d a response a r e considered compatible when t h e i r respective positions can b e matched according t o some spatial code, as suggested by t h e f a c t t h a t spatial-compatibility effects can b e observed even in t h e absence of an o v e r t spatial correspondence between stimulus displays and responses. For example, Nicoletti, Anzola, Luppino, Rizzolatti, and Umiltd (1982) measured choice R T in a situation in which t h e r e were t w o possible stimuli horizontally a r r a n g e d in m e visual f i e l d a n d t w o horizont a l l y disposed response keys on t h e side o f t h e midline opposite t h e visual f i e l d (see also Nicoletti e t al., 1984; Umiltd & Nicoletti, 1985, f o r similar experiments). Because in b o t h t h e stimulus set a n d t h e response set one member could b e classified as right a n d t h e o t h e r as left, independent o f t h e position o f t h e set w i t h t h e respect o f t h e midline, subjects were f a s t e r in responding w i t h t h e right k e y t o t h e right light a n d w i t h t h e l e f t k e y t o t h e l e f t light, as compared t o r i g h t - l e f t a n d l e f t right associations. T h e spatial -codi ng hypot hes is seems t o e x p la i n t h e pr e f e r e nt i a I association between t h e dominant hand w i t h t h e u p p e r stimuli a n d t h e non-dominant hand w i t h t h e lower stimuli b e t t e r than does t h e hypothesis t h a t subjects t e n d t o respond in t h e direction o f t h e stimulus. It is possible t o postulate t h a t t h e t w o hands a r e coded n o t only o n t h e h o r i zontal dimension (i.e., right a n d left), b u t also o n t h e v e r t i c a l dimension, w i t h t h e dominant hand b e i n g assigned a " h i g h e r " position compared t o t h e non-dominant hand. When bimanual activities a r e c a r r i e d o u t u n d e r visual control, t h e o p e r a t i n g dominant hand is most f r e q u e n t l y above t h e h o l d i n g non-dominant hand. Repeated bimanual a c t i v i t y o f t h i s kind may g r a d u a l l y build u p a combined i n t e r n a l representation of t h e t w o hands, whereby t h e image o f t h e p r e f e r r e d hand i s placed above t h a t o f t h e n o n - p r e f e r r e d hand. T h e notion t h a t t h e mental images o f t h e t w o hands may d i f f e r in several respects is s u p p o r t e d by Sekiyama (19821, who showed t h a t t h e right hand is assigned a " h i g h e r " position compared t o t h e l e f t hand. In right-handers, t h e right hand's image may b e generated more easily t h a n t h e l e f t hand's image, because judgments f o r right hands are faster than those f o r l e f t hands. T h e "higher" position o f t h e dominant hand in t h i s mental representation may express i t s e l f i n simple motor behavior, such as t h e c r o s s i n g of t h e extended and pronated forearms. When subjects a r e r e q u i r e d t o cross t h e extended and pronated forearms, r i g h t - h a n d e r s consistently place t h e right hand above t h e l e f t hand, whereas l e f t handers consistently place t h e l e f t hand above t h e right hand.

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While t h e spatial-compatibility effects r e p o r t e d h e r e a r e most p a r simoniously explained by t h e above hypothesis o f an asymmetry in t h e coding of t h e dominant a n d non-dominant hands along t h e v e r t i c a l dimension, o t h e r f a c t o r s can e x p l a i n such effects. In a r e c e n t study, LBdavas (1988) has shown t h a t in t h e mental representation o f space in r i g h t - h a n d e r s t h e r e i s a p r e f e r e n t i a l l i n k a g e of t h e concepts "right" a n d "above" a n d o f t h e concepts " l e f t " a n d "below, w h i l e in l e f t - h a n d e r s t h i s asymmetrical l i n k a g e is r e v e r s e d . T h e r e s u l t s o f t h i s s t u d y showed t h a t r i g h t - h a n d e r s were f a s t e r in v e r i f y i n g a n d f a l s i f y i n g t h e statements cont a i n i n g t h e terms RIGHT a n d ABOVE as compared t o statements containing t h e terms LEFT a n d BELOW, whereas l e f t - h a n d e r s were f a s t e r in v e r i f y i n g a n d f a l s i f y i n g statements c o n t a i n i n g t h e terms LEFT a n d ABOVE as compared t o statements c o n t a i n i n g t h e terms R I G H T a n d BELOW. In o t h e r words, t h e p o s i t i v e t e r m on t h e horizontal dimension in r i g h t - h a n d e r s a n d l e f t - h a n d e r s is t h a t c o r r e s p o n d i n g t o t h e dominant hand, whereas t h e p o s i t i v e t e r m on t h e v e r t i c a l dimension is independent o f handedness. Therefore, it is possible t o i n t e r p r e t t h e S-R compatibility e f f e c t f o u n d in right handers w i t h p e r p e n d i c u l a r a r r a n g e ment o f stimuli a n d respo7:es as a !ink of,,the concepts "right" a n d "above" a n d t h e concepts l e f t " a n d below, and vice versa f o r lefthanders. T h i s p r e f e r e n t i a l - l i n k i n g i n t e r p r e t a t i o n o f t h e S-R compatibility effects w i t h p e r p e n d i c u l a r arrangement o f stimuli a n d responses seems t o b e less plausible t h a n t h e i n t e r p r e t a t i o n in terms o f an asymmetry in t h e coding o f t h e dominant a n d non-dominant h a n d along t h e v e r t i c a l dimension, because t h e f i r s t i n t e r p r e t a t i o n implies a v e r b a l labeling stage. Most a u t h o r s (Chase & C l a r k , 1971; Sholl & Egeth, 1981) agree t h a t t h e d i f f e r e n t d i s c r i m i n a b i l i t y o f t h e terms r i g h t - l e f t a n d above-below o c c u r s when t h e v e r b a l I!bels a r e used in t h e comparison stage o f processing. I n o t h e r words, above" positions appear t o b e processed f a s t e r t h a n " l e f t " positions o n l y when v e r b a l labels a r e applied t o judgments o f r e l a t i v e locations. I n contrast, t h e d i f f e r e n t d i s c r i m i n a b i l i t y between "right" a n d " l e f t " positions a n d "above" a n d "below" positions disappears when t h e t a s k r e q u i r e s perceptual encoding o f t h e r e l a t i v e location. T h e p r e f e r e n t i a l - l i n k i n g i n t e r p r e t a t i o n cannot explain t h e r e s u l t s obtained by LBdavas (1987), because t h e l i n g u i s t i c d e s c r i p t i o n o f t h e position o f t h e stimuli a n d responses does n o t appear t o p l a y a role in t h e compatibility effects. T h i s is demonstrated by t h e following s t u d y where c h i l d r e n show S-R compatibility effects, a l t h o u g h t h e y have n o t y e t learned t o t e l l right f r o m l e f t a n d t h e r e f o r e cannot use v e r b a l labels t o code t h e positions o f stimuli a n d responses. Spatial S-R C o m p a t i b i l i t y E f f e c t s In Y o u n g C h i l d r e n Wallace (1971, 1972) p o i n t e d o u t t h a t t h e c r u c i a l f a c t o r t h a t facilitates compatible responses is t h e correspondence between t h e code d e s c r i b i n g t h e position o f t h e stimulus a n d t h e code d e s c r i b i n g position o f t h e responding p a r t of t h e body. In t h e case o f symb6lic compatibility, t h e n a t u r e o f t h e codes i s v e r y l i k e l y t o b e verbal, whereas in t h e case o f b o t h spatial compatibility a n d t h e Simon effect, it c o u l d b e e i t h e r spatial o r v e r b a l . In o t h e r words, it is n o t a p p a r e n t w h e t h e r t h e subjects make use o f spatial codes o r o f t h e c o r r e s p o n d i n g v e r b a l descriptions in comparing t h e position o f t h e stimulus w i t h t h a t o f t h e response. A d i r e c t way t o t e s t t h e n a t u r e o f t h e codes i n v o l v e d in spatial compatibility i s t o assess t h e presence o f compatibility effects in

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c h i l d r e n who cannot discriminate between l e f t a n d right and, therefore, cannot use v e r b a l labels t o code t h e positions o f stimuli a n d responses. T h e aim o f t h e p r e s e n t s t u d y was t o assess t h e presence o f spatial comp a t i b i l i t y effects along t h e horizontial a n d v e r t i c a l dimensions a n d t h e a b i l i t y t o discriminate between l e f t a n d right in 4-5 y e a r - o l d chil.dren. Many studies have f o u n d a p r o g r e s s i v e development o f l e f t - r i g h t discrimination s k i l l s t h r o u g h t h e age r a n g e f r o m 6 t o 9 years, w i t h maturity in t h i s discrimination reached o n l y a r o u n d t h e 12th y e a r (Belmont & Birch, 1963; Benton, 1959; Boone & Prescott, 1968). More precisely, Belmont a n d B i r c h (1963), u s i n g t h e Piaget b a t t e r y o f l e f t - r i g h t awareness items, f o u n d t h a t discrimination o f l e f t - r i g h t followed a sequence w h i c h f i r s t includes one's own b o d y p a r t s , t h e n those o f another person f a c i n g t h e subject, a n d f i n a l l y object relations in t h e environment. Studies c o n c e r n i n g t h e motor a b i l i t y o f y o u n g c h i l d r e n in reaching f o r objects have f o u n d a p r e f e r e n c e f o r an ipsilateral o v e r a contralateral response a n d a difficulty in p e r f o r m i n g movements t h a t cross t h e b o d y midline ( B r u n e r , 1969; Kephart, 1971; Provine & Westerman, 1979). These l a t t e r studies suggest t h a t ipsilateral S - R l i n k s a r e p r e s e n t b e f o r e contralateral S-R l i n k s a n d t h a t these effects a r e independent o f t h e a b i l i t y t o discriminate between l e f t a n d right. T h i s suggestion was v e r i f i e d in t h e following s t u d y . S-R C o m p a t i b i l i t y E f f e c t F o r Horizontal Dimension E i g h t subjects, 4 males a n d 4 females, between t h e ages 4 a n d 5 p a r t i c i p a t e d in t h e experiment. T h e y h a d normal o r corrected-to-normal v i s i o n a n d were n a i v e as t o t h e p u r p o s e o f t h e experiment. Handedness was assessed by a s k i n g t h e subjects t o p e r f o r m some actions described in a h a n d p r e f e r e n c e questionnaire (Annet, 1970) w i t h t h e i r p r e f e r r e d hand. T h e subjects were asked t o p e r f o r m s i x actions: T o w r i t e a l e t t e r legibly, t o t h r o w a b a l l t o hit a t a r g e t , t o c u t w i t h scissors, t o h o l d a t o o t h b r u s h while cleaning teeth, t o draw, t o c u t food w i t h k n i f e . T h e h a n d used by t h e subject f o r each a c t i v i t y was recorded. Awareness o f r i g h t - l e f t relations was assessed by a s k i n g t h e s u b j e c t t o make r i g h t - l e f t discriminations in relation (a) t o his own b o d y parts, (b) t o those o f another person, ( c ) t o object relations in t h e e n v i ronment, a n d (d) t o t h e capacity t o o r i e n t himself t o l e f t a n d right in one d i r e c t i o n a n d t h e n r e v e r s e t h e process in t h e opposite d i r e c t i o n . In t h e f i r s t l e f t - r i g h t discrimination, t h e subjects were asked t o show t h e i r right o r l e f t hand, leg, ear, in t h e second one t h e same b o d y p a r t o f a person f a c i n g t h e subject, in t h e third one t h e r e l a t i v e position o f t w o objects displayed along t h e horizontal dimension, a n d f i n a l l y in t h e last one t h e i r capacity t o o r i e n t themselves t o l e f t a n d right u n d e r t h e ins t r u c t i o n s o f t h e experimenter. Each o f t h e f o u r conditions consisted o f s i x items, e q u a l l y d i v i d e d between l e f t a n d right items. Each subject sat w i t h b o t h eyes open i n f r o n t o f a panel d i s p l a y i n g t h r e e l i g h t - e m i t t i n g diodes (LEDs) . T h e distance between t h e eyes a n d t h e m i d p o i n t o f t h e panel was 50 cm. A central, g r e e n LED s e r v e d as a f i x ation point, a n d t w o r e d LEDs s e r v e d as stimuli. T h e y were located o n e i t h e r side, a t a n d above t h e f i x a t i o n p o i n t a t a 45' angle f r o m t h e h o r i zontal plane a n d passing t h r o u g h it. Each LED was 11.5 cm f r o m t h e f i x a t i o n p o i n t a n d subtended a v i s u a l angle o f about 0.57'. When activated, it emitted a pul e of r e d light t h a t lasted 100 ms a n d h a d an Each h a n d h e l d a p l a s t i c c y l i n d e r equipped i n t e n s i t y o f 27.22 cd/m .

1

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w i t h a p u s h b u t t o n on i t s t o p . T h e t w o hands h o l d i n g t h e c y l i n d e r s rested o n t h e desk, w i t h t h e right h a n d in f r o n t o f t h e right light a n d t h e l e f t h a n d in f r o n t o f t h e l e f t light (uncrossed condition), o r t h e right h a n d in f r o n t o f t h e l e f t light a n d t h e l e f t h a n d in f r o n t o f t h e right light (crossed condition). A special-purpose computer f o r t h e presentation o f t h e stimuli a n d t h e r e c o r d i n g a n d analysis o f t h e r e sponses was located in t h e same room, b e h i n d t h e subject. T h e t r i a l s were a r r a n g e d in a quasi-random sequence so t h a t t h e p r o b a b i l i t y o f o c c u r r e n c e o f a l e f t o r right light on each t r i a l was equal; t h e o n l y r e s t r i c t i o n was t h a t no more t h a n t h r e e consecutive t r i a l s c o u l d o c c u r on one side. A w a r n i n g signal was p r o v i d e d by lighting t h e f i x a t i o n LED 1-3 sec p r i o r t o each stimulus presentation. T h e subject was i n s t r u c t e d t o p r e s s one o f t h e k e y s upon t h e appearance o f a g i v e n light. A l l possible combinations between l i g h t s a n d k e y s w e r e tested. Each subject attended t w o experimental sessions f o r each condition (uncrossed a n d crossed condition) t h a t were run o n consecutive days. Each session consisted o f s i x blocks o f 10 p r a c t i c e t r i a l s a n d 60 experimental t r i a l s p e r In t h e t w o b l o c k w i t h a 10-min r e s t p e r i o d between consecutive blocks. conditions o f S-R pairings, one S-R p a i r i n g i n v o l v e d p r e s s i n g t h e right k e y f o r t h e right light a n d t h e l e f t k e y f o r t h e l e f t light; t h e o t h e r S-R p a i r i n g i n v o l v e d p r e s s i n g t h e right k e y t o t h e l e f t light a n d t h e l e f t k e y t o t h e right light. T h e o r d e r o f S-R p a i r i n g conditions was counterbalanced across sessions a n d across subjects. O n l y R T s longer t h a n 100 ms a n d s h o r t e r t h a n 2000 ms were collected. T r i a l s in w h i c h subjects responded i n c o r r e c t l y a n d / o r RTs were o u t s i d e t h e above limits w e r e n o t repeated. T h e reduced v e r s i o n o f t h e h a n d p r e f e r e n c e questionnaire (Annet, 1970) showed t h a t s i x subjects h a d a right h a n d p r e f e r e n c e f o r all t h e manual activities a n d o n l y t w o subjects h a d a l e f t h a n d preference. The r i g h t - l e f t awareness t e s t showed t h a t o n l y f o u r subjects c o u l d make a discrimination between t h e i r own l e f t a n d right p a r t s o f t h e body, a n d n o one subject was able (a) t o discriminate between t h e l e f t a n d right side o f t h e person located in f r o n t of them, (b) t o d i s t i n g u i s h between t h e r e l a t i v e position o f t w o objects aligned along t h e horizontal dimension, a n d (c) t o t u r n t o t h e l e f t o r t o t h e right u n d e r t h e experimenter's instructions. Mean R T was computed across sessions f o r each subject f o r each o f t h e conditions r e s u l t i n g f r o m t h e combinations between t h e side o f t h e stimulus ( l e f t a n d right), t h e r e s p o n d i n g h a n d ( l e f t a n d right), a n d h a n d position (uncrossed a n d crossed positions). T h e means across subjects f o r t h e e i g h t basic conditions in uncrossed a n d crossed conditions a r e shown in Figures 1 a n d 2. A n analysis o f variance u s i n g Condition, Side o f t h e Stimulus, a n d Responding Hand as main f a c t o r s showed a s i g n i f i c a n t e f f e c t of Condition [ F ( l , 7)= 8.49, p < .025], w i t h uncrossed condition b e i n g f a s t e r (860 ms) t h a n crossed condition (993 ms). No o t h e r main e f f e c t was significant. T h e o n l y s i g n i f i c a n t i n t e r a c t i o n was t h a t between condition, side o f t h e stimulus a n d r e s p o n d i n g h a n d [ F ( l , 7) = 26.48, p < .005]. Paired t tests showed t h a t in t h e uncrossed condition, t h e 283 ms advantage o f t h e l e f t o v e r t h e right h a n d in r e s p o n d i n g t o t h e l e f t stimulus was s i g n i f i c a n t [ t ( 7 ) = 3.86, p < .01], as well as t h e 274 ms advantage o f t h e r i g h t o v e r t h e l e f t h a n d in r e s p o n d i n g t o t h e right stimulus [ t ( 7 ) = 3.75,

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Left Stimulus Right Stimulus

1200

I

I

T

- 1000 --E aoo -u)

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F i g u r e 1. Reaction times as a f u n c t i o n of stimulus position ( l e f t and right) and responding hand ( l e f t and r i g h t ) i n uncrossed condition.

HORIZONTAL DIMENSION: CROSSED CONDITION

Left Stimulus Right Stimulus

= . 1

1

-&j 1000

E

F c .-0 m

600 400

200 Left Hand

Right Hand

Reaction times as a f u n c t i o n of stimulus position ( l e f t and F i g u r e 2. r i g h t ) and responding hand ( l e f t and r i g h t ) i n crossed condition.

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p < .01]. In t h e crossed condition, t h e 260 ms advantage o f t h e l e f t o v e r t h e right h a n d in r e s p o n d i n g t o t h e right stimulus was s i g n i f i c a n t [t(7)=3.08, p .02], as well as t h e 284 ms advantage o f t h e right o v e r t h e l e f t h a n d in r e a c t i n g t o t h e l e f t stimulus [ t (7) = 2.75, p < . 0 5 ] .

A similar analysis o f variance was c a r r i e d o u t o n t h e a r c - s i n e transformation o f percentage o f e r r o r s . These data a r e g r a p h i c a l l y dep i c t e d in Figures 3 a n d 4. O n l y t h e interaction between Condition, Side o f t h e Stimulus a n d Responding Hand was significant [F(l, 7) = 41.72, p < .001]. Paired t tests showed t h a t in uncrossed condition t h e l e f t h a n d was more accurate t o respond t o t h e l e f t t h a n t h e right stimulus, [t(7) = 2.99, p < .02], whereas t h e advantage o f t h e right h a n d i n r e s p o n d i n g t o t h e right t h a n t o t h e l e f t stimulus was n o t s i g n i f i c a n t [t(7) = 1.721. In t h e crossed condition, t h e right hand, located o n t h e l e f t side o f t h e body, was more accurate t h a n t h e l e f t hand, located o n t h e right side o f t h e body, t o respond t o t h e l e f t stimulus [t(7) = 2.36, p < .05], whereas t h e l e f t h a n d was more accurate t h a n t h e right t o respond t o t h e right stimulus [ t ( 7 ) = 4.66, p < .001]. T h e p r e s e n t s t u d y shows t h a t t h e r e is n o t a simple relation between t h e establishment o f lateral p r e f e r e n c e in h a n d usage a n d t h e development o f t h e a b i l i t y t o discriminate right f r o m l e f t . The results pointed out t h a t in 4 - 5 - y e a r - o l d c h i l d r e n h a n d preference is stabilized, whereas t h e a b i l i t y t o make accurate r i g h t - l e f t identifications on t h e i r own body, o n t h e b o d y o f t h e examiner, a n d among environmental objects i s n o t y e t developed. T h e r e s u l t s obtained on l e f t - r i g h t awareness a r e in agreement w i t h those obtained by Belmont a n d B i r c h (1963). I n t h e i r s t u d y , t h e age o f 7 appears t o b e c r i t i c a l f o r t h e development o f t h e a b i l i t y t o d i s t i n g u i s h l e f t a n d right i n relation t o one's own b o d y p a r t s , although it is f r o m age 10 o n w a r d t h a t all c h i l d r e n respond c o r r e c t l y t o all questions. Moreover, Belmont a n d B i r c h showed t h a t when t h e t a s k r e q u i r e d t o d i s criminate between l e f t a n d right in relation t o objects in t h e e x t e r n a l environment, f u l l y accurate r i g h t - l e f t awareness was n o t stabilized b e f o r e t h e age o f 1 1 . T h e novel f i n d i n g o f t h e p r e s e n t experiment is t h e presence o f S - R compatibility effects, similar t o those showed by adults, in c h i l d r e n who were unable t o discriminate between l e f t a n d r i g h t . T h e r e s u l t s showed that, as i n adults, speed a n d accuracy o f response was f a c i l i t a t e d by t h e correspondence between side o f t h e stimulus a n d side o f t h e response key, r a t h e r t h a n t h e correspondence between t h e side o f t h e stimulus a n d t h e responding hand. In o t h e r words, t h e r i g h t k e y was f a s t e r f o r t h e r i g h t stimulus, a n d t h e l e f t k e y was f a s t e r f o r t h e l e f t stimulus, r e g a r d less o f w h e t h e r t h e right k e y was pressed w i t h t h e right h a n d a n d t h e l e f t k e y was pressed w i t h t h e l e f t h a n d (uncrossed condition) o r t h e right k e y was pressed w i t h t h e l e f t h a n d a n d t h e l e f t k e y was pressed w i t h t h e right h a n d (crossed condition). Furthermore, RTs w e r e f a s t e r i n t h e uncrossed condition t h a n in t h e crossed condition (crossed-hand effect), w i t h t h i s e f f e c t b e i n g g r e a t e r f o r t h e c h i l d r e n t h a n it u s u a l l y i s f o r adults. In children, R T s in t h e uncrossed condition a r e 133 ms f a s t e r t h a n in t h e crossed condition, whereas in a d u l t s t h e advantage o f t h e uncrossed condition o v e r t h e crossed condition t y p i c a l l y is a r o u n d 50 ms .

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c W

B %

7-6--

5-4--

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F i g u r e 3. Percentage o f e r r o r s as a f u n c t i o n o f stimulus position ( l e f t a n d right) a n d r e s p o n d i n g h a n d ( l e f t a n d r i g h t ) i n uncrossed condition.

H0R IZONTAL DIMEN S I0N: CROSSED C0NDIT I0N

Left Stimulus Right Stimulus

Left Hand

I I

Right Hand

Percentage of e r r o r s a s a f u n c t i o n of stimulus position ( l e f t F i g u r e 4. a n d right) a n d responding h a n d ( l e f t a n d right) in crossed condition.

Spatial

S-R Compatibility in Adults and Children

A n o t h e r d i f f e r e n c e t h a t emerges g r e a t e r d i f f e r e n c e between compatible c h i l d r e n t h a n in adults. In adults, R T s f o r compatible S-R p a i r i n g s t h a n f o r c h i l d r e n t h e d i f f e r e n c e is a r o u n d 270 ms.

155

from t h e present study is t h e a n d incompatible responses in a r e between 40 a n d 80 ms f a s t e r incompatible ones, whereas in

T h e r e a r e t w o possible explanations f o r these r e s u l t s . T h e f i r s t i s t h a t t h e crossed-hand e f f e c t i s g r e a t e r in c h i l d r e n t h a n in a d u l t s because t h e mismatch between t h e spatial code d e s c r i b i n g t h e r e s p o n d i n g h a n d a n d t h a t d e s c r i b i n g t h e position o f t h e response h a n d is more difficult t o resolve f o r c h i l d r e n t h a n f o r adults. T h i s explanation also can b e applied t o t h e d i f f e r e n c e between compatible a n d incompatible responses, w i t h c h i l d r e n h a v i n g more d i f f i c u l t y t h a n a d u l t s when t h e r e is 'a lack o f correspondence between t h e code d e s c r i b i n g t h e spatial position o f t h e stimulus a n d t h a t d e s c r i b i n g t h e spatial position o f t h e r e s p o n d i n g hand. T h e second explanation i s t h a t t h e o v e r a l l longer R T s in c h i l d r e n t h a n in a d u l t s may m a g n i f y t h e differences between crossed a n d uncrossed conditions a n d between compatible a n d incompatible responses. T h e most d i r e c t way t o d i s t i n g u i s h between t h e t w o explanations is t o a r t i f i c i a l l y inflate t h e RTs o f adults, f o r instance by r e d u c i n g t h e i n t e n s i t y o f t h e v i s u a l stimuli. In t h i s case, RTs should b e slowed overall, and, if t h e second explanation is correct, t h e differences between conditions should increase t o magnitudes similar t o those shown by c h i l d r e n . T h e f i r s t explanation, t h a t c h i l d r e n have e x t r a difficulty in r e s o l v i n g mismatches between stimulus a n d response codes, seems plausible when we consider t h e d i f f i c u l t y t h a t y o u n g i n f a n t s have in r e a c h i n g across t h e midline ( B r u n e r , 1969). Provine a n d Westerman (1979) have shown t h a t , in i n f a n t s o f 9 t o 20 weeks of age, v i s u a l l y d i r e c t e d h a n d extension a n d reaching develop p r o g r e s s i v e l y f r o m t h e ipsilateral domain t o t h e midline a n d l a t e r t o t h e contralateral domain. Thus, it seems t h a t ipsilaterat S-R l i n k s a r e p r e s e n t b e f o r e contralateral ones, a n d in y o u n g c h i l d r e n S-R p a i r i n g s on t h e same side o f t h e b o d y midline a r e predomin a n t o v e r S-R p a i r i n g s t h a t c r o s s t h e b o d y midline (Wapner E Cirillo, If we consider t h a t t h i s e f f e c t decreases w i t h age (Wapner & 1968). Cirillo, 1968), it is n o t s u p r i s i n g t h a t t h e d i f f e r e n c e between compatible a n d incompatible responses i s g r e a t e r f o r c h i l d r e n t h a n f o r adults. T h e second s t u d y was i n t e n d e d t o assess w h e t h e r spatial compatibility effects in c h i l d r e n a r e confined t o t h e horizontal dimension or o c c u r in t h e v e r t i c a l dimension as well. Several studies (Nicoletti & Umilta, 1984; Simon, Mewaldt, Acosta, & Hu, 1976) have demonstrated in adults above-below compatibility effects similar t o those f o u n d f o r t h e horizontal dimension. T h a t is, RTs a r e s h o r t e r when t h e t o p stimulus r e q u i r e s a response w i t h t h e t o p h a n d t h a n when it r e q u i r e s a response w i t h t h e bottom hand, a n d v i c e versa f o r t h e bottom stimulus.

S-R Compatibility E f f e c t For Vertical Dimension T h i s s t u d y i s essentially a r e p e t i t i o n of t h e p r e v i o u s one t h a t h a d shown t h e spatial compatibility f o r t h e r i g h t h e f t dimension. The only notable d i f f e r e n c e is t h a t t h e l i g h t stimuli a n d t h e response k e y s a r e a r r a n g e d v e r t i c a l l y . Six new subjects, 3 males a n d 3 females, selected as in t h e p r e v i o u s experiment, took p a r t in t h e experiment. T h e apparatus was t h e same as t h a t described f o r p r e v i o u s one, e x c e p t t h a t t h e t w o LEDs were above a n d below t h e f i x a t i o n p o i n t a t a distance o f 11.5 cm. T h e hands were aligned w i t h t h e midsagittal plane o f t h e b o d y a n d placed

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one above t h e t a b l e a n d t h e o t h e r below t h e table. T h e experiment comp r i s e d t w o conditions o f S-R p a i r i n g . In one condition t h e subject was r e q u i r e d t o respond t o t h e t o p light w i t h t h e t o p h a n d a n d t o t h e bottom light w i t h t h e bottom hand, whereas in t h e o t h e r condition t h e assignment was r e v e r s e d so t h a t t h e t o p light corresponded t o t h e bottom h a n d a n d t h e bottom light t o t h e t o p hand. In each condition, t h e responses were g i v e n w i t h t h e right h a n d above a n d t h e l e f t below. Mean R T was computed across sessions f o r each subject f o r each o f t h e conditions r e s u l t i n g f r o m t h e combination between t h e position o f t h e stimulus (above a n d below) a n d t h e position o f t h e c o r r e s p o n d i n g h a n d (above a n d below). These data a r e depicted g r a p h i c a l l y in F i g u r e 5. A n analysis o f variance u s i n g position o f t h e stimulus a n d position of t h e r e s p o n d i n g h a n d as main f a c t o r s showed t h a t o n l y t h e interaction was s i g n i f i c a n t [F(1,5) = 25.16, p < .005]. Paired t - t e s t s showed t h a t t h e 296 ms advantage o f t h e t o p h a n d o v e r t h e bottom h a n d i n responding t o t h e t o p stimulus was s i g n i f i c a n t [ t ( 5 ) = 3.11, p < .05], as well as t h e 367 ms advantage o f t h e bottom h a n d o v e r t h e t o p h a n d in responding t o t h e bottom stimulus [ t ( 5 ) = 4.92, p < .01].

A similar analysis o f variance was c a r r i e d o u t on t h e arc-sine transformation o f percentage o f e r r o r s . These data a r e depicted g r a p h i cally in F i g u r e 6. T h e analysis o f variance showed a s i g n i f i c a n t e f f e c t o f stimulus position [ F ( I , 5) = 8.99, p < .05], d u e t o f e w e r e r r o r s f o r t h e bottom stimulus (7.4) t h a n f o r t h e t o p stimulus (9.2). T h e interaction between stimulus position a n d h a n d position was s i g n i f i c a n t [F(1,5) = 9.77 p < .05] a n d showed t h a t t h e t o p h a n d was more accurate t h a n t h e bottom h a n d in r e s p o n d i n g t o t h e t o p stimulus a n d t h e bottom h a n d was more accurate t h a n t h e t o p h a n d in r e s p o n d i n g t o t h e bottom stimulus ( p < .05 in b o t h comparisons). T h e r e s u l t s o f t h e p r e s e n t s t u d y showed t h a t in c h i l d r e n spatial compatibility is n o t confined t o t h e horizontal dimension but occurs in t h e v e r t i c a l dimension as well. RTs were s h o r t e r when t h e t o p stimulus r e q u i r e d a response w i t h t h e t o p h a n d t h a n when it r e q u i r e d a response w i t h t h e bottom hand, a n d v i c e versa f o r t h e bottom stimulus. As f o r t h e horizontal dimension, t h e d i f f e r e n c e between compatible a n d incomp a t i b l e responses was g r e a t e r f o r c h i l d r e n t h a n f o r a d u l t s . F o r adults, R T s t y p i c a l l y a r e a r o u n d 50 ms f a s t e r f o r compatible S-R p a i r i n g s t h a n f o r incompatible ones, whereas f o r c h i l d r e n t h e d i f f e r e n c e is 331 ms. Conclusions From C o m p a t i b i l i t y Studies With C h i l d r e n T h e p r e s e n t s t u d y showed t h a t (a) 4-5-year-old c h i l d r e n have spatial compatibility effects i n b o t h t h e horizontal a n d t h e v e r t i c a l dimension a n d (b) spatial compatibility effects a r e g r e a t e r f o r c h i l d r e n t h a n f o r adults. These f i n d i n g s allow us t o reject t h e hypothesis t h a t t h e codes i n v o l v e d in spatial compatibility a r e v e r b a l (e.g., t h a t t h e y c o r r e spond t o t h e v e r b a l labels d e s c r i b i n g t h e r e l e v a n t positions o f stimuli a n d responses), because y o u n g c h i l d r e n a r e unable t o a p p l y v e r b a l labels t o t h e positions o f stimuli aligned along t h e horizontal dimension. Therefore, these codes a r e l i k e l y t o b e spatial. In conclusion, t h e compatibility e f f e c t s f o u n d w i t h a p e r p e n d i c u l a r arrangement o f stimuli a n d r e sponses by Lgdavas (1987) in right handers cannot b e i n t e r p r e t e d as

Spatial S-R Compatibility in Adults and Children

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Figure 5. Reaction times as a function of position of the stimulus (top and bottom) and position of t h e responding hand (top and bottom).

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Figure 6. Percentage of e r r o r s as a function of position of t h e stimulus (top and bottom) and position of t h e responding hand (top and bottom).

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S-R l i n k s o f t h e v e r b a l codes f o r "right" a n d "above" a n d t h e v e r b a l codes f o r " l e f t " a n d "below." Rather, t h e effects a r e b e t t e r explained as an asymmetry in t h e coding of t h e dominant h a n d a n d non-dominant h a n d along t h e v e r t i c a l dimension, w i t h t h e dominant h a n d b e i n g assigned a "higher" position compared t o t h e non-dominant hand.

Some D e t e r m i n i n g Factors I n P r o d u c i n g S-R Compatibility E f f e c t s Before d i s c u s s i n g t h e relevance o f t h e coding o f t h e anatomical status o f t h e h a n d in d e t e r m i n i n g t h e spatial compatibility effects, it i s necessary t o discuss t h e relevance o f t w o o t h e r f a c t o r s . So f a r i we have considered t h e response set by making reference t o t h e positions o f t h e effectors in general. However, t w o positional features o f t h e response can b e distinguished: one is t h e position o f t h e e f f e c t o r a n d t h e o t h e r is t h e position o f t h e response goal. I n spatial S-R compatibility effects b o t h positional features have been t a k e n i n t o consideration. Brebner et al. (1972) stressed t h e importance of t h e spatial coding o f t h e location o f t h e response goal, whereas Wallace (1971, 1972) emphasized t h e spatial c o d i n g o f t h e location o f t h e e f f e c t o r . Because in all p r e v i o u s studies effectors a n d response goals were confounded, Riggio e t al. (1986) f o u n d an experimental s i t u a t i o n in w h i c h each e f f e c t o r (hand) h a d a response goal ( k e y ) t h a t shared (uncrossed condition) o r did n o t share (crossed conditions) t h e same locational code In t h i s study, t h e subjects operated t w o s t i c k s t h a t o f t h e effectors. were e i t h e r uncrossed o r crossed. I n t h e uncrossed condition, t h e effectors ( t h e t w o hands) a n d t h e response goals ( t h e t w o keys) occupied t h e same location in b o t h absolute ( i . e . , i n relation t o t h e b o d y midline) a n d r e l a t i v e (i.e., in relation t o t h e o t h e r e f f e c t o r o r response goal) terms. In t h e crossed condition, t h e effectors a n d response goals occupied opposite locations, again absolutely o r r e l a t i v e l y . The results were clear in showing t h a t spatial compatibility depended on t h e location o f t h e response goal, whereas t h e location o f t h e e f f e c t o r h a d no influence. In t h e crossed condition, t h e right hand, which was on t h e right side but operated t h e l e f t key, was f a s t e r f o r t h e l e f t t h a n f o r t h e right stimulus, whereas t h e l e f t hand, which was o n t h e l e f t side but operated t h e right key, was f a s t e r f o r t h e right t h a n f o r t h e l e f t stimulus. In o t h e r words, t h e locational coding o f t h e response goal dominated t h a t o f t h e location o f t h e e f f e c t o r . It is s u r p r i s i n g t h a t also in t h i s experiment t h e responses w i t h t h e s t i c k s crossed were slower t h a n those w i t h t h e s t i c k s uncrossed, although t h e hands were always in t h e anatomical uncrossed positions. It seems t h a t t h e r e a r e t h r e e factors t h a t a r e responsible for S-R compatibility effects: (a) t h e spatial coding o f t h e goal, (b) t h e spatial coding o f t h e response effectors, a n d (c) t h e c o d i n g o f t h e anatomical status o f t h e response (i.e., w h i c h h a n d emits t h e response). These f a c t o r s seem t o b e h i e r a r c h i c a l l y o r d e r e d . T h e h i g h e r - l e v e l f a c t o r dominates a n y f a c t o r o f lower r a n k , but a lower-level f a c t o r emerges if t h e h i g h e r - l e v e l f a c t o r s a r e n o t available. A s shown by Riggio e t al. (1986), t h e location o f t h e response goal dominates t h e location o f t h e effector, whereas t h e location o f t h e effector dominates t h e anatomical status o f t h e r e s p o n d i n g h a n d (Anzola e t al., 1977; B r e b n e r e t al., 1972).

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However, t h e anatomical status o f t h e r e s p o n d i n g h a n d dominates t h e o t h e r t w o f a c t o r s when t h e spatial coding o f t h e stimulus a n d t h e spatial coding o f t h e response goal and/or t h e location o f response e f fectors belong t o d i f f e r e n t spatial dimensions (i.e., horizontal a n d v e r t i cal). In o t h e r words, t h e r e s p o n d i n g h a n d i s t h e c r u c i a l f a c t o r in det e r m i n i n g t h e S-R compatibility effects, when t h e spatial relation between stimulus locations a n d response locations is considered t o b e n e u t r a l . T h i s was t h e case in a s t u d y by Klapp, Greim, Mendicino, a n d Koenig (19791, in w h i c h t h e spatial position o f t h e stimuli ( l e f t o r right) was u n r e l a t e d t o t h e spatial position o f t h e hands (up o r down). However, since t h e r e was a relationship between t h e t h u m b (right o r l e f t ) a n d stimulus location (right o r l e f t ) , t h e y f o u n d S - R Compatibility e f f e c t s simi l a r t o those obtained between stimulus location a n d response location. LBdavas a n d Moscovitch (1984) a n d LBdavas (1987) f o u n d similar r e s u l t s in an experimental situation in w h i c h t h e stimuli were displayed v e r t i c a l l y (above a n d below) a n d t h e hands h o r i z o n t a l l y ( l e f t a n d right). In a recent study, P r o c t o r a n d Reeve (1985) also f o u n d t h a t t h e spatial locat i o n is n o t t h e o n l y response f e a t u r e t h a t can b e used t o p r o d u c e S-R compatibility effects, but t h a t responses can b e coded also o n t h e basis o f a d i s t i n c t i o n between hands. I n an experimental condition w h e r e t h e hands were overlapped, t h a t is, one above t h e other, a n d symbolic stimuli were assigned t o f i n g e r s such t h a t t h e salient f e a t u r e o f t h e stimuli corresponded t o t h e h a n d d i s t i n c t i o n , subjects used h a n d coding instead o f spatial-location c o d i n g t o p r o d u c e S-R compatibility effects. Therefore, we can conclude t h a t when t h e spatial position o f t h e stimuli a n d t h e spatial position o f t h e response a r e unrelated, t h e r e s p o n d i n g h a n d replaces t h e response goal a n d t h e location o f response e f f e c t o r as a determinant f a c t o r i n p r o d u c i n g S - R compatibility effects. Summary In t h i s chapter, I examine t h e p o s s i b i l i t y t h a t t h e r e a r e t h r e e factors h i e r a r c h i c a l l y o r d e r e d t h a t a r e responsible f o r spatial S - R comp a t i b i l i t y effects: (a) t h e spatial coding o f t h e goal, (b) t h e spatial coding o f t h e response effectors, a n d (c) t h e c o d i n g o f t h e anatomical status o f t h e response (i.e., w h i c h h a n d emits t h e response). T h e location of t h e response goal dominates t h e location o f t h e effector, whereas t h e location of t h e e f f e c t o r dominates t h e anatomical status o f t h e responding h a n d . However, t h e anatomical status o f t h e r e s p o n d i n g h a n d dominates t h e o t h e r t w o f a c t o r s when t h e r e is n o t dimensional o v e r l a p between stimulus locations a n d response locations. A n o t h e r question t h a t has been addressed i n t h e p r e s e n t c h a p t e r is Two t h e n a t u r e o f t h e codes i n v o l v e d in spatial compatibility effects. experiments were described t h a t show t h e presence o f spatial compatibility effects i n c h i l d r e n who cannot discriminate between l e f t a n d right and, therefore, cannot use v e r b a l labels t o code t h e positions o f stimuli a n d responses. T h e r e s u l t s showed t h a t (a) 4-5-year-old c h i l d r e n have spatial compatibility effects in b o t h t h e horizontal a n d t h e v e r t i c a l dimension, a n d (b) spatial compatibility effects a r e g r e a t e r f o r c h i l d r e n than for adults. Therefore, these f i n d i n g s allows u s t o reject t h e hypothesis t h a t t h e codes i n v o l v e d i n spatial compatibility a r e v e r b a l .

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Acknowledgement T h i s research was s u p p o r t e d by a CNR g r a n t t o E. LBdavas and by Natural Sciences a n d Engineering Research Council o f Canada G r a n t 48347 t o M o r r i s Moscovitch. I t h a n k Carlo UmiltB f o r h i s h e l p o n t h e discussion of t h e manuscript. References Annet, M. (1970). A classification o f hand preference by analysis. British Journal of Psychology, 61, 303-321. Anzola, G. P . , Bertoloni, G . , Buchtel, H. A., & Rizzolatti, G. (1977). Spatial compatibility a n d anatomical factors in simple a n d choice r e action time. Neuropsychologia, 15, 295-302. Belmont, L., E Birch, H. G. (1963). Lateral dominance a n d r i g h t - l e f t awareness in normal c h i l d r e n . Child Development, 34, 257-270. Benton, A. L. (1959). Right-left discrimination and finger localization. New Y o r k : Hoeber-Harper. Berlucchi, G., Crea, F., D i Stefano, M., & Tassinari, G. (1977). Influence o f spatial stimulus-response compatibility o n reaction time o f ipsilateral a n d contralateral h a n d t o lateralized light stimuli. Journal of Expertmental Psychology: Human Perception and Performance, 3,

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(1988). Asymmetries in processing horizontal a n d v e r t i c a l LBdavas, E. dimensions. Memory & Cognition, 16, 377-382. (1984). M u s t egocentric a n d e n v i r o n LBdavas, E., & Moscovitch, M. mental frames o f reference b e aligned t o p r o d u c e spatial S-R comp a t i b i l i t y effects? Journal of Experimental Psychology: Human Perception and Performance, 10, 205-215. Myers, R. E. (1965). T h e neocortical commissures a n d interhemispheric transmission o f information. In E. G. E t t l i n g e r , A . V. S. d e Reuch, & R. P o r t e r (Eds.), Function of the corpus callosum. London: Churchill. Nicoletti, R., Anzola, G. P. , Luppino, G . , Rizzolatti, G . , & UmiltB, C. (1982). Spatial compatibility e f f e c t s on t h e same side of t h e body midline. Journal of Experimental Psychology: Human Perception and Performance, 8 , 664-673. Nicoletti, R., & UmiltB, C. (1984). R i g h t - l e f t prevalence in spatial comp a t i b i l i t y . Perception & Psychophysics, 3 5 , 333-343. (1984). Compatibility d u e t o Nicoletti, R., UmiltB, C., & LBdavas E. Acta Psychot h e coding o f t h e r e l a t i v e position of t h e effectors. logica. 5 7 , 133-143. Proctor, R. W., & Reeve, T. G . (1985). Compatibility effects in t h e assignment o f symbolic stimuli t o d i s c r e t e f i n g e r response. Journal of Experimental Psychology: Human Perceptlon and Performance, 1 1 , 623639. C r o s s i n g t h e midline: Provine, R. R. & Westerman, J. A . (1979). Limits o f e a r l y eye-hand behavior. Child Development, 50, 437-441. Riggio, L., Gawryszewski, L. G., E UmiltB, C. (1986). What is crossed in crossed-hand effects? Acta Psychologica, 62, 89-100. Sekiyama, K . (1982). Kinesthetic aspects o f mental representation in t h e identification o f l e f t a n d right h a n d s . Perception & Psychophysics, 32, 89-95. Simon, J. R . (1968). E f f e c t o f ear stimulated o n reaction time a n d Journal of Experimental Psychology, 7 8 , 344-346. movement time. Simon, J. R., Hinrichs, J. V., & C r a f t , J. L. (1970). A u d i t o r y S-R compatibility: Reaction time as a f u n c t i o n o f e a r - h a n d correspondence Journal of Experimental a n d ear- response-location correspondence, Psychology, 86, 97-102. Simon, J. R . , Mewaldt, S . P., Acosta, E., Jr., & Hu, J. M. (1976). Processing a u d i t o r y information : I n t e r a c t i o n of t w o population stereot y p e s . Journal of Applied Psychology, 60, 354-358. Simon,

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h i l t & , C., & Nicoletti, R. (1985). A t t e n t i o n a n d coding effects i n S - R compatibility d u e t o i r r e l e v a n t spatial cues. I n M. I . Posner, & 0. S. M. Marin, (Eds.), Attention ond performance X I (pp. 457-471). Hillsdale, NY: Erlbaum.

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S TIMULUS-RESPONSE COMPATIfllUW R.W. Proctor and EG. Reeve (Editors 0 Elsevier Science Publishers 6.V. (Jorth-Holland), 1990

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THE SALIENT-FEATURES CODING PRINCIPLE FOR SPATIALAND SYMBOLIC-COMPATI B I L l T Y EFFECTS

T. GILMOUR REEVE Motor Behavior C e n t e r Auburn University ROBERT W. PROCTOR Department o f Psychological Sciences Purdue University Stimulus-response (S-R) compatibility effects typically are a t t r i b u t e d t o a stage o f human-information processing t h a t is r e f e r r e d t o as e i t h e r S-R translation, response selection, o r decision (e.g., T e i c h n e r & Krebs, 1974). T h e p r i m a r y f u n c t i o n o f t h i s stage i s t o t r a n s l a t e between t h e codes t h a t a r e used t o r e p r e s e n t t h e stimulus a n d response sets. Situations f o r w h i c h S-R t r a n s l a t i o n is minimal a r e r e g a r d e d as compatible, whereas situations f o r w h i c h it is n o t a r e r e g a r d e d as incompatible. With spatial-location stimuli, S-R compatibility is a f u n c t i o n o f t h e e x t e n t t o w h i c h t h e assignment o f stimulus locations t o response locations maintains a d i r e c t relation. F o r example, when t w o l e f t a n d r i g h t spatially a r r a n g e d stimuli a r e assigned t o t h e l e f t a n d right index f i n g e r s , reaction times (RTs) a r e fastest if t h e right stimulus is assigned t o t h e right response a n d t h e l e f t stimulus t o t h e l e f t response. With hands placed i n a normal manner, t h e relation o f stimulus locations t o response locations is confounded w i t h t h e relation of stimulus locations t o t h e l e f t a n d r i g h t hands. However, t h e t w o relations can b e dissociated by c r o s s i n g t h e hands, such t h a t t h e l e f t - i n d e x f i n g e r i s placed a t t h e right response location a n d t h e r i g h t - i n d e x f i n g e r a t t h e l e f t location. Even w i t h t h i s crossed hands placement, t h e R T s s t i l l a r e f a s t e s t when t h e right stimulus is assigned t o t h e right response location a n d t h e l e f t stimulus t o t h e l e f t response location (e.g., Nicoletti, Anzola, Luppino, Rizzolatti, & U m i l t i , 1982). Thus, t h e relation between stimulus locations. a n d response locations is crucial, r a t h e r t h a n t h e relation between stimulus locations a n d t h e hands. T h i s f i n d i n g indicates t h a t t h e compatibility effects r e f l e c t t r a n s l a t i o n between codes t h a t a r e based o n t h e spatial characteristics o f t h e stimulus a n d response sets (see C h a p t e r 3, by U m i l t i & Nicoletti). Spatial coding also i s a p p a r e n t when symbolic stimuli (e.g., letters, colors) a r e assigned t o l e f t a n d right response locations, but t h e stimuli o c c u r i n e i t h e r of t w o locations t h a t a r e i r r e l e v a n t f o r d e t e r m i n i n g t h e c o r r e c t response (see C h a p t e r 2 , by Simon). T h a t is, in such situations, responses t y p i c a l l y a r e f a s t e r when t h e i r r e l e v a n t location o f t h e symbolic stimulus corresponds t o t h e response f o r t h a t stimulus t h a n when it does not. T h i s relation between i r r e l e v a n t stimulus locations a n d response locations holds even when t h e hands a r e crossed (Umilt6 8 Nicoletti, 1985; Wallace, 19711, w h i c h is consistent w i t h t h e r e s u l t s obtained when t h e locations o f t h e stimuli a r e r e l e v a n t t o t h e responses.

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A l t h o u g h S-R compatibility effects were s t u d i e d i n i t i a l l y in relat i v e l y complex t a s k s (e.g., F i t t s & Seeger, 1953; G a r v e y & Knowles, 19541, most o f t h e r e c e n t evidence f o r spatial coding has been r e s t r i c t e d t o two-choice t a s k s similar t o those j u s t described. Moreover, few studies have examined situations f o r w h i c h t h e responses a r e made a t d i s t i n c t spatial locations, but t h e stimuli d o n o t d i f f e r o n e i t h e r a r e l e v a n t o r i r r e l e v a n t spatial-location dimension (i.e., symbolic stimuli a r e presented a t o n l y one location). A n exception o f t h e r e s t r i c t i o n t o t w o choice t a s k s involves studies t h a t have used a movement-precuing procedure, in w h i c h parameters o f limb movements, such as arm, direction, o r extent, a r e specified in advance (Rosenbaum, 1983). The i n t e n t o f t h e movement-precuing studies has been t o evaluate t h e n a t u r e o f response p r e p a r a t i o n t h r o u g h t h e p a t t e r n s o f R T benefits t h a t a r e obtained when v a r i o u s movement parameters, o r combinations o f movement parameters, a r e precued. In such studies, t h e potential f o r S-R compatibility effects must b e evaluated, because t h e effects have t h e i r basis in S-R t r a n s l a t i o n processes r a t h e r t h a n in response-preparation processes. Several studies have demonstrated t h a t S-R compatibility does in f a c t i n f l u e n c e t h e r e s u l t s obtained i n movement-precuing t a s k s (e.g., Goodman 8 Kelso, 1980; Larish, 1986). However, these studies have been concerned p r i m a r i l y w i t h t h e necessary procedures t o c o n t r o l comp a t i b i l i t y effects, r a t h e r t h a n w i t h t h e u n d e r l y i n g n a t u r e o f S-R compatibility (see C h a p t e r 11, by Zelaznik & F r a n z ) . A l t h o u g h i n t e r e s t i n S-R compatibility effects in t h e movementp r e c u i n g t a s k has focused p r i m a r i l y o n c o n t r o l l i n g such effects, t h e t a s k i t s e l f may b e used as a vehicle f o r i n v e s t i g a t i n g t h e n a t u r e o f S - R compatibility. T h a t is, t h e p r e c u i n g t a s k maintains some o f t h e characteristics o f t h e basic two-choice t a s k but i s more complex. Its complexity in terms o f t h e stimulus a n d response sets, t h e number o f S-R alternatives, a n d t h e use o f advance information allows examination o f a b r o a d e r r a n g e o f compatibility issues. Recently, we have conducted systematic investigations o f S-R compatibility in variations o f t h e movement-precuing t a s k t h a t have used b o t h spatial-location a n d symbolic stimuli assigned t o keypress responses. T h e investigations w i t h spatiallocation stimuli have indicated a r o l e f o r spatial coding in t h e movementp r e c u i n g t a s k t h a t is similar t o t h e role t h a t such coding p l a y s in t h e two-choice t a s k . B u t o u r studies also have shown t h a t coding can o c c u r on t h e basis o f o t h e r features, such as t h e d i s t i n c t i o n between hands, when those features a r e salient. Moreover, coding o f stimulus a n d response sets based o n salient features occurs when t h e stimuli a r e symbolic. T h e r e s u l t s o f o u r studies have c o n v e r g e d t o s u p p o r t a basic p r i n c i p l e , t h e salient-features coding p r i n c i p l e , f o r spatial- a n d symboliccompatibility effects. In t h e following sections, we p r e s e n t t h e p r i m a r y evidence f o r t h i s p r i n c i p l e . Coded Representations o f Spatial-Location Stimuli a n d Responses O u r i n i t i a l studies used a v a r i a t i o n o f t h e movement-precuing task, i n t r o d u c e d by M i l l e r (1982), t h a t involves f i n g e r movements r a t h e r t h a n limb movements. T h e t a s k uses f o u r potential keypress responses t h a t a r e made w i t h t h e i n d e x a n d middle f i n g e r s o f each hand. Each t r i a l begins w i t h a w a r n i n g r o w o f f o u r p l u s signs, followed one-half second l a t e r by a p r e c u e t h a t is presented immediately below t h e w a r n i n g row

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(see Table 1). For an u n p r e p a r e d condition, t h e p r e c u e consists o f another r o w o f f o u r p l u s signs. F o r t h e p r e p a r e d conditions, t h e p r e c u e consists o f p l u s signs in o n l y t w o o f t h e f o u r locations. Following a v a r i a b l e p r e c u i n g i n t e r v a l of 0-3000 ms, a t a r g e t stimulus occurs in a third row a t one o f t h e c u e d locations, a n d t h e subject is t o make t h e c o r r e s p o n d i n g response as q u i c k l y as possible. Table 1 Stimulus Displays f o r Each Preparation Condition When t h e T a r g e t Indicated L e f t Middle-Finger Response Finger

LM

LI

RI

RM

+ +

+

+

+

Unprepared Warning Precue Target

+ +

+ +

Warning Precue Target

+

+

+

+

Warning Precue Target

+

+

+ Prepared: Hand

+

Prepared: F i n g e r + +

+ + Prepared: N e i t h e r

Warning Precue Target Note. finger.

+

+

+

+

+

L = l e f t hand;

R = right hand;

M = middle f i n g e r ;

I = index

T h e p r e p a r e d conditions a r e o f t h r e e d i f f e r e n t t y p e s (see Table 1). I n t h e p r e p a r e d : h a n d condition, t h e p r e c u e specifies t w o f i n g e r s on t h e same h a n d (e.g., t h e left-middle a n d l e f t - i n d e x f i n g e r s ) ; in t h e p r e p a r e d : f i n g e r condition, t h e p r e c u e specifies t h e same f i n g e r on d i f f e r e n t hands (e.g., t h e l e f t - i n d e x a n d r i g h t - i n d e x f i n g e r s ) ; a n d in t h e p r e pared: n e i t h e r condition, t h e p r e c u e specifies d i f f e r e n t f i n g e r s on d i f f e r e n t hands (e.g., t h e left-middle a n d r i g h t - i n d e x f i n g e r s ) . When t h e hands a r e placed i n a normal, adjacent manner ( t h e adjacent-hand placement), a p a t t e r n o f d i f f e r e n t i a l p r e c u i n g b e n e f i t s is obtained. This patt e r n i s t h a t R T s a r e fastest f o r t h e p r e p a r e d : h a n d condition a n d slowest f o r t h e p r e p a r e d : n e i t h e r condition, w i t h t h e p r e p a r e d : f i n g e r condition

T.G. Reeve and R. W.

166 b e i n g intermediate Proctor, 1984).

(Miller,

1982;

Proctor

Proctor &

Reeve,

198813;

Reeve

&

O u r studies indicate t h a t t h i s p a t t e r n o f d i f f e r e n t i a l p r e c u i n g benef i t s is an S-R compatibility e f f e c t t h a t reflects how r a p i d l y t h e subject can determine the. p r e c u e d p a i r o f responses. One finding 'consistent w i t h t h i s compatibility i n t e r p r e t a t i o n is t h a t t h e advantage f o r t h e p r e pared: h a n d condition is a p p a r e n t p r i m a r i l y a t s h o r t p r e c u i n g i n t e r v a l s (Reeve & Proctor, 1984). As shown in F i g u r e 1, when t h e i n t e r v a l is extended t o 3 s, all p a i r s o f responses show approximately equivalent benefits r e l a t i v e to t h e u n p r e p a r e d condition (see also Reeve & Proctor, 1988). Thus, c e r t a i n p a i r s o f p r e c u e d responses can b e selected f a s t e r t h a n others, w i t h n o i n h e r e n t differences in t h e p r e p a r a t i o n of t h e diff e r e n t p a i r s once t h e responses have been selected.

As f o r t h e compatibility e f f e c t t h a t occurs w i t h a normal h a n d placement in two-choice tasks, t h e relation between stimulus locations a n d response locations i s confounded w i t h t h e relation between stimulus locations a n d f i n g e r s when o n l y t h e adjacent-hand placement is used. To determine w h e t h e r response locations o r specific f i n g e r s a r e crucial, we used an overlapped-hand placement (Reeve & Proctor, 1984, Experiment 3 ) . Relative t o t h e adjacent placement, t h e overlapped placement dissociates t h e spatial locations o f t h e stimuli a n d responses f r o m t h e p a r t i c u l a r

-

800

Prepared: Hand Prepared :Finger Prepared: Neither Unprepared

5 700

E W 2 Iz 0

I-

2 600 W

a

500

I

0

I

375

I

750

I

1500 PRECUING INTERVALS lMSl

I

3000

F i g u r e 1. Mean R T s f o r u n p r e p a r e d a n d p r e p a r e d conditions as a f u n c t i o n o f p r e c u i n g i n t e r v a l ( f r o m Reeve & Proctor, 1984, Experiment 1 ) .

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p a i r s o f f i n g e r s t h a t a r e cued. T h a t is, t h e c u e d locations f o r t h e t w o extreme conditions, t h e p r e p a r e d : h a n d a n d p r e p a r e d : n e i t h e r conditions, a r e switched f o r t h e o v e r l a p p e d placement. Thus, if t h e p r e c u i n g advantage arises f r o m t h e relation between stimulus locations a n d t h e f i n g e r s o n t h e l e f t a n d right hands, it s t i l l should o c c u r f o r t h e p r e p a r e d : h a n d condition. However, if t h e advantage is a f u n c t i o n o f t h e relation between stimulus locations a n d response locations, it should o c c u r f o r t h e p r e p a r e d : n e i t h e r condition. As shown in Table 2, t h i s l a t t e r r e s u l t was obtained, t h u s i n d i c a t i n g t h a t t h e advantage r e a l l y is one f o r t h e t w o leftmost a n d t w o r i g h t m o s t spatial locations, regardless o f w h e t h e r t h e f i n g e r s a r e f r o m t h e same o r d i f f e r e n t hands. Table 2 Mean Reaction Times (in ms) f o r t h e Hand Placement X Preparation Condition I n t e r a c t i o n ( f r o m Reeve & Proctor, 1984, Experiment 3)

Hand placement Preparation condition

U n p repa r e d Prepared: Hand Prepared: F i n g e r Prepared: N e i t h e r

Adjacent hands

Overlapped hands

551 502 516 547

771 765 746 721

One problem w i t h i n t e r p r e t i n g t h e r e s u l t s obtained w i t h t h e o v e r lapped placement as s u p p o r t f o r spatial coding i s t h a t t h e responses w i t h t h a t placement a r e considerably slower o v e r a l l t h a n t h e y a r e w i t h t h e adjacent placement. Thus, t h e comparison between t h e p a t t e r n s o f RTs f o r t h e t w o placements p o t e n t i a l l y is confounded w i t h t h e o v e r a l l level o f RTs (Miller, 1985). We have a r g u e d t h a t t h e p a t t e r n s o f d i f f e r e n t i a l p r e c u i n g benefits a r e independent f r o m t h e r e l a t i v e d i f f i c u l t i e s o f e x e c u t i n g responses w i t h t h e adjacent a n d overlapped placements (Reeve & Proctor, 1985). T o evaluate o u r position r e g a r d i n g t h e o v e r a l l levels o f RT, we conducted experiments t h a t used v e r t i c a l stimulus a n d response a r r a n g e ments (Proctor & Reeve, 1986). With t h e v e r t i c a l stimulus arrangement, t h e warning, precue, a n d t a r g e t stimuli a r e p r e s e n t e d in t h r e e columns o f f o u r locations, r a t h e r t h a n in t h r e e rows (see Table 3 ) . T h e responses a r e made on a row o f f o u r response keys, w h i c h i s s i t u a t e d p e r p e n d i c u l a r t o t h e subject. T o place t h e f i n g e r s on these keys, subjects must turn t h e i r hands i n w a r d .

T.G. Reeve and R. W. Proctor

168

Table 3 A n Example o f t h e V e r t i c a l Stimulus a n d Response Arrangements, With T a r g e t I n d i c a t i n g t h e T o p Response Location

Stimulus arrangement Warning

Precue

+ +

Hand placement

Target

+

Adjacent LM LI RM RI

Overlapped LM RM LI RI

L = l e f t hand; R = right hand; M = middle f i n g e r , I = i n d e x Note. finger. T h e response arrangements a r e indicated f o r t h e l e f t h a n d b e i n g assigned t o t h e t o p position. With t h e right h a n d on top, t h e h a n d assignments a r e r e v e r s e d . F o r t h e adjacent placement w i t h t h e v e r t i c a l arrangement, t h e t w o f i n g e r s f r o m one h a n d a r e placed on t h e t o p t w o response locations, a n d t h e t w o f i n g e r s f r o m t h e o t h e r h a n d a r e placed o n t h e bottom t w o locations (see Table 3). F o r t h e o v e r l a p p e d placement, t h e f i n g e r s f r o m t h e t w o hands a r e alternated. However, because one h a n d is n o t p h y s i c a l l y o n t o p o f t h e other, as is t h e case w i t h t h e horizontal response a r r a n g e ment, t h e f i n g e r movements a r e n o t o b s t r u c t e d . With t h e v e r t i c a l arrangement, no o v e r a l l d i f f e r e n c e i n R T e x i s t e d between t h e t w o h a n d placements (see F i g u r e 2 ) . Thus, t h e goal of eliminating t h e problem o f o v e r a l l R T differences between t h e adjacent a n d overlapped placements was met. Yet, t h e p a t t e r n o f p r e c u i n g benefits s t i l l followed t h e spatial locations. T h a t is, as indicated by t h e c r o s s i n g o f t h e filled-symbol f u n c t i o n s in F i g u r e 2 , t h e maximum b e n e f i t was obtained when e i t h e r t h e t w o top-most o r t w o bottom-most locations were precued, regardless o f w h e t h e r t h e f i n g e r s were f r o m t h e same o r d i f f e r e n t hands. Thus, w i t h b o t h horizontal a n d v e r t i c a l arrangements, t h e spatial locations o f stimuli a n d responses a r e c r u c i a l t o t h e p a t t e r n s o f r e l a t i v e p r e c u i n g benefits. A d d i t i o n a l evidence f o r spatial coding is apparent in a comparison o f t h e p r e c u i n g p a t t e r n s obtained w i t h t h e horizontal a n d v e r t i c a l arrangements. With t h e horizontal stimulus a n d response arrangements, t h e p r e p a r e d : f i n g e r condition corresponds t o t h e t w o innermost a n d t w o outermost locations, regardless o f w h e t h e r t h e h a n d placement is adjacent o r overlapped. Thus, even t h o u g h t h e r e l a t i v e benefits f o r t h e p r e p a r e d : h a n d a n d p r e p a r e d : n e i t h e r conditions a r e switched f o r t h e t w o h a n d placements, t h e p r e p a r e d : f i n g e r condition is intermediate f o r b o t h placements (Reeve & Proctor, 1984). I n contrast, w i t h t h e v e r t i c a l stimu l u s a n d response arrangements, t h e prepared: n e i t h e r condition c o r r e sponds t o t h e t w o innermost a n d t w o outermost locations. With t h i s arrangement, t h e p r e p a r e d : n e i t h e r condition shows intermediate p r e c u i n g

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Prepored:Hond

72 0

4

Prepored: Finger

H

Prepared:Neither

A 0

Unprepared

E w 700

3E

c.

I

ADJACENT

OVERLAPPED

Figure 2. Mean RTs w i t h t h e v e r t i c a l stimulus a n d response arrangements as a f u n c t i o n o f h a n d placement a n d p r e p a r a t i o n condition ( f r o m Proctor & Reeve, 1986, Experiment 1). b e n e f i t s for b o t h t h e adjacent a n d overlapped h a n d placements ( P r o c t o r & Reeve, 1986). Therefore, t h e r e s u l t s indicate t h a t p r e c u i n g t h e t w o innermost o r t w o outermost locations produces an intermediate benefit, regardless o f t h e specific p a i r s o f f i n g e r s t h a t a r e cued. A l t h o u g h spatial coding of stimulus and response sets predominates, some f l e x i b i l i t y e x i s t s in t h a t t h e d i s t i n c t i o n between t h e hands can b e used in t h e coding o f t h e response set. Hand coding was a p p a r e n t in t h e experiments t h a t used v e r t i c a l stimulus a n d response sets. T h a t is, although t h e r e l a t i v e p r e c u i n g b e n e f i t s in those experiments w e r e p r i m a r ily a f u n c t i o n o f spatial locations, t h e top-bottom advantage was l a r g e r f o r t h e adjacent placement t h a n f o r t h e o v e r l a p p e d placement. In other words, t h e advantage was l a r g e r when t h e t w o c u e d f i n g e r s were f r o m t h e same h a n d (see F i g u r e 2 ) . T h e l a r g e r top-bottom advantage f o r t h e adjacent-hand placement l i k e l y i s d u e t o t h e relation between t h e h a n d d i s t i n c t i o n a n d t h e t o p bottom spatial d i s t i n c t i o n . T h e h a n d d i s t i n c t i o n c o u l d inhibit t h e t o p bottom advantage w i t h t h e overlapped placement because t h e h a n d d i s t i n c t i o n i s inconsistent w i t h t h e top-bottom d i s t i n c t i o n . A l t e r n a t i v e l y , t h e h a n d d i s t i n c t i o n c o u l d f a c i l i t a t e t h e top-bottom advantage w i t h t h e adjac e n t placement because t h e t w o d i s t i n c t i o n s a r e consistent. We conducted

170

T.G. Reeve and R. W. Proctor

t w o experiments t o d i s t i n g u i s h between these alternatives ( P r o c t o r & Reeve, 1986). In one experiment, a single-hand placement was used, in w h i c h subjects responded w i t h t h e f o u r f i n g e r s o f t h e i r right hand; t h e f i n g e r s were aligned v e r t i c a l l y as f o r t h e o t h e r h a n d placements. This single-hand placement showed a top-bottom advantage o f similar magnitude t o t h a t obtained w i t h t h e overlapped placement, t h u s indicatirig t h a t t h e h a n d d i s t i n c t i o n did n o t h a v e an i n h i b i t o r y e f f e c t f o r t h e l a t t e r placement. In t h e o t h e r experiment, an adjacent-side placement was used f o r some subjects. These subjects placed b o t h hands t o t h e right side o f t h e body, in a normal, adjacent manner. T h e logic o f t h i s placement was t h a t t h e v e r t i c a l correspondence between stimulus a n d response locations was maintained, but t h e hands n o longer h a d t o b e t u r n e d i n w a r d . T h i s adjacent-side placement y i e l d e d a smaller top-bottom advantage t h a n did t h e adjacent placement w i t h t h e hands t u r n e d i n w a r d . T h i s finding indicates t h a t t h e h a n d d i s t i n c t i o n h a d a f a c i l i t a t o r y e f f e c t f o r t h e adjacent placement. O u r i n t e r p r e t a t i o n o f t h i s finding is t h a t turning t h e hands i n w a r d makes t h e h a n d d i s t i n c t i o n more salient. T h i s saliency o f t h e hands r e s u l t s in an added p r e c u i n g b e n e f i t when it corresponds t o t h e salient top-bottom spatial f e a t u r e o f t h e stimulus a n d response sets.

Because b o t h t h e horizontal a n d v e r t i c a l arrangements t h a t we have used in t h e p r e c u i n g studies a r e linear, an i n t e r e s t i n g issue is how a stimulus set in one orientation is t r a n s l a t e d t o a response set in t h e o t h e r . We r e c e n t l y conducted an experiment in w h i c h t h e orientations o f t h e stimulus a n d response arrangements were manipulated factorially, w i t h o n l y adjacent h a n d placements b e i n g used (Proctor & Reeve, 1988a). In all situations, a p r e c u i n g advantage was obtained when e i t h e r t h e t w o leftmost (bottom-most) o r t w o r i g h t m o s t ( t o p m o s t ) locations were cued. A mismatch in stimulus- a n d response-set orientations added o n l y a constant t o RTs, w i t h o u t c h a n g i n g t h e p a t t e r n o f p r e c u i n g benefits t h a t was obtained. Thus, an additional orientation t r a n s l a t i o n was r e q u i r e d when t h e orientations o f t h e stimulus a n d response sets d i f f e r e d , but t h i s t r a n s l a t i o n was independent o f t h e spatial coding t h a t produces t h e relative precuing benefits. In summary, t h e r e s u l t s f r o m t h e s p a t i a l - p r e c u i n g t a s k s a r e cons i s t e n t w i t h those f r o m t h e two-choice reaction t a s k s in showing t h e stimu l u s a n d response locations t o b e important. T h e coding o f t h e stimulus a n d response sets occurs on t h e basis o f salient features o f t h e linear arrangements, w i t h t h e predominant f e a t u r e b e i n g t h e l e f t - r i g h t spatial locations. Additionally, t h e d i s t i n c t i o n between t h e t w o hands can b e used f o r coding. Hand c o d i n g i s e v i d e n t when the h a n d d i s t i n c t i o n is made more salient by turning t h e hands inward, but o n l y when t h e h a n d d i s t i n c t i o n coincides w i t h t h e salient spatial f e a t u r e .

Coded Representations o f Symbolic Stimuli a n d Spatial- Location Responses

Discussions o f S-R compatibility t y p i c a l l y s t r e s s t h e relation between t h e spatial locations o f stimuli a n d responses. Thus, most studies o f S-R compatibility t h a t have used symbolic stimuli (such as color patches o r l e t t e r s ) a n d keypress responses have presented t h e stimuli in d i f f e r e n t locations a n d examined t h e influence of t h e i r r e l e v a n t location d i s t i n c t i o n (e.g., Simon, 1969). A s a consequence, the

The Salient-Features Coding Principle p o s s i b i l i t y t h a t some assignments of symbolic stimuli responses may b e more compatible t h a n o t h e r s has t e n d e d looked (see F i t t s & Biederman, 1965, f o r an exception). have obtained evidence f o r such compatibility e f f e c t s f r o m have used two-dimensional, symbolic stimuli.

171 to keypress t o be overHowever, we studies t h a t

I n one experiment (Proctor & Reeve, 19851, t h e stimuli w e r e consonant-vowel p a i r s c o n s t r u c t e d f r o m t h e consonants B a n d M a n d t h e vowels E a n d 0, w i t h t h e l e f t - t o - r i g h t assignment o f stimuli t o response f i n g e r s b e i n g BE, 80, ME, a n d MO (see also Miller, 1982). In t h i s experiment, t h e p r e c u e consisted o f p r e s e n t i n g one letter, w i t h t h e second l e t t e r t h e n presented a f t e r a v a r i a b l e p r e c u i n g i n t e r v a l . A s in t h e p r e v i o u s l y described s p a t i a l - p r e c u i n g experiments, an R T advantage f o r t h e p r e p a r e d : h a n d condition was obtained when t h e hands were adjacent (see Figure 3 ) . T h a t is, responses were f a s t e r when t h e consonant, which precues f i n g e r s f r o m e i t h e r t h e l e f t o r right h a n d ( w h i c h a r e o n t h e t w o leftmost a n d t w o r i g h t m o s t response locations, respectively), was presented f i r s t . Moreover, when t h e hands were overlapped, allowing t h e consonant s t i l l t o cue t h e t w o leftmost o r t w o r i g h t m o s t locations, but w i t h t h e cued f i n g e r s now b e i n g on d i f f e r e n t hands, a c o n s o n a n t - f i r s t advantage s t i l l was obtained. Thus, as w i t h t h e spatial-precuing tasks, spatial locations a r e implicated as b e i n g c r u c i a l i n t h e c o d i n g o f stimuli t o responses in t h i s symbolic-precuing t a s k . The differential precuing benefits obtained w i t h t h e consonant-vowel stimuli seem t o b e S-R compatibility effects of t h e t y p e obtained w i t h spatial-location stimuli.

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F i g u r e 3. Mean RTs w i t h t h e BEBOMEMO assignment as a f u n c t i o n o f l e t t e r presented f i r s t a n d h a n d placement ( f r o m Proctor E Reeve, 1985, Experiment 1 ) . Evidence for spatial coding also was apparent when b o t h features of two-dimensional, symbolic stimuli were p r e s e n t e d simultaneously, a n d t h e relation t o spatial locations was v a r i e d by manipulating S-R assignments. In these experiments (Proctor & Reeve, 19851, t h e stimuli were composed f r o m t w o l e t t e r identities (0, Z) o f t w o sizes (large, small). Pilot experiments showed t h a t l e t t e r i d e n t i t y was more salient t h a n size. T h a t

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is, a l e t t e r discrimination could b e made more r a p i d l y t h a n a size d i s crimination. U n l i k e t h e experiments described t o t h i s point, t h e e x p e r i ments u s i n g t h e letter/size stimuli did n o t i n v o l v e an o v e r t precue. T h e subject's t a s k simply was t o i d e n t i f y t h e stimulus that o c c u r r e d by making t h e assigned keypress response. T h e manipulation o f importance i n v o l v e d t h e assignments o f stimuli t o responses. For one assignment, l e t t e r i d e n t i t y distinguished between t h e t w o t h e OozZ assignment, leftmost a n d t w o rightmost response locations, whereas f o r t h e o t h e r With the assignment, t h e OzoZ assignment, it did n o t (see F i g u r e 4). adjacent-hand placement, t h e results were similar t o those o f earlier experiments. Responses were f a s t e r w i t h t h e OozZ assignment t h a n w i t h t h e OzoZ assignment, i n d i c a t i n g an advantage when t h e salient stimulus feature, l e t t e r identity, d i s t i n g u i s h e d between t h e t w o leftmost a n d t w o However, with the rightmost response locations (see F i g u r e 5). overlapped placement, t h e RTs were equivalent f o r t h e OozZ and OzoZ assignments.

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F i g u r e 5. Mean RTs f o r t h e OozZ a n d OzoZ assignments as a f u n c t i o n o f h a n d placement ( P r o c t o r & Reeve, 1985, Experiment 2 ) . A c r u c i a l d i f f e r e n c e exists between t h e adjacent a n d overlapped With t h e overlapped placement, placements f o r t h e OzoZ assignment. l e t t e r i d e n t i t y distinguishes between responses on t h e t w o hands. Howe v e r , w i t h t h e adjacent placement, l e t t e r i d e n t i t y does n o t d i s t i n g u i s h between e i t h e r t h e hands o r t h e t w o leftmost a n d t w o r i g h t m o s t locations. Because responses a r e no slower f o r t h e OzoZ assignment t h a n f o r t h e OozZ assignment when hands a r e overlapped, h a n d coding a p p a r e n t l y is used w i t h t h e OzoZ assignment To t e s t t h i s account, we i n c l u d e d a h a n d placement t h a t removed t h e d i s t i n c t i o n between hands. For this placement, subjects responded w i t h t h e f o u r f i n g e r s o n a single hand. With t h e single-hand placement, t h e 0022 assignment again showed a l a r g e advantage o v e r t h e OzoZ assignment (see F i g u r e 6). Our conclusion, then, is t h a t S-R compatibility is greatest when t h e r e is a consistent relation between t h e salient stimulus f e a t u r e ( l e t t e r i d e n t i t y ) a n d e i t h e r t h e salient l e f t - r i g h t f e a t u r e o f t h e response arrangement o r the hand distinction. Additional s u p p o r t f o r h a n d coding was obtained in another p r e c u ing experiment t h a t used a d i f f e r e n t mapping o f t h e consonant-vowel stimulus set described p r e v i o u s l y ( P r o c t o r & Reeve, 1985). Because M i l l e r (1982) h a d f o u n d a c o n s o n a n t - f i r s t advantage when t h e l e f t - t o - r i g h t assignment was BE, BO, ME, a n d MO, we h a d i n i t i a l l y evaluated o n l y t h a t assignment. T h e a l t e r n a t i v e assignment, f o r w h i c h Miller h a d n o t f o u n d a c o n s o n a n t - f i r s t advantage, was BE, ME, 6 0 , a n d MO. Miller concluded that, w i t h t h i s BEMEBOMO assignment, t h e c o n s o n a n t - f i r s t advantage was n o t obtained because t h e consonants specified d i f f e r e n t f i n g e r s on d i f f e r e n t hands. However, w i t h an o v e r l a p p e d - h a n d placement, t h e consonants

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F i g u r e 6. Mean RTs f o r t h e OozZ a n d OzoZ assignments as a f u n c t i o n o f h a n d placement ( P r o c t o r & Reeve, 1985, Experiment 3 ) . would c o r r e s p o n d t o t h e h a n d d i s t i n c t i o n b u t n o t t o t h e l e f t - r i g h t spatial d i s t i n c t i o n . Thus, if h a n d coding can b e used independently f r o m spatial coding, it should b e e v i d e n t when t h e BEMEBOMO assignment is used w i t h t h e overlapped placement. T h r e e aspects o f t h e r e s u l t s obtained w i t h t h e BEMEBOMO assignment a r e o f importance. F i r s t , in c o n t r a s t t o o u r o t h e r experiments, R T s did n o t d i f f e r r e l i a b l y f o r t h e adjacent- a n d overlapped-hand placements, w i t h o n l y a t e n d e n c y f o r responses t o b e f a s t e r w i t h t h e f o r m e r placement (compare t h e o v e r a l l levels o f t h e f u n c t i o n s in F i g u r e 7 ) . This relative lack o f a d i f f e r e n c e was p r e d i c t e d because t h e salient consonant f e a t u r e o f t h e stimulus set corresponds w i t h t h e l e f t - r i g h t h a n d d i s t i n c t i o n f o r t h e o v e r l a p p e d placement, but w i t h n e i t h e r t h e l e f t - r i g h t h a n d n o r t h e spatial d i s t i n c t i o n f o r t h e adjacent placement. I n fact, t h e BEMEBOMO assignment is comparable, i n terms o f t h e relation o f t h e salient stimulus f e a t u r e t o t h e response set, w i t h t h e OzoZ assignment used f o r t h e Similarly, t h e BEBOMEMO l e t t e r / s i z e stimuli in o t h e r experiments. assignment described p r e v i o u s l y is comparable w i t h t h e OozZ assignment. Comparison o f t h e r e s u l t s obtained w i t h t h e BEBOMEMO a n d BEMEBOMO assignments shows t h a t t h e p a t t e r n of r e s u l t s is equivalent t o t h a t f o r t h e OozZ a n d OzoZ assignments, p r o v i d i n g f u r t h e r evidence t h a t coding can o c c u r o n t h e basis o f t h e h a n d d i s t i n c t i o n . Second, a c o n s o n a n t - f i r s t advantage was obtained when t h e BEMEBOMO assignment was used w i t h t h e overlapped-placement (see t h e right panel o f F i g u r e 7 ) . T h i s advantage was o f a similar magnitude t o t h a t obtained when t h e BEBOMEMO assignment was used p r e v i o u s l y w i t h t h e adjacent placement (see t h e l e f t panel o f F i g u r e 3 ) . Thus, t h e c o n s o n a n t - f i r s t advantage can b e obtained when t h e salient consonant f e a t u r e o f t h e stimulus set corresponds e i t h e r t o t h e l e f t - r i g h t spatial d i s t i n c t i o n o r t o t h e h a n d d i s t i n c t i o n o f t h e response set.

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LETTER PRESENTED FIRST F i g u r e 7. Mean RTs w i t h t h e BEMEBOMO assignment as a f u n c t i o n o f l e t t e r presented f i r s t a n d h a n d placement ( f r o m Proctor & Reeve, 1985, Experiment 4 ) . T h i r d , a c o n s o n a n t - f i r s t advantage also was obtained when t h e BEMEBOMO assignment was used w i t h t h e adjacent-hand placement, although t h e advantage t e n d e d t o b e smaller in magnitude. This advantage was unexpected because Miller (1982) h a d n o t obtained it a n d because t h e r e was no correspondence between t h e salient stimulus f e a t u r e a n d a salient response f e a t u r e . However, t h e c o n s o n a n t - f i r s t advantage t h a t we obtained f o r t h e BEMEBOMO assignment w i t h t h e adjacent placement was s i g n i f i c a n t l y smaller t h a n t h a t obtained p r e v i o u s l y f o r t h e BEBOMEMO assignment w i t h t h a t placement. Thus, t h e r e s u l t s indicate t h a t p r e c u i n g benefits a r e obtained when t h e p r e c u e i s t h e salient f e a t u r e o f t h e stimulus set, but t h e benefits a r e g r e a t e r when t h i s salient stimulus f e a t u r e corresponds w i t h a salient f e a t u r e of t h e response set. In summary, t h e r e s u l t s f r o m t h e studies t h a t used symbolic stimuli show p a t t e r n s similar t o those obtained w i t h spatial-location stimuli. Responses a r e fastest when t h e salient f e a t u r e o f t h e stimulus set corresponds t o t h e salient l e f t - r i g h t f e a t u r e o f t h e response set. Moreover, a correspondence w i t h t h e h a n d f o r responding i s used when t h e salient stimulus f e a t u r e distinguishes between t h e hands but n o t between t h e t w o leftmost a n d t w o r i g h t m o s t response locations.

Practice a n d T r a n s f e r Effects Translation processes customarily a r e assumed t o p l a y a decreasing Teichner E role in performance as subjects become p r a c t i c e d (e.g., Krebs, 1974). Thus, if t h e d i f f e r e n t i a l p a t t e r n s o f RTs w i t h t h e spatiallocation a n d symbolic stimuli r e f l e c t t r a n s l a t i o n processes, t h e p a t t e r n s should disappear w i t h practice. We i n i t i a l l y tested t h i s p r e d i c t i o n w i t h t h e spatial-precuing t a s k (Proctor & Reeve, 1988b). Subjects received t h r e e sessions o f 310 t r i a l s , w i t h e i t h e r t h e adjacent o r overlapped-hand placements. T h e r e s u l t s o f session 1 replicated b o t h t h e o v e r a l l slower RTs f o r t h e overlapped placement a n d t h e location-determined p a t t e r n o f d i f f e r e n t i a l p r e c u i n g benefits. I n t h e third session, RTs s t i l l were

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slower f o r t h e o v e r l a p p e d placement, a l t h o u g h t h e magnitude o f t h e More d i f f e r e n c e was less t h a n in t h e f i r s t session (see F i g u r e 8). importantly, in t h e third session, a l l p r e c u i n g conditions w i t h i n each o f t h e h a n d placements showed equivalent p r e c u i n g benefits r e l a t i v e t o t h e In o t h e r words, t h e l e f t - r i g h t u n p r e p a r e d condition (see F i g u r e 8). advantage t h a t is. obtained in a single session i s n o longer e v i d e n t following p r a c t i c e .

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SESSION F i g u r e 8. Mean R T s as a f u n c t i o n p r e p a r a t i o n condition, h a n d placement, a n d session (from P r o c t o r & Reeve, 1988b, Experiment 1 ) . To evaluate f u r t h e r t h e n a t u r e o f t h e p r a c t i c e effects, experiments In w e r e conducted t h a t used t r a n s f e r designs (Proctor 8 Reeve, 198813). these experiments, subjects p a r t i c i p a t e d in t h r e e sessions u s i n g e i t h e r In t h e adjacent o r overlapped placement, as in t h e p r e v i o u s experiment. a f o u r t h session, h a l f of t h e subjects were t r a n s f e r r e d t o t h e a l t e r n a t i v e h a n d placement. T h i s t r a n s f e r maintains t h e spatial correspondence between stimuli a n d response locations f r o m t h e e a r l i e r sessions, but a l t e r s t h e assignment o f f i n g e r s t o response locations. Thus, if p r a c t i c e r e s u l t s in t h e acquisition o f procedures t h a t d i r e c t l y link t h e stimuli t o

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f i n g e r s , t h e n t h e p a t t e r n o f d i f f e r e n t i a l p r e c u i n g b e n e f i t s should b e reinstated in t h e t r a n s f e r session. A l t e r n a t i v e l y , t h e p a t t e r n should n o t reappear if p r a c t i c e r e s u l t s in more e f f i c i e n t spatial codings o f t h e stimulus a n d response sets. T h e r e s u l t s obtained when subjects t r a n s f e r r e d f r o m t h e overlapped t o t h e adjacent placement were unambiguous. The pattern of differential p r e c u i n g b e n e f i t s was reinstated, w i t h t h e magnitude b e i n g comparable t o t h a t obtained in an i n i t i a l session. T h e p a t t e r n also t e n d e d t o b e reinstated when subjects t r a n s f e r r e d f r o m t h e adjacent t o t h e overlapped placement, but t h e magnitude o f t h e differences was less t h a n in an i n i t i a l session a n d was nonsignificant. However, an additional experiment i n w h i c h subjects t r a n s f e r r e d f r o m t h e adjacent placement t o a- crossedhands placement (in w h i c h t h e l e f t - t o - r i g h t o r d e r i n g of f i n g e r s was right index, r i g h t middle, l e f t middle, a n d l e f t i n d e x ) showed a complete reinstatement. A p p a r e n t l y , subjects did r e v e r t t o t r a n s l a t i v e c o d i n g operations when t r a n s f e r r e d t o t h e overlapped placement, but t h e y i n c o r p o r a t e d t h e h a n d f e a t u r e i n t o t h e coding o f t h e response set t o b e n e f i t t h e normally less compatible conditions. Thus, o u r conclusion i s t h a t t h e salient-features coding used t o t r a n s l a t e between stimuli a n d responses i s no longer used when procedures t h a t d i r e c t l y relate stimuli t o response f i n g e r s have been acquired. However, when t h e assignment of f i n g e r s t o locations i s changed, a n d t h e a c q u i r e d procedures a r e n o longer applicable, subjects r e v e r t t o coded representations. More recently, we have b e g u n i n v e s t i g a t i n g p r a c t i c e effects w i t h t h e symbolic stimulus sets. In an i n i t i a l experiment, we have examined t h e change i n t h e R T s f o r subjects who used t h e OozZ assignment o r t h e OzoZ assignment o f t h e l e t t e r / s i z e stimuli. In t h e f i r s t session, R T s f o r t h e subjects w i t h t h e OozZ assignment were approximately 90 ms f a s t e r t h a n those w i t h t h e OzoZ assignment, r e p l i c a t i n g t h e d i f f e r e n c e obtained p r e v i o u s l y ( P r o c t o r & Reeve, 1985). However, by t h e third session, t h e d i f f e r e n c e was eliminated, b e i n g o n l y a n o n s i g n i f i c a n t 2 ms. T h e p r a c t i c e e f f e c t w i t h t h e symbolic stimuli again suggests t h a t t h e i n i t i a l d i f f e r e n c e between t h e OozZ a n d OzoZ assignments reflects t r a n s l a t i o n processes t h a t p l a y a decreasing role as subjects become p r a c t i c e d . In summary, t h e r e s u l t s f r o m these studies demonstrate t h e role o f t r a n s l a t i o n processes diminishes w i t h practice. The results from the t r a n s f e r studies indicate t h a t subjects r e v e r t t o t h e t r a n s l a t i o n processes when t h e assignment o f f i n g e r s t o locations is changed. Thus, when t h e relation between stimuli a n d responses is novel, t r a n s l a t i o n processes based on coded representations a r e used t o p e r f o r m t h e choice-reaction task. Salient-Features C o d i n g P r i n c i p l e O u r research p r o g r a m p r o v i d e s a coherent p i c t u r e o f S-R compatibility effects t h a t incorporates spatial-location stimuli, symbolic stimuli, o v e r t precues, a n d stimulus-response assignments. A t short p r e c u i n g intervals, t h e r e l a t i v e p r e c u i n g benefits f o r t h e d i f f e r e n t p a i r s o f responses a r e a f u n c t i o n o f S - R t r a n s l a t i o n processes, o r in o t h e r words, o f how f a s t t h e responses indicated by t h e c u e can b e determined. Moreover, similar t r a n s l a t i o n processes appear t o account f o r assignment effects when two-dimensional symbolic stimuli a r e assigned t o keypress responses. In a l l situations, t h e t r a n s l a t i o n processes operate

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according t o a salient-features c o d i n g p r i n c i p l e . This principle is that t h e stimulus a n d response sets a r e coded i n terms o f t h e salient features o f each, w i t h response determination o c c u r r i n g most r a p i d l y when t h e salient features o f t h e r e s p e c t i v e sets correspond. With spatial-location stimuli, spatial c o d i n g predominates. The c o d i n g is based on a h i e r a r c h y of salience f o r t h e spatial locations o f t h e patterns. T h e most salient f e a t u r e f o r linear a r r a y s is location r e l a t i v e t o center. Regardless o f w h e t h e r t h e linear a r r a y s a r e o r i e n t e d h o r i z o n t a l l y o r vertically, o r o f w h e t h e r t h e orientations o f t h e stimulus a n d response arrangements a r e consistent o r inconsistent, precues t h a t indicate t h e p a i r s o f locations t o e i t h e r side o f c e n t e r a r e most effective. T h e t w o i n n e r a n d t w o o u t e r locations a r e o f intermediate salience, w i t h t h e a l t e r n a t i n g locations b e i n g less salient. Also, coding can b e based o n t h e d i s t i n c t i o n between t h e hands, when t h e salience o f t h e h a n d d i s t i n c t i o n is increased a n d is consistent w i t h t h e salient spatial distinction. With two-dimensional, symbolic stimuli, t h e coding is in terms o f salient features o f t h e stimulus set a n d t h e correspondence w i t h t h e salient f e a t u r e o f t h e response set. Responses a r e fastest when t h e salient stimulus f e a t u r e discriminates t h e t w o leftmost a n d t w o r i g h t m o s t response locations o f a horizontal response arrangement. Similarly, a p r e c u e consisting o f t h e salient f e a t u r e o f t h e stimulus set i s most beneficial w i t h such an assignment. Hand coding i s used when t h e salient f e a t u r e o f t h e symbolic stimulus corresponds t o t h e d i s t i n c t i o n between t h e t w o hands but n o t t o t h e l e f t - r i g h t spatial d i s t i n c t i o n . T h e r e s u l t s obtained by u s w i t h spatial-location a n d symbolic stimuli in four-choice t a s k s a r e g e n e r a l l y consistent w i t h t h e r e s u l t s obtained in two-choice t a s k s . In b o t h situations, t h e coding is predominantly spatial, a l t h o u g h evidence f o r h a n d coding is a p p a r e n t in c e r t a i n circumstances (e.g., Ladavas & Moscovitch, 1984). Moreover, o u r studies p r o v i d e an i m p o r t a n t extension o f t h e two-choice f i n d i n g s t o more complex situations a n d indicate how compatibility effects can arise f r o m stimulus a n d response sets t h a t v a r y on seemingly u n r e l a t e d dimensions. A coherent theoretical framework f o r S - R compatibility effects has y e t t o emerge f r o m t h e e x t e n s i v e research on t h e topic. T h e salientfeatures c o d i n g p r i n c i p l e p r o v i d e s a framework t h a t p o t e n t i a l l y could s e r v e t h i s o r g a n i z i n g role. If so, t h e p r i n c i p l e represents a s i g n i f i c a n t advance in o u r knowledge r e g a r d i n g t h e fundamental c o g n i t i v e processes t h a t u n d e r l i e compatibility effects. Acknowledgement Preparation o f t h i s c h a p t e r was s u p p o r t e d in p a r t by g r a n t AFOSR88-0002 f r o m t h e A i r Force O f f i c e of Scientific Research.

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References (1965). S-R compatibility a n d information Fitts, P. M., & Biederman, I . reduction. Journal of Experimental Psychology, 69, 408-412. (1953). S-R compatibility: Spatial c h a r Fitts, P. M., & Seeger, C . M. Journal of Experlmental acteristics o f stimulus a n d response codes. Psychology, 46, 199-210. Garvey, W. D., & Knowles, W. B. (1954). Response time p a t t e r n s assoJournal of Experlciated w i t h v a r i o u s d i s p l a y - c o n t r o l relationships. mental Psychology, 47, 315-322. Goodman, D., E Kelso, J. A. S . (1980). A r e movements p r e p a r e d in parts? Not u n d e r compatible (naturalized) conditions. Journal of Experimental Psychology: General, 109, 475-495. LBdavas, E., & Moscovitch, M. (1984). M u s t egocentric a n d e n v i r o n mental frames o f reference b e a l i g n e d t o p r o d u c e spatial S-R compatibility effects? Journal of Experimental Psychology: Human Perception and Performance, 10, 205-215. Larish, D. D . (1986). I n f l u e n c e o f stimulus-response t r a n s l a t i o n o n response programming: Examining t h e relationship o f arm, direction, a n d e x t e n t o f movement. Acta Psychologica, 61, 53-70. Miller, J. (1982). Discrete v e r s u s continuous models of human informat i o n processing: In search o f p a r t i a l o u t p u t . Journal of Experimental Psychology: Human Perception and Performance, 8 , 273-296. (1985). A h a n d advantage in p r e p a r a t i o n o f simple keypress Miller, J. Journal of Experiresponses: Reply t o Reeve a n d Proctor (1984). mental Psychology: Human Perception and Performance, 1 1 , 221 -233. Nicoletti, R . , Anzola, G. P. Luppino, G . , Rizzolatti, G., & Umilta, C . (1982). Spatial compatibility effects on t h e same side o f t h e bodymidl ine. Journal of Experimental Psychology: Human Perception and Performance, 8 , 664-673. Proctor, R. W., & Reeve, T. G . (1985). Compatibility effects in t h e assignment o f symbolic stimuli t o d i s c r e t e f i n g e r responses. Journal of Experimental Psychology: Human Perception and Performance, 7 1 , 623-639. Salient-feature c o d i n g operaProctor, R. W., & Reeve, T. G . (1986). Journal of Experimental Psychology: t i o n s in spatial p r e c u i n g t a s k s . Human Perception and Performance, 12, 277-285. (1988a, A u g u s t ) . Correspondence of Proctor, R. W., & Reeve, T. G . stimulus and response set orientations i n preculng tasks. Poster p r e sented a t t h e 96th annual convention o f t h e American Psychological Association, Atlanta, GA. T h e acquisition o f task-speProctor, R. W., & Reeve, T . G . (1988b). c i f i c p r o d u c t i o n s a n d modification o f declarative representations in s p a t i a l - p r e c u i n g t a s k s . Journal of Experimental Psychology: General, 117, 182-196.

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Reeve, T . G., & Proctor, R . W. (1984). On t h e advance p r e p a r a t i o n of d i s c r e t e f i n g e r responses. Journal of Experimental Psychology: Human Perception and Performance, 10, 541 -553. (1985). Non-motoric t r a n s l a t i o n p r o Reeve, T. G . , & Proctor, R . W. cesses in t h e p r e p a r a t i o n o f d i s c r e t e f i n g e r responses: A r e b u t t a l o f Journal of Experimental Psychology: Human Miller's (1985) analysis. Perception and Performance, 1 1 , 234-240. Reeve, T . G . , & Proctor, R . W. (1988). Determinants o f two-choice reaction-time p a t t e r n s f o r same-hand a n d d i f f e r e n t - h a n d f i n g e r p a i r i n g s . Journal of Motor Behavior, 20, 317-340. Rosenbaum, D. A . (1983). T h e movement p r e c u i n g technique: AssumpI n R. A. Magill (Ed.), Memory tions, applications, a n d extensions. and control of action (pp. 231-274). Amsterdam: N o r t h Holland. Simon, J. R . (1969). Reactions t o w a r d t h e source o f stimulation. nal of Experimental Psychology, 81, 174-176.

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U m i l t i , C., & Nicoletti, R . (1985). A t t e n t i o n a n d c o d i n g effects in S-R compatibility d u e t o i r r e l e v a n t spatial cues. In M. I . Posner t 0. S . M. M a r i n (Eds.), Attention and performance X I (pp. 457-471). Hillsdale, NJ: Lawrence Erlbaum. Wallace, R . J. (1971). S-R compatibility a n d t h e idea of a response code. Journal of Experimental Psychology, 88, 354-360.

P A R T Ill PSYCHOPHYSIOLOGICAL INDICES A N D NEUROPHYSIOLOGICAL MECHANISMS T h e f o u r chapters o f P a r t I l l examine t h e relation between stimulus-response compatibility a n d psychophysiological measures o f b r a i n a c t i v i t y , as well as t h e neurophysiological mechanisms t h a t may u n d e r l i e compatibility effects. C h a p t e r 7, by Bashore, examines t h e role o f s t imuIus - response cornp a t ibiI it y in meas u res o f interhemis ph e r ic t r a nsmis sion time. It also p r o v i d e s a t u t o r i a l on t h e use o f event-related b r a i n potentials a n d discusses t h e ways in w h i c h s u c h potentials illuminate t h e n a t u r e o f compatibility effects. Chapter 8, by Ragot, describes r e c e n t studies in w h i c h situations t h a t p r o d u c e similar compatibility effects on reaction time y i e l d disparate effects on e v e n t - r e l a t e d potentials. The c h a p t e r discusses when a n d w h y e v e n t - r e l a t e d potentials can b e used t o discriminate t h e bases f o r compatibility effects. C h a p t e r 9, by B r e b n e r , examines t h e effects on performance o f v a r i o u s recoding processes a n d attempts t o relate p a r t i c u l a r electrophysiological a c t i v i t y t o such p r o cesses. C h a p t e r 10, by Verfaellie, Bowers, a n d Heilman, compares spatial coding a n d attentional hypotheses f r o m a neuropsychological perspective.

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STIMULUS-RESPONSE COMPATI 6IL lT Y VIEWED FROM A COGNITIVE PSYCHOPHYSIOLOGICAL PERSPECTIVE THEODORE R . BASHORE Department o f P s y c h i a t r y T h e Medical College o f Pennsylvania a t EPPl T h e f i r s t systematic studies o f t h e relation between t h e elements o f a stimulus d i s p l a y and t h e responses r e q u i r e d t o them were u n d e r t a k e n in t h e e a r l y 1950s (e.g., F i t t s & Seeger, 1953). T h e aim o f much o f t h i s w o r k was t o solve problems c o n f r o n t i n g t h e human engineer in designing stimulus-display/control-device configurations t h a t optimize t h e operator's performance. These configurations were conceptualized as v a r y i n g along a dimension o f stimulus-response ( S - R ) compatibility. Operationally, t h e degree o f S-R compatibility was defined on t h e basis o f response latency and accuracy. Configurations that p r o d u c e d t h e fastest response times and lowest e r r o r rates were defined as compatible, whereas configurations t h a t produced t h e slowest response times a n d h i g h e s t e r r o r rates were defined as incompatible. These differences in compatibility were i n t e r p r e t e d in information theoretic terms (i.e., i n b i t s o f information t r a n s m i t t e d o v e r t h e i n p u t channel). T h e pioneering research in t h e f i e l d was done by Paul F i t t s a n d h i s colleagues. I n t h e f i r s t p a p e r f r o m t h e group, F i t t s a n d Seeger (1953) defined a compatible response as one in which ". t h e ensemble o f stimulus a n d response combinations comprising t h e t a s k results,, in a high r a t e o f a information t r a n s f e r " (p. 199). T h e y made use o f . . . t h e idea o f a hypothetical process o f information transformation o r recoding in t h e course o f perceptual-motor a c t i v i t y . . ." which assumed t h a t ". . . t h e degree o f compatibility is a t a maximum when recoding processes a r e a t a minimum" (p. 199). It is important t o note t h a t F i t t s a n d Seeger regarded S - R compatibility effects as r e f l e c t i n g t h e degree ,,of correspondence between a stimulus set a n d a response set. Thus, . . . effect i v e performance depends t o a l a r g e e x t e n t upon t h e u n i q u e charact e r i s t i c s o f S - R ensembles r a t h e r t h a n on specific aspects o f p a r t i c u l a r stimulus or response sets" (p. 201).

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Although an e a r l y goal o f compatibility research was t o articulate t h e mechanisms o f information processing t h a t mediate S-R compatibility effects ( F i t t s & Deininger, 1954; F i t t s & Seeger, 19531, manipulations o f S-R compatibility have been used generally t o t e s t a n d r e f i n e models o f human information processing, r a t h e r t h a n t o f u r t h e r o u r u n d e r s t a n d i n g o f t h e processes u n d e r l y i n g S-R compatibility. T h i s may r e f l e c t a widely held belief t h a t S-R compatibility effects a r e mediated a t t h e response translation stage o f mental processing (see T e i c h n e r & Krebs, 1974, f o r example) a n d t h a t no f u r t h e r delineation is e i t h e r necessary o r possible. I n t h i s chapter, I a r g u e t h a t t h e concept o f S-R compatibility can b e r e f i n e d u s i n g c o g n i t i v e psychophysiological measures. I demonstrate how these measures n o t o n l y can help d i f f e r e n t i a t e t h e neuroanatomical f r o m t h e c o g n i t i v e constituents o f a reaction, b u t also can articulate t h e l a t t e r w i t h more precision t h a n can behavioral measures alone. One

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problem t h a t lends i t s e l f r a t h e r nicely t o such a n analysis is estimating t h e time r e q u i r e d t o convey information f r o m one cerebral hemisphere t o the other. T h e experimental procedures used t o a r r i v e a t estimates o f these transmission rates have revealed a dissociation between t h e simple reaction a n d more complex reactions, which suggests t h e utility o f electrophysiological measures in delineating t h e reaction processes t h a t u n d e r l i e S-R compatibility. Hence, my discussion w i l l begin w i t h a review o f t h e research t h a t has estimated interhemispheric transmission time. Because o f m y predilections, t h e b u l k o f t h i s c h a p t e r w i l l b e devoted t o reviewing t h e role o f S-R compatibility in visuo-motor reactions. Reaction Time Estimates

of Interhemispheric Transmission Time

T h e i n t e n t o f these studies is t o d e r i v e an estimate o f t h e time r e q u i r e d t o t r a n s m i t information f r o m one cerebral hemisphere t o t h e o t h e r v i a t h e commissural pathways connecting them (see review in Bashore, 1981). T h i s w o r k i s g u i d e d by t h e organizational characterist i c s o f t h e nervous system: Sensory i n p u t i s mediated p r i m a r i l y in t h e cerebral hemisphere opposite t h e stimulated side o f t h e body (via p a t h ways t h a t cross t h e neural axis during t h e i r ascent) a n d commands t o execute hand a n d f i n g e r movements o r i g i n a t e f r o m t h e hemisphere oppos i t e t h e responding d i s t a l limb ( v i a pathways t h a t cross t h e n e u r a l axis during t h e i r descent). T h e w o r k i s also motivated by t h e assumption t h a t t h e transmission o f simple sensory information and t h e execution o f uncomplicated manual responses, when b o t h c o g n i t i v e a n d response demands a r e minimal, a r e mediated o v e r f i x e d a n d reasonably well-isolated neuroanatomical pathways. T h e l a t t e r has led t o t h e use o f a s t r a i g h t f o r w a r d procedure, t h e subtraction method, t o estimate interhemispheric transmission time ( I H T T ) in behavioral studies. T h e experimental paradigm developed t o d e r i v e t h e estimate involves t h e presentation o f an u n p a t t e r n e d stimulus (e.g., p o i n t source o f light) i n t o t h e visual h a l f - f i e l d and t h e execution o f a simple, f i x e d manual response t o it (e.g., depression o f a response b u t t o n w i t h t h e right index f i n g e r ) . T h e reasoning, f i r s t enunciated by Poffenberger (1912), is t h a t if b o t h reception of t h e stimulus i n p u t a n d execution o f t h e motor o u t p u t occur w i t h i n t h e same hemisphere, t h e n reaction times (RTs) should b e s h o r t e r t h a n when cross-commissural communication is r e q u i r e d t o mediate t h e response f r o m t h e o t h e r hemisphere. T h e d i f f e r e n c e between t h e t w o response latencies i s t h o u g h t t o r e f l e c t t h e time r e q u i r e d t o convey information to t h e opposite hemisphere, f r o m which t h e motor command i s t h e n i n i t i a t e d (i.e., I H T T ) . Simple Reaction-Time Tasks Reliable estimates o f I H T T have o n l y been p r o d u c e d u s i n g simple R T tasks (Bashore, 1981). In these tasks, t h e subject makes an uncomplicated manual response t o t h e h a l f - f i e l d presentation o f an unpatterned, salient stimulus t h a t occurs on every, trial, w i t h o u t exception. The d u r a t i o n o f t h e stimulus must b e s u f f i c i e n t l y b r i e f (less t h a n 200 ms) t o minimize t h e likelihood o f saccadic eye movements. Presentation o f t h i s imperative stimulus i s usually preceded by a w a r n i n g stimulus (e.g., a

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tone) t h a t is t h e n followed by a c e n t r a l f i x a t i o n stimulus (e.g., a cross). T h e d u r a t i o n o f t h e f i x a t i o n stimulus v a r i e s randomly f r o m 1 t o 3 s a n d allows t h e subject t o p r e p a r e t h e response. Random f i x a t i o n d u r a t i o n s a r e used t o e n s u r e against lateral s h i f t s in gaze timed t o t h e onset o f t h e t a r g e t stimulus a n d t o reduce t h e likelihood o f a n t i c i p a t o r y responses. Immediately following t h e o f f s e t o f t h e f i x a t i o n stimulus, t h e t a r g e t stimulus is p r e s e n t e d i n t o t h e v i s u a l h a l f - f i e l d . T h e subject is t y p i c a l l y r e q u i r e d t o respond w i t h a f i n g e r on one h a n d during a n y block o f trials. However, t h e subject may also make a bimanual f i n g e r response (DiStefano, Morelli, Marzi, & Berlucchi, 1980; Jeeves, 1969; Jeeves & Dixon, 1970). T h e imperative stimulus is p r e s e n t e d e i t h e r pseudo-randomly in t h e l e f t o r right v i s u a l h a l f - f i e l d during a b l o c k o f t r i a l s (i.e., w i t h c o n s t r a i n t s o n t h e number o f consecutive presentations in - t h e same half-field; DiStefano e t al., 1980; Lines & Milner, 1983; McKeever & Hoff, 1979; Milner & Lines, 19821, o r in t h e same v i s u a l h a l f - f i e l d f o r t h e e n t i r e b l o c k o f t r i a l s (Anzola, Bertoloni, Buchtel, & Rizzolatti, 1977; Berlucchi, Heron, Hyman, Rizzolatti, & UmiltB, 1971; Berlucchi, Crea, DiStefano, & Tassinari, 1977; Jeeves, 1969; Jeeves E Dixon, 1970; Poffenberger, 1912; Smith, 1938). T h e f a s t e s t response latencies a r e p r o d u c e d in t h e simple reaction by t h e response h a n d ipsilateral t o t h e v i s u a l h a l f - f i e l d stimulated, w i t h e i t h e r blocked o r randomized h a l f - f i e l d presentations (Anzola e t al., 1977; Berlucchi e t al., 1971, 1977; DiStefano e t al., 1980; Jeeves, 1969; Jeeves & Dixon, 1970; Lines & Milner, 1983; McKeever & Hoff, 1979; M i l n e r & Lines, 1982; Poffenberger, 1912). Thus, t h e h a n d c o n t r o l l e d by t h e hemisphere t h a t is activated i n i t i a l l y by t h e sensory input has t h e s h o r t e s t response latency. T h i s ipsilateral response h a n d advantage is t h o u g h t t o r e f l e c t intrahemispheric processing o f t h e sensory input a n d motor o u t p u t t h a t obviates t h e need f o r interhemispheric communication t o execute t h e motor response. T h e advantage is r e p o r t e d t o r a n g e f r o m 1 t o 6 ms a n d p r o v i d e s t h e behavioral estimate o f I H T T . Poffenberger's (1912) estimates, however, a r e t h e longest (5.6 ms, 6.0 ms), w i t h contemporary estimates f a l l i n g t y p i c a l l y between 2 a n d 3 ms f o r unimanual responses a n d 1 t o 2 ms f o r bimanual responses. O f i n t e r e s t f o r o u r purposes is t h e consistent observation t h a t spat i a l compatibility does n o t a l t e r t h e response-hand s u p e r i o r i t y in a simple reaction (Anzola e t al., 1977; B e r l u c c h i e t al., 1977). T h a t is, t h e h a n d controlled by t h e hemisphere t h a t receives t h e i n i t i a l sensory input p r o duces t h e fastest RT, w h e t h e r t h e response device is positioned on t h e same side o f t h e b o d y as t h e source o f stimulation o r on t h e opposite side ( s o t h a t t h e h a n d is crossed o v e r t h e body's midline). T h i s suggests t h a t simple R T estimates o f I H T T r e p r e s e n t t h e transmission o f information o v e r fixed, p r i m a r y neuroanatomical pathways whose activation may n o t b e influenced by h i g h e r o r d e r c o g n i t i v e ( i . e . , c e n t r a l ) variables, such as S R compatibility. A similar inference is suggested by t h e observation t h a t attentional biases induced by p r e s e n t i n g t h e stimulus in t h e same h a l f f i e l d during a block o f t r i a l s d o n o t a l t e r t h e estimate o f I H T T in t h e simple reaction, whereas t h e y do i n more complicated d i s j u n c t i v e reactions (see discussion below; Ledlow, Swanson, & Kinsbourne, 1978a,b). Stimulus-Detection Tasks U n l i k e t h e simple R T task, in w h i c h an imperative stimulus is presented a n d a response is executed on e v e r y t r i a l , a stimulus-detection

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task has blank ("catch") trials inserted on which t h e response is withheld. This procedure was designed t o reduce t h e likelihood of anticipatory responding and, as such, t o obtain a p u r e r measure of t h e rate of t h e simple reaction (Woodworth, 1938). However, insertion of these "catch" t r i a l s transforms t h e task. It is no longer a simple RT task, but a "go/no go" task 'in which a stimulus must be detected before a response is executed. I n t h e simple RT task, stimulus detection is not actually required because t h e subject knows t h a t on e v e r y t r i a l a stimulus will be presented as soon as t h e fixation stimulus leaves t h e central visual field; t h e subject need only prepare and execute t h e response. In a stimulus-detection task, the subject must, i n addition, detect t h e imperative stimulus following t h e fixation stimulus and withhold a response if none is detected. Thus, detection and motor-execution processes are added. This added task complexity is reflected i n longer RTs and greater variability i n estimates of IHTT, ranging from 222 t o 307 ms and from 5.0 t o 28.5 ms, respectively (Kleinman, Carron, Cloninger, & Halvachs, 1976; Moscovitch & Smith, 1979; Smith & Moscovitch, 1979; Swanson & Ledlow cited in Swanson, Ledlow, & Kinsbourne, 1978). Unfortunately, t h e relevant parametric analyses t o assess t h e effects of variables l i k e S-R comp a t i b i l i t y on performance i n t h i s task and the concomitant influence on estimates of I H T T have not been done. Two-Choice Reaction-Time Tasks

A two-choice RT task designed t o estimate I H T T requires t h e subject t o discriminate between two stimuli, each of which signals a d i f f e r e n t response. As in t h e simple RT and stimulus-detection tasks, simple unpatterned stimuli and uncomplicated manual responses are used. The subject may be required t o make a unimanual digit selection (e.g., press one response button w i t h t h e index f i n g e r on t h e r i g h t hand t o t h e t a r get stimulus when it i s presented i n t h e l e f t visual half field and press another button w i t h t h e middle f i n g e r on t h e r i g h t hand if t h e stimulus is shown i n t h e r i g h t visual half field), o r t o make a response-hand decision (e.g., press a response button w i t h t h e index f i n g e r on t h e left hand t o l e f t visual half-field stimulation and press another response button w i t h t h e index f i n g e r on t h e r i g h t hand t o r i g h t visual half-field stimulation). Thus, a two-choice RT task d i f f e r s from a stimulus-detection task i n t h a t stimulus location is mapped t o response selection. Further, unlike the stimulus-detection task, t h e choice task requires a response on every trial. Estimates of I H T T t h a t derive from two-choice RT tasks are generally longer than those derived from t h e simpler tasks. Bradshaw and Perriment (1970) reported an I H T T of 20 ms favoring t h e response hand ipsilateral t o t h e visual half-field stimulated, and Harvey (1978) estimated the I H T T t o be 25 ms. However, with t h e introduction of location d i s crimination and response selection, the influence of spatial compatibility assumes t h e plenary role in determining response latencies and, as a result, most of t h e choice RT research has centered on identifying t h i s effect, rather than on estimating I H T T .

S-R Compatibility and I H T T The task most often used in choice RT analyses of S-R compatibility effects on interhemispheric transmission requires a manual response by

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t h e l e f t o r right h a n d t o a stimulus presented in t h e l e f t o r right v i s u a l half-field. T h e subject's hands a r e positioned e i t h e r a t t h e side o f t h e b o d y o r a r e crossed o v e r t h e b o d y ' s midline. U n d e r b o t h conditions, responses a r e always fastest when t h e response h a n d a n d stimulus a r e on t h e same side o f t h e body, w h e t h e r t h e hands a r e crossed o r uncrossed ( i . e . , t h e y a r e spatially compatible; Anzola e t al., 1977; Brebner, 1973; Brebner, Shephard, & Cairney, 1972; Wallace, 1971, 1972). This reflects, o f course, t h e e f f e c t o f spatial compatibility on t h e response a n d suggests, in c o n t r a s t t o t h e simple reaction, t h a t t h e influence o f t h i s variable on t h e f i n a l response o u t p u t is s t r o n g e r t h a n is t h e i n f l u e n c e o f t h e u n d e r l y i n g anatomical pathways. Studies designed t o assess t h e impact o f S-R compatibility o n estimates o f I H T T have been restricted, however, t o manipulations o f spatial relations. T h e r e a r e o t h e r forms o f S-R compatibility t h a t influence processing latency as well. In t h e n e x t section, I d e s c r i b e some t y p e s o f S - R compatibility t h a t a r e p e r t i n e n t t o t h i s discussion. Varieties of S-R C o m p a t i b i l i t y

S - R compatibility can b e g r o u p e d b r o a d l y i n t o t w o classes, spatial a n d symbolic (Simon, Sly, & Vilapakkam, 1981). T h e f o r m e r is based on t h e locational correspondence between t h e positions o f t h e imperative stimuli a n d o f t h e response devices, r e l a t i v e t o t h e b o d y ' s midline. Thus, when response h a n d selection decisions a r e made, a compatible c o n f i g u r a t i o n involves responding w i t h t h e h a n d placed on a response device positioned on t h e same side o f t h e b o d y as t h e source o f stimulation, a n d an incompatible arrangement r e q u i r e s r e s p o n d i n g w i t h t h e h a n d placed on a response device positioned on t h e side o f t h e b o d y opposite t h e source o f stimulation. As I discussed i n t h e p r e c e d i n g section, t h i s response advantage is a p p a r e n t w h e t h e r t h e compatible response is made by a h a n d placed in i t s normal anatomical position o r by a h a n d crossed o v e r t h e b o d y ' s midline (Anzola e t al., 1977; B r e b n e r , 1973; B r e b n e r e t al, 1972; Wallace, 1971, 1972; see also L i d a v a s & Moscovitch, 1984; Nicoletti, U m i l t i , & L i d a v a s , 1984; U m i l t i & Nicoletti, 1985). These same response device-stimulus location relations d o n o t obtain, however, when t h e head is t i l t e d 90O i n t h e horizontal plane; r a t h e r , t h e response handstimulus location correspondence is most i n f l u e n t i a l in d e t e r m i n i n g compatibility effects ( L i d a v a s & Moscovitch, 1984).

A directional advantage similar t o t h a t seen when response h a n d selection is r e q u i r e d is e v i d e n t when unimanual digit selection choices a r e made, whether t h e h a n d is i n t h e crossed o r uncrossed position (Bradshaw & Perriment, 1970; Harvey, 1978). T h a t is, t h e r e l a t i v e positions o f t h e d i g i t s a n d o f t h e sources of stimulation comprise t h e compatib l e arrangement, i r r e s p e c t i v e o f t h e side o f t h e b o d y o n which t h e h a n d is placed. Compatibility effects a r e also o b s e r v e d when t h e stimuli sign a l i n g a l e f t o r r i g h t response a r e presented in one v i s u a l hemifield a n d each response device is in a d i f f e r e n t lateral hemispace o r b o t h devices a r e in t h e same hemispace, w i t h hands crossed o r uncrossed a n d t h e head o r i e n t e d s t r a i g h t ahead o r r o t a t e d s l i g h t l y to t h e side (Nicoletti e t al, 1982, 1984; U m i l t i 8 Nicoletti, 1985). A s in t h e case o f t h e compatibility effects seen w i t h unimanual digit selection, t h e r e l a t i v e positions of t h e response devices a n d t h e imperative stimuli establish t h e compatible relations.

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Locational compatibility effects a r e manifest as well when t h e c o r r e spondence is between v i s u a l stimuli shown above o r below f i x a t i o n in t h e horizontal plane a n d up o r down response requirements ( L i d a v a s & Moscovitch, 1984; Proctor & Reeve, 1986; Umilt6 & Nicoletti, 1984). A compatible response i n t h i s c o n f i g u r a t i o n involves p r e s s i n g b u t t o n s in t h e v e r t i c a l plane a t t h e subject's midline t o a stimulus shown above o r below f i x a t i o n in t h e c e n t r a l v i s u a l f i e l d . These v e r t i c a l compatibility relations can b e d i s r u p t e d , however, by p r e s e n t i n g t h e stimuli above o r below t h e horizontal plane in t h e l e f t o r right hemifield, a n d r e q u i r i n g e i t h e r a unimanual response on a device positioned a t t h e midline (Cotton, Tzeng, & H a r d y c k , 1977, 1980) o r a response h a n d selection on devices in separate hemispaces positioned above a n d below one another (Umilta & Nicoletti, 1984). T h e subject is i n s t r u c t e d t o p r e s s t h e t o p b u t t o n t o t h e u p p e r stimulus a n d t h e bottom b u t t o n t o t h e lower stimulus, w i t h no mention made o f t h e l e f t - r i g h t d i s t i n c t i o n . Compatible responses when unimanual responses a r e made i n v o l v e p r e s s i n g t h e t o p o r bottom b u t t o n o f a response device w i t h t h e l e f t index f i n g e r i n response t o a f l a s h o f l i g h t in t h e u p p e r p o r t i o n o f t h e l e f t v i s u a l h a l f - f i e l d o r lower p o r t i o n o f t h e right v i s u a l half-field, respectively; a n d incompatible responses i n v o l v e p r e s s i n g t h e t o p o r b o t tom b u t t o n w i t h t h e l e f t index f i n g e r i n response t o t h e flash in t h e u p p e r right o r lower l e f t v i s u a l half-field, respectively. T h e lateral c o r respondence i s r e v e r s e d when t h e right h a n d is responding. When response h a n d decisions a r e made a n d t h e response devices a r e in separ a t e hemispaces, t h e l e f t - r i g h t d i s t i n c t i o n s determine t h e d i r e c t i o n o f t h e compatibility e f f e c t and, indeed, an incompatible above-below c o n f i g u r a t i o n can p r o d u c e a s h o r t e r response latency t h a n i t s compatible c o u n t e r p a r t (UmiltB & Nicoletti, 1984). Symbolic compatibility has a number o f v a r i a n t s as well. It can b e divided into three types. I n one, t h e c r i t i c a l element i s t h e correspondence between t h e v e r b a l label f o r t h e stimulus a n d t h e response r e q u i r e d t o it. In t h i s case, f o r example, a compatible response would i n v o l v e p r e s s i n g a r e d b u t t o n t o a r e d light o r a g r e e n b u t t o n t o a g r e e n light, a n d an incompatible response would i n v o l v e t h e opposite stimulusresponse device mapping (see C h a p t e r 2, by Simon, in t h i s volume). A n o t h e r t y p e o f symbolic compatibility involves t h e correspondence between a directional l i n g u i s t i c label (e.g., LEFT) o r symbol (e.g., -->) a n d t h e r e q u i r e d response t o it ( l e f t o r right directional response, respectively, f o r a compatible response; a n d t h e opposite directional movement f o r an incompatible response; Bashore & Osman, 1987; Magliero, Bashore, Coles, & Donchin, 1984; M c C a r t h y & Donchin, 1981). And, another v a r i a n t o f symbolic compatibility comprises t h e correspondence between t h e negation o f a l i n g u i s t i c label ( e . g . , NOT LEFT) o r symbol (e.g., -/->) a n d t h e r e q u i r e d response t o it (right o r left, respectively, f o r a compatible response; a n d t h e converse mapping f o r an incompatible response (Whitaker,1980, 1982). T h e r e l a t i v e prepotence o f spatial compatibility o v e r symbolic comp a t i b i l i t y is revealed in what has come t o b e called t h e "Simon effect" (Hedge & Marsh, 1975) a f t e r J. R. Simon, who f i r s t i d e n t i f i e d t h i s e f f e c t a n d whose research has been l a r g e l y responsible f o r a r t i c u l a t i n g i t s parameters (see C h a p t e r 2 , by Simon, in t h i s volume). U n d e r circumstances i n w h i c h a symbolic cue (e.g., t h e w o r d LEFT) is presented i n t o t h e lateral hemispace (e.g., l e f t o r r i g h t ear) a n d t h e subject must p r e s s a response button t o t h e l e f t o f t h e body's midline w i t h the l e f t index

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f i n g e r (i.e., t h e symbolic stimulus a n d t h e location o f t h e response device/response h a n d a r e anatomically compatible), t h e presentation o f t h e stimulus i n t o t h e spatially incompatible hemispace (in t h i s case, t h e right ear) p r o l o n g s t h e response latency. T h a t is, t h e spatial o r directional cue slows t h e response t o t h e imperative symbolic stimulus even t h o u g h it is i r r e l e v a n t . T h i s e f f e c t obtains w h e t h e r a bimanual response h a n d selection o r a unimanual directional movement (e.g., move a l e v e r t o t h e l e f t o r right w i t h t h e right hand) is r e q u i r e d . A l t h o u g h t h e Simon e f f e c t suggests t h a t spatial relations between t h e stimulus a n d response a r e more compelling t h a n t h e symbolic c o n t e n t of a stimulus a n d t h e response it signals in d e t e r m i n i n g response latencies, i n i t i a l characterizations o f t h e r e l a t i v e c o n t r i b u t i o n s of each t o t h e ultimate processing time have been made in t h e research o f Reeve a n d Proctor (1984) a n d Proctor a n d Reeve (1985, 1986). Their work is b e g i n n i n g t o a r t i c u l a t e t h e p r o p e r t i e s of stimulus a n d response sets t h a t i n t e r a c t t o determine v a r y i n g degrees o f S-R compatibility. It has revealed t h a t t h e most compatible configurations a r e determined by t h e mapping of c e r t a i n features o f a stimulus set o n t o c e r t a i n features o f a response set, w h e t h e r o r n o t a p r e c u e r e d u c i n g t h e response choices is presented. T h u s f a r t h e response features t h e y have i d e n t i f i e d i n c l u d e spatial components, l e f t - r i g h t positioning o f t h e response b u t t o n s ( r e l a t i v e t o t h e b o d y ' s midline) a n d response h a n d decisions, a n d symbolic components, consonant ( v s . vowel) a n d l e t t e r i d e n t i t y ( v s . size) distinctions. Reeve a n d Proctor's f i n d i n g s obtained w i t h spatial-location stimuli a r e consistent w i t h much p r e v i o u s w o r k in showing t h e dominant r o l e o f t h e spatial components (see t h e I H T T section o f t h i s c h a p t e r a n d t h e coding chapters i n t h i s volume). Most important, however, is t h e i r demonstration w i t h symbolic stimuli t h a t t h e p a r t i c u l a r features o f t h e response set i n t e r a c t w i t h features o f t h e stimulus t o p r o d u c e compatibility effects. F o r example, when t h e stimulus set consists o f t h e stimuli 0 , 0, z, a n d Z, a n d t h e response set consists o f t h e index a n d middle f i n g e r s o f each hand, responses a r e f a s t e r w i t h an assignment f o r w h i c h l e t t e r i d e n t i t y distinguishes between t h e t w o leftmost a n d t w o rightmost responses t h a n w i t h an assignment f o r w h i c h it does not. However, when t h e f i n g e r s f r o m t h e t w o hands a r e alternated, response latencies w i t h an assignment f o r w h i c h l e t t e r i d e n t i t y distinguishes between responses o n t h e t w o hands a r e equal t o those w i t h t h e assignment f o r w h i c h i d e n t i t y distinguishes t h e t w o leftmost a n d t w o rightmost responses. Thus, w i t h t h e same assignments of stimuli t o response locations, d i f f e r e n t r e s u l t s a r e obtained when t h e f i n g e r s a r e alternated. In t h e f o r m e r case, t h e assignment f o r w h i c h t h e salient l e t t e r - i d e n t i t y f e a t u r e i s consistent w i t h t h e l e f t - r i g h t spatial component o f t h e response set shows a b e n e f i t i n r e s p o n d i n g . B u t in t h e l a t t e r case, both t h a t assignment a n d t h e assignment f o r w h i c h t h e l e t t e r i d e n t i t y f e a t u r e distinguishes between t h e hands show equivalent benefits, p r o v i d i n g evidence t h a t e i t h e r t h e spatial component o r t h e anatomical component of t h e response set can b e used if it coincides w i t h t h e l e t t e r - i d e n t i t y f e a t u r e o f t h e stimulus set. A t t h i s point, discussion w i l l t u r n t o ERPs a n d t o t h e i r role i n t h e analysis o f mental chronometry in general a n d S-R compatibility in p a r t i c ular.

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Event-Related B r a i n Potentials Data Collection and Basic Description Cognitive psychophysiologists w i t h interests i n mental chronometry study two classes o f dependent variables, traditional behavioral measures, response latency (i.e., RT) and accuracy, and measures of b r a i n electrical a c t i v i t y recorded a t t h e scalp, commonly r e f e r r e d t o as event-related b r a i n potentials (ERPs). ERPs are recorded by a f f i x i n g electrodes on t h e subject's scalp, t y p i c a l l y a t placement sites used f o r both clinical and research purposes (10-20 o r International Electrode Placement System, Jasper, 1958; see Figure 1 ) . These potentials represent positive o r negative deflections (called components) i n the voltage of t h e on-going electroencephalographic (EEG) signal t h a t are produced by t h e presentation of a stimulus t o which t h e subject either sits passively o r makes a decision and initiates a response. However, t h e r e is a t y p e of movementrelated activity, t h e readiness potential (RP), t h a t is evident when a subject makes a series of spontaneous (i.e., unsignaled), repetitive movements (e.g., f i n g e r tapping) o r responds t o t h e presentation of an imperative stimulus in simple o r more complicated R T tasks. This scalp a c t i v i t y i s presumed t o reflect t h e activation of cortical motor neurons t h a t mediate t h e movement (see discussion in Bashore, McCarthy, Heffley, T h e electrical signals emanating from t h e Clapman, & Donchin, 1982). b r a i n are analog. However, i n most research applications these signals are not sampled continuously, b u t are digitized a t rates v a r y i n g typically from 100 t o lo00 Hz (i.e., e v e r y 1-10 ms). The digitized a c t i v i t y is stored on magnetic tape and all of t h e analyses on t h i s a c t i v i t y are done o f f - Iine

.

Components of t h e ERP t h a t are elicited by stimulus events are labeled on t h e basis of t h e i r electrical p o l a r i t y and t h e minimum latency at which t h e i r peak amplitude i s reached (Donchin, Ritter, & McCallum, 1978; Picton & Stuss, 1980; see Figure 2). For example, N140 refers t o a negative change i n voltage whose peak is achieved at about 140 ms a f t e r presentation of a stimulus. Analyses are done most often on t h e amplitude, latency, and scalp distribution (i.e., relative amplitudes a t d i f f e r e n t scalp sites) of these potentials. Because the voltages of ERPs are small relative t o those of t h e EEG, these signals are obscured by t h e background EEG and are most evident i n a signal t h a t represents t h e average of a large number of repetitions of t h e stimulus. However, multivariate techniques, such as discriminant functions and lag cross-correlations, have allowed t h e detection of larger ERP components on single t r i a l s w i t h a high degree of success (Horst & Donchin, 1980; Squires F. Donchin, 1976). T o perform these single t r i a l analyses, however, t h e digitized signal must be f i l t e r e d off-line t o extract t h e signal from the noise (Farwell, Martinerie, Bashore, 8 Rapp, i n preparation; Ruchkin 8 Glaser, 1978).

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FRONT

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TOP OF HEAD

F i g u r e 1. T h e placement sites used in t h e I n t e r n a t i o n a l (10-20) Electrode Placement System. Recording sites o v e r t h e l e f t hemisphere a r e designated by o d d numbers a n d o v e r t h e right hemisphere by even numbers. T h e l e t t e r p r e c e d i n g t h e number r e f e r s t o t h e p a r t of t h e cerebral c o r t e x o v e r w h i c h t h e electrode is placed (F=frontal; Czcentral; Pzparietal; O=occipital; a n d T=temporal), F p l a n d Fp2 designate sites on t h e forehead a n d A l / A 2 i d e n t i f y mastoid sites commonly used as references f o r scalp r e c o r d i n g s . T h i s d r a w i n g is f r o m H a r n e r & Sannit (1974). It is r e p r i n t e d w i t h t h e p u b l i s h e r ' s permission.

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HUMAN VEP ~~

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HUMAN SEP RMN SmI&i

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Figure 2. Schematic representations o f ERPs as t h e y would b e evoked in t h e a u d i t o r y (labeled AEP), v i s u a l (VEP), a n d somatosensory (SEP) modalities f r o m a normal, awake a d u l t u n d e r conditions of passive stimulation (i.e., t h e subject is n o t p e r f o r m i n g a n y specific t a s k ) . Components a r e n o t necessarily l a r g e s t a t t h e sites shown in these drawings. N in t h e SEP ERP r e f e r s t o an electrbde located o n t h e p o s t e r i o r p a r t of t h e neck, a n d RMN indicates right median n e r v e . These d r a w i n g s were t a k e n from Goff, Allison, & Vaughan (19781, a n d a r e r e p r i n t e d w i t h t h e a u t h o r s ' a n d p u b l i s h e r ' s permission.

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Exogenous a n d Endogenous Components of t h e ERP Exogenous components. T h e earliest components o f t h e ERP a r e o b l i g a t o r y electrical responses t o stimulation recorded t y p i c a l l y in circumstances designed t o assess t h e i n t e g r i t y o f p r i m a r y a f f e r e n t pathways (i.e., f o r c l i n i c a l . evaluations) o r t o characterize f a c t o r s t h a t influence transmission rates in these systems (e.g., age, b o d y temperature). T h e i r p r o d u c t i o n is dependent on e x t e r n a l stimulation, a n d t h e amplitudes a n d latencies of these signals v a r y l a r g e l y as a f u n c t i o n o f t h e p h y s i c a l p r o p e r t i e s o f t h e e l i c i t i n g stimulus (e.g., an increase in t h e i n t e n s i t y o f t h e stimulus w i l l p r o d u c e an increase in t h e amplitude a n d decrease in t h e latency o f t h e component t o asymptote). Hence, these components a r e o f t e n r e f e r r e d t o as "exogenous" o r evoked potentials (EPs). Moreover, t h e scalp s i t e w h e r e t h e amplitude o f t h e EP i s l a r g e s t a n d latency s h o r t a v i s u a l stimulus e s t is determined by t h e modality stimulated (e.g., evokes t h e most r o b u s t response o v e r o c c i p i t a l - - p r i m a r y v i s u a l c o r t e x - r e c o r d i n g sites). Consequently, electrode placement i s determined by t h e Examples of sensory modality in w h i c h t h e stimulus is delivered. exogenous potentials e l i c i t e d u n d e r passive stimulation conditions in t h e auditory, visual, a n d somatosensory systems a r e shown in F i g u r e 2. Endogenous components. When decisions must b e made a b o u t t h e stimuli presented (e.g., c o u n t t h e high p i t c h e d tones, i g n o r e t h e low p i t c h e d tones), exogenous components a r e manifested w i t h i n t h e f i r s t 100150 ms following t h e stimulus. A t longer latencies, a series o f components, r e f e r r e d t o as endogenous, is e v i d e n t . T h e amplitude, latency, a n d scalp d i s t r i b u t i o n o f these components a r e r e l a t i v e l y i n s e n s i t i v e to Instead, t h e amplichanges in t h e p h y s i c a l p r o p e r t i e s of t h e stimulus. t u d e a n d latency o f these l a t e r components a r e v e r y sensitive t o changes in t h e psychological demands o f t h e task, a n d t h e i r scalp d i s t r i b u t i o n i s r e l a t i v e l y constant across t a s k s a n d sensory modalities. Thus, these components a r e believed t o b e generated by n e u r a l events associated w i t h h i g h e r o r d e r processing o f t h e ,?timulus i n p u t a n d response o u t p u t . Hence, t h e y a r e r e f e r r e d t o as endogenous" components, examples of which a r e t h e N200 a n d P300 discussed below. Because t h e scalp d i s t r i b u t i o n of these components is n o t tightly coupled t o t h e sensory modality in w h i c h stimulation occurs, c o g n i t i v e psychophysiological investigations o f mental processing t y p i c a l l y u t i l i z e a l a r g e r a r r a y o f electrode placements t h a n d o investigations o f p r i m a r y a f f e r e n t systems. Electrodes a r e usually placed a t locations along t h e midline o f t h e scalp ( r e f e r r e d t o as Fz [frontal], Cz [central], Pz [parietal], a n d Oz [occipital]), a n d a t a v a r i e t y o f lateral r e c o r d i n g sites (e.g., temporal sites T3, T5; c e n t r a l sites C3, C4; parietal sites P3, P4; see F i g u r e 1 ) .

It is i m p o r t a n t t o note t h a t t h e n e u r a l o r i g i n s o f t h e e a r l y evoked potentials a r e characterized f a r b e t t e r t h a n a r e those of t h e l a t e endogeIndeed, it i s n o t nous components (Goff, Allison, 8 Vaughan, 1978). u n f a i r t o assert t h a t t h e sources o f t h e late potentials a r e u n k n o w n It i s (Wood, McCarthy, Squires, Vaughan, Woods, & McCallum, 1984). f a i r t o claim, however, t h a t t h e elements o f mental processing i n d e x e d by a v a r i e t y o f these ERPS a r e becoming i n c r e a s i n g l y well-characterized (Donchin, Karis, Bashore, Coles, & Gratton, 1986; Picton & Stuss, 1980; R i t t e r , Simson, & Vaughan, 1983). Hence, we can u t i l i z e these measures in tandem w i t h behavioral measures t o h e l p a r t i c u l a t e t h e s t r u c t u r e a n d Before d i s c u s s i n g this, however, I shall t i m i n g of c o g n i t i v e processing. review t h e electrophysiological studies of IHTT t o see how t h e y i n f o r m us. I shall t h e n discuss t h e endogenous components of t h e ERP, u s i n g

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(Note t h a t f o r purposes t h e P300 component of t h e ERP as t h e example. of t h i s discussion I have not differentiated what many r e f e r to as the middle latency potentials [100-200 ms poststimulus], thought t o have both exogenous and endogenous properties .) This tutorial provides t h e necessary backdrop f o r t h e discussion t h a t follows of ERP estimates o f I H T T . Electrophysiological Estimates of I H T T A small number of ERP studies have been done t o estimate I H T T . The same logic f o r estimating t h i s value t h a t motivates behavioral studies drives electrophysiological studies. The assumption is t h a t activation o f t h e primary cortical area opposite t h e source of stimulation will be manifest i n t h e latency of a component of the ERP. This information will then be conveyed t o t h e homologous receptive area in the other hemisphere where t h e same component will be evident b u t a t a longer latency, and t h e difference in component latency will provide t h e estimate of I H T T . The earliest such studies were n o t influenced by t h e behavioral work; rather, estimates were derived under passive stimulation conditions. Latency differences between contralateral and ipsilateral scalp sites i n t h e generation o f an evoked response t o monaural clicks were reported t o be about 2 ms (Wolpaw & Penry, 1977); t o tactile stimulation of t h e f i n g e r from 3 t o 8 ms (Salamy, 1978); and t o visual stimulation from 8 t o 33 ms (Andreassi, Okamura, & Stern, 1975). Rugg and associates have completed a series of studies t h a t is s t r o n g l y influenced by t h e earlier behavioral work (Lines, Rugg, & Milner, 1984; Rugg, Lines, & Milner, 1984, 1985). They have utilized simple and go/no go choice reactions t o derive estimates of I H T T from both behavioral and electrophysiological measures. The choice task required a left- o r right-index f i n g e r response (with t h e response b u t tons placed t o t h e l e f t and right of t h e body's midline) t o a l e f t o r right visual half-field presentation of t h e target o r go stimulus (determined by duration), whose probability of occurrence was .24. In one s t u d y (Rugg et a!., 1984), spatial compatibility was varied. I n all of t h e other studies, subjects were required only t o make compatible responses. ERPs were recorded from standard lateral central (C3, C4) and occipital (01, 02) sites, a t a standard midline site (Pz), and at two non-standard sites lateral t o t h e standard occipital placements ( r e f e r r e d t o as LO1 and LO2 by t h e authors). T h e i r work has focused primarily on t h e N160 component and has revealed t h a t t h e latency of t h i s component is always shorter a t contralateral (i.e., directly stimulated) than at ipsilateral occipital scalp sites. This directional asymmetry is apparent consistently a t central sites, but i t s magnitude i s smaller than t h a t observed a t occipital sites and is not always significant. Lines e t al. (1984, Experiment 2 ) reported t h a t the N160 evident i n a simple reaction had shorter latencies and larger amplitudes a t central and occipital sites over t h e directly stimulated hemisphere than at the homologous sites over t h e other hemisphere. They also found t h a t these effects varied as a function of t h e intensity of t h e light stimulus a t occipital but not a t central sites ( b r i g h t flashes produced shorter latenThe estimated IHTT, cies and larger amplitudes than did dim flashes). collapsed across electrode sites and stimulus intensity, is 7.9 ms f o r l e f t visual half-field stimulation and 11.1 ms f o r right visual half-field stimulation (my calculations). Estimates of I H T T varied, however, a t d i f f e r e n t scalp sites; collapsed across visual half-field of stimulation and stimulus

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t h e estimate is 11.73 ms a t t h e s t a n d a r d occipital sites (01,

0 2 ) , 14.23 ms a t t h e n o n - s t a n d a r d occipital sites (LO1, L02), a n d 2.58 ms a t t h e c e n t r a l sites (C3, C4) (my calculations). Stimulus i n t e n s i t y altered t h e estimate o f I H T T a t t h e occipital sites but n o t a t t h e c e n t r a l sites; a t LO sites it was 17.6 ms w i t h dim stimuli a n d 10.9 ms w i t h bright stimuli, a n d a t 0 sites it was 15.1 ms a n d 8.4 ms f o r dim a n d bright stimuli, respectively. Amplitude effects paralleled these latency effects; t h e l a r g e s t amplitude N160 was apparent o v e r t h e contralateral hemisphere a n d it was l a r g e s t t o t h e b r i g h t stimulus. T h e R T estimate o f I H T T , 1.8 ms, r e p o r t e d by Lines e t al. is consistent w i t h p r e v i o u s studies. A l t h o u g h R T s were f a s t e r t o t h e bright stimuli, t h e estimate o f I H T T did n o t d i f f e r as a f u n c t i o n of stimulus i n t e n s i t y .

A similar p a t t e r n of asymmetries was revealed in t h e i r analyses o f t h e choice reaction (Lines e t a l . , 1984, Experiment 1; R u g g e t al., 1984, 1985; see F i g u r e 3). Thus, t h e estimate o f I H T T was smaller a t c e n t r a l

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(3.2 ms, Lines e t al., 1984, Experiment 1; 3.7 ms, R u g g e t al., 1984; 2.0 ms, R u g g e t al., 1985) t h a n a t occipital sites (15.7 ms (LO), Lines e t al., 1984, Experiment 1; 15.7 ms (LO), 12.4 ms (O),R u g g e t al., 1984; 14.0 ms, (LO) R u g g e t al., 1985), a n d t h e amplitude o f t h e signal was As in t h e simple reaclargest o v e r t h e d i r e c t l y stimulated hemisphere. tion, t h e latency a n d amplitude o f t h e N160 v a r i e d w i t h stimulus. i n t e n s i t y a t occipital b u t n o t a t c e n t r a l electrode sites, as did t h e estimate o f I H T T . N e i t h e r t h e latency n o r t h e amplitude o f t h e N160 o r t h e estimate o f I H T T d e r i v e d f r o m i t s contralateral-ipsilateral differences in latency were influenced by t h e requirement t o make a n incompatible response ( R u g g e t al., 1984). R u g g a n d h i s associates did n o t present behavioral data in these studies t h a t p e r m i t R T estimates o f I H T T t o b e derived, b u t R u g g e t al. (1984) did r e p o r t t h a t R T was prolonged s i g n i f i c a n t l y when an incompatible response was executed.

It i s p a r t i c u l a r l y i n t e r e s t i n g t h a t t h e latency o f N160 a n d t h e estimates o f I H T T d e r i v e d f r o m it were n o t influenced by e i t h e r t h e complexity o f t h e reaction (simple o r go/no g o choice reaction, w i t h t h e u n p a t t e r n e d stimuli these authors used), o r by S - R compatibility, while a t t h e same time t h e y were affected a t occipital sites by t h e i n t e n s i t y o f t h e stimulation. T h i s p a t t e r n suggests t h a t t h e N160 component is an exogenous component whose characteristics are responsive t o t h e physical parameters o f stimulation b u t n o t t o t h e psychological demands o f t h e t a s k . T h i s fact can b e u t i l i z e d t o dissect t h e basic neuroanatomical components f r o m t h e c o g n i t i v e components o f S - R compatibility, as I discuss below. R u g g a n d h i s associates have d i r e c t e d t h e i r attention t o t h e N160 in p a r t because earlier components have been d i f f i c u l t f o r them t o i d e n t i f y In one s t u d y , however, t h e y have r e p o r t e d that an earlier reliably. component, t h e P120, resembles t h e N160 in that it has a s h o r t e r latency (by 14 ms) o v e r t h e d i r e c t l y stimulated hemisphere (at lateral occipital sites), b u t d i f f e r s f r o m it by b e i n g l a r g e r in amplitude o v e r t h e ipsiIn a d i s j u n c t i v e reaction, Ledlow lateral hemisphere ( R u g g e t al., 1985). e t al. (1978a) also i d e n t i f i e d an e a r l y positive component, t h e y called t h e P130, whose latency is s h o r t e r a t occipital sites o v e r t h e d i r e c t l y stimulated hemisphere, b u t whose amplitude is largest o v e r t h e ipsilateral hemisphere. They, too, i d e n t i f i e d a negative component, r e f e r r e d t o by them as t h e N170, whose amplitude was largest a n d latency shortest a t non-standard sites midway between 0 1 and P3, a n d between 0 2 and P4. T h e d i s j u n c t i v e R T t a s k t h a t t h e y used r e q u i r e d t h e subject t o make o n l y one response (index f i n g e r , blocked by l e f t o r right hand) t o a t a r g e t stimulus t h a t o c c u r r e d on 50% o f t h e t r i a l s and t o r e f r a i n f r o m responding when t h e a l t e r n a t i v e stimulus was presented ( a square w i t h a n X i n it was t h e t a r g e t a n d an empty square was t h e nontarget, o r vice versa). T h e y used t w o .locations f o r t h e response hand, a t t h e midline o r placed lateral t o t h e midline. T h e R T estimate o f I H T T was influenced by t h e location o f t h e response hand, whereas t h e estimates f r o m t h e ERP latencies were not. A nonsignificant estimate o f about 2 ms was observed f o r t h e behavioral measure when t h e response h a n d was in t h e c e n t r a l position, but a s i g nificant estimate o f about 24 ms was evident when t h e response hand was This signifiplaced in t h e lateral position (my estimates f r o m a g r a p h ) . c a n t e f f e c t was restricted, however, t o a randomized visual h a l f - f i e l d presentation. When t h e stimulus was presented in t h e same h a l f - f i e l d f o r

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a n e n t i r e block o f trials, an advantage f o r t h e d i r e c t l y stimulated hemisphere was seen, b u t it was n o t significant f o r e i t h e r response position. In contrast, t h e contralateral advantage f o r t h e P130 a n d N170 did n o t vary e i t h e r with blocked o r randomized h a l f - f i e l d stimulus presentations o r w i t h t h e location of t h e response hand. Estimates o f I H T T d e r i v e d from latencies of t h e P130 were 18 ms a t b o t h t h e c e n t r a l a n d lateral response positions; f o r t h e N170, t h e y were 15 a n d 19.5 ms, respectively (my calculations f r o m Table 1 in t h e i r p a p e r ) .

In another s t u d y i n v o l v i n g a d i s j u n c t i v e reaction (Ledlow e t al., 1978b), subjects made a unimanual b u t t o n press ( w i t h t h e i n d e x f i n g e r o n t h e l e f t o r right hand, blocked) t o laterally presented l e t t e r p a i r s w i t h t h e same name (half o f which were p h y s i c a l l y identical as well) a n d withheld t h e response t o l e t t e r p a i r s w i t h d i f f e r e n t names. T h r e e e a r l y components were i d e n t i f i e d whose latencies were s h o r t e r o v e r t h e cont r a l a t e r a l hemisphere (again, a t t h e occipital sites used in t h e i r o t h e r study). These components, labeled by t h e authors as N70, P130, a n d N170, had estimated I H T T s o f 4, 21, and 13 ms. T h e amplitude o f t h e N70 and N170 did n o t d i f f e r w i t h stimulation, b u t t h e P130 had a smaller amplitude o v e r t h e ipsilateral hemisphere. In summary, t h i s research has i d e n t i f i e d e a r l y evoked potentials whose amplitude a n d latency a r e influenced by t h e source o f stimulation (e.g., t h e visual half-field) a n d by t h e i n t e n s i t y o f t h e stimulus, b u t whose amplitude and latency a r e unaffected by t h e complexity o f t h e reaction, S-R compatibility, position o f t h e response hand, a n d o r d e r of t h e h a l f - f i e l d stimulation (i.e., blocked o r random). Moreover, t h e estimates o f I H T T d e r i v e d f r o m these components d o n o t seem t o v a r y w i t h changes in processing demands. In contrast, R T i s prolonged by increases in processing demands, by t h e requirement t o make an incompatible response, a n d by t h e location o f t h e response device. In addition, t h e R T estimate o f I H T T was r e p o r t e d by Ledlow e t al. (1978a) t o b e sensitive t o hand position, even t h o u g h o n l y uncrossed positions were used. And, as we have seen, behavioral estimates o f I H T T a r e longer in stimulus detection a n d two-choice R T tasks t h a n t h e y a r e in simple R T tasks, a n d t h e y cannot b e d e r i v e d when incompatible responses a r e made. It should b e noted h e r e t h a t R u g g e t al. (1984) f o u n d t h a t t h e latency o f a late component, t h e P300, v a r i e d w i t h t h e t y p e o f stimulus presented (go v s n o go), b u t n o t w i t h t h e t y p e o f response (compatible, incornpatible). R T was prolonged, however, by t h e need t o make an incompatible response t o t h e imperative stimulus. These f i n d i n g s a r e in accord w i t h o t h e r s I shall p r e s e n t in t h e sections on P300 t h a t follow.

T h e P300

My discussion has t h u s f a r been o f e a r l y exogenous components of t h e ERP. Next, I t u r n t o l a t e r endogenous components as a preface t o discussing S - R compatibility f r o m a c o g n i t i v e psychophysiological p e r spective. P300 w i l l b e used as t h e exemplar o f these components. Early investigations o f t h e P300 determined t h a t it i s manifest in simple i n f o r mation-processing tasks in which t h e subject is r e q u i r e d t o monitor a series o f stimulus events comprised o f t w o stimuli, one o f which signals an o v e r t (e.g., b u t t o n press) o r c o v e r t ( e . g . , r u n n i n g mental count) response a n d t h e o t h e r o f which indicates t h a t n o response i s r e q u i r e d ( f o r reviews, see Donchin e t al., 1986; Duncan-Johnson G Donchin, 1982; Johnson, 1986; Pritchard, 1981). Whenever t h e c r i t i c a l e v e n t is

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presented, a P300 i s elicited, t h e amplitude o f w h i c h is i n v e r s e l y p r o p o r tional t o t h e p r o b a b i l i t y o f o c c u r r e n c e o f t h a t e v e n t . T h a t is, t h e less f r e q u e n t t h i s event, t h e l a r g e r t h e amplitude o f t h e P300 it elicits. Indeed, it is now well-established t h a t t h e amplitude o f t h e P300 i s determined by t h e t a s k relevance o f t h e stimulus a n d by t h e f r e q u e n c y o f occurrence o f t h e c r i t i c a l event, s u c h t h a t r e l e v a n t stimuli p r o d u c e P300s t h a t a r e l a r g e s t when t h e e v e n t i s r a r e s t . More precisely, t h e amplitude of t h e P300 i s t i e d closely t o t h e subjective p r o b a b i l i t y o f t h e t a r g e t e v e n t (see Donchin & Coles, 1988, f o r an i n - d e p t h discussion o f t h i s relation). T h e finding t h a t low p r o b a b i l i t y events e l i c i t t h e l a r g e s t P300s has g i v e n r i s e t o t h e name "oddball" f o r t h e basic t a s k i n w h i c h t h e P300 is generated. In t h i s task, t h e subject i s presented one o f t w o possible lo00 Hz a n d 2000 Hz tones), which d i f f e r in t h e i r stimuli (e.g., f r e q u e n c y of presentation. T h e i n f r e q u e n t stimulus (e.g., p = .15) is designated as t h e "target," a n d t h e f r e q u e n t stimulus (e.g., p = .85) is designated as t h e " n o n - t a r g e t . T h e subject's t a s k is t o keep a r u n n i n g mental c o u n t o f t h e r a r e t a r g e t stimulus ( o r t o p r e s s a b u t t o n when it occurs) but t o keep n o mental c o u n t o f presentations o f t h e f r e q u e n t n o n - t a r g e t ( o r t o w i t h h o l d t h e b u t t o n p r e s s ) . A l a r g e P300 is elicited by t h e counted stimulus a n d l i t t l e o r n o P300 is p r o d u c e d by t h e uncounted stimulus. I n t e r e s t i n g l y , as t h e p r o b a b i l i t y o f occurrence o f t h e t a r g e t increases, t h e amplitude o f t h e P300 decreases a n d a P300 begins t o emerge t o t h e n o n - t a r g e t . A t e q u i p r o b a b i l i t y , b o t h stimuli p r o d u c e a P300 but t h e P300 t o t h e t a r g e t is s l i g h t l y l a r g e r . T h i s is t h e t a r g e t Most o f t h e effect. These relations a r e d e p i c t e d in Figures 4 a n d 5. e a r l y w o r k o n P300 u t i l i z e d t h e oddball t a s k o r some v a r i a n t o f it. More r e c e n t w o r k has shown t h a t t h e P300 i s elicited in complex informationprocessing t a s k s in w h i c h decisions must b e made a n d actions i n i t i a t e d about r e l e v a n t classes o f stimulus events (e.g., Callaway, 1983; Donchin, 1981; Donchin e t al., 1986; Duncan-Johnson & Donchin, 1982; Mulder, Gloerich, Brookhuuis, v a n Dellen, & Mulder, 1984; R i t t e r e t al., 1983). It has also been shown t h a t t h e scalp d i s t r i b u t i o n of t h e P300 is reasonably consistent across stimulus modalities a n d information-processing tasks; in y o u n g adults, it is l a r g e s t i n amplitude a t Pz, intermediate a t Cz, a n d smallest a t Fz. b e noted now t h a t t h e designation "300" r e f e r s t o t h e latency o fItt hshould i s component i n t a s k s u s i n g simple stimuli, w i t h easy discriminations between t a r g e t s a n d n o n - t a r g e t s . As processing difficulty increases, t h e latency of t h e "P300" can b e prolonged by as much as 300 o r 400 ms. U n d e r these circumstances, identification o f t h i s component as a P300 i s done on t h e basis o f i t s scalp d i s t r i b u t i o n a n d r e s p o n s i v i t y t o experimental manipulations. Hence, some investigat o r s p r e f e r t o number t h e components serially a n d label t h i s component t h e P3, because it is t h e third p o s i t i v e component in t h e ERP elicited i n mental processing t a s k s ( a n d t h e N100, f o r example, as t h e N l ) .

A good demonstration of t h e endogenous n a t u r e o f t h e P300 is p r o v i d e d in t h e omitted stimulus t a s k (Ruchkin, Sutton, & T u e t i n g , In t h i s task, t h e subject i s presented a series o f identical stimuli 1975). (e.g., 1000 Hz tones), w i t h an i n t e r s t i m u l u s i n t e r v a l o f 1000 t o 1500 ms, t h a t i s i n t e r r u p t e d i n f r e q u e n t l y ( e . g . , on 15% o f t h e t r i a l s ) by an omisT h e subject's t a s k is t o c o u n t occurrences o f t h i s sion of t h e stimulus. omission. T h e ERP generated t o t h e omission contains a l a r g e P300, whereas t h e ERP p r o d u c e d by t h e stimulus does not. T h u s , t h e P300 can

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F i g u r e 4. T h e relationship between P300 amplitude a n d stimulus p r o b a b i l ity across t h e midline scalp a t sites Fz, Cz, a n d Pz is shown in t h i s figure. A p p a r e n t is t h e reduction in amplitude o f t h e P300 t o presentations of t h e t a r g e t stimulus (high tone) as i t s p r o b a b i l i t y increases a n d a concomitant increase i n amplitude o f P300 t o t h e nontarget stimulus as i t s p r o b a b i l i t y increases. Note t h a t a t 50/50 p r o b a b i l i t y t h e t a r g e t stimulus elicits a P300 whose amplitude is s l i g h t l y T h i s is t h e t a r g e t e f f e c t . l a r g e r t h a n t h a t elicited by t l i e n o n - t a r g e t . Note also t h a t t h e P300 t e n d s t o b e l a r g e s t a t t h e Pz electrode, especially a t t h e lower probabilities f o r t h e t a r g e t stimulus. T h i s f i g u r e is taken f r o m Donchin E M c C a r t h y (1980) a n d is r e p r i n t e d w i t h permission o f t h e authors and p u b l i s h e r s . b e generated i n t h e absence of an e v o k i n g p h y s i c a l stimulus. is shown in F i g u r e 5.

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P300 Latency in t h e Analysis of Mental C h r o n o m e t r y in t h e p r e c e d i n g discussion, t h e conditions As is apparent necessary f o r i n v o k i n g t h e P300 have been reasonably well-characterized. However, if t h i s b r a i n e v e n t is t o b e used i n analyses o f components of mental processing, such as S - R compatibility, we must know those

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elements o f mentation t h a t a r e engaged when t h e P300 is generated. A f a i r amount o f research in t h e p a s t 10 years has been devoted t o achieving t h i s end. R i t t e r , Simson, a n d Vaughan (1972) a n d Kutas, McCarthy, a n d Donchin (1977) suggested t h a t t h e latency o f t h e P300 reflects t h e time t a k e n t o evaluate a n d categorize stimulus events a n d is r e l a t i v e l y independent o f t h e time r e q u i r e d t o select a n d execute a response. In s u p p o r t o f t h i s hypothesis, Kutas e t al. (1977) f o u n d t h a t t h e latency o f P300 increased as t h e d i f f i c u l t y in c l a s s i f y i n g a stimulus increased and that variations in response set (i.e., speed,

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+4----+796 msec F i g u r e 5. In t h e t o p p a r t o f t h e f i g u r e is an example o f t h e p r o b a b i l i t y e f f e c t measured a t Pz f o r t h e t a r g e t stimulus (a t o n e ) . T h i s is t a k e n f r o m Johnson (1986) a n d i s r e p r i n t e d w i t h permission o f t h e a u t h o r a n d publisher. In t h e lower p a r t o f t h e f i g u r e is an example o f t h e ERPs p r o d u c e d in t h e omitted stimulus t a s k . T h e solid l i n e is t h e ERP elicited by t h e stimulus omission a n d t h e dashed l i n e i s t h e ERP p r o d u c e d by t h e Note h e r e t h e presence o f a P300 t o t h e omission presentation o f a tone. of t h e tone. T h i s p a r t o f t h e f i g u r e i s adapted f r o m Donchin & M c C a r t h y (1980) a n d i s r e p r i n t e d w i t h permission o f t h e a u t h o r s a n d p u b l i s h e r .

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speed/accuracy i n s t r u c t i o n s ) p r o d u c e d l a r g e changes in R T but l i t t l e change in P300 latency. T h i s dissociation between t h e effects o f response set on R T a n d P300 latency has also been r e p o r t e d by Pfefferbaum, Ford, Johnson, Wenegrat, a n d Kopell (19831, Coles, Gratton, Bashore, Eriksen, a n d Donchin (19851, a n d S t r a y e r , Wickens, a n d B r a u n e (1987).

A v e r y c o n v i n c i n g demonstration o f t h e relation o f P300 latency t o stimulus processing was p r o v i d e d by M c C a r t h y a n d Donchin (1981). T h e y v a r i e d stimulus d i s c r i m i n a b i l i t y a n d response compatibility o r t h o g o n a l l y a n d f o u n d t h a t t h e stimulus manipulation influenced b o t h P300 latency a n d RT, but t h a t t h e response manipulation altered R T a n d h a d l i t t l e effect on P300 latency. T h e i n f e r e n c e d r a w n f r o m t h i s s t u d y was t h a t t h e t i m i n g o f P300 is influenced s t r o n g l y by stimulus processing a n d o n l y weakly, if a t all, by response processing. T h i s basic finding has been replicated a n d extended in w o r k by Magliero e t al. (1984). Other demonstrations o f t h e dependence o f P300 latency on t h e r a t e o f stimulus processing have been p r o v i d e d by Callaway (19831, Ford, Roth, Mohs, Hopkins, a n d Kodell (19791, M u l d e r e t al. (19841, a n d others, as well as in t h e w o r k o f R u g g e t al. (1984) a n d Ledlow e t a l . (1978a), discussed e a r l i e r . A l t h o u g h evidence is c o n v e r g i n g in s u p p o r t o f t h e stimulus-processing hypothesis, some c o n t r o v e r s y remains as t o t h e r e l a t i v e c o n t r i b u t i o n o f response processing t o t h e latency o f P300 (Bashore & Osman, 1987; Ragot, 1984; see discussion below a n d C h a p t e r 8 by Ragot in t h i s volume. 1 N200, P300, a n d S - R C o m p a t i b i l i t y Bashore a n d Osman (in p r e p a r a t i o n ) have r e c e n t l y completed t w o experiments t h a t p r o d u c e d a set o f unexpected f i n d i n g s t h a t a r e incons i s t e n t w i t h t h e stimulus-processing hypothesis. First, t h e timing and amplitude o f an ERP component, t h e N200, were f o u n d t o b e sensitive t o variations in S-R compatibility b u t n o t t o variations in stimulus-processing complexity. And, second, P300 latency was o b s e r v e d t o v a r y w i t h SR compatibility. These t w o experiments a r e p a r t o f a series o f studies w e h a v e u n d e r t a k e n t o examine t h e continuous flow model u s i n g an experimental paradigm developed by Charles E r i k s e n a n d h i s colleagues ( e . g . , E r i k s e n & Eriksen, 1974; E r i k s e n & Schultz, 1979). T h e basic E r i k s e n t a s k is a two-choice reaction t h a t r e q u i r e s t h e subject t o respond t o t h e c e n t r a l c h a r a c t e r of a f i v e character a r r a y . T h e r e a r e t w o possible c e n t r a l characters (e.g., H o r S ) , a n d each signals a d i f f e r e n t response (e.g., l e v e r movement t o t h e l e f t o r t o the right). T w o classes of stimulus arrays, c o n g r u e n t a n d incongruent, a r e presented. In a c o n g r u e n t a r r a y t h e c e n t r a l c h a r a c t e r i s f l a n k e d by i t s e l f ( e . g . , HHHHH), whereas in an i n c o n g r u e n t a r r a y it is f l a n k e d by t h e c h a r a c t e r d e s i g n a t i n g t h e alternat i v e response (e.g., SSUSS). T h e subject i s i n s t r u c t e d t o i g n o r e t h e f l a n k i n g elements i n t h e a r r a y a n d t o respond o n l y t o t h e c e n t r a l stimul u s . U n d e r these circumstances, R T has always been f o u n d t o b e longer (40-80 ms) a n d e r r o r rates h i g h e r t o i n c o n g r u e n t t h a n t o c o n g r u e n t a r r a y s , even a f t e r substantial p r a c t i c e ( e . g . , Proctor & Fober, 1988). Eriksen a n d Schultz (1979) hypothesized t h a t these effects s u p p o r t a "continuous flow" model o f information processing. In t h i s model, response activation i s i n i t i a t e d c o n c u r r e n t l y w i t h perceptual analysis a n d

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gains cumulative s t r e n g t h by t h e continuous t r a n s f e r o f new stimulus information t r o m t h e perceptual system, as information in t h i s system g r a d u a l l y accumulates. Moreover, u n d e r c e r t a i n conditions, a process o f response competition is hypothesized t o b e i n i t i a t e d simultaneously w i t h response activation, t h e e f f e c t of which is t o inhibit t h e activation o f alternative responses. T h e increases in R T a n d e r r o r r a t e to t h e incong r u e n t a r r a y s a r e t h o u g h t t o arise because t h e e a r l y perceptual evidence f a v o r s t h e i n c o r r e c t response a n d activates response competition processes more t h a n when a c o n g r u e n t a r r a y i s presented. Greater activation of t h e i n c o r r e c t response leads, in t u r n , t o g r e a t e r i n h i b i t i o n o f t h e c o r r e c t response a n d slower, less accurate R T s t o t h e incongruent arrays. C o g n i t i v e psychophysiological tests o f t h e continuous flow model have been conducted by Coles, Eriksen, a n d colleagues (Coles e t al., 1985; Gratton, Coles, Sirevaag, Eriksen, & Donchin, 1988) u s i n g t h e t r a ditional E r i k s e n t a s k described above. T h e i r results p r o v i d e general s u p p o r t f o r t h e response activation and competition elements o f t h e model, as t h e y p e r t a i n to t h e congruency effect. However, all o f t h e tests o f t h e model, u s i n g b o t h behavioral and psychophysiological measures, have been r e s t r i c t e d t o manipulations o f stimulus processing, although import a n t inferences have been d r a w n about response-related processes. For t h i s reason, Osman a n d I investigated t h e interaction between stimulus congruency a n d S-R compatibility. We chose t o v a r y S-R compatibility because, as discussed in t h i s volume, a l a r g e b o d y o f l i t e r a t u r e indicates t h a t it influences response processes, and, as reviewed earlier, t h i s fact o r has been observed t o p r o l o n g R T while h a v i n g no e f f e c t on P300 latency. Thus, we developed a version o f t h e basic Eriksen t a s k t h a t permits S-R compatibility t o b e manipulated. Preliminary analyses of data f r o m t h e t w o experiments have been completed, and t h e following discussion o f these results should b e read accordingly.

In t h e f i r s t experiment, subjects were shown a f i v e element a r r a y t h a t consisted of arrows p o i n t i n g t o t h e l e f t o r t o t h e right. A c o n g r u e n t a r r a y h a d f i v e arrows t h a t p o i n t in t h e same direction (e.g., -->-->*-->-->I a n d an incongruent a r r a y h a d a c e n t r a l a r r o w t h a t points i n one direction and f l a n k i n g arrows t h a t p o i n t i n t h e opposite d i r e c t i o n (e.g., <- - < - - - - > < - - < - - ) . Subjects were i n s t r u c t e d t o a t t e n d t o t h e c e n t r a l arrow, a n d o n t h e basis o f i t s direction, t o move a response l e v e r away f r o m t h e midline w i t h t h e l e f t o r t h e right hand (each response device was located comfortably a t t h e subject's side). S-R compatibility was v a r i e d by changing t h e mapping between t h e t w o responses a n d t h e direction o f t h e c e n t r a l a r r o w . On compatible t r i a l s , subjects responded by moving t h e l e v e r on t h e side t o which t h e a r r o w pointed (e.g., a right a r r o w signaled a movement t o t h e right w i t h t h e right l e v e r ) . On incompatible t r i a l s , subjects responded by moving a l e v e r on t h e side opposite t o t h a t which t h e a r r o w pointed (e.g., a right a r r o w signaled a movement t o t h e l e f t w i t h t h e l e f t hand). For a response t o b e recorded, t h e l e v e r had t o b e moved beyond a c r i t e r i o n threshold. R T was defined as t h e time a t which t h i s c r i t e r i o n was achieved. A t r i a l was i n i t i a t e d by a w a r n i n g word, SAME o r OPPOSITE, whose o f f s e t was followed s h o r t l y by t h e b r i e f presentation o f an arrow a r r a y (200 ms). SAME signaled a compatible response a n d OPPOSITE signaled a n incompatible response. I n s t r u c t i o n s placed a s l i g h t emphasis o n accuracy. Variations in stimulus congruency and S-R compatibility o c c u r r e d

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randomly ( a n d e q u i p r o b a b l y ) w i t h i n a b l o c k o f t r i a l s . ERPs w e r e recorded f r o m Fz, Cz, Pz, Oz, C3' a n d C4' ( t h e l a t t e r t w o sites a r e nons t a n d a r d placements approximately 4 cm lateral t o Cz along t h e i n t e r a u r a l line). Twelve y o u n g a d u l t s were tested in t h r e e 2f h o u r sessions. In t h e f i r s t session, READY was t h e w a r n i n g w o r d on e v e r y t r i a l , a n d subjects o n l y made compatible responses. T h i s is a v a r i a n t o f t h e t r a d i t i o n a l Eriksen t a s k and, therefore, s e r v e d as a c o n t r o l t o i n s u r e t h a t t h e inclusion o f incompatible responses did n o t have a q u a l i t a t i v e e f f e c t o n t h e subject's performance o f compatible responses. In t h e last t w o sessions, SAME a n d OPPOSITE were t h e w a r n i n g words, a n d subjects were r e q u i r e d to make t h e a p p r o p r i a t e compatible or incompatible responses. Experimental data were collected i n 8 blocks o f 96 t r i a l s during each session. Subjects were g i v e n t w o p r a c t i c e blocks o f 96 t r i a l s each in t h e f i r s t t w o sessions, a n d an i n i t i a l warm-up b l o c k of 24 t r i a l s in t h e third session. O u r p r e l i m i n a r y analyses of 10 of t h e 12 subjects have revealed some discernible t r e n d s . T h e ERPs recorded a t Pz a r e shown in F i g u r e 6, a n d t h e s i g n i f i c a n t component latency a n d R T data a r e summarized in Table 1. First, note t h a t t h e RTs a r e similar in t h e c o n t r o l t a s k a n d in t h e compatible conditions of t h e experimental task, a n d t h e magnitude o f t h e e f f e c t o f i n c o n g r u e n c y on R T r e p o r t e d by Eriksen's g r o u p has been T h e r e is a s l i g h t slowing o f R T in t h e compatireplicated i n b o t h tasks, b l e conditions of t h e experimental task, w h i c h may r e f l e c t a conservative

B I

C

i

F i g u r e 6. ERPs recorded a t Pz. Time is o n t h e abscissa a n d amplitude is o n t h e o r d i n a t e . T h e v e r t i c a l dashed l i n e indicates t h e p o i n t in time when t h e a r r o w a r r a y was presented. ERPs in solid lines were elicited by c o n g r u e n t stimulus a r r a y s a n d ERPs in dashed lines were i n v o k e d by incongruent arrays. A: C o n t r o l task; 6 : Experimental task--compatible response; C : Experimental tas k--incompatible response.

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s h i f t i n strategy w i t h t h e increase i n task demands. Second, visual inspection of t h e ERPs reveals t h a t t h e experimental manipulations i n f l u enced two components, t h e P300 and t h e N200. Note t h a t t h e latencies of these components are reasonably comparable i n t h e control and experimental (congruent condition) tasks. An interesting set o f findings is emerging from t h e experimental task. Both R T and P300 latency were prolonged when an incongruent a r r a y was presented and when an incompatible response was made, and these effects produced an underadditive interaction. I n contrast, N200 latency was unaffected by incongruency b u t prolonged by incompatibility. R T was increased i n t h e control and experimental tasks by 64 ms and 49 ms, respectively, when an incongruent a r r a y was presented, and by 32 ms when an incompatible response was made. T h e cost of incompatibility on RT was reduced, however, when the response was made t o an incong r u e n t a r r a y (37 vs 28 ms). Like RT, P300 latency was prolonged i n t h e control and experimental tasks (by 72 ms and 67 ms, respectively) when an incongruent a r r a y was presented. I n contrast t o w o r k cited above, P300 latency was also prolonged by 57 ms when an incompatible response was made. As w i t h RT, t h e cost of incompatibility was reduced when an Unlike P300, variations incongruent a r r a y was presented (74 v s 41 ms). Table 1 Reaction Time (MS) and ERP Data from Arrow Study ~~

N200

~~

P300 Amplitude Latency

Amplitude

Latency

RT

Cong

38

220

89

372

425

I ncong

36

221

83

444

489

Com/Cong

41

214

85

364

450

Com/lncong

37

218

82

448

503

Incom/Cong

48

224

77

438

487

Incom/lncong

44

223

76

489

531

Control

Experimental

Note. Results from preliminary analysis of Experiment 1 by Bashore and Congzcong ruent , I ncong=i ncongruent , Com=compati ble , Osman ( 1987) . Incom=incompatible; amplitude is given i n a r b i t r a r y units. N200 amplitude is measured from t h e peak of t h e P200 t o the peak of t h e N200, and P300 amplitude i s estimated by comparing i t s maximum peak against t h e average value f o r t h e 100 ms p r i o r t o t h e presentation of t h e warning stimulus. The values f o r t h e N200 actually represent a negative change i n value from t h e P200.

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i n stimulus c o n g r u e n c y did n o t a f f e c t N200 latency in e i t h e r t h e c o n t r o l o r t h e experimental task; l i k e P300, N200 latency was lengthened (by 8 ms) when an incompatible response was made. B o t h experimental f a c t o r s also p r o d u c e d d i f f e r e n t i a l effects on N200 a n d P300 amplitude: T h e N200 increased in amplitude when an incompatible response was made, but was unchanged when an i n c o n g r u e n t a r r a y was presented; whereas P300 amplitude was reduced when e i t h e r an i n c o n g r u e n t a r r a y was p r e s e n t e d o r an incompatible response was executed. T h e influences t h a t we have o b s e r v e d o f variations in stimulus congruency a n d S-R compatibility on R T a r e consistent w i t h p r e v i o u s research. A u n i q u e observation, however, is t h a t presentation o f a n i n c o n g r u e n t stimulus a r r a y reduces t h e cost o f incompatibility, as revealed in R T . T h i s e f f e c t is predicted, however, by t h e continuous f l o w model. T h a t is, i n c o n g r u e n t noise in t h e stimulus a r r a y i s h y p o t h e sized t o activate t h e response opposite t o t h a t signaled by t h e c e n t r a l a r r o w . T h i s activation should increase response competition when a cornp a t i b l e response i s made, but reduce it when an incompatible response is made. In o t h e r words, an incompatible response should b e f a c i l i t a t e d when an i n c o n g r u e n t stimulus a r r a y i s p r e s e n t e d because t h e opposite response is activated. T h e f a c t t h a t t h e effects o f t h e stimulus-congruency a n d S-R comp a t i b i l i t y manipulations p r o d u c e d parallel effects on P300 latency is more d i f f i c u l t t o i n t e r p r e t . T h e w o r k o f Coles, Eriksen, a n d colleagues (e.g., Coles e t al., 1985) revealed t h a t P300 latency increased when an i n c o n g r u e n t a r r a y was presented. O n t h i s basis, a n d o n t h e o b s e r v e d s e n s i t i v i t y o f P300 t o stimulus but n o t t o response manipulations, w e expected P300 latency t o b e increased by i n c o n g r u e n c y but n o t by S-R incompatibility. Some studies, l i k e those o f Ragot reviewed in C h a p t e r 8 (see also F o r d & Pfefferbaum, 19851, have r e p o r t e d t h a t t h e t i m i n g o f t h e P300 component i s sensitive t o b o t h stimulus a n d response-selection factors. O u r f i n d i n g s a r e consistent w i t h these l a t t e r studies a n d s u p p o r t t h e inference t h a t t h e P300 is sensitive t o b o t h stimulus a n d response-selection processes. T h e r e is, however, a fundamentally i m p o r t a n t d i f f e r e n c e in t h e stimulus a r r a y s p r e s e n t e d i n t h e t w o sets o f studies. Those studies t h a t r e p o r t a s t r o n g stimulus e f f e c t a n d l i t t l e response-selection e f f e c t on P300 latency have v a r i e d elements o f stimulus processing u s i n g manipulations t h a t a r e n o t associated w i t h e i t h e r response, whereas those studies t h a t r e p o r t a response e f f e c t on P300 latency have used manipulations in w h i c h t h e stimulus a r r a y includes elements associated w i t h b o t h responses. T h e E r i k s e n paradigm, by i t s v e r y aim, must c o n f o u n d stimulus a n d response-selection elements. T o disentangle t h e stimulus a n d response-selection elements o f t h e stimulus a r r a y , we conducted a second experiment. Six subjects comp l e t e d one t e s t session t h a t i n c l u d e d 12 blocks o f 96 trials, t h e f i r s t 2 blocks o f w h i c h were practice. T h e y were r e q u i r e d t o make compatible o r incompatible responses, signaled by t h e w o r d SAME o r OPPOSITE, t o t h e same stimulus a r r a y s used in t h e f i r s t experiment, as well as t o an a r r a y w i t h a l e f t - o r r i g h t - p o i n t i n g c e n t r a l a r r o w f l a n k e d by a n e u t r a l stimulus ( t w o asterisks on each side, <--*). Stimulus c o n g r u e n c y (congruent, incongruent, n e u t r a l ) a n d S-R compatibility were v a r i e d randomly a n d equiprobably within a block of trials. T h e experimental e f f e c t s seen in t h e f i r s t experiment f o r a r r o w a r r a y s w e r e replicated. However, a l t h o u g h p r o d u c t i o n of an incompatible response p r o l o n g e d R T when a

*

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n e u t r a l a r r a y was presented, P300 latency appeared t o b e unaffected. Hence, t h e e f f e c t of S - R compatibility on P300 latency when an i n c o n g r u e n t a r r a y is p r e s e n t e d may r e f l e c t t h e c o n f o u n d i n g o f stimulus a n d response-selection elements in t h e noise stimuli. Response-Channel A c t i v a t i o n O u r observation t h a t t h e t i m i n g a n d amplitude o f t h e N200 v a r i e d as a f u n c t i o n o f changes in S-R compatibility is consistent w i t h o t h e r w o r k t h a t indicates t h a t t h i s component reflects t h e activation o f responserelated processes (Renault, Ragot, Lesevre, & Remond, 1982). Its a p p a r e n t independence o f stimulus-related processing, when considered in conjunction w i t h t h e seeming dependence o f P300 on stimulus-related p r o cessing, suggests t h e presence o f separate stimulus- a n d responseprocessing systems. T h e i n t e r a c t i v e effects o f stimulus c o n g r u e n c y a n d S-R compatibility o n P300 latency t h a t we o b s e r v e d suggests f u r t h e r t h a t information is t r a n s f e r r e d between these t w o systems as t h e reaction emerges. T h e w o r k o f Coles, Eriksen, a n d associates has revealed an import a n t relation between v a r i a t i o n in t h e asymmetry o f t h e readiness potential, d e s c r i b e d above, t h a t can b e used t o i n f e r t h e cumulative activation o f response channels in t h e response system. Recall t h a t t h e continuous flow model postulates t h e co-existence o f response activation a n d response competition mechanisms. U n d e r circumstances in w h i c h a stimulus a r r a y contains information c o n c e r n i n g b o t h t h e c o r r e c t a n d t h e i n c o r r e c t response, each response channel i s t h o u g h t t o b e a c t i v a t e d c o n c u r r e n t l y . Coles e t al. have d i s t i n g u i s h e d t h e response- (conceptualized in terms o f response channels; see G r a t t o n e t al., 1988, for a complete discussion) a n d stimulus-processing systems as follows. T h e f u n c t i o n o f t h e l a t t e r is t o analyze stimulus information a n d to convey t h e r e s u l t s o f t h i s analysis t o t h e a p p r o p r i a t e response channel, so t h a t t h e response can b e p r e p a r e d . Partial information about t h e stimulus is conveyed c o n t i n u o u s l y t o t h e response system as it accumulates. T h e level o f activation o f t h e response channel also varies c o n t i n u o u s l y as a f u n c t i o n o f factors such as priming (i.e., response bias) or response competition (i.e., c o n c u r r e n t activation o f t h e c o r r e c t a n d i n c o r r e c t response channels). Hence, t h e activation level o f t h e response system, l i k e t h a t o f t h e stimulus system, can v a r y a t a n y moment. Response p r e p a r a t i o n produces an increase i n level o f activation o f a response channel, a n d response competition p r o duces a simultaneous decrease in t h e level o f activation o f one channel a n d an increase in t h e o t h e r . T h e activation o f response channels can b e i n f e r r e d , f o r example, f r o m p e r i p h e r a l sources such as electromyographic (EMG) activity r e c o r d e d f r o m t h e muscles i n v o l v e d in t h e response (Coles e t al., 1985) o r f r o m scalp r e c o r d i n g s o f c e n t r a l a c t i v i t y such as t h e readiness potent i a l (RP) ( G r a t t o n e t al., 1988). T h e RP, shown in F i g u r e 7, is classic a l l y d e f i n e d in association w i t h spontaneous movements o f t h e d i s t a l extremities ( K o r n h u b e r & Deecke, 1965). It is a slow, negative change in electrical p o l a r i t y t h a t i s l a r g e s t a t c e n t r a l scalp sites o v e r motor areas contralateral t o t h e d i s t a l e x t r e m i t y e x e c u t i n g t h e movement. T h i s negativity is a p p a r e n t several h u n d r e d milliseconds p r i o r t o t h e movement a t b o t h lateral c e n t r a l scalp sites a n d increases in a slow, r a m p - l i k e fashion until about 100 ms b e f o r e t h e response. A t t h i s point, t h e n e g a t i v i t y becomes l a r g e r a t t h e scalp s i t e contralateral t o t h e moving e x t r e m i t y a n d accelerates r a p i d l y in amplitude t o peak s h o r t l y a f t e r t h e movement is

A Psychophysiological Perspective

LEFT

HAND

RIGHT

207

HAND

A-

F i g u r e 7. On t h e l e f t side o f t h e f i g u r e is shown g r a n d averages o f t h e readiness potentials recorded a t C3' a n d C4' ( i d e n t i f i e d as C1' a n d C2' in t h e f i g u r e , a d i f f e r e n c e t h a t reflects a change in o u r terminology) f r o m g r o u p s o f y o u n g a d u l t males who were e i t h e r l e f t - ( L I , LN) o r righthanded ( R N ) as t h e y executed a self-paced b a l l i s t i c squeeze. Movement onset is indicated by t h e v e r t i c a l line; t h e a c t i v i t y a t C3' is represented by a solid l i n e a n d a t C4' by a dashed line. Difference waves f o r each o f these g r o u p s a r e shown on t h e right side. T h e solid a n d dashed lines i d e n t i f y d i f f e r e n c e waves f o r l e f t a n d right h a n d squeezes, respectively. Time 0 ( a n d t h e v e r t i c a l line) indicates i n i t i a t i o n of t h e movement. Negative changes in electrical p o l a r i t y a r e shown as u p w a r d deflect i o n s f o r b o t h t h e RPs a n d d i f f e r e n c e waves. T h i s f i g u r e i s a composite o f t w o f i g u r e s t a k e n f r o m Bashore e t al. (19821, a n d i s r e p r i n t e d w i t h t h e permission o f t h e p u b l i s h e r .

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executed. T h i s n e g a t i v i t y resolves i n t o a positive-going signal f o r a few h u n d r e d milliseconds a f t e r t h e response. A r e l a t i v e n e g a t i v i t y (i.e., less p o s i t i v i t y ) p e r s i s t s a t t h e contralateral scalp s i t e during t h i s p e r i o d . A substantial l i t e r a t u r e suggests t h a t t h e RP p r o v i d e s an i n d e x o f t h e activation o f specific motor systems in choice reactions, b o t h b e f o r e a n d a f t e r presentation o f an imperative stimulus, when response-hand decisions m u s t b e made (e.g., Kutas & Donchin, 1980; see discussion in G r a t t o n e t al., 1988). T h e Coles-Eriksen g r o u p have used t h i s t o assess b o t h p r e - a n d poststimulus activation in response channels as a choice reaction is e v o l v i n g . M y discussion w i l l b e restricted, however, t o t h e poststimulus p e r i o d . T o determine t h e d i r e c t i o n a n d degree o f l a t e r a l i t y in t h e RP, a d i f f e r e n c e wave i s obtained by t h e p o i n t - b y - p o i n t s u b t r a c t i o n o f t h e digit i z e d values of t h e signals recorded a t electrodes sites C3'and C4' ( w h e r e Since t h e RP t h e RP is largest, Kutas & Donchin, 1974; see F i g u r e 7). is negative in p o l a r i t y a n d l a r g e s t a t t h e c e n t r a l s i t e contralateral t o t h e d i s t a l e x t r e m i t y e x e c u t i n g t h e movement, t h e d i f f e r e n c e wave can b e used t o i n f e r w h i c h response channel, (i.e., l e f t o r right hand) i s b e i n g activ a t e d in t h e motor c o r t e x . More importantly, it p r o v i d e s a continuous measure o f t h i s activation. T o o b t a i n t h i s wave, a c t i v i t y recorded a t t h e scalp s i t e ipsilateral t o t h e c o r r e c t movement (i.e., o v e r t h e motor sites t h a t d o n o t c o n t r o l t h e movement) is s u b t r a c t e d f r o m t h e a c t i v i t y r e c o r d e d a t t h e scalp s i t e contralateral t o t h e movement (i.e., o v e r t h e motor sites t h a t c o n t r o l t h e movement). Thus, t h e d i f f e r e n c e wave is negative f o r a c o r r e c t response a n d p o s i t i v e f o r an i n c o r r e c t response in a choice R T t a s k ( f o r l e f t hand, C4'-C3' = d i f f e r e n c e wave; f o r right hand, C3'-C4' = d i f f e r e n c e wave). B y u s i n g t h i s measure, then, i n f e r ences can b e d r a w n about t h e activation of response channels as choice reactions a r e emerging. G r a t t o n e t al. (1988) have i d e n t i f i e d an asymmetry in t h e poststimulus RP t h a t v a r i e s w i t h changes in stimulus c o n g r u e n c y a n d accords n i c e l y w i t h conditional accuracy measures on response latency (in t h e i r case, activation o f t h e forearm EMG recorded f r o m t h e muscles c o n t r o l l i n g a squeeze response). F i g u r e 8, t a k e n f r o m t h e i r paper, shows v a r i a t i o n s in t h e shape of t h e d i f f e r e n c e wave f o r compatible responses t o c o n g r u e n t a n d i n c o n g r u e n t a r r a y s e i t h e r f o r a l l t r i a l s (middle panel) o r o n l y f o r t r i a l s w i t h slow, highly accurate responses t o t h e t a r g e t stimuli (see bottom panel) in t h e E r i k s e n t a s k . Note t h a t t h e d i f f e r e n c e wave is negative-going following presentation o f a c o n g r u e n t a r r a y , but it is p o s i t i v e - g o i n g f o r t h e f i r s t 150-250 ms a f t e r presentation o f an i n c o n g r u e n t a r r a y , b e f o r e becoming negative. T h e e a r l y p o s i t i v i t y in response t o t h e i n c o n g r u e n t a r r a y indicates t h a t c o r t i c a l motor a c t i v i t y p r e p a r i n g t h e i n c o r r e c t response was activated. Note f u r t h e r t h a t t h i s o c c u r r e d f o r slow responses w i t h high levels o f accuracy (i.e., when o v e r t responses were c o r r e c t ) . T h e suggestion f r o m t h i s l a t t e r f i n d i n g is t h a t when an i n c o n g r u e n t a r r a y is presented, activation o f those cortical elements o f t h e motor system w h i c h mediate t h e i n c o r r e c t response occurs even when a c o r r e c t response i s executed a n d t h e r e i s no evidence ( a t t h e EMG level) o f i n c o r r e c t movement. Note also t h a t t h e shapes o f t h e d i f f e r e n c e waves f o r b o t h f a s t a n d slow responses resemble t h e shape of a conditional accuracy f u n c t i o n G r a t t o n e t al. d e r i v e d f o r response latency (shown in t h e t o p panel).

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F i g u r e 8. T h e u p p e r panel shows conditional accuracy f u n c t i o n s f o r comp a t i b l e (what Bashore & Osman call c o n g r u e n t ) a n d incompatible ( w h a t Bashore 8 Osman call i n c o n g r u e n t ) t r i a l s . T h e d i f f e r e n c e wave f o r a l l c o n g r u e n t a n d i n c o n g r u e n t t r i a l s is shown i n t h e middle panel, a n d o n l y f o r c o r r e c t t r i a l s w i t h response latencies between 300 a n d 349 ms in t h e bottom panel. T h i s f i g u r e is t a k e n f r o m G r a t t o n e t al. (1988). It is r e p r i n t e d w i t h t h e p u b l i s h e r ' s a n d authors' permission.

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Where A r e We?

I n t h i s review, I have described e a r l y components o f t h e ERP, whose amplitude a n d latency p r o v i d e indices o f transmission o v e r p r i m a r y sensory system pathways, t h a t a r e n o t affected by variations in h i g h e r o r d e r c o g n i t i v e demands imposed by factors such as S-R compatibility. F u r t h e r , I have suggested t h a t t h e r e a r e late components o f t h e ERP whose p r o p e r t i e s a r e d i f f e r e n t i a l l y sensitive t o manipulations o f h i g h e r o r d e r stimulus a n d response processes. I n p a r t i c u l a r , t h e N200 has been i d e n t i f i e d as one component t h a t i s sensitive t o manipulations of responseselection processes, but n o t t o manipulations of stimulus processes. As a result, it can b e s t u d i e d i n conjunction w i t h P300, whose latency is a p p a r e n t l y sensitive t o manipulations o f stimulus processes, but n o t t o manipulations o f response-selection processes, t o h e l p a r t i c u l a t e t h e s t r u c t u r e a n d t i m i n g o f mental processing. I m p o r t a n t t o t h e argument a r e t h e observations t h a t t h e effects o f a number o f experimental f a c t o r s o n R T a n d components o f t h e ERP can b e dissociated. T h i s suggests, o f course, t h a t something more t h a n r e d u n d a n t information is b e i n g p r o v i d e d by t h e addition of these psychophysiological measures. I n addition, t h e d i f f e r e n t i a l s e n s i t i v i t y of t h e P300 a n d N200 t o stimulus a n d response manipulations suggests t h a t t h e y a r e manifestations o f t w o d i s t i n c t processing systems. T h i s view is consonant with t h a t o f f e r e d by Coles, Eriksen, a n d associates i n t h e i r psychophysiological analyses o f response activation elements o f t h e continuous f l o w model ( G r a t t o n e t al., 1988; see also Ragot & Lesevre, 1986). T h i s b o d y o f f i n d i n g s can now b e used t o g u i d e a c o g n i t i v e p s y chophysiological dissection of the mechanisms underlying S-R compatibility.

T h e Somatosensory System a n d S - R C o m p a t i b i l i t y A t t h i s p o i n t I am g o i n g t o make w h a t a t f i r s t b l u s h may seem t o b e a reasonably a b r u p t t r a n s i t i o n i n my discussion. T h u s f a r it has centered on v i s u a l information processing. I n t h i s section, however, I am g o i n g t o a r g u e t h a t many o f t h e issues raised in t h e visuomotor studies can b e addressed most e f f e c t i v e l y by s t u d y i n g S-R compatibility I am aware of o n l y one such s t u d y , effects in t h e somatosensory system. a n d it demonstrated t h a t manipulations o f S - R compatibility in t h i s modali t y p r o d u c e behavioral effects resembling those seen i n t h e a u d i t o r y a n d Further, I will argue that the v i s u a l modalities ( G a r d n e r & Ward, 1979). anatomical p r o p e r t i e s o f t h e somatosensory system make it a more suitable modality t h a n e i t h e r t h e v i s u a l o r a u d i t o r y f o r d i s t i n g u i s h i n g t h e p r i m a r y sensory f r o m t h e h i g h e r o r d e r c o g n i t i v e consistitutents o f S - R compatibility i n c o g n i t i v e psychophysiological s t u d i e s . T h e somatosensory system has t h r e e anatomical characteristics t h a t make it especially a t t r a c t i v e f o r use i n c o g n i t i v e psychophysiological studies o f S-R compatibility (Carpenter, 1978): (i) it has a long, wellisolated p e r i p h e r a l component; (ii) i t s p r i m a r y receptive areas l i e on t h e s u r f a c e r a t h e r t h a n i n t h e medial depths of c o r t e x (as in t h e a u d i t o r y a n d v i s u a l systems); a n d (iii) these cortical receptive areas a r e located in t h e hemisphere contralateral t o t h e p e r i p h e r a l receptors a n d receive a f f e r e n t input f r o m pathways t h a t a r e almost, if n o t completely, crossed in t h e dorsal column-lemniscal pathways. S t u d y o f t h e somatosensory

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system is appealing from an electrophysiological perspective, because i t s structural properties have p e r m i t t e d a reasonably s t r a i g h t f o r w a r d differentiation o f components o f t h e ERP t h a t a r e generated f r o m p e r i p h e r a l a n d c e n t r a l sources i n response t o t a c t i l e stimulation o f t h e lemniscal pathways. In addition, f a i r l y good p r o g r e s s has been achieved in i d e n t i f y i n g t h e c e n t r a l (i.e., spinal cord, brainstem, subcortical, a n d T h e electrophysiological cortical) o r i g i n of c e r t a i n e a r l y components. p r o p e r t i e s o f t h e somatosensory system have been s t u d i e d e x t e n s i v e l y by Desmedt a n d colleagues (e.g., Desmedt, 1981; Desmedt, 1986; Desmedt & Brunko, 1980; Desmedt & Huy, 1984; Desmedt, H u y , & Bourguet, 1983; Desmedt & Robertson, 1977; Mauguiere, Desmedt, & Courjon, 1983). T h e discussion t h a t follows is d r a w n l a r g e l y f r o m t h e i r w o r k . T h e p a t h o f an a f f e r e n t volley p r o d u c e d by m i l d electrical stimulat i o n t o t h e f i n g e r s o f a h a n d can b e t r a c e d w i t h v e r y good precision t h r o u g h t h e e x t e n t of t h e somatosensory pathways i n t o p r i m a r y r e c e p t i v e cortex. T h e i n i t i a l 25 ms following stimulation o f t h r e e f i n g e r s o n t h e l e f t h a n d (thumb, index, middle) is shown i n F i g u r e 9 ( f r o m Desmedt & Cheron, 1980). T h i s stimulation activates t h e median n e r v e in t h e l e f t arm, as revealed in sensory n e r v e action potentials recorded a t t h e w r i s t , forearm, elbow, axilla ( a x i l l a r y n e r v e near t h e shoulder), a n d Erb's p o i n t ( o v e r t h e brachial p l e x u s a t t h e mid-clavicle). These action potentials a r e seen w i t h i n t h e f i r s t 9-11 ms a f t e r t h e stimulus i s delivered. Entry of t h e afferent impulse i n t o t h e spinal c o r d occurs a t t h e component labeled as t h e N11 a n d reception i n p r i m a r y somatosensory c o r t e x is marked by t h e N20 (see also Wood, Cohen, C u f f i n , Yarita, & Allison, 1985). T h e l a t t e r component is l a r g e s t a t t h e scalp s i t e o v e r p a r i e t a l somatosensory c o r t e x contralateral t o t h e stimulated f i n g e r s . These ERPs were evoked u n d e r passive stimulation conditions (i.e., t h e subject sat q u i e t l y as stimulation was d e l i v e r e d a n d h a d n o specific t a s k t o do.) A n example of somatosensory ERPs recorded in an oddball t a s k a r e shown i n F i g u r e 10 ( f r o m Desmedt e t al., 1983). Stimulation was d e l i v e r e d t o t h e l e f t t h u m b o r l e f t index f i n g e r , a n d t h e subject was r e q u i r e d t o p r e s s a b u t t o n w i t h h i d h e r right index f i n g e r when one of t h e d i g i t s was Stimulated ( e . g . , p r e s s when t h u m b stimulated, w i t h h o l d p r e s s when index f i n g e r stimulated). T h e stimulation c a l l i n g f o r a response o c c u r r e d randomly on 15% o f t h e t r i a l s . These ERPs were elicited by stimulation of t h e t h u m b when it was t h e i n f r e q u e n t e v e n t ( t h i c k line) o r when it o c c u r r e d on e v e r y t r i a l i n a passive c o n t r o l condition (thin l i n e ) . I n t h e u p p e r half of t h e f i g u r e , labeled A a n d B, a r e t h e signals recorded a t a scalp s i t e o v e r t h e contralateral p a r i e t a l c o r t e x ( A ) o r o v e r t h e p r i m a r y motor c o r t e x (B; labeled as p r e r o l a n d i c by Desmedt's g r o u p ) i n t h e f i r s t 160 ms a f t e r stimulus onset. T h e ERP recorded f o r 700 ms post-stimulation is shown as C . T h e p o s i t i v e component, P14, seen i n A a n d B, is t h o u g h t t o r e p r e s e n t a f f e r e n t volleys in t h e f i b e r t r a c t t h a t conveys t h e somesthetic information t h r o u g h t h e brainstem t o t h e thalamus ( t h e medial lemniscus; Desmedt 8 Huy, 1984). As indicated above, t h e N20 t h a t follows r e p r e sents t h e i n i t i a l activation o f p r i m a r y somatosensory c o r t e x . The two succeeding p o s i t i v e components, P27 a n d P45, seen o n l y in A, a r e also t h o u g h t t o o r i g i n a t e i n p r i m a r y somatosensory c o r t e x (Desmedt & B r u n k o , 19801, whereas t h e P22 a n d N30, seen o n l y in 6, a r e believed t o b e generated i n p r i m a r y motor c o r t e x (Mauguiere e t al., 1983). A p p a r e n t in

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F i g u r e 9. T h e i n i t i a l 25 ms following stimulation (mild shock) t o t h e f i r s t t h r e e f i n g e r s o f t h e l e f t h a n d u n d e r passive conditions. T h e pathway of t h e a f f e r e n t volley p r o d u c e d by t h i s stimulation i s t r a c e d t o p r i m a r y sensory c o r t e x . T h e s i t e o f activation is labeled i n t h e d r a w i n g ( f r o m Desmedt 8 Cheron, 1980). R e p r i n t e d w i t h permission o f a u t h o r s a n d publisher. each o f t h e ERPs recorded in t h i s t a s k is an e a r l y d i v e r g e n c e o f compon e n t amplitude t h a t occurs a f t e r t h e P14, N20, P22, o r N30 components a n d p e r s i s t s t h r o u g h o u t t h e balance of t h e r e c o r d i n g epoch. Divergence is e v i d e n t f i r s t a t t h e contralateral parietal scalp site, indexed by t h e component labeled as P40 (whose onset is about 26 ms post-stimulation). A similar d i v e r g e n c e i s n o t a p p a r e n t a t t h e p r e r o l a n d i c scalp s i t e u n t i l t h e N60 (whose onset is about 36 msec).

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F i g u r e 10. Examples o f ERPs recorded f r o m a single subject i n an oddball t a s k i n w h i c h stimulation o f t h e l e f t t h u m b was t h e counted r a r e e v e n t (pz.15). T h e t h i c k t r a c e is t h e ERP elicited by i n f r e q u e n t stimulation t o t h e t h u m b t h a t t h e subject counted, a n d t h e t h i n t r a c e i s identical stimulation t o t h e t h u m b t h a t o c c u r r e d on e v e r y t r i a l a n d was i g n o r e d by t h e subject. T h e ERPs labeled as A a n d B a r e f r o m t h e f i r s t 100 ms o f t h e r e c o r d i n g epoch, where t h e ERPs shown in C a r e f o r the f i r s t 700 ms. A c t i v i t y a t t h e r i g h t parietal electrode is shown in A a n d C, a n d a t t h e r i g h t p r e - r o l a n d i c i n B ( f r o m Desmedt e t al., 1983). R e p r i n t e d w i t h permission o f a u t h o r s a n d p u b l i s h e r .

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T h i s p a t t e r n suggests t h a t selection of t h e stimulation as t a r g e t o r n o n - t a r g e t is i n i t i a t e d in p r i m a r y somatosensory cortex, s h o r t l y a f t e r t h e a f f e r e n t signal i s received, a n d t h a t involvement o f p r i m a r y motor c o r t e x in t h e process does n o t o c c u r f o r several milliseconds. Indeed, i d e n t i f i cation o f these v e r y e a r l y components whose p r o p e r t i e s v a r y w i t h cognit i v e demands c o u l d lead t o a refinement in t h e exogenous-endogenous d i s t i n c t i o n I described e a r l i e r . Desmedt e t a l . (1983) r e f e r t o these t w o components as t h e "cognitive" P40 a n d N60. Subsequent t o these v e r y e a r l y "cognitive" components a r e middle latency components t h a t a r e elicited by t h e target, b u t n o t t h e non-target, t h e PlOO a n d N140 t h a t were most prominent a t t h e parietal a n d p r e r o l a n d i c sites, respectively; a n d t h e late p o s i t i v e component, t h e P300. I m p o r t a n t f o r o u r purposes is t h a t t h e ERPs elicited by somatosensory stimulation have v e r y c l e a r l y i d e n t i f i e d lateral components. F i g u r e 11 shows somatosensory ERPs recorded in a selective attention t a s k ( f r o m Desmedt & Robertson, 1977). In t h e conditions shown here,

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F i g u r e 11. T h e ERPs recorded in a selective attention t a s k a t t h e i p s i a n d contralateral parietal electrodes (labeled as B a n d C ) as a subject counted ( t h i c k line) o r i g n o r e d (thin line) stimulation t o t h e third f i n g e r o f t h e l e f t hand. Shown in E a n d F a r e t h e ERPs elicited by uncounted stimulation t o t h e second f i n g e r o f t h e same h a n d ( f r o m Desmedt & Robertson, 1977). R e p r i n t e d w i t h permission of authors a n d p u b l i s h e r .

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stimulation was presented t o t h e second o r third f i n g e r s o f t h e l e f t o r right hand. A r u n n i n g mental c o u n t o f stimulation t o t h e third f i n g e r of one h a n d was maintained by t h e subject, but stimulation t o t h e o t h e r f i n g e r s was n o t counted. T h e ERPs recorded a t t h e contralateral a n d ipsilateral parietal electrodes a r e shown i n 6 a n d C, respectively, when stimulation o f t h e third f i n g e r o f t h e l e f t h a n d was e i t h e r t h e counted ( t h i c k line) o r uncounted ( t h i n line) e v e n t . Comparison o f t h e ERPs a t contralateral a n d ipsilateral scalp sites reveals t h a t t h e P45 is comparable f o r b o t h t a r g e t s a n d nontargets, but it is o n l y e v i d e n t a t t h e contralateral site. T h e P300 (labeled as P400 by Desmedt, 1981), o n t h e o t h e r hand, is n o t o n l y apparent, but i t s amplitude i s similar a t b o t h r e c o r d i n g sites. Also, as would b e expected, t h e P300 is elicited o n l y by targets. T h e N140, l i k e t h e P300, is p r e s e n t a t b o t h contralateral a n d ipsilateral scalp sites f o r counted stimulation but, u n l i k e t h e P300, it i s substantially l a r g e r a t t h e contralateral site. Moreover, a small N140 i s e v i d e n t t o uncounted stimulation a t t h e contralateral scalp site, but n o t a t t h e ipsilateral site. Since t h e N140 may o r i g i n a t e f r o m sources in t h e parietal association c o r t e x t h a t a r e closely connected t o t h e p r i m a r y somatosensory c o r t e x (Desmedt, 1981), t h i s component may have b o t h exogenous a n d endogenous elements. T h e various dissociations among t h e components o f t h e somatosensory ERP elicited by these t a s k s suggest t h a t n o t o n l y can p r i m a r y afferent input b e d i f f e r e n t i a t e d f r o m secondary c o g n i t i v e components o f a mental reaction, but t h a t t h e t r a n s i t i o n f r o m p r i m a r y t o secondary may b e identifiable. T h e observation t h a t t h e P300 is symmetrical a t lateral p a r i etal scale sites in a selective attention t a s k in w h i c h no o v e r t response i s r e q u i r e d a n d stimulation i s d e l i v e r e d t o o n l y one hemispace d i f f e r s f r o m t h e p a t t e r n e v i d e n t when o v e r t response choices a r e r e q u i r e d a n d (visual) stimulation is presented randomly t o e i t h e r t h e l e f t o r t h e right hemispace. U n d e r these conditions, P300 amplitude i s reduced a t c e n t r a l scalp sites contralateral t o t h e responding hand, i r r e s p e c t i v e o f t h e v i s u a l f i e l d stimulated o r t h e t y p e o f response (compatible, incompatible; Ragot, 1984: Ragot & Lesevre, 1986; R u g g e t at., 1984). This asymmetry may r e f l e c t t h e superimposition o f a negative-going RP on t h e p o s i t i v e - g o i n g P300, t h e effect o f w h i c h is t o reduce t h e l a t t e r (Ragot, 1984). However, t h a t non-motoric asymmetries may e x i s t is suggested by t h e observations t h a t a t occipital sites t h e P300 is smaller o v e r t h e hemisphere contralateral t o t h e v i s u a l h a l f - f i e l d stimulated ( R u g g e t at., 1984) a n d i t s amplitude i s g r e a t e r o v e r t h e right hemisphere (Ledlow e t at., 1978). In summary, t h e transmission o f a f f e r e n t information in t h e somatosensory system can b e t r a c e d w i t h some precision. Hence, i n s i g h t can b e p r o v i d e d i n t o t h e t i m i n g o f t h e constituents of stimulus processing as t h e y a r e engaged. T h e research o f Coles-Eriksen a n d colleagues suggests f u r t h e r t h a t t h e continuous activation of response mechanisms can b e o b s e r v e d as well in t h e emergence o f t h e movement-related readiness potential. Moreover, a g r o w i n g l i t e r a t u r e indicates t h a t t h e stimulus- a n d response-related c o g n i t i v e elements o f a mental reaction can b e d i s t i n guished by t h e effects o f manipulations o f factors such as S-R compatibility a n d stimulus d i s c r i m i n a b i l i t y on t h e p r o p e r t i e s o f various endogenous components o f t h e ERP. It is n o t d i f f i c u l t t o imagine, as a s t a r t i n g point, a series o f experiments in w h i c h interhemispheric transmission time

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is assessed in t h e somatosensory system a n d t h e e f f e c t o f S-R compatibility o n t h i s communication process is evaluated w i t h respect t o b o t h i t s anatomical a n d i t s c o g n i t i v e constituents u t i l i z i n g components o f t h e ERP. T h i s research c o u l d lead, f o r example, t o resolution o f an important issue in t h e field: I s t h e Simon e f f e c t p r o d u c e d by a n a t u r a l tendency t o respond t o t h e source o f stimulation, as Simon has suggested, o r is it a manifestation of t h e establishment o f c o g n i t i v e codes t h a t d i s t i n g u i s h t h e r e l a t i v e spatial positions o f stimuli a n d response effectors, as Nicoletti e t al. (1982, 1984) a n d U m i l t i a n d Nicoletti (1985) have argued? Simon's position has been characterized as a classical o r i e n t i n g position. However, u n l i k e a classical o r i e n t i n g response, t h e Simon e f f e c t does n o t habituate o v e r trials, even t h o u g h t h e r e l a t i v e position o f t h e stimulus It may b e t h a t r a t h e r t h a n d i s t i n g u i s h i n g has no obvious signal value. o r i e n t i n g a n d spatial code positions, it would b e more a p p r o p r i a t e t o pursue t h e suggestion by Nicoletti e t al. (1982) t h a t t h e response tendency hypothesis implicates response selection mechanisms w h e r e t h e spatial code hypothesis implicates stimulus-response t r a n s l a t i o n mechanisms. Analysis o f t h e effects o f variations in S-R compatibility o n t h e t i m i n g o f components o f t h e ERP in t h e somatosensory modality c o u l d complement behavioral measures in assessing t h e m e r i t o f t h i s d i s t i n c t i o n . References Andreassi, J. L., Okamura, H., f, Stern, M. (1975). Hemispheric asymmetries in t h e v i s u a l cortical evoked potential as a f u n c t i o n o f stimulus location. Psychophysiology, 12, 541 -546. Anzola, G . P., Bertoloni, G . , Buchtel, H. A., & Rizzolatti, G . (1977). Spatial compatibility a n d anatomical f a c t o r s in simple a n d choice react i o n . Neuropsychologia, 15, 295-305. (1981). Vocal a n d manual reaction time estimates o f Bashore, T . R. interhemispheric transmission time. Psychological Bulletin, 89, 352368. On the temporal relation between Bashore, T. R . , & Osman, A . (1987). perceptual analysis and response selection: A psychophysiological investigation of stimulus congruency and S- R compatibility effects on human information processing. Poster presented a t t h e F o u r t h I n t e r national Congress of C o g n i t i v e Neuroscience, Dourdan, France.

Bashore, T. R., McCarthy, G . , Heffley, E. F., Clapman, R. M., & Donchin, E. (1982). Is h a n d w r i t i n g p o s t u r e associated w i t h d i f f e r ences in motor c o n t r o l ? A n analysis o f asymmetries in t h e readiness potential. Neuropsychologia, 20, 327-346. InfluBerlucchi, G . , Crea, F., DiStefano, M., & Tassinari, G . (1977). ence o f spatial stimulus-response compatibility on reaction time o f ipsilateral a n d contralateral h a n d t o lateralized light stimuli. Journal of Experimental Psychology: Human Perception and Performance, 3 , 505-517.

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Simon, J. R., Sly, P. E., & Vilapakkam, S . (1981). Effects o f compatibility o f S-R mapping o n reactions t o w a r d t h e stimulus source. Acta Psychologica, 47, 63-81. A n experimental s t u d y o f t h e reaction time o f t h e Smith, F. 0. (1938). c e r e b r a l hemispheres i n relation t o handedness a n d eyedness. Journal of Experimental Psychology, 22, 75-83. (1979). Writing posture, hemispheric Smith, L. C., & Moscovitch, M. c o n t r o l of movement a n d cerebral dominance in i n d i v i d u a l s w i t h i n v e r t e d a n d n o n i n v e r t e d postures d u r i n g w r i t i n g . Neuropsychologia, 17, 637-644.

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Whitaker, L. A . (1982). Stimulus-response compatibility f o r l e f t - r i g h t discriminations as a function o f stimulus position. Journal of Experlmental Psychology: Human Perception and Performance, 8 , 865-874. Wolpaw, J. R., & Penry, J. K. a u d i t o r y evoked potential. Neurophysiology, 4 3 , 99-103.

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Wood, C . C., Cohen, D., C u f f i n , 6. N., Yarita, M., & Allison, T. (1985). Electrical sources in human somatosensory c o r t e x : IdentificaScience, 227, t i o n by combined magnetic a n d potential recordings. 1051 - 1053. Wood, C . C., McCarthy, G . , Squires, N. K . , Vaughan, H . G . , Woods, D. L., & McCallum, W. C . (1984). Anatomical and physiological subIn R . K a r r e r , strates o f event-related potentials: T w o case studies. J. Cohen, & P. T u e t i n g (Eds.), Brain and information processing: event-related potentials (pp. 681 -721 1 . New Y o r k : NYAS. Woodworth,

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STIMULUS-RESPONSE COMPATIBILITY R. W. Proctor and T.G. Reeve (Editors

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CEREBRAL EVOKED POTENTIALS: E A R L Y INDEXES OF C O M P A T I B I L I T Y EFFECTS RICHARD RAGOT C e n t r e National d e la Recherche Scientifique LENA--Hopital d e la Salpetriere, France I n s o f a r as t h e y r e f l e c t stimulus-response (S-R) relations, S-R comp a t i b i l i t y effects s t a n d a t a c r u c i a l node between stimulus i d e n t i f i c a t i o n a n d response i n i t i a t i o n . A n evaluation o f t h e global performance ( f r o m stimulus reception t o motor execution) is classically g i v e n by t w o behavioral indexes, reaction time ( R T ) a n d e r r o r r a t e . However, p s y c h o p h y siological indexes o f performance also can b e used. Among these a r e c e r e b r a l electrophysiological potential variations t h a t r e f l e c t b r a i n operations as t h e y proceed f r o m t h e earliest stages o f stimulus evaluation t o motor programming, a n d t h u s a r e liable t o g i v e a more p r e c i s e i n s i g h t i n t o t h e mechanisms involved. B u t , as we shall see when r e v i e w i n g seve r a l publications devoted t o electrophysiological analyses o f S-R compatibility effects, t h e i n t e r p r e t a t i o n s o f t h e r e s u l t s a r e f a r f r o m b e i n g straightforward. T h e Functional Significance of C e r e b r a l E v o k e d Potentials T h e electroencephalogram (EEG) recorded on t h e scalp in man r e p resents t h e global electrical a c t i v i t y o f t h e u n d e r l y i n g b r a i n . While t h e so-called "spontaneous" EEG reflects c h i e f l y v a r i o u s states" o f t h e cent r a l n e r v o u s system such as degrees o f arousal o r sleep stages, t h e cereb r a l "evoked potentials" (EPs) a r e specific p a t t e r n s o f b r a i n responses t h a t a r e elicited by i n t e r n a l o r by e x t e r n a l events (stimuli). EPs a r e u s u a l l y o f small amplitude in comparison t o t h e spontaneous EEG. Therefore, in o r d e r t o i m p r o v e t h e sig:al t o noise ratio, by t h e use o f data-processing techniques, such as averaging" (Dawson, 1954) a n d "Woody f i l t e r i n g " (Woody, 1967), most experimental paradigms r e q u i r e t h e numerous repetitions o f each stimulus. E a r l y a n d medium-latency EPs (0 t o 200 ms) r e f l e c t t h e a r r i v a l o f sensory information in subcortical areas, in p r i m a r y o r secondary specific cortical areas, a n d t h e f i r s t stages o f stimulus evaluation. These components a r e mainly dependent on t h e p h y s i c a l a t t r i b u t e s of t h e stimulus, such as i n t e n s i t y a n d c o n t r a s t . Late E P s (200 ms onwards), o f t e n called event-related potentials (ERPs), a r e less specific a n d more dependent on t h e relevance o f t h e These late EPs can b e p r e s e n t information contained in t h e stimulus. even if t h e y a r e elicited by v e r y weak stimuli, p r o v i d i n g t h a t t h e stimuli a r e "task relevant" (i.e., if t h e subject is requested t o p e r f o r m a Among t h e components o f l a t e specific t a s k in response t o such stimuli). EPs, t h e N200 is g e n e r a l l y considered as r e f l e c t i n g o n - l i n e perceptual However, t h e processing (Renault, Ragot, Lesevre, & Remond, 1982). s t u d y o f t h e N200 is r e n d e r e d difficult by t h e f a c t t h a t t h i s component i s

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often intermingled w i t h t h e last component "P200" o f t h e medium-latency sensory potential a n d with t h e motor readiness potential (see F i g u r e l ) , t h a t occurs j u s t b e f o r e t h e movement (Ragot, 1984; Ragot, Renault, & Remond, 1980). T h e "P300" o r "P3" component (see F i g u r e 2) has been much more widely s t u d i e d because i t s measurements a r e easier, w i t h i t s amplitude usually b e i n g l a r g e r t h a n t h a t o f o t h e r "cognitive" components. It is p r e s e n t whenever a subject i s asked t o detect i n f r e q u e n t events (Sutton, Braren, Zubin, 8 John, 1965), to select events t h a t have to b e responded t o (such as in choice R T tasks, c o u n t i n g tasks), o r t o memor i z e information. Donchin (1977) f o u n d t h a t P300 components t e n d t o increase f o r items t h a t have been memorized. T h a t finding, added t o t h e fact t h a t P300 i s o f t e n considered as signaling t h e e n d o f t h e perceptual processes i n v o l v e d in a g i v e n t a s k ( t h e P300 is sometimes seen t o occur a f t e r t h e motor response), has l e d Donchin t o propose t h a t P300 could represent "context updating" processes. F u r t h e r relations o f P300 t o memory have been established by studies with intracerebral probes t h a t have shown one of t h e probable generators o f P300 t o b e w i t h i n t h e hippocamus (Halgren e t al., 1980; Y i n g l i n g & Hosobuchi, 1984). However, t h e functional significance o f t h e P300 is s t i l l n o t fuliy known. Three main factors have been r e p o r t e d t h a t a f f e c t t h e presence a n d amplitude o f t h e parietal P300: (1) information transmission, (2) subjective p r o b a b i l i t y , a n d (3) stimulus meaning (Johnson, 1986). Because factors 1 a n d 3 a r e i n v o l v e d in S-R compatibility situations, t h e P300 component is present in these situations. But, clearly, experiments a r e needed t o specify more accurately what t h e P300 "probe" i s measuring. I n fact, on t h e one hand, we a r e s t i l l p e r f o r m i n g experiments in o r d e r t o t r y t o understand what P300 is r e a l l y measuring, b u t , on t h e o t h e r hand, we t r y t o use what we already know about t h e P300 specificity t o i n t e r p r e t o u r e x p e r i ments. Therefore, all EP studies stand a t a midpoint between u s i n g results obtained w i t h an i l l - d e f i n e d probe, a n d a t t h e same time, trying t o improve t h e probe. T o i n t e r p r e t results concerning t h e relative t i m i n g o f P300 t o RT, it is necessary t o make hypotheses about t h e organization o f information processing f r o m stimulus reception t o response production. Serial-stage models have been mostly considered, p r i n c i p a l l y because t h e y p r o v i d e a theoretical framework f o r t e s t i n g t h e i r v a l i d i t y in a r e l a t i v e l y simple way, t h r o u g h t h e use o f Sternberg's (1969) a d d i t i v e factors method. The number o f t h e stages a n d t h e names g i v e n t o these stages may d i f f e r according t o t h e model. Since stages can e x i s t as such o n l y if t h e y a r e d i s t i n c t a n d independent, t h e y can o n l y b e defined precisely t h r o u g h a number o f experimental conditions o r variables known t o ( o r supfosed to) act upon them. F o r example, Frowein' (1981) defined t h e response selection" stage as depending on S-R compatibility and r e l a t i v e S - R f r e q u e n c y . Such an approach i s also exemplified by Sanders (1980). In several experiments, P300 latency has been shown t o v a r y to a l a r g e e x t e n t w i t h S-R compatibility. However, in a few o t h e r e x p e r i ments, l i t t l e or n o variations o f P300 w i t h compatibility were seen, despite t h e f a c t t h a t R T varied. These discrepancies may reveal t h a t compatibility phenomena t h a t produce similar R T effects may b e a f u n c t i o n o f diff e r e n t processes. Such processes may depend on t h e t a s k and also on t h e t y p e o f S-R relation involved.

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F i g u r e 1. Cerebral electrical potentials recorded during a self-paced (no stimulus) b u t t o n p r e s s (microswitch), performed w i t h t h e r i g h t - i n d e x a n d left-index finger. 0 ms on t h e time scale marks microswitch closure. G r a n d mean (across 7 subjects) o f 80 flexions p e r subject. Above each map, t h e evoked potential recorded a t t h e v e r t e x is shown by a solid l i n e (Cz), a n d t h e v e r t i c a l electro-oculogram (which i s meant t o b e f l a t ) by a d o t t e d l i n e (vEOG). Spatiotemporal maps show amplitude in t h e f o r m o f isopotential lines (1.6 UV between t w o adjacent lines), p l o t t e d against time (on t h e abscissa), a n d space (on t h e o r d i n a t e ) . Positive potentials a r e shown by d o t t e d lines, n e g a t i v e ones by thin p l a i n lines. Thick p l a i n lines indicate zero potential. T h e n e g a t i v e potential maxima a r e indicated by a thick l i n e running along t h e peaks. T h e slow-rising negative motor p r e p a r a t i o n potential is, j u s t b e f o r e t h e movement, cont r a l a t e r a l t o t h e h a n d t h a t p e r f o r m s t h e movement. (Reprinted from Ragot, 1984).

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In6

R

P ma.

F i g u r e 2. Cerebral potentials recorded during a spatial compatibility paradigm, in w h i c h subjects responded with t h e right- o r l e f t - i n d e x f i n g e r a c c o r d i n g t o t h e color o f t h e stimulus, i r r e s p e c t i v e o f i t s position. T h e t w o t o p t r a c i n g s c o r r e s p o n d t o incompatible situations (spatial conflict), whereas t h e t w o bottom t r a c i n g s correspond t o compatible situat i o n s (no spatial c o n f l i c t ) . "S" indicates stimulus onset, "R" t h e average reaction time, w i t h i t s s t a n d a r d deviations on each side. Each o f these f o u r responses r e p r e s e n t t h e g r a n d mean (across t h e same 7 subjects as in F i g u r e 1) o f 80 i n d i v i d u a l t r i a l s p e r subject. T h e N140 component o f t h e sensory evoked response is contralateral t o t h e stimulus. T h e P300 peak, o n t h e c o n t r a r y , is positioned i p s i l a t e r a l l y t o t h e r e s p o n d i n g hand; t h i s is d u e t o t h e presence o f an u n d e r l y i n g motor p r e p a r a t i o n potential which, b e i n g negative, p a r t i a l l y cancels t h e P300 c o n t r a l a t e r a l l y t o t h e r e s p o n d i n g hand. B o t h R T a n d P300 peak latency a r e l o n g e r in t h e spat i a l l y non-compatible situations. ( R e p r i n t e d f r o m Ragot, 1984.)

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S-R Compatibility Paradigms U n d e r t h e b r o a d t e r m of S-R compatibility, v e r y d i f f e r e n t e x p e r i mental paradigms can b e used. F o r instance, compatibility can b e p r e sented e i t h e r in an e x p l i c i t o r in an implicit way, according t o w h e t h e r t h e subject i s aware of t h e compatible o r incompatible c h a r a c t e r i s t i c o f t h e situation. F o r example; if a subject has t o p r e s s a b u t t o n on t h e right when :eeing :he w o r d RIGHT" o r a b u t t o n o n t h e l e f t when seeing t h e w o r d LEFT, p r i n t e d in u p p e r case, but t o s w i t c h t h e responses when t h e same words a r e in lowercase, compatibility can b e considered as explicit. It is e x p l i c i t because t h e response is dependent on t h e meaning o f t h e w o r d a n d w h e t h e r t h e response w i l l b e compatible o r n o t i s designated by l e t t e r size. Conversely, if a subject has t o i g n o r e w o r d meaning a n d respond o n l y t o l e t t e r size (such as p r e s s i n g t h e right b u t t o n t o an uppercase w o r d a n d t h e l e f t b u t t o n t o a lowtrcase word), t h e p r e s s i n g of t h e right b u t t o n in response t o t h e w o r d RIGHT" in u p p e r case i s implicitly, but n o t e x p l i c i t l y , compatible. T h e t e r m "S-R compatibility" m i g h t b e used by d i f f e r e n t authors, o r sometimes by t h e same author, t o designate d i f f e r e n t S-R relations. F o r example, w h i l e summarizing Hedge a n d Marsh's (1975) results, Simon, Sly, a n d Vilapakkam (1981) characterized t h e experimental condition during w h i c h subjects h a d t o p r e s s a b u t t o n o f t h e same color as t h e stimulus as "compatible," a n d t h e condition d u r i n g w h i c h subjects h a d t o p r e s s a b u t t o n o f t h e opposite color as "incompatible." Hedge a n d Marsh's p a p e r also i n v o l v e d t h e t y p e of spatial compatibility situations t h a t h a d been p r e v i o u s l y characterized as such by Simon, Hinrichs, a n d K r a f t (1970), but w h i c h Simon e t al. (1981) designated as "corresponding" a n d "non-corresponding . I ' Because these experimental situations w e r e r e p o r t e d w i t h g r e a t precision by Simon e t al., t h e terminology w i l l n o t b e misleading t o t h e a t t e n t i v e r e a d e r . B u t it does show t h a t one m u s t b e aware o f what is actually manipulated when t h e t e r m "S-R compatibility" is used. We shall now examine recent r e s u l t s obtained t h r o u g h electrophysiological investigations, f o c u s i n g o u r attention on t h e relations between R T a n d t h e t i m i n g o f t h e P300 c o g n i t i v e component. EP studies t h a t have addressed S-R compatibility can b e classified b r o a d l y i n t o t w o groups: those i n v o l v i n g a spatial f a c t o r a n d those i n v o l v i n g a semaf;ltic f a c t o r . For simplification, we shall r e f e r t o these t w o g r o u p s as spatial" o r "semantic" compat ibiIit y studies . Spatial a n d Semantic Compatibility ERP Studies Spatial Compat ibiIity We o b s e r v e d in a bimanual task, similar t o t h e one used by Simon a n d Small (19691, s i g n i f i c a n t increases o f b o t h P300 latency a n d P300 amplitude w i t h S-R incompatibility (Ragot & Remond, 1979). In t h i s experiment, S-R compatibility was presented i m p l i c i t l y . T h a t is, stimul u s color was t h e c u e i n d i c a t i n g on which side t h e response was t o b e made, a n d incompatibility o c c u r r e d when t h e stimulus location was oppos i t e t o t h e response location. Because, f o r technical reasons, t h e ERP average responses in t h i s s t u d y were calculated u s i n g t h e behavioral response ( b u t t o n press) as a t r i g g e r ( a n d n o t t h e stimulus), it c o u l d n o t

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b e concluded f r o m these r e s u l t s t h a t t h e P300 latency increase was d u e o n l y t o S-R incompatibility. Ragot (1984) replicated Ragot a n d Remond's (1979) experiment, t h i s time u s i n g t h e stimulus as a t r i g g e r f o r t h e a v e r a g i n g process. The r e s u l t s confirmed that, in parallel w i t h a 30 ms increase o f RT, a small (20 ms), but nevertheless significant, increase o f P300 latency was T h e experiment o b s e r v e d in t h e incompatible situation (see F i g u r e 2 ) . also manipulated motor programming, t h r o u g h t h e compatibility between t h e r e s p o n d i n g h a n d a n d i t s usual position in space, by a s k i n g subjects t o p e r f o r m t h e t a s k w i t h t h e hands crossed a n d uncrossed. Crossing hands p r o d u c e d a l a r g e increase in R T (56 ms) but a negligible (2 ms) T h i s r e s u l t suggests t h a t P300 increase in P300 latency (see F i g u r e 3a). indexes t h e d u r a t i o n of t h e stimulus evaluation stages but n o t t h e d u r a In a s l i g h t l y d i f f e r e n t experimental t i o n of t h e motor o u t p u t stages. situation, Ragot a n d Lesevre (1986) o b s e r v e d t h a t spatial compatibility also h a d an e f f e c t on P300 latency when S-R compatibility effects were elicited separately o n t h e right a n d on t h e l e f t sides o f t h e b o d y (see F i g u r e 3b). More recently, Bashore a n d Osman (1987; see also Chapter 7 by Bashore) also o b s e r v e d a notable (15 ms) increase of P300 latency w i t h spatial S-R compatibility ( l e f t - o r r i g h t - h a n d responses t o a r r o w s In t h e i r s t u d y , t h e N2M) component, o c c u r r i n g p o i n t i n g l e f t o r right). b e f o r e t h e P300 a n d considered as r e f l e c t i n g o n - l i n e perceptual processing In sum(Renault e t al., 19821, was also delayed by S-R compatibility. mary, a l l these studies r e p o r t e d a P300 latency l e n g t h e n i n g w i t h incomp a t i b i l i t y . T o t h e c o n t r a r y , as we shall now discuss, n o s i g n i f i c a n t P300 latency variations w i t h incompatibility were o b s e r v e d ( a t least a t f i r s t ) in o t h e r studies. Semantic C o m p a t i b i l i t y M c C a r t h y a n d Donchin (1981) investigated EPs recorded during t h e Subjects h a d t o respond t o r e a d i n g o f t h e c u e in a compatibility t a s k . t h e w o r d s "RIGHT" a n d "LEFT" by p r e s s i n g one o f t w o b u t t o n s located t o A p r e c u e (SAME/DIFFERENT) message t o l d them t h e right o r l e f t . w h e t h e r t h e y h a d t o g i v e a "compatible" (SAME) o r an incompatible (DIFFERENT) response. In these conditions (precued, e x p l i c i t compatibility), n o s i g n i f i c a n t P300 latency variations w i t h S - R compatibility were observed. Later, M c C a r t h y a n d Donchin (1983) obtained similar results, but w i t h t h e compatibility information d e l i v e r e d a t t h e same time as t h e cue, t h r o u g h t h e l e t t e r size o f t h e cue w o r d s "LEFT" a n d "RIGHT" Neither did ( u p p e r case = compatible, lower case = incompatible). Duncan-Johnson a n d Kopell (1981) o b t a i n a n y P300 latency variations w i t h S-R compatibility in a S t r o o p t a s k f o r w h i c h responses were g i v e n vocally (see F i g u r e 4). Several reasons may e x p l a i n t h e differences between t h e r e s u l t s of t h e spatial-compatibility studies a n d o f t h e semantic-compatibility studies. F i r s t , t h e stimuli were w o r d s in t h e M c C a r t h y a n d Donchin (1981) a n d t h e Duncan-Johnson a n d Kopell (1981) studies, whereas t h e y were colored flashes o f light in all o u r experiments (Ragot, 1984; Ragot 8 Lesevre, 1986; Ragot & Remond, 1979). Second, i n M c C a r t h y a n d Donchin, compatibility was explicit, whereas in o u r w o r k it was always implicit. Third, M c C a r t h y a n d Donchin used Woody f i l t e r i n g f o r evaluating P300 latency, whereas we did n o t . However, Woody f i l t e r i n g can p r o b a b l y b e r u l e d o u t as a possible explanation o f these d i f f e r e n t P300 latency results, because s i g n i f i c a n t increases o f P300 latency w i t h spatial

Early Indexes of Compatibility Effects

23 1

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F i g u r e 3. R T a n d P300 latency variations w i t h S-R compatibility, obtained: (a) in Ragot (1984), a n d (b) in Ragot a n d Lesevre (1986). In a l l cases, an increase o f b o t h R T a n d P300 latency w i t h S-R spatial incompatibility is observed. In (a), R T i s globally lengthened when subjects respond w i t h t h e i r hands crossed, whereas P300 latency is n o t modified. In (b), t h e effects o f spatial compatibility on P300 latency a n d o n R T d o n o t depend o n which hemisphere is i n v o l v e d in t h e t a s k .

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Figure 4. Cerebral potentials recorded f r o m a parietal electrode (Pz) d u r i n g a Stroop task, a n d averaged o v e r subjects. Whether ink color a n d color name coincide, n o variations o f P300 latency a r e observed, e i t h e r w i t h a latency adjustment o f t h e data w i t h a Woody f i l t e r , o r w i t h o u t such a n adjustment (”raw” averages). (Reprinted f r o m DuncanJohnson and Kopell, 1981, w i t h permission. C o p y r i g h t 1981 by AAAS.)

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incompatibility were r e c e n t l y obtained in o u r l a b o r a t o r y b o t h with a n d w i t h o u t Woody f i l t e r i n g (Renault, Fiori, & Giami, in p r e s s ) . Ragot (1984) concluded t h a t it was p r i n c i p a l l y t h e n a t u r e of t h e i n t e r f e r i n g f a c t o r (non-cue information) t h a t seemed t o b e t h e main f a c t o r responsible f o r p r o d u c i n g P 3 0 latency v a r i a t i o n s w i t h S-R compatibility: When t h e i n t e r f e r i n g f a c t o r c o u l d b e classified as "spatial, " P300 latency would v a r y w i t h compatibility, whereas when it c o u l d b e classified as "semantic, " no P300 latency variations would b e observed. A t present, t h e dichotomy between semantic a n d spatial S-R comp a t i b i l i t y situations appears less clear-cut. Magliero, Bashore, Coles, a n d Donchin (1984) replicated M c C a r t h y a n d Donchin's (1981) semanticcompatibility s t u d y a n d o b s e r v e d P300 latency increases w i t h S-R incomp a t i b i l i t y in all t h e e i g h t conditions o f t h e i r experiment. B u t these increases were n o t s i g n i f i c a n t when considered i n d i v i d u a l l y , causing Magliero e t al. t o conclude t h a t P300 latency did n o t increase w i t h S - R incompatibility. However, because these small increases were p r e s e n t in all e i g h t conditions, a s i g n t e s t is s u f f i c i e n t t o show a s i g n i f i c a n t P300 increase w i t h S-R incompatibility. T h i s f a c t (which was overlooked by t h e a u t h o r s i n t h e i r own article) was discussed in Ragot a n d Renault (1985). In a similar way, r e s u l t s obtained by Polich a n d M c C a r t h y (1983) seem t o c o n t r a d i c t t h e above-mentioned t e n t a t i v e conclusion t h a t semantic incompatibility would n o t e l i c i t P300 latency variations. Although not s t r i c t l y an S-R compatibility situation, t h e t a s k t h a t Polich a n d M c C a r t h y employed (subjects h a d t o decide w h e t h e r p a i r s o f words matched, o r not, o n t h e basis o f t h e i r o r t h o g r a p h i c o r phonological differences) u n d o u b t e d l y i n v o l v e d semantic factors, a n d t h e a u t h o r s o b s e r v e d t h a t P300 latency did v a r y in a manner consistent w i t h t h e R T p r o f i l e s f o r each match/non-match condition.

Pfefferbaum, Christensen, Ford, a n d Kopell (1986) also f o u n d P300 latency variations w i t h semantic compatibility, a t least in some conditions. Pfefferbaum e t al. h a d subjects a t t e n d t o f o u r randomly o c c u r r i n g stimuli "right," "LEFT," "RIGHT") a n d p e r f o r m according t o e i t h e r o f ("left," t h e t h r e e following conditions: 1. "Respond t o words," t h a t is, a c c o r d i n g t o what t h e w o r d indicated f o r t h e e n t i r e b l o c k of t r i a l s ("compatible" case) a n d t o t h e oppos i t e f o r another b l o c k ("incompatible" case).

2 . "Respond t o case," t h a t is, according t o w h e t h e r t h e w o r d was w r i t t e n in u p p e r o r lower case, i g n o r i n g t h e meaning o f t h e w o r d . 3. "Respond t o case/word," t h a t is, according t o b o t h t h e case a n d t h e meaning o f t h e word, in a manner similar t o t h a t o f M c C a r t h y a n d Donchin's (1983) experiment. In condition 1, Pfefferbaum e t al. (1986) f o u n d t h a t t h e r e was indeed an increase o f b o t h P300 latency a n d R T f o r t h e incompatible case, (It i s o f i n t e r e s t to w i t h P300 b e i n g delayed by as much as 70 ms. mention h e r e t h a t Pfefferbaum e t al. o b s e r v e d these P300 latency v a r i a tions despite t h e i r use o f Woody f i l t e r i n g , which is an additional argument f o r ruling o u t Woody f i l t e r i n g as a possible cause f o r n o t o b s e r v i n g P300 In condition 3, t h e r e s u l t s were similar t o those o f latency changes.)

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M c C a r t h y a n d Donchin (1983). In s p i t e o f an increase o f R T w i t h incomEven more p a t i b i l i t y , t h e r e was no equivalent increase o f P300 latency. s u r p r i s i n g was condition 2, for w h i c h t h e r e was n o increase w i t h incomp a t i b i l i t y f o r e i t h e r P300 latency o r R T . Because condition 2 i s an analog t o t h e S t r o o p task, it c e r t a i n l y does seem s u r p r i s i n g n o t t o find a n y R T increase with incompatibility. (The authors' interpretation o f t h e result was t h a t subjects a r e able t o d i s r e g a r d t h e meaning o f t h e w o r d when r e s p o n d i n g o n t h e basis o f w o r d case.) A r e T h e r e "P300-Latency Sensitive" a n d "P300-Latency Non-Sensitive" Paradigms, a n d W h y ? We a r e now f a c i n g a d i f f e r e n t question: What is t h e actual factor, besides a semantic o r a spatial i n t e r f e r i n g factor, t h a t is responsible f o r e l i c i t i n g e i t h e r strong, weak, o r null P300 variations in t h e experiments discussed above? T h e d i f f e r e n t P300 latency effects o f incompatibility r e p o r t e d in t h e above-mentioned experiments may b e d u e t o several phenomena t h a t c o u l d e i t h e r c o n v e r g e o r w o r k against each o t h e r t o p r o d u c e t h e p a t t e r n o f effects t h a t were observed. When S-R compatibility is p r e s e n t e d implicitly, such as in t h e Stroop t a s k o r in t h e Simon effect, t h e n a t u r e o f t h e i n t e r f e r i n g f a c t o r plays a major role in t h e generation o f S-R compatibility effects, p r o b a b l y t h r o u g h t h e influence o f i t s r e l a t i v e t i m i n g w i t h t h e processing o f t h e information t o w h i c h t h e subject is t o respond. Based o n t h e n a t u r e o f t h e i n t e r f e r i n g f a c t o r a n d t h e t y p e o f S-R compatibility, t h e i n t e r f e r e n c e t h u s would o c c u r a t d i f f e r e n t times. Because t h e i n t e r f e r e n c e w o u l d t a k e place e i t h e r b e f o r e o r a f t e r t h e processing stage i n d e x e d by P300, t h e effects on P300 latency a n d R T would d i f f e r .

I n t h e t y p e o f S - R compatibility s t u d i e d by F i t t s a n d Seeger (1953), t h e stimulus c h a r a c t e r i s t i c making up t h e t a r g e t stimlus is o f t h e same n a t u r e (spatial, i n t h i s case) as t h a t o f t h e response. In t h a t respect, these effects can b e considered as t r u e S-R compatibility effects. S-R compatibility is t h e n manipulated e x p l i c i t l y by a s k i n g subjects t o make e i t h e r a spatially c o r r e s p o n d i n g (compatible) response o r a s p a t i a l l y n o n - c o r r e s p o n d i n g (incompatible) response. It has been r e c e n t l y a r g u e d (Hasbroucq, 1987) t h a t c e r t a i n categories o f S - R comp a t i b i l i t y effects, u s u a l l y i n t e r p r e t e d i n terms of S - R relations, m i g h t in f a c t b e caused by t h e i n t e r n a l congruence o r non-congruence o f t w o c o n f l i c t i n g f a c t o r s w i t h i n t h e stimulus i t s e l f . F o r example, in t h e Simon e f f e c t (Simon & Small, 1969), t h e response has a spatial dimension, whereas t h e stimulus is characterized by b o t h a spatial a n d a non-spatial dimension. Because t h e response indicated by t h e non-spatial characteri s t i c of t h e stimulus is spatial, t h e response can b e i n t e r f e r e d w i t h by t h e spatial aspect o f t h e stimulus. Thus, it appears t h a t t h e compatibility relation t h a t i s responsible f o r t h e Simon e f f e c t is already p r e s e n t as suggested by w i t h i n t h e stimulus. Therefore, it would b e b$ter, G u i a r d (1988), t o classify t h e Simon e f f e c t as a stimulus incongruence" e f f e c t . Because P300 latency is normally considered as i n d e x i n g t h e time r e q u i r e d t o evaluate a n d categorize an e v e n t (see Johnson, 1986, p. 379), it is n o t s u r p r i s i n g t o o b s e r v e P300 latency variations w i t h compatibility when compatibility is d u e t o incongruences w i t h i n t h e stimulus itself. When t h e compatibility effects a r e n o t d u e t o t w o c o n f l i c t i n g aspects o f t h e stimulus, t h e n no P300 latency variations should b e observed.

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Therefore, no variations of P300 latency should t h e n b e o b s e r v e d when S-R compatibility is manipulated e x p l i c i t l y w i t h no within-stimulus incongruence. T h i s i s indeed t h e case in M c C a r t h y a n d Donchin's (1981, 1983) experiments, as well as in t h e "respond t o case/word" condition o f Pfefferbaum e t al. (1986).

In t h e case o f i m p l i c i t compatibility conditions, such as t h o s e t h a t p r o d u c e t h e Simon effect (Simon & Small, 19691, b o t h w i t h i n - s t i m u l u s incongruence a n d S-R compatibility a r e p r e s e n t . Given o u r interpretation, P300 latency should t h e n r e f l e c t S-R compatibility. T h i s is indeed t h e case in Ragot a n d Remond (19791, Ragot (1984), Ragot a n d Lesevre (1986), Polich a n d M c C a r t h y (1983), a n d Renault e t a l . (in p r e s s ) . However, t h i s i n t e r p r e t a t i o n does n o t explain w h y no P300 latency variations w i t h implicit S-R compatibility were o b s e r v e d in t h e Stroop t a s k o f Duncan-Johnson a n d Kopell (1981). T h e implications o f these P300 latency variations according t o comp a t i b i l i t y a r e by n o means t r i v i a l o r useless because:

1. it has n o t actualty been resolved w h e t h e r P300 indexes stimulus evaluation time alone o r also response-selection processes.

2 . Whatever t h e answer, t h e f a c t t h a t S-R compatibility can p r o duce e i t h e r large, small, or null variations o f P3M) latency a c c o r d i n g t o experimental conditions indicates t h a t t h e chronological organization o f information processing v a r i e s a c c o r d i n g t o experimental conditions. The Use of EPs in S-R C o m p a t i b i l i t y Experiments: When a n d Why? Whereas R T measures indicate t h e global time t a k e n to process information between stimulus d e l i v e r y a n d response production, cerebral evoked responses r e f l e c t t h e intermediate stages o f b r a i n processing t h a t t a k e place between stimulus a n d response. In t h a t respect, cerebral evoked responses p r o v i d e additional information on t h e t i m i n g a n d o r d e r i n g o f b r a i n operations. Because we d o n o t know e x a c t l y w h i c h b r a i n mechanisms a r e reflected by long latency EPs, we cannot y e t use EPs to d r a w accurate conclusions about t h e neural mechanisms i n v o l v e d in d i f f e r e n t S-R compatibility paradigms. However, EP data make it possible t o d i f f e r e n t i a t e situations t h a t would otherwise appear t o b e identical on t h e basis o f R T r e s u l t s alone. T h r e e ways i n w h i c h EP data can b e used a r e as follows: 1. Too l i t t l e is known about t h e mechanisms g o v e r n i n g t h e generat i o n o f t h e late EP components t o b e able t o t e l l w h i c h ones r e f l e c t processing in real time a n d which ones d o n o t . However, s t r o n g assertions can b e made f r o m t h e simple f a c t t h a t t h e cause m u s t precede t h e effect. If a g i v e n ERP o c c u r r i n g a t time t a f t e r stimulus d e l i v e r y e x h i b i t s variations between experimental conditions A a n d 6, t h e b r a i n processes t h a t t a k e i n t o account t h e d i f f e r e n c e between condition A a n d condition B must have t a k e n place b e f o r e time t . In o t h e r words, t h e processing by t h e b r a i n o f a g i v e n c h a r a c t e r i s t i c o f t h e stimulus must necessarily o c c u r e i t h e r a t t h e same time as o r b e f o r e t h e ERP component t h a t reflects variations o f t h i s c h a r a c t e r i s t i c .

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2. T h e peak amplitude of each ERP component is also a relevant parameter, usually i n t e r p r e t e d as r e f l e c t i n g t h e amount o f processing t h a t goes on o r t h e level o f activation o r motor preparation, depending o n t h e component t h a t i s b e i n g studied.

3. T h e scalp d i s t r i b u t i o n o f t h e various E R P components can help t o i d e n t i f y and separate components t h a t occur a t v e r y close time i n t e r vals. For example, t h e lateralization o f t h e readiness potential, which precedes a manual movement, represents a n accurate index of motor preparation in a bimanual task, insofar as t h i s component has been shown t o always culminate o n t h e hemisphere contralateral t o t h e hand which is p r e p a r e d t o p e r f o r m t h e movement (see Figures 1 a n d 2). However, several points must b e considered when u s i n g E P s :

1. It is well known t h a t t h e scalp location o f a peak does n o t necessarily indicate t h a t t h e neural s t r u c t u r e generating t h e peak is located in t h e c o r t e x immediately below. Potentials, generated e i t h e r i n o r below t h e cortex, g i v e birth t o passive c u r r e n t s which propagate i r r e g u l a r l y t h r o u g h o u t t h e head, mainly along low-resistance pathways, such as blood vessels, ventricles containing cerebro-spinal f l u i d , a n d sulci, a n d t h e r e f o r e may g i v e r i s e on t h e scalp t o potential p a t t e r n s that e i t h e r severely d i s t o r t o r even have n o t h i n g t o d o w i t h t h e geometry o f generators. In t h i s respect, neuromagnetism i s a b e t t e r tool f o r i n v e s t i g a t i n g neural sources: A t t h e r e l a t i v e l y low frequencies involved, magnetic f i e l d s a r e n o t d i s t o r t e d a n d propagate in t h e neural a n d s u r r o u n d i n g tissues in t h e same way as t h e y would d o in a vacuum, a n d t h u s generators can b e located accurately. However, t h e localization problem does n o t p r e v e n t researchers f r o m u s i n g t h e t o p o g r a p h y o f E P s as an "abstract" dependent variable, needing n o hypotheses concerning t h e location o f generators. If, f o r example, E P s appear in t w o d i f f e r e n t experimental situations w i t h d i f f e r e n t scalp distributions, it can b e i n f e r r e d t h a t d i s t i n c t mechanisms i n v o l v i n g d i f f e r e n t neural s t r u c t u r e s a r e c e r t a i n l y i n v o l v e d in these t w o situations.

2 . E P topography, amplitude, a n d latency o f t e n i n t e r a c t in a complex manner. In p a r t i c u l a r , t h e latency v a r i a b i l i t y o f i n d i v i d u a l nonaveraged responses may modify t h e amplitude o f t h e averaged response. T h e l a r g e r t h e latency j i t t e r , t h e lower t h e amplitude o f t h e averaged response. T h i s renders amplitude comparisons unreliable, unless one has r u l e d o u t t h i s i n d i v i d u a l latency j i t t e r . Another way around t h i s problem is t o use Woody f i l t e r i n g (Woody, 19671, whose p r i n c i p a l b e n e f i t is t o synchronize all t h e i n d i v i d u a l responses i n such a way that t h e y a l l add u p nicely a t t h e same moment on t h e average. B u t t h i s procedure may have undesirable side effects because it requires additional parameters, such as t h e choice o f t h e time window b e i n g analyzed. I n s p i t e o f these (well-known) p i t f a l l s a n d although t h e mental p r o cesses indexed by ERPs a r e s t i l l n o t f u l l y documented, we shall e n d u p by s t r e s s i n g t h a t these electrophysiological indexes d o p r o v i d e information concerning mental chronometry t h a t is c e r t a i n l y n o t r e d u n d a n t w i t h regard t o RT.

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Summary I n a d d i t i o n t o t h e measurement o f RT, cerebral event-related potentials p r o v i d e data o n t h e processes i n v o l v e d during t h e completion of choice R T tasks. Recent studies have shown t h a t S-R compatibility s i t u ations t h a t p r o d u c e similar effects on R T can p r o d u c e d i f f e r e n t ERP p a t terns, according t o t h e t y p e o f compatibility i n v o l v e d a n d t h e e x p e r i mental p r o c e d u r e employed.

E a r l y electrophysiological investigations o f S-R compatibility effects, which i n v o l v e d e i t h e r t h e Stroop e f f e c t o r a semantic t y p e o f compatibility, showed t h a t t h e latency o f t h e P300 ERP component did n o t v a r y w i t h S-R compatibility. T o t h e c o n t r a r y , studies i n v o l v i n g spatial S-R relations r e p o r t e d s i g n i f i c a n t increases o f P300 latency w i t h spatial incomp a t i b i l i t y . Thus, S-R compatibility phenomena c o u l d a p p a r e n t l y b e separ a t e d i n t o t w o groups, as a f u n c t i o n of w h e t h e r compatibility affected P300 latency. However, f u r t h e r studies demonstrated t h a t small increases of P300 c o u l d b e obtained w i t h semantic compatibility effects, a n d t h e p i c t u r e is now less clear. Because these discrepancies d o n o t seem t o depend solely on t h e t y p e o f S-R compatibility, t h i s c h a p t e r reviewed t h e l i t e r a t u r e concerning these results, explained t h e reasons t h a t may b e responsible f o r such discrepancies, a n d showed how ERPs can b e used in f u t u r e experiments in o r d e r t o p r o v i d e a b e t t e r u n d e r s t a n d i n g o f S-R compatibility effects.

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Woody, C . D . (1967). Characterization o f an adaptive f i l t e r for t h e Medical and analysis o f variable latency neuroelectric signals. Biological Engineering, 5 , 539-553. Yingling,

C.

D.,

P300 in man. 59. 72-76.

& Hosobuchi,

Y. (1984). A subcortical correlate o f Electroencephalography and Clinical Neurophysiology,

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STIMULUS-RESPONSECOMPATIBILITY R. W. PIoctor and T.G. Reeve (Editors Q ~ ~ s e v iscience er Ph\isbrs B. v. (Jorth-Holland). 1990

24 1

PSYCHOLOGICAL A N D NEUROPHYS IOLOG I C A L FACTORS I N STIMULUS-RESPONSE C O M P A T I B I L I T Y JOHN BREBNER Department o f Psychology U n i v e r s i t y o f Adelaide, A u s t r a l i a T h e t e r m "stimulus-response ( S - R ) compatibility" is p o t e n t i a l l y applicable t o a v e r y wide r a n g e o f situations in which responses have t o b e made t o stimuli ( e . g . , see Brebner, Shephard, & Cairney, 1972). However, studies o f S-R compatibility have t e n d e d t o b e concerned w i t h t h e speed a n d accuracy o f performance f o r situations in which spatial o r semantic p r o p e r t i e s of stimuli a n d responses do o r d o n o t correspond. Frequently, in S-R compatibility studies o f t h i s nature, correspondence a n d non-correspondence mean " t h e same as" a n d " t h e oppositf of," respectively, because responses l i k e "up-down,'' "right-left, and "toward-away'' have been employed. A good reason f o r d e f i n i n g c o r r e spondence in t h i s way is t o s t u d y t h e g e n e r a l i t y o f r u l e s g o v e r n i n g t h e relation between stimuli a n d responses. If it is easier t o p e r f o r m a task, such as finding a p o i n t on t h e s u r f a c e o f a sphere, w i t h a l l t h e usual spatial relations r e v e r s e d t h a n w i t h one o r more o f t h e t h r e e dimensions unchanged, t h i s may suggest t o us t h a t a l l t h r e e dimensions can b e encoded in a single, i n t e g r a t e d process r a t h e r t h a n in separate processes, p r o v i d e d one r u l e ( + / - 180) can b e applied t o a l l dimensions. T h e compatibility-incompatibility of S-R relations i s d e f i n e d by t h e parameters o f s k i l l e d performance, which i n c l u d e faster, more accurate performance t h a t is r e s i s t a n t t o f a t i g u e a n d d i s t r a c t i o n a n d generalizes t o o t h e r t a s k s commensurately w i t h t h e i r degree o f similarity. However, it is n o t always clear in S-R compatibility experiments w h y "opposite" responses a r e chosen as t h e o n l y incompatible conditions, t h u s leaving unanswered t h e i n t e r e s t i n g question o f w h e t h e r increasing s i m i l a r i t y implies increasing compatibility o r w h e t h e r compatibility i s an all-or-none matter. One negative e f f e c t o f c o n c e n t r a t i n g on "same-opposite" relations i s t o limit t h e range o f applied situations i n which S-R compatibility f i n d i n g s can b e used. A n associated r i s k is t h a t S-R compatibility studies may become encapsulated if t h e i r experimental arrangements become too stereotyped a n d t h e relevance o f advances i n related areas is lost. One example o f t h i s r i s k can b e f o u n d in Norman's (1976) d e s c r i p t i o n o f an experiment by Brooks (1968). In t h a t experiment, subjects learned sentences a n d t h e n went t h r o u g h t h e sentence mentally w o r d f o r w o r d while p e r f o r m i n g one o f t w o tasks. Tas!, one i n v o l v e d p o i n t i n g t o Y f o r "yes" In t a s k two, t h e t h e w o r d was a noun o r N f o r no" if it was not. yes-no" response was made v e r b a l l y . Brooks' subjects also memorized s t r a i g h t - l i n e f i g u r e s and, again, mentally followed t h e outline, p o i n t i n g t o Y f o r outside c o r n e r s a n d N f o r i n s i d e c o r n e r s i n one task, a n d s a y i n g "yes" o r "no" i n t h e o t h e r . Brooks f o u n d that, w i t h t h e sentences, t h e v e r b a l responses averaged 13.8 s a n d t h e p o i n t i n g responses averaged 9.8 s, but w i t h t h e outlines t h e v e r b a l responses averaged 11.3 s a n d

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t h e p o i n t i n g responses averaged 28.2 s . T h a t is, t h e v e r b a l responses were s l i g h t l y slower when dealing w i t h memorized v e r b a l material, but t h e p o i n t i n g responses were 2.5 times longer i n t h e spatial o u t l i n e t a s k t h a n in t h e v e r b a l sentence t a s k .

In short, Brooks' (1968) experiment showed t h a t l i k e t a s k s i n t e r f e r e d a n d u n l i k e t a s k s did not. Norman's (1976) comment o n Brooks' r e s u l t s was, " T h e incompatibilities must r e s u l t f r o m interaction o f t h e processes o f maintaining t h e mental representations a n d selecting a n d d o i n g t h e responses. T h u s , t h e r e i s clear evidence f o r d i f f e r e n t t y p e s o f encoding f o r t h e t w o d i f f e r e n t t a s k s " (Norman, 1976, p . 161). Of more importance for S-R compatibility studies is t h e clear evidence o f t h e relevance o f encoding in s h o r t - t e r m memory t o compatibility effects. Principles of S-R Compatibility A n o t h e r detectable r i s k in S-R compatibility research is t o l i m i t attention t o responses whose features mimic too e x a c t l y those o f t h e Again, t h e e f f e c t o f d o i n g so may b e t o d i v o r c e t h e research "stimulus." f r o m t h e s o r t s o f stimuli people actually d o i n t e r a c t w i t h o u t s i d e t h e labor a t o r y , t h e r e b y r e d u c i n g t h e applied significance a n d impact o f S-R comp a t i b i l i t y research. A p p l i e d research s u r v i v e s t h i s d i v o r c e because it is possible t o o b s e r v e w h i c h c o n t r o l - d i s p l a y relations a r e acceptably "compatible" w i t h o u t a more,,general u n d e r s t a n d i n g o f w h y t h i s is t h e case. A t the time t h e t e r m S - R compatibility" e n t e r e d t h e psychological l i t e r a t u r e in t h e e a r l y 195os, it was common f o r c o n t r o l - d i s p l a y arrangzments w i t h t h e characteristics o f compatibility t o b e described as n a t u r a l " while arrangements d e f i c i e n t i n them were termed " u n n a t u r a l . " T h e r e were less c i r c u l a r explanations f o r v e r y many specific effects, most o f w h i c h d e r i v e d f r o m an u n d e r s t a n d i n g o f how sensory organs a n d effectors functioned, (see, e.g., Boring, 1945, o r Hausner, 1951). But, f o r the combination o f information displays a n d c o n t r o l mechanisms, explanations t e n d e d t o b e v e r y general. In 1957, u n d e r t h e CONTROLS Compatibility, Woodson's w i d e l y used "Human E n g i n e e r i n g Guide f o r Equipment Designers" simply stated, " T h e r e a r e c e r t a i n 'expected relationships' between c o n t r o l a n d d i s p l a y t h a t should b e p r o v i d e d so t h a t movement e r r o r s w i l l b e a t a minimum." T h e g u i d e also p r o v i d e d some examples o f "expected" relations. What was " n a t u r a l " or "expected" was determined by convention, o r f r o m trying t o avoid competiCg response tendencies b e i n g possible, o r f r o m "population stereotypes, o r f r o m t h e a p p l i c a b i l i t y o f a v e r y few p r i n c i p l e s ascribable t o individuals, chief o f w h i c h was Warrick's P r i n c i p l e (Warrick, 1947). F o r r o u n d c o n t r o l knobs associated w i t h s t r a i g h t scales, t h i s p r i n c i p l e asserts t h a t t h e p o i n t e r / i n d i c a t o r o f t h e scale w i l l b e expected t o move in t h e same d i r e c t i o n as t h e p o i n t on t h e c o n t r o l w h i c h is closest t o it. Hence, w i t h a v e r t i c a l scale, a clockwise rotation o f t h e c o n t r o l knob w i l l b e expected t o make t h e p o i n t e r move u p w a r d s if t h e k n o b is s i t u a t e d t o t h e right o f t h e scale, but downwards if t h e k n o b is on t h e l e f t . A n y potential confusions could b e avoided by u s i n g a c o n t r o l t h a t moved in a s t r a i g h t line, r a t h e r t h a n u s i n g a r o u n d c o n t r o l knob. T h e p o i n t e r on a s t r a i g h t , v e r t i c a l scale i s expected t o move i n t h e same d i r e c t i o n as an upwards o r downwards movement o f a s l i d i n g

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switch o r lever, regardless o f t h e position o f these controls. But, f o r many reasons (e.g., t h a t r o u n d knobs a r e easily associated w i t h potentiometers), these c o n t r o l s w i l l c o n t i n u e t o b e used. Warrick's P r i n c i p l e shows p r o x i m i t y b e i n g used t o select which side of a r o u n d c o n t r o l i s associated w i t h t h e p o i n t e r ( o r w h i c h p a r t s of a stimulus a r e attended t o ) . Once t h a t has been established, however, t h e operator's expectation is one of s i m i l a r i t y f o r t h e d i r e c t i o n o f movement. T h e operation o f t h i s p r i n c i p l e makes possible t h e use of r o u n d controls w i t h s t r a i g h t d i s p l a y s . In some arrangements, s u c h as w h e r e a c o n t r o l i s located a t t h e t o p o r bottom e n d o f a s t r a i g h t , v e r t i c a l scale, Warrick's Principle f a i l s . F i g u r e 1 i l l u s t r a t e s these effects. Figure 1 shows confusion when t h e c o n t r o l k n o b is a t t h e t o p o f t h e scale. However, Woodson (1957), showing t h e k n o b a t t h e bottom o f t h e scale, c i t e d a clockwise t u r n as c r e a t i n g an "expectation" t h a t it w i l l move t h e p o i n t e r upwards, see F i g u r e 2 . If t h i s is correct, w h a t p r i n c i p l e g o v e r n s t h i s

0

13 1 b

C

a

Illustration of 'Warrick's Principle'. To move indicators upwards, Warrick's Principle rules that: (a) A n t i c l o c k w i s e r o t a t i o n of control (b) C l o c k w i s e r o t a t i o n of control (c) No clear a p p l i c a t i o n of t h e p r i n c i p l e i s p o s s i b l e .

F i g u r e 1. Examples o f situations f o r w h i c h Warrick's P r i n c i p l e applies (a b) a n d f o r which it does n o t ( c ) .

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FOR POINTER MOVEMENT UP F i g u r e 2. Example o f t h e "expected" e f f e c t o f rotation a knob a t t h e bottom o f t h e scale in a clockwise direction. "expectation"? One can i n v o b a "clockwise f o r increase" p r i n c i p l e (Bradley, 1959) if one assumes increase" is expected t o occur f r o m t h e bottom t o t h e t o p o f t h e scale. B u t t h e r e a r e t w o features o f F i g u r e 2 , which have n o t y e t been mentioned, t h a t may b e relevant in developing "expectations." The f i r s t is t h e pointer/indicator on t h e right side of t h e scale. T h e second is t h e t w o arrows, which a r e t h e r e o n l y t o i n f o r m t h e reader a n d would n o t b e Nevertheless, t h e a y o w s do convey informap r e s e n t in actual displays. tion, a n d it is much easier t o accept Woodson's expectations" when t h e y a r e p r e s e n t t h a n if t h e y a r e removed. Simply considering what t h e effect o f an anticlockwise t u r n should be, may remove o r reduce t h e e f f e c t of t h e a r r o w s on t h e viewer's "expectation," w i t h several possible outcomes. One p o s s i b i l i t y is t h a t t h e same "expectation" w i l l generalize along t h e lines "if clockwise is up, anticlockwise is down," a n d t h i s f i t s t h e "anticlockwise f o r decrease" extension o f "clockwise f o r increase. " A n o t h e r p o s s i b i l i t y is t h a t t h e position o f t h e pointer/indicator on t h e right side o f t h e scale w i l l b e associated w i t h t h e right side o f t h e knob, so t h a t what happens t o t h a t side o f t h e k n o b should b e reflected in t h e movement o f t h e p o i n t e r . I n t h i s case, c o n t r a r y t o Woodson, anticlockwise means an upwards p o i n t e r movement and c!pckwise means a downwards movement. A n inability t o develop a n y expectation" i s a third poss ibiIity .

With these simple f i g u r e s , we have t r i e d t o demonstrate t h a t behavior o r "expectation" results f r o m t h e interaction o f several possible p r i n c i p l e s r a t h e r t h a n j u s t one principle, although some p r i n c i p l e s a r e s t r o n g e r t h a n others. Experimentally, these matters were investigated by B r e b n e r a n d Sandow (1976) in a paper and pencil study, a n d by

Psychological and Neurophysiological Factors Petropoulos a n d B r e b n e r (1981) actual turning responses.

245

u s i n g a real r o t a r y c o n t r o l k n o b a n d

I n o u r studies, t h r e e p r i n c i p l e s were implicated: (a) Warrick's Principle; (b) clockwise f o r increase/anything; a n d (c) a scale-side principle, such t h a t t h e p o i n t e r w i l l b e expected t o move in t h e same d i r e c t i o n as t h e side o f t h e c o n t r o l k n o b w h i c h corresponds w i t h t h e side o f t h e scale m a r k i n g s o r p o i n t e r . T h e second a n d third p r i n c i p l e s operate even when t h e c o n t r o l k n o b is located a t t h e t o p o r bottom o f vertical, linear displays. We expected t h a t when t h e p r i n c i p l e s mapped o n t o t h e same response a n d r e i n f o r c e d each other, t h e r e would b e concordance, a n d almost e v e r y o n e would make t h e same response., T h a t is, a populat i o n stereotype would emerge. When t h e p r i n c i p l e s were in conflict, however, t h i s would r e d u c e t h e degree o f concordance. Chance levels o f clockwise a n d anticlockwise response would show competing response tendencies t o b e equally s t r o n g . T h e r e s u l t s o f t h e p a p e r a n d pencil s t u d y , w h i c h a r e based o n t h e responses o f 152 adults, a r e shown in F i g u r e 3. The results from t h e Petropoulos a n d B r e b n e r (1981) s t u d y u s i n g a real c o n t r o l k n o b t u r n e d clockwise o r anticlockwise i n response t o a computer-controlled d i s p l a y (100 subjects) a r e shown in parentheses. Statistical analyses, such as ndimensional C h i - s q u a r e d t e s t s (Kendall t. S t u a r t , 1973), p e r f o r m e d on t h e r a w data c o n f i r m w h a t is obvious f r o m F i g u r e 3. When Warrick's P r i n c i p l e a n d Scale-side r e i n f o r c e each other, t h e r e is concordance across subjects in t h e d i r e c t i o n o f t h e i r responses. But when these p r i n c i p l e s a r e in conflict, t h e agreement disappears in t h e actual t e s t a n d is reduced in t h e p a p e r a n d p e n c i l v e r s i o n . While t h e r e is good agreement between t h e t w o versions, t h e discrepancy in F i g u r e 3c i s a reminder of t h e d a n g e r o f r e l y i n g e n t i r e l y on paper a n d pencil tests, a p o i n t made by v a r i o u s researchers (Loveless, 1962; Warrick, 1947). T h e r i s k o f u s i n g as stimuli o n l y those whose fea!ures can b e closely mimicked in t h e response f a i l s t o consider w h a t a stimulus" is. Whether in theoretical o r applied S - R compatibility studies, a stimulus i s t h e information conveyed by t h e experimental materials o r t h e r e a l - l i f e c o n t r o l - d i s p l a y arrangements. T h i s information may i n c l u d e a number o f response possibilities o r tendencies. It may b e t h a t spatial correspondence between p a r t s o f a d i s p l a y a n d a c o n t r o l ( t h e response made) w i l l determine w h i c h response is more compatible (Gottsdanker & Senders, 1959) and, therefore, more probable. B u t t h e conceptualization o f t h e situation may also a f f e c t t h e response. F o r example, whatever t h e national convention o r s t a n d a r d is, t h e movement made in turning o n a w a t e r t a p involves a response t h a t is r e l e v a n t f o r releasing a f o r c e t h a t is b e i n g contained. In many countries, t h e conventional response for releasing or c o n t a i n i n g a f o r c e is t h e same as t h a t f o r loosening a screw - " l e f t is loosen, r i g h t is t i g h t e n . These conventions a r e overlearned t o a degree t h a t can b e dangerous if t h e y a r e c o n t r a d i c t e d in p o t e n t i a l l y hazardous situations because o f c r o s s - c u l t u r a l differences. In b r i e f , human operators bring t h e i r learned contingency rules, t h e i r sets of abstracted relations, a n d t h e i r generalizations w i t h them w h e t h e r t h e y a r e e n t e r i n g an e v e r y d a y s e t t i n g i n v o l v i n g c o n t r o l s a n d d i s p l a y s o r an experimental l a b o r a t o r y .

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:I. Y

15

15

10

10

5

a

a

5

5

b

C

d

Proportions of 'clockwise'/'anticlockwise' responses observed where the scale side principle contradicts (in a and c ) or reinforces (in b and d ) the 'clockwise for everything' stereotype and Warrick's Principle respective1y.

Penci.1 Actually ( a ) Scale side predicts anticlockwise 6, Paper turning b o b 43% (45%) Percentage responding "clockwise" Percentage responding "anticlockwise" 57% (55%) (b) Scale side predicts clockwise 80% (72%) Percentage responding "clockwise" (28%) Percentage responding "anticlockwise" 20% (c) Warrick's Principle predicts c l o c k w i s e 73% (57%) Percentage responding "clockwise" Percentage responding "anticlockwise" 27% (43%) ( d ) Warrick's Principle predicts clockwise Percentage responding "clockwise" 86% (85%) 14% (15%) Percentage responding "anticlockwise"

F i g u r e 3. Comparison o f stereotypical responses f o r effects o f k n o b rotation determined f r o m pencil-and-paper tests a n d f r o m actual performance test. Spatial v e r s u s Anatomical Correspondence T h e same i n t e r e s t in t h e e f f e c t on performance o f spatially similar o r dissimilar relations between d i f f e r e n t components o f a t a s k prompted some o t h e r S-R compatibility experiments (Brebner, 1973, 1979; B r e b n e r , In t h e f i r s t o f Shephard, & Cairney, 1972; Smith & B r e b n e r , 1983). these, t h e aim was t o manipulate t h e relations between (a) t h e signals ( t w o l i g h t s ) a n d response mechanisms ( t w o keys), (b) t h e response

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mechanisms (two keys) a n d e f f e c t o r (two hands) positions, a n d (c) signals (two l i g h t s ) a n d e f f e c t o r positions ( t w o hands). T h e experiment was performed w i t h t h e hands crossed a t t h e w r i s t a n d w i t h t h e hands uncrossed. Table 1 details t h e experimental conditions. Table 1 T h e Experimental Conditions o f B r e b n e r e t al. (1972)

Condition

Left light

R i g h t light

Key Hand

K e y Hand

cu* c c*

L L R R

Iu*

K*

L R R L

R R L

R L L

L

R

*CU: compat., hands uncrossed. *IU: incomp., hands uncrossed.

Spatial Relations Compatible o r Incompatible Light - K e y Light - Hand Hand - K e y Compat. Compat. Incomp. Incomp .

*CC: *IC:

compat., incomp.,

Compat. Incomp. Incomp. Compat.

Compat. Incomp. Compat. Incomp.

hands crossed. hands crossed.

These conditions were i n t e n d e d t o u n c o n f o u n d as f a r as possible t h e several spatial relations i n v o l v e d in what is basically a two-choice R T task. Assuming t h a t incompatible relations would g i v e r i s e t o slower RTs, a n d t h a t t h i s e f f e c t would b e t h e same f o r all relations, t h e r e l a t i v e speed o f responding in each condition can b e r a n k o r d e r e d i n terms o f what would b e expected if o n l y a p a r t i c u l a r relation, o r a combination o f relations, h a d an e f f e c t on R T (see Table 2). Table 2 R T Rank O r d e r f o r Particular/Combined Spatial Relations

Relation(s) a f f e c t i n g R T

Light- key Light-hand Hand- k e y Light- k e y + L i g h t - h a n d L i g h t - k e y +Hand - k e y L i g h t - h a n d + H a n d -k e y Light-key+Light-hand+Hand-key

*CU : compat., hands uncrossed. *IU:

incomp.,

hands uncrossed.

cu

cc

1.5

1.5 3.5 3.5 2.5 2.5

1.5 1.5 1 .o 1 .0 1 .o

1 .O

4.0

3.0

Conditions IU

3.5 3.5 1.5

4.0 2.5 2.5 3.0

* C C : compat., hands crossed. incomp., hands crossed.

*IC:

IC

3.5 1.5 3.5 2.5

4.0 2.5 3.0

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T h e r e s u l t s showed t h a t o n l y t h e d i f f e r e n c e in R T between t h e CC a n d IU conditions was n o t s i g n i f i c a n t a n d t h a t t h e p a t t e r n o f R T across conditions indicated t h a t it was t h e compatibility-incompatibility o f t h e L i g h t - k e y + H a n d - k e y combination, rather than that of the Light-hand relation, t h a t g a v e r i s e t o these data. T h i s finding argues t h a t t h e spat i a l relation between t h e light a n d h a n d is ineffective, a n d t h i s agrees w i t h Simon e t al. (1970) a n d Wallace (1971), b o t h o f whom f o u n d t h a t it was t h e relation between t h e spatial position o f t h e e f f e c t o r t h a t mattered, r a t h e r t h a n w h i c h h a n d was used. T h e mean R T s f r o m t h e t h r e e studies a r e shown in Table 3. Table 3 Mean R T (ms) in T h r e e Separate Experiments Condition

cu Hand:

Simon e t al. (1970) Wallace (1971) B r e b n e r e t al. (1972)

cc

L

R

L

R

386

385

432

405

N-D

D 346 245

N-D

D 386 285

373 255

426 288 Condition IC

IU

Hand: Simon e t al. (1970) Wallace (1971) B r e b n e r e t al. (1972)

L

R

L

R

457

445

482

465

N-D

D 408 293

N-D

D 428 309

419 292

485 329

( L = left, R = right, D = dominant, N - D = Non-dominant) *CU: compat., hands uncrossed. *CC: compat., hands crossed. *IU: incomp., hands uncrossed. *IC: incomp., hands crossed. Apart. f r o m t h e o v e r a l l agreement across studies, it is n o t e w o r t h y t h a t t h e B r e b n e r e t al. (1972) s t u d y p r o d u c e d considerably f a s t e r RTs. T h i s presumably reflects t h e considerable p r a c t i c e undergone by t h e subjects in t h a t s t u d y b e f o r e these data were collected, a l t h o u g h t h e pace o f t h i s computer-controlled experiment was also much f a s t e r a n d t h a t c o u l d have p r o d u c e d g e n e r a l l y f a s t e r performance. What happens during p r a c t i c e in S - R compatibility studies is potent i a l l y v e r y informative, a l t h o u g h asymmetric p r a c t i c e effects may p r o v e difficult t o i n t e r p r e t . B r e b n e r (1973) tested subjects f o r 10 t e s t sessions (120 stimulus presentations in each) w i t h each o f t h e f o u r conditions in t h e e a r l i e r B r e b n e r e t al. (1972) experiment. F o r t h e non-dominant

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hand, t h e r e s u l t s f r o m t h e last session showed t h e same r a n k - o r d e r as in the first. B u t t h e p a t t e r n f o r t h e dominant h a n d h a d changed. Table 4 shows t h e r a n k positions.

It was t e n t a t i v e l y suggested t h a t t h e r e s u l t s m i g h t show t h e e a r l y developing o f a d i f f e r e n t spatial schema. Because t h e CU condition, in w h i c h all t h r e e spatial relations w e r e compatible, c o n t i n u e d t o g i v e r i s e to f a s t e r RTs, t h e s i m i l a r i t y o f t h e relations m i g h t b e i m p o r t a n t if t h a t simi l a r i t y allows t h e application o f a single r e c o d i n g r u l e . T h i s c a r r i e s w i t h it t h e implication t h a t if all relations a r e incompatible, performance should b e f a s t e r t h a n when some a r e compatible a n d o t h e r s incompatible. Table 4 Mean R a n k Position of R T in F i r s t a n d L a s t T e s t Sessions

Condition

cu

cc

F i r s t session Final session

1.0

2.3 2.6

1.2

N-D h a n d IU

IC

cu

cc

2.9 2.6

3.8 3.6

1.0

2.1 2.5

1.2

D hand IU

IC

3.2

3.7 2.7

3.6

(D = dominant, N-D = Non-dominant) *CU:

*IU:

cornpat., hands uncrossed. incomp., hands uncrossed.

*CC: cornpat., hands crossed. *IC: incomp., hands crossed.

T h i s i n t e r p r e t a t i o n may b e applicable t o a number-, o f o t h e r findings. F o r example, B r e b n e r (1979) suggested t h a t t h e Simon E f f e c t " - w h i c h is t h a t when t h e spatial location o f t h e stimulus i s i r r e l e v a n t , RT is f a s t e r when responding in t h e d i r e c t i o n o f t h e stimulus t h a n when responding in t h e opposite d i r e c t i o n (Simon e t al., 1970)--might o c c u r because a// t h e spatial relations i n v o l v e d a r e ipsilateral.

A v e r y similar explanation was proposed by Hedge a n d Marsh (1975), who used t w o stimulus locations, t w o stimulus colors (red/green) t h a t c o u l d o c c u r a t eithej; location, t w o colored response b u t t o n s (red/green), and a white home" k e y f r o m which responses s t a r t e d . When subjects were i n s t r u c t e d t o p r e s s t h e b u t t o n o f t h e same color as t h e stimulus (regardless o f w h i c h side t h e stimulus appeared on), t h e "Simon Effect" was o b s e r v e d a n d R T was f a s t e r when t h e stimulus was on t h e same side as t h e response b u t t o n . But when t h e i n s t r u c t i o n s were changed t o r e q u i r e subjects t o p r e s s t h e b u t t o n o f t h e a l t e r n a t i v e color t o t h a t o f t h e stimulus, regardless o f i t s side, R T was f a s t e r when t h e a p p r o p r i a t e l y colored b u t t o n was also in t h e a l t e r n a t i v e location to t h a t o f t h e stimulus. Hedge a n d Marsh called t h i s a negative version o f t h e "Simon E f f e c t . " T h e f a c t t h a t t h e e f f e c t reverses in t h i s manner accords w i t h a single spatial r u l e ("same" or "alternative") b e i n g used, a n d Hedge a n d Marsh concluded t h a t performance was f a c i l i t a t e d when t h e same logical process c o u l d b e applied t o b o t h a t t r i b u t e s o f t h e stimulus. A question n o t addressed by Hedge a n d Marsh (1975) is w h e t h e r a non-spatial a t t r i b u t e e x e r t s t h e same degree o f i n f l u e n c e as a spatial one. B r e b n e r (1979) examined t h i s question by r e v e r s i n g t h e i n s t r u c t i o n s used

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by Hedge a n d Marsh (1975) a n d making t h e i r r e l e v a n t v a r i a b l e color, a n d t h e r e l e v a n t v a r i a b l e stimulus location. For both RT and RT+MT (movement time), t h e same p a t t e r n emerged. Responses were f a s t e r when t h e y were t o w a r d t h e location o f a stimulus a n d were n o t facilitated when t h e i n s t r u c t i o n was t o respond t o t h e spatial a n d color alternatives, even t h o u g h these a r e l o g i c a l l y similar. Spatial location, therefore, seems t o b e t h e more p o w e r f u l determinant o f RT, because it o v e r r i d e s color. T h e mean R T s obtained f o r t h e four conditions were:

1. Same Color, Same Location

328 ms

2.

Same Color, A l t e r n a t i v e Location

380 ms

3.

A l t e r n a t i v e Color, Same Location

330 ms

4.

A l t e r n a t i v e Color, A l t e r n a t i v e Location

388 ms

In a f u r t h e r experiment, Smith a n d B r e b n e r (1983) compared t h e speed o f response in r e l a t i v e l y p r a c t i c e d subjects o p e r a t i n g u n d e r one o r t w o possible recoding rules. Basically, a light ( r e d o r g r e e n ) appeared a t one o f t w o locations ( l e f t o r right), a n d subjects responded by p r e s s ing one o f t w o response b u t t o n s ( l e f t o r right) t h a t were also colored r e d o r green. One r u l e c o u l d b e applied where subjects c o u l d respond b o t h t o t h e same side ISS) a n d t h e same color ( S C ) - ("same" rule), a n d where t h e y c o u l d respond t o t h e a l t e r n a t i v e side (AS) a n d t h e a l t e r n a t i v e color (AC) - ("alternative" r u l e ) . Mixing t h e rules involved responding t o t h e same side but a l t e r n a t i v e color (SSAC), o r t h e a l t e r n a t i v e side but same color (ASSC). Each subject performed u n d e r all f o u r possibilities w i t h i n t h e same condition. T h i s was achieved by v a r y i n g t h e i n s t r u c tions, w h i c h were:

1. If a r e d light appears, p r e s s t h e b u t t o n on t h e same side as t h e light; if a g r e e n light appears, p r e s s t h e b u t t o n on t h e a l t e r n a t i v e side t o t h e light. 2. If a r e d l i g h t appears p r e s s t h e b u t t o n on t h e a l t e r n a t i v e side t o t h e light, if a g r e e n light appears p r e s s t h e b u t t o n on t h e same side as t h e light. These i n s t r u c t i o n s a r e t o map f r o m color t o position a n d exchanged t h e roles o f r e d a n d green. T w o o t h e r forms o f i n s t r u c t i o n s p r o d u c e d position t o color mapping: 3. If a light appears on t h e r i g h t , p r e s s t h e b u t t o n o f t h e same color as t h e stimulus; if a light appears on t h e left, p r e s s t h e b u t t o n o f t h e a l t e r n a t i v e ,color.

4. If a light appears o n t h e right, p r e s s t h e b u t t o n of t h e a l t e r n a t i v e color t o t h e stimulus; if a light appears on t h e left, p r e s s t h e button o f t h e same color. O f 20 subjects, 10 performed u n d e r color-to-position mapping a n d 10 u n d e r position-to-color mapping. For each o f t h e i n s t r u c t i o n s , subjects p r a c t i c e d until t h e y made seven out o f e i g h t c o r r e c t responses. As expected, in mapping color t o position, where t h e c o r r e c t responses could b e determined d i r e c t l y f r o m t h e recoding rule, performance was f a s t e r

Psychological and Neurophysiological Factors

25 1

t h a n i n mapping position t o color, where t h e spatial location o f t h e approp r i a t e colored object s t i l l has t o b e determined. T h e r e l e v a n t mean RTs a r e shown i n Table 5.

Table 5 Mean R T (ms) f r o m Smith a n d B r e b n e r (1983)

1 Recoding Rule SSSC

ASAC

Mapping color t o position

454

Mapping position t o color

558

2 Recoding Rules SSAC

ASSC

474

470

470

604

614

586

Note. SS = Same Side; SC = Same Color, AS = A l t e r n a t i v e Side; AC = A l t e r n a t i v e Color Table 5 also shows t h a t t h e r e is no advantage o f a single r e c o d i n g r u l e when it is " A l t e r n a t i v e Side, A l t e r n a t i v e Color." So, t h e negative "Simon Effect" observed by Hedge a n d Marsh (1975),,was n o t f o u n d here. in t h a t r e s p o n d i n g Equally, however, n e i t h e r was t h e Simon Effect, SSAC was n o t f a s t e r th an ASAC o r ASSC. O n t h i s evidence, t h e o r i g i n a l explanation o f f e r e d by Simon f o r t h e "Simon Effect," w h i c h was t h a t t h e r e is a s t r o n g t e n d e n c y t o r e s p o n d towards a s o u r c e o f stimulation, does n o t a p p l y u n d e r these experimental conditions. While variations o f t h e experiment, p o s s i b l y i n v o l v i n g g r e a t e r p r a c t i c e o r comparing subjects who o n l y p e r f o r m u n d e r one o f t h e rules, a r e c e r t a i n l y possible, t h e f a c t remains t h a t r e l a t i v e l y p r a c t i c e d subjects who a r e d o i n g t h e same actions t o t h e same o r v e r y similar stimuli a n d w i t h t h e same apparatus, p r o d u c e v e r y d i f f e r e n t R T s depending upon t h e instructions/mapping rules t h e y a r e u s i n g . F i g u r e 4 shows t h i s in more detail.

P300 a n d Compatibility Because t h e responses a r e t h e same i n F i g u r e 4, "compatibility/ incompatibility" seems t o r e f l e c t differences i n processing stimulus-related information a t a stage b:fore response programming. Unfortunately, despite psychology's long stimulus-response" h i s t o r y , it has n o t p r o v e d easy t o i d e n t i f y t h e n e u r a l events t h a t a r e "stimulus-related" f r o m those t h a t a r e "response-related, " even i n theories t h a t propose i n d i v i d u a l differences in these events (e.g., B r e b n e r , 1983). Recently, however, t h e r e has been a f o c u s i n g of i n t e r e s t on t h e p o s i t i v e wave o c c u r r i n g a r o u n d 300 ms (P300) post-stimulus by a number o f psychophysiological researchers.

J. Brebner

252

REAS G

1

A

A

RT.4 7 4 m . s ~ .

S.D. 92.3

(3)

R\

A

P n

A

KT.469m.5ec

S.D. 86.1 ( 5 )

I

A

A

2

n

fi. 478m.sec.

S.D. 81.3 (4)

/R

A ’ A \n fi.475m.sec S.D. 80.6 (7)

RESS G

R

A \A 7

A A \a

n CT. 471 m.sec.

S.D. 84.8 (3)

R

G

FT 462m S ~ C 8 3 0 (3)

SD

/- A

A’A

A 7 n

‘\n KT472m sec

ITT 435m sec

S D 8 4 9 (1)

SD

7 3 2 (4)

RIAC G

I

A

A

R?. 565rn.sec. S.D. 97.1 (7)

KT. 598 msec. S.D. 106.2 (23)

fi.606 m.sec

S.D. 104.4 (21)

KT 604 m.sec S.D. 96.8(19)

RISC G

A

R

R

I 2 n

A\

A

‘a

KT.W m . s e c .

KT 615 m.sec.

S.D. 97.4 (7)

S.D. 106.1 (9)

A/ G A

“n R T-; 577 m.sec S.D. 99.0 (9)

A

A

7

n

RI. 551 m.sec S.D. 94.9 ( 2 3 )

Figure 4 . Reaction time data from Hedge and Marsh (1975). (REAS r e d alternate side; RESS - r e d same side; R I A C = r i g h t alternate color; RISC = r i g h t same color; G = green stimulus; R = r e d stimulus; filled symbols = green response button; unfilled symbols = r e d response buttons; number of e r r o r s given in parentheses).

Psychological and Neurophysiological Factors

253

In many studies, changes o r differences in t h e amplitude and/or latency of P300 have been related t o p a r t i c u l a r t a s k conditions in attempts t o i n f e r t h e n a t u r e o f t h e psychological processes reflected by P300. Similarly, P300 has been compared in v a r i o u s p a t i e n t a n d non-patient groups, a n d o b s e r v e d differences a t t r i b u t e d t o features o f t h e diagnosed condition. G i v e n t h a t P300 has been implicated in a wide v a r i e t y o f c l i n i cal conditions a n d across dissimilar experimental arrangements, it seems l i k e l y t h a t it represents some n e u r a l process fundamental t o many forms o f interaction w i t h t h e environment.

A f t e r considerable research i n t o how p r e c i s e l y changes in P300 f o l low manipulations o f t a s k demands a n d experimental arrangements, a few generalizations seem reasonably safe. First, P300 represents t h e c o r t i c a l a c t i v i t y associated w i t h general information processing (e.g., Donchin e t al., 1973; Lacey & Lacey, 1980). Second, P300 reflects a c t i v i t y o c c u r ring post-stimulus. T h i s does n o t mean t h a t P300 is unaffected by p r i o r factors such as expectation. It can r e f l e c t stimulus p r o b a b i l i t y (Johnson E Donchin, 1980; Karis, Chesney, & Donchin 1983) a n d i s affected by novel stimuli (Kok, 1978), o r even t h e absence o f expected stimuli 1981). B u t it does mean t h a t P300 follows t h e ( R u c h k i n e t al., information f r o m some s o r t o f "stimulus-event,'' t h o u g h t h i s has t o b e i n t e r p r e t e d v e r y widely. Third, P300 seems t o b e i n v o l v e d w i t h t h e process o f "categorizing" or c l a s s i f y i n g stimuli (e.g., Polich, 1985) o r what A l e x a n d r o v a n d Maksimova (1985a,b) r e g a r d as t h e c e n t r a l reorganization t h a t occurs when t h e "subject o f behavior" changes. T h i s , p a r t i c u l a r l y , i s not independent o f stimulus p r o b a b i l i t y because P300 has t e n d e d t o b e enhanced f o r i n f r e q u e n t o r u n p r e d i c t a b l e stimuli a n d t o "relevant" rather than "irrelevant" stimuli (e.g., Kaufman, Kaufman, & Salapatek, 1982).

P300 also has been shown t o have g r e a t e r amplitude when t h e informatjon t h a t was p r e s e n t e d disconfirmed r a t h e r t h a n confirmed subjects expectations o f b e i n g c o r r e c t o r i n c o r r e c t in a paired-associate l e a r n i n g t a s k (Horst, Johnson, & Donchin, 1980). T h i s e f f e c t is a k i n t o p r e s e n t i n g novel o r unexpected stimuli. T h e e f f e c t of "relevance" on P300 emerges across d i f f e r e n t s o r t s o f studies; f o r example, g r e a t e r amplitudes a n d reduced latencies f o r P300 were f o u n d t o t h e w o r d "vodka" compared t o n e u t r a l words in a sample o f c h r o n i c alcoholics (Genkina & Shostakovich, 1983). A n d Goto (1978), in a Japanese study, found t h e amplitude of P300 was increased t o k e y characters t h a t determined w h e t h e r a sentence was meaningful o r not. T h e latency o f P300 also was longer t o k e y characters t h a n t o o t h e r s . A fourth generalization, therefore, a n d one p a r t i c u l a r l y important here, is t h a t P300 seems t o r e f l e c t t h e process of stimulus analysis r a t h e r t h a n t h a t o f response organization. T h i s p o i n t is made by Kantowitz (1987), who states: Some o f t h e g r e a t e s t potential ( p a r d o n t h e pun) f o r p s y chophysiology centers on t h e P300 component of t h e ERP. T h e r e is considerable s u p p o r t f o r t h e hypothesis t h a t t h e P300 is related t o psychological processes related t o stimulus identification r a t h e r t h a n t o response selection (p. 240). For example, i n f r e q u e n t t a r g e t s e l i c i t smaller P300 amplitudes (p. 235), a n d P300 latency is constant in a Stroop task, a l t h o u g h behavioral reaction time is n o t (p. 2 6 4 ) . Such data suggests t h a t P300 is u s e f u l when one wishes t o a v o i d limitation o f response bottlenecks [p. 229) a n d f o r decomposition o f

254

J. Brebner i n f e r r e d mental processing stages (p. 708). T h i s gives t h e experimental psychologist a valuable tool. Instead o f p e r f o r m i n g many experiments w i t h c o n v e r g i n g operations t o determine whether assorted independent variables e x e r t effects a t e a r l y o r late processing stages, it may b e possible t o instead monitor P300 t o answer t h i s question w i t h considerable savings in time a n d e f f o r t . (pp. 1003-1004)

T h e results o f semantic studies l i k e those above seem t o imply t h a t P300 represents more t h a n a n o r i e n t i n g response produced by t h e p h y s i cal aspects o f t h e stimulus. O'Gorman (1979) has discussed t h e o r i e n t i n g response, a n d Donchin e t al. (1984) reviewed t h e relation between it and P300 w i t h o u t b e i n g able t o conclude specifically how t h e y relate. The t i m i n g o f P300 may f a v o r t h e notion t h a t it represents a c t i v i t y t h a t f o l lows t h e i n i t i a l o r i e n t i n g t o w a r d a source o f stimulation. T h e o r i e n t i n g response could b e i n v o k e d t o explain t h e "Simon Simon's own explanation is that t h e r e i s an Effect" in S-R compatibility. i n h e r e n t tendency t o respond t o w a r d a stimulus t h a t has t o b e overcome if responding away f r o m it. T h i s explanation d i f f e r s f r o m t h e suggestion t h a t t h e orientation response encompasses t h e process o f locating t h e d i r e c t i o n o f a response when responding towards a stimulus, but does not when responding away f r o m it. However, t h e t w o explanations a r e comp a t i b l e w i t h each o t h e r . T h e e f f e c t o n P300 o f responding t o w a r d a n d away f r o m stimuli is o f i n t e r e s t here. Ragot (1984), in a s t u d y o f S-R compatibility, came v e r y close t o t h e suggestion above. However, he related t h e "Simon Effect" t o i n t e r ? d i z e d spatial relations u s i n g Paillard's (1980) t e r m "schema corporel. What Ragot f o u n d was t h a t b o t h R T and P300 latency were longer when t h e relation between stimulus position and responding hand was contralateral r a t h e r t h a n ipsilateral, except when t h e hands were crossed. W i t h hands crossed, o n l y R T was lengthened. On these grounds, Ragot also concluded t h a t P300 reflects t h e processing o f i n f o r mation concerned w i t h t h e spatial relations between stimulus and response, r a t h e r t h a n t h e motor programming o f t h e response. Ragot's (1984) experiment, a n d t h e Stroop t a s k r e s u l t o f DuncanJohnson a n d Koppell (1981), p o i n t t o t h e potential value o f t h e P300 measure in d i s c o v e r i n g what it i s t h a t makes tasks S-R compatible o r incompatible. P300's relevance extends t o cases where t h e arrangements a r e d i r e c t l y p e r t i n e n t t o control-display relations t h a t a r e widely used (e.g., B r e b n e r & Sandow, 1976, a n d Petropoulos & Brebner, 1981). P300 a n d Personality Personality factors have been related t o P300 i n several studies, b u t w i t h o u t a n y v e r y s t r o n g theoretical basis (e.g., Ward e t al., 1984; Amabile, 1984). However, if P300 reflects stimulus analysis, t h e n it may b e used t o t e s t t h e c e n t r a l proposition in Brebner's a n d Cooper's (1974) model o f e x t r a v e r s i o n . T h a t proposition is t h a t i n t r o v e r t s d e r i v e "excitation" f r o m stimulus analysis but " i n h i b i t i o n " f r o m response organization, a n d t h e opposite is t r u e f o r e x t r a v e r t s . "Excitation" and "inhibition" a r e operationalized, respectively, t o mean "continuing in a n a c t i v i t y o r augmenting it" and "ceasing an a c t i v i t y o r attenuating it." Behavioral studies have s u p p o r t e d t h e model (see Brebner, 1983), but

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t h i s is n o t s u r p r i s i n g because it began as a w a y o f unifying v a r i o u s i n f l u e n t i a l theories, each o f w h i c h h a d considerable experimental s u p p o r t . However, t h e t h e o r y proposes t h a t i n t r o v e r t s a n d e x t r a v e r t s d i f f e r in how t h e y process information, because t h e y have i n h e r e n t l y d i f f e r e n t biases t o w a r d excitation f r o m c e n t r a l stimulus-analysis a n d response-organization processes, respectively ( a n d conversely t o w a r d i n h i b i t i o n f r o m t h e o t h e r process). Thus, it may b e i m p o r t a n t t o measure e x t r a v e r s i o n in S-R compatibility experiments, w h e r e compatibility/incompatibility effects a r e d u e e i t h e r t o stimulus analysis (Ragot, 1984) o r t o competing response tendencies (Duncan-Johnson & Kopell, 1981). Results r e c e n t l y obtained u n d e r experimental arrangements similar t o B r e b n e r e t al. (1972) a n d Ragot (1984) suggest t h a t i n t r o v e r t - e x t r a v e r t differences in P300 may b e so r o b u s t t h a t t h e y should n o t b e i g n o r e d . F i g u r e 5 shows t h a t i n t r o v e r t s have much s h o r t e r P300 latencies t h a n e x t r a v e r t s , but much l o n g e r RTs. T h e b r a i n wave a c t i v i t y in t h i s s t u d y was recorded f r o m C3 a n d C4 o f t h e 10/20 I n t e r n a t i o n a l Electrode System, r e f e r r e d t o l i n k e d mastoid reference electrodes (see C h a p t e r 7, by Bashore). In l a t e r experiments, 100 K r e s i s t o r s were placed in a series w i t h each mastoid electrode (Ragot, 1984). T h e electrodes were s i l v e r - s i l v e r c h l o r i d e dome electrodes, 9 mm i n diameter; t h e scalp electrodes were f i x e d in place w i t h Grass EC2 electrode cream a n d t h e mastoid electrodes w i t h adhesive tape. Eye movements w e r e monitored w i t h an electrode above t h e l e f t eyebrow, v e r t i c a l l y above t h e pupil in f o r w a r d gaze, a n d another a t l e f t o u t e r canthus. T h e electrical signals were amplified t h r o u g h a NEOMEDIX "Neotrace 800ZF" physiological recorder; t h e -3dB b a n d pass limits o f t h e EEG preamplifiers were set t o 2 a n d 100 Hz. A " g r o u n d " electrode was connected t o t h e l e f t w r i s t . A MANOVA o f t h e data in F i g u r e 5 reveals a s i g n i f i c a n t d i f f e r e n c e between t h e i n t r o v e r t e d ( n = 7) a n d e x t r a v e r t e d ( n = 8) g r o u p s (Wilks lambda, p = .032, Exact F = 4.25, e r r o r df = 11) a n d a s i g n i f i c a n t e f f e c t o f conflict (Wilks lambda, p = .015, Exact F = 5.49, e r r o r df = 11). T h e c o n f l i c t effect appears t o b e mainly d u e t o t h e R T data, as F i g u r e 5 indicates. T h e interaction o f c o n f l i c t w i t h hands crossed-uncrossed approaches significance (Wilks lambda, p = .052, Exact F = 3.54, e r r o r df = ll),a n d t h e r e is a s i g n i f i c a n t C o n f l i c t x Hand i n t e r a c t i o n t h a t does n o t appear in F i g u r e 5 (Wilks lambda, p = .034, Exact F = 4.15, e r r o r df = ll),but w h i c h reflects slower R T values f o r t h e l e f t h a n d u n d e r c o n f l i c t . In t h e p r e s e n t context, t h e most important aspects o f these r e s u l t s are, f i r s t , t h e interrelatedness o f t h e R T a n d evoked potential data, showing t h a t b o t h index S-R compatibility-incompatibility. T h i s also makes m u l t i v a r i a t e analysis a p p r o p r i a t e . Second, t h e robustness a n d coherence o f t h e i n t r o v e r t - e x t r a v e r t differences, w i t h i n t r o v e r t s h a v i n g s h o r t e r P300 a n d longer R T values t h a n e x t r a v e r t s , in l i n e w i t h t h e Brebner-Cooper model, shows t h e importance o f i n d i v i d u a l differences in S-R compatibility research. From these results, measures o f b r a i n a c t i v ity a n d o f i n d i v i d u a l differences may p r o v i d e new d i r e c t i o n s a n d impetus in S-R compatibility research.

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Summary This paper touches on f o u r aspects of t h e research i n t o S-R compatibility t h a t has been conducted over t h e last 40 years. The f i r s t aspect is t h e relation between S-R compatibility and applied areas, such as control-display relations. Second, and d i v e r g i n g from that, is t h e study of S-R compatibility t o discover t h e effects on performance of v a r i ous recoding processes. Third, and more recently still, are studies using S-R compatibility in an attempt to relate particular electrophysiological a c t i v i t y t o cognitive processes, such as t h e resolution of spatial uncertainty. Fourth, and most recently, t h i s paper suggests t h a t individual differences may modulate t h e strength of effects o f spatially compatible o r incompatible arrangements. Acknowledgement The help of my colleagues Christopher Cooper, Roger Dubois, Con Stough, and Bob Wilson i n various areas of t h e studies presented here is gratefully acknowledged. References Alexandrov, I. O., & Maksimova, N. E. (1985a). Psychological meaning of P300 component. Psikologicheskii Zhurnal, 6, 86-95. Alexandrov, I . O., & Maksimova, N. E. (1985b). P300 and psychological analysis of t h e s t r u c t u r e of behavior. Electroencephalography 6 Clinlcal Neurophysiology, 61, 548-558. (1984). Electrophysiological (CNV) analysis of personality Amabile, G . t r a i t s . Neuropsychobiology, 12, 260-264. Boring, E. G . ( E d . ) . (1945). Psychology for the Armed Services, Washington, DC: Washington I n f a n t r y Journal Press. Bradley, T. V. (1959). Direction of knob t u r n stereotypes. Applied Psychology, 4 3 , 21 -24.

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Brebner, J., Shephard, M., & Cairney, P. T . (1972). Spatial relationships a n d S-R compatibility. Acta Psychologica, 36, 1-15. Brooks, L. R. (1968). Spatial a n d v e r b a l components o f t h e a c t o f recall. Canadian Journal of Psychology, 22, 349-350. Donchin, E., Heffley, E., H i l l y a r d , S. A , , Loveless, N., Maltzman, I . , Ohman, A., Rosler, F., Ruchkin, D., & Siddle, D. (1984). Cognition a n d ERPs: T h e o r i e n t i n g r e f l e x a n d P300. Annals of the New Y o r k Academy of Sciences. 425, 39-57.

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427-439. Horst, R. L., Johnson, R., a n d Donchin, E. (1980). Event-related b r a i n potentials a n d subjective p r o b a b i l i t y in a l e a r n i n g t a s k . Memory & Cognition, 8, 476-488. Johnson, R . , & Donchin, E. (1980). P300 a n d stimulus categorisation: PsychoT w o p l u s one is n o t so d i f f e r e n t f r o m one p l u s one. physiology, 1 7 , 167-178. Kantowitz, B . H . (1987). Premises and promises of psychophysiology. Contemporary Psychology, 32, 1002-1004. Karis, D., Chesney, G. L . , & Donchin, E . (1983). ' I . . . 'twas t e n t o one; A n d y e t we v e n t u r e d . . . " : P300 a n d decision making. Psychophysiology, 20, 260-268.

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Kaufman, F., Kaufman, R., 6 Salapatek, P. (1982). The effect o f change in a stimulus sequence on P300. Neuropsychologia, 20, 439445. Kendall, M. G., E Stuart, A . (1973). T h e advanced theory of statistics (Vol. 2, 3rd e d . ) . London: G r i f f i n Press. Kok, A . (1978). T h e e f f e c t o f w a r n i n g stimulus n o v e l t y o n t h e P300 a n d components o f t h e c o n t i n g e n t negative variation. Biologlcal Psychology, 6 , 219-233. Lacey, B . C . , & Lacey, J. I . (1980). C o g n i t i v e modulation in timedependent p r i m a r y b r a d y c a r d i a . Psychophysiology, 17, 209-221. Loveless, N. E. 5, 357-383.

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Ruchkin, D . S . , Sutton, S . , Munson, R., Silver, K., E Macar, F. (1981). P300 a n d feedback p r o v i d e d by absence o f t h e stimulus. Psychophysiology, 18, 271 -282. Simon, J. R., Hinrichs, J. V., E C r a f t , J. L. (1970). A u d i t o r y S-R compatibility: T h e e f f e c t of an i r r e l e v a n t cue on information processing. Journal of Experimental Psychology, 86, 97-102. (1983). S - R compatibility: T h e r e l a t i v e Smith, P., E Brebner, J. effects of "relevant" spatial a n d non-spatial variables. Australian Journal of Psychology, 3 5 , 1-10. Wallace, R. (1971). S-R compatibility a n d t h e idea o f a response code. Journal of Experimental Psychology, 8 8 , 354-360. (1984). Ward, P., Catts, S . V., Armstrong, M . S . , & McConaghy, N . P300 a n d p s y c h i a t r i c v u l n e r a b i l i t y in u n i v e r s i t y students. Annals of the New York Academy of Sciences, 425, 645-652.

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STIMULUS-RESPONSECOMPATIBILITY R. W. Proctor and T.G. Reeve (Editors Elsevier Science Publishers B. V. (~orth-Hoiiand).1990

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A T T E N T I O N A L PROCESSES I N S P A T I A L STIMULUS- RES PONSE COMPAT I B IL IT Y MIEKE VERFAELLIE Memory Disorders Research C e n t e r Boston Veterans Administration Medical C e n t e r and Boston U n i v e r s i t y School o f Medicine DAWN BOWERS Department o f N e u r o l o g y U n i v e r s i t y o f Florida KENNETH M. HEILMAN Department o f Neurology, U n i v e r s i t y o f Florida and Gainesville Veterans Administration Medical Center In choice reaction time ( R T ) tasks, t h e time needed to r e s p o n d to a stimulus depends n o t o n l y on t h e characteristics o f t h e stimulus a n d characteristics o f t h e response, but also on t h e relation between t h e two. T h i s phenomenon is known as stimulus-response (S-R) compatibility ( F i t t s & Seeger, 1953). When t h e f a c t o r d e t e r m i n i n g t h e speed o f response is t h e spatial relation between stimulus a n d response, t h i s is r e f e r r e d t o as spatial compatibility. F o r example, when a flash o f light is p r e s e n t e d t o t h e l e f t o r right o f a c e n t r a l f i x a t i o n p o i n t a n d manual responses a r e made t o t h e l e f t o r right o f b o d y midline, responses a r e f a s t e r when t h e l e f t k e y has t o b e pressed in response t o t h e l e f t light a n d t h e right k e y has t o b e pressed in response t o t h e right light, as opposed t o t h e oppos i t e p a i r i n g s (Anzola, Bertoloni, Buchtel, & Rizzolatti, 1977; Brebner, Shephard, & Cairney, 1972). In t h i s context, t h e more e f f i c i e n t S-R combinations (i .e., l e f t - l e f t a n d right-right) a r e termed compatible, a n d t h e less e f f i c i e n t ones (i.e., l e f t - r i g h t a n d r i g h t - l e f t ) , incompatible.

Spatial compatibility effects also o c c u r when a t t r i b u t e s o f t h e stimulus o t h e r t h a n i t s location d e f i n e response selection. F o r example, t h e color o f a stimulus p r e s e n t e d on t h e l e f t o r t h e right side o f a d i s p l a y may indicate w h i c h h a n d t o use f o r responding. Here as well, responses a r e f a s t e r when t h e stimulus a n d t h e response a r e on t h e same side t h a n when t h e y a r e o n opposite sides, even t h o u g h location i t s e l f i s i r r e l e v a n t to t h e t a s k . T h i s phenomenon is known as t h e Simon e f f e c t (Hedge & Marsh, 1975) a n d i s u s u a l l y viewed as a v a r i a t i o n o f spatial compatibility. A l t h o u g h it is t h e o r e t i c a l l y possible t h a t stimulus location m i g h t e f f e c t response latency d i f f e r e n t l y depending o n i t s relevance t o t h e experimental task, r e c e n t studies have shown experimental manipulations t o a f f e c t t h e basic spatial compatibility e f f e c t a n d t h e Simon e f f e c t similarly (Nicoletti, Anzola, Luppino, Rizzolatti, & U m i l t i , 1982; Umiltd & Nicoletti, 1985). Spatial compatibility a n d t h e Simon e f f e c t should also b e d i s t i n In g u i s h e d from t h e l a t e r a l i t y effects o b s e r v e d in simple R T t a s k s . simple R T tasks, ipsilateral stimuli a r e responded to, on t h e average, 2

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t o 10 ms f a s t e r t h a n contralateral stimuli. Such f i n d i n g s can b e i n t e r p r e t e d in t h e c o n t e x t of an anatomical pathway-transmission model. T h i s model states t h a t t h e R T is longer when t h e h a n d opposite t h e stimulated f i e l d responds because, in t h i s situation, information must b e t r a n s m i t t e d between t h e hemispheres, a n d t h e i n t e g r a t i o n o f information takes more time t h a n when information can b e i n t e g r a t e d w i t h i n - t h e same hemisphere. Differences between ipsilateral a n d contralateral responses t h u s can b e t a k e n as a measure o f interhemispheric transmission time (Berlucchi, Heron, Hyman, Rizzolatti, & Umilta, 1971; Jeeves, 1969, 1972). Compatibility effects o b s e r v e d i n choice R T situations cannot b e i n t e r p r e t e d in terms o f anatomical c o n n e c t i v i t y . First, these effects a r e considerably l a r g e r t h a n t h e l a t e r a l i t y effects o b s e r v e d in simple R T tasks, w i t h t h e differences between compatible a n d incompatible conditions generally v a r y i n g between 40 a n d 80 ms. More importantly, R T t o a lateralized stimulus is s i g n i f i c a n t l y affected by c h a n g i n g t h e position o f t h e response hand, a manipulation w h i c h affects t h e spatial environmental coordinates but n o t t h e anatomical connections (Anzola e t al., 1977; Berlucchi, Crea, Di Stefano, & Tassinari, 1977; B r e b n e r e t at., 1972; Wallace, 1971, 1972). When t h e hands a r e crossed in a choice R T task, so t h a t t h e l e f t h a n d presses t h e r i g h t k e y a n d t h e r i g h t h a n d presses t h e l e f t key, f a s t e r responses a r e made by t h e h a n d positioned in t h e same side o f space as t h e stimulus, even t h o u g h t h i s information must cross t h e c o r p u s callosum. Based o n these f i n d i n g s , Heister a n d Schroeder-Heister (1985) have suggested a d e f i n i t i o n o f S-R compatibility t h a t is r e s t r i c t e d t o o n l y those effects t h a t can b e a t t r i b u t e d t o d i f f e r e n t response positions a n d n o t t o those t h a t can b e a t t r i b u t e d t o neuroanatomical differences. Whether neuroanatomical effects should b e excluded f r o m t h i s d e f i n i t i o n i s debatable a n d may depend o n t h e p a r t i c u l a r experimental situation one conside r s (see, e.g., Klapp, Greim, Mendicino, & Koenig, 1979). For the t r a d i t i o n a l l e f t / r i g h t compatibility effects we a r e considering here, however, t h i s d e f i n i t i o n c a p t u r e s t h e i m p o r t a n t notion t h a t compatibility effects a r e p r e s e n t o n l y in tasks t h a t i n v o l v e a decision as t o w h i c h response is t o b e emitted. In t h i s chapter, we w i l l consider various accounts o f spatial S-R compatibility and, in p a r t i c u l a r , of t h e Simon e f f e c t . More specifically, t h e notion t h a t these compatibility effects may b e t h e r e s u l t o f a hemispatially-mediated attentional bias w i l l b e elaborated. New evidence consistent w i t h t h i s i n t e r p r e t a t i o n w i l l b e advanced, a n d i t s significance f o r o u r u n d e r s t a n d i n g o f compatibility effects w i l l b e discussed.

Accounts of Spatial Compatibility a n d t h e Simon E f f e c t T w o main t y p e s o f hypotheses, which may b e r e f e r r e d t o as attena n d coding hypotheses, respectively, have been advanced t o account f o r b o t h spatial S-R compatibility a n d t h e Simon e f f e c t (Nicoletti e t al., 1982). A n attentional hypothesis was i n i t i a l l y o f f e r e d by Simon a n d h i s colleagues (Simon, 1968, 1969; Simon, C r a f t , & Small, 1971), who suggested t h a t t h e r e is a n a t u r a l tendency t o respond t o w a r d t h e source of stimulation. T h i s tendency is conceived as b e i n g a stereotype comparable t o t h e o r i e n t i n g reaction; when a stimulus is presented, a response in t h e d i r e c t i o n o f t h e stimulus is automatically elicited. When tional

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t h e position o f t h e stimulus a n d t h e position o f t h e response match, t h i s t e n d e n c y i s consistent w i t h t h a t commanded by t h e stimulus, hence t h e s h o r t RTs. In contrast, when t h e position o f t h e stimulus does n o t match t h e position o f t h e response, an i n a p p r o p r i a t e t e n d e n c y t o r e s p o n d t o w a r d t h e stimulus m u s t b e i n h i b i t e d , a n d consequently, RTs are r e l a t i v e l y long.

A n i n t e r e s t i n g situation arises when an i r r e l e v a n t stimulus competes f o r attention w i t h t h e t a r g e t stimulus t o w h i c h t h e subject has t o respond. Simon a n d colleagues (Simon & C r a f t , 1970; Simon, Sly, & Vilapakkam, 1981) showed t h a t t h e t e n d e n c y t o respond t o w a r d s t h e source of a stimulus can b e counteracted by simultaneously p r e s e n t i n g a competing stimulus t o t h e contralateral ear. T h e y suggested t h a t in t h i s situation, compatibility effects a r e determined by a t e n d e n c y t o r e s p o n d t o " t h e major source o f stimulation." In o t h e r words, if a stimulus elicits a s t r o n g t e n d e n c y t o respond, even if it is i r r e l e v a n t t o t h e task, it may i n t e r f e r e w i t h r e s p o n d i n g t o t h e r e l e v a n t stimulus. T h e hypothesis t h a t a lateralized stimulus can create an attentional asymmetry has more r e c e n t l y been elaborated by Heilman a n d h i s colleagues (Heilman & Valenstein, 1979; Heilman, Watson, & Valenstein, 1985) a n d has been i n c o r p o r a t e d w i t h i n a t h e o r y o f u n d e r l y i n g hemispheric organization. A c c o r d i n g t o these authors, each hemisphere is responsible n o t o n l y f o r r e c e i v i n g stimuli in contralateral space a n d f o r i n i t i a t i n g movements o f t h e contralateral limbs but also f o r c o n t r o l l i n g d i f f e r e n t aspects of a t t e n t i o n in contralateral hemispace. In t h i s model, a t t e n t i o n is i n v o l v e d n o t o n l y in t h e selection a n d p r o c e s s i n g of incoming stimuli, but also in t h e selection a n d p r e p a r a t i o n o f motor responses. When stimulus processing a n d response p r e p a r a t i o n a r e mediated by t h e same hemisphere, responses w i l l b e f a s t e r t h a n when t h e y a r e mediated by a d i f f e r e n t hemisphere. Before discussing t h i s attentional hypothesis in f u r t h e r detail, however, an a l t e r n a t i v e hypothesis ( t h e c o d i n g hypothesis), w h i c h incorporates compatibility effects w i t h i n a more general model of spatial representation, should b e considered. Wallace (1971, 1972; see also B r e b n e r e t al., 1972) proposed t h a t t h e c r u c i a l f a c t o r in o b t a i n i n g comp a t i b i l i t y effects is t h e correspondence between t h e coding of t h e stimulus position a n d t h e coding of t h e position o f t h e e f f e c t o r . When a stimulus is presented, i t s position is coded i n t o a set o f spatial coordinates a n d compared w i t h t h e spatial coordinates o f t h e response. If t h e stimulus a n d response share t h e same spatial code, responses w i l l b e f a s t e r t h a n when t h e positions a r e coded in d i f f e r e n t ways. S u p p o r t f o r t h e coding hypothesis has been o f f e r e d by Nicoletti a n d his colleagues (see Umilth & Nicoletti, C h a p t e r 3 ) , b o t h i n situations in w h i c h t h e stimulus location is t h e relevant c u e f o r subsequent r e s p o n d i n g (Nicoletti e t al., 1982) a n d in situations in w h i c h t h e position o f t h e c u e is i r r e l e v a n t (Umilth & Nicoletti, 1985). In these studies, t h e t w o stimuli were presented on t h e same side o f t h e b o d y midline, a n d subjects responded by p r e s s i n g one o f t w o response keys, b o t h o f w h i c h were located i n t h e same side o f space. T h i s experimental condition, it was argued, allows one t o d i s t i n g u i s h between t h e attentional a n d coding hypotheses. T h e attentional hypothesis, as o u t l i n e d above, p r e d i c t s t h a t compatibility effects w i l l b e absent, because d i f f e r e n t i a l hemispatial o r i e n t i n g is eliminated. T h e spatial coding hypothesis, i n contrast, p r e d i c t s t h a t compatibility effects w i l l b e present, because t h e r e is a

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match between t h e r e l a t i v e spatial position o f stimuli a n d response k e y s . In s u p p o r t o f t h e c o d i n g hypothesis, s i g n i f i c a n t compatibility effects were indeed observed. To conclude t h a t these f i n d i n g s a r e inconsistent w i t h an attentional hypothesis, however, may b e premature. L a t e r in t h i s chapter, we w i l l a r g u e t h a t a b e t t e r u n d e r s t a n d i n g o f how attention is mediated may resolve some a p p a r e n t inconsistencies between t h e t w o hypotheses. Components of A t t e n t i o n a n d T h e i r D i s t r i b u t i o n in Space T h e manner in w h i c h attention is allocated is modulated by a number o f processes. These processes i n c l u d e arousal, selective a t t e n tion, a n d i n t e n t i o n (Bowers & Heilman, 1980). Arousal r e f e r s t o t h e degree o f general, nonspecific r e c e p t i v i t y o f t h e organism t o stimulation. Behaviorally, arousal occupies a continuum r a n g i n g f r o m t h e state o f coma, in w h i c h t h e organism may n o t respond t o a n y stimuli, t o various degrees o f waking. Physiologically, arousal describes t h e e x c i t a b i l i t y o f t h e n e r v o u s system (Heilman, Watson, Valenstein, & Goldberg, 1987). Selective a t t e n t i o n i s t h e guiding selective e f f e c t superimposed on t h e arousal reaction (Posner, 1978). It r e f e r s t o t h e mechanisms by which one decides w h a t t o process a n d a t w h a t level t o process incoming information. Selective a t t e n t i o n r e s u l t s in increased responsiveness a n d processing e f f i c i e n c y f o r a subset o f available stimuli. A t t e n t i o n may b e d i r e c t e d t o specific stimulus locations o r t o specific stimulus a t t r i b u t e s . I n t e n t i o n i s also superimposed o n t h e arousal reaction a n d r e f e r s t o t h e mechanisms by w h i c h one selectively prepares f o r action ( P r i b r a m & McGuinness, 1975). I n t e n t i o n encompasses t h e p r e p a r a t i o n o f b o t h cognit i v e a n d motor systems t o i n t e r a c t w i t h t h e environment. In t h e more r e s t r i c t e d sense o f motor preparation, it is synonymous w i t h t h e terms motor set o r response readiness. J u s t as attention may b e d i r e c t e d t o a specific location o r stimulus a t t r i b u t e , i n t e n t i o n may p r e p a r e one t o a c t i n o r t o w a r d a c e r t a i n location o r t o p e r f o r m a specific set o f movements. While s p a t i a l l y - d i r e c t e d attention a n d i n t e n t i o n a r e highly i n t e r a c t i v e processes, a r e c e n t s t u d y in o u r l a b o r a t o r y has shown t h a t these processes can b e dissociated, n o t o n l y conceptually, but also experiment a l l y (Verfaellie, Bowers, & Heilman, 1988b). Attention refers t o a process o f input selection a n d can b e manipulated by selectively cuing t h e subject f o r c e r t a i n stimuli. Intention, on t h e o t h e r hand, r e f e r s t o a process o f o u t p u t selection a n d can b e manipulated by i n d u c i n g a select i v e response set. B y manipulating these processes simultaneously in a choice R T task, we f o u n d t h a t t h e y a f f e c t information processing indep e n d e n t l y o f each o t h e r . Behavioral studies in humans have suggested t h a t each hemisphere i s i m p o r t a n t f o r mediating selective attention a n d i n t e n t i o n in contralateral hemispace. Patients w i t h unilateral neglect, a unilateral d i s o r d e r o f t h e attentional mechanisms, may f a i l t o detect o r respond t o stimuli in t h e contralesional side o f space ( C r i t c h l e y , 1966; Heilman & Valenstein, 1979). F o r example, t h e y may f a i l t o cross o u t lines on t h e l e f t side o f t h e page o r may f a i l t o move a limb when it is positioned in t h e contralesional side o f space. A l t h o u g h u n i l a t e r a l neglect occurs more f r e q u e n t l y following right hemisphere lesions, it also occurs in an attenuated f o r m following lesions o f t h e l e f t hemisphere ( A l b e r t , 1973; Ogden, 1985, 19871, t h u s

Attentional Processes in Spatial Compatibility suggesting a special contralateral hemispace.

relation

between

each

hemisphere

265 and

its

T h e concept o f hemispace, as it has been defined in t h e c o n t e x t o f t h e behavioral abnormalities observed in hemispatial neglect, is a complex one. It r e f e r s t o t h e corporeal a n d extracorporeal space t o t h e l e f t a n d right of t h e body, as defined according t o e i t h e r eye, head, o r t r u n k position. When all t h r e e reference axes a r e aligned, t h e hemispaces defined in these ways a r e congruent. However, w i t h t h e eyes deviated t o t h e f a r left, f o r instance, t h e right visual f i e l d falls l a r g e l y w i t h i n l e f t hemispace as defined by b o d y and head midline. Similarly, w i t h t h e head a n d eyes deviated t o t h e right, l e f t head hemispace falls l a r g e l y w i t h i n r i g h t b o d y hemispace. Because hemispace can b e defined according t o a number of coordinate systems, it provides a highly f l e x i b l e representation o f space. More importantly, however, it also defines a perceptual f i e l d in which stimuli a r e attended t o a n d processed by t h e contralateral hemisphere, as well as a behavioral f i e l d in which stimuli a r e selectively acted upon by t h e contralateral hemisphere. According to t h i s hemispatial hypothesis, t h e hemisphere t h a t mediates sensory-motor processing and t h e hemisphere t h a t mediates attention and intention may b e dissociated. For example, when t h e hand is crossed i n t o t h e incompatible (i.e., contralateral) hemispace, t h e processors t h a t control movements a n d receive a f f e r e n t i n p u t s a r e mediated by a d i f f e r e n t hemisphere t h a n t h e hemisphere t h a t mediates attention a n d intention w i t h i n t h a t hemispace. For these sensory, motor, a n d a t t e n t i v e - i n t e n t i v e processes t o b e coordinated, interhemispheric communication v i a t h e corpus callosum appears necessary. Thus, when t h e c o r p u s callosum i s injured, one m i g h t expect t h a t each h a n d would have a p r o p e n s i t y t o act in i t s own (i.e., ipsilateral) hemispace. Heilman, Bowers, a n d Watson (1984) studied a patient w i t h a p a r t i a l callosal disconnection by a s k i n g h e r t o bisect lines in contralateral a n d ipsilateral hemispace. As expected, when w o r k i n g in contralateral hemispace, each hand e r r e d towards i t s own hemispace. Hemispatial-attentional f a c t o r s also operate in t h e normal nondamaged b r a i n . Traditionally, however, these effects have l a r g e l y been overlooked in studies of normal b r a i n - b e h a v i o r relations. When one considers t h e experimental paradigms used in t r a d i t i o n a l l a t e r a l i t y research, one notes a paradigmatic confound (Heilman, Bowers, Valenstein, & Watson, 1987). Namely, t h e r e is a p e r f e c t one t o one relation between t h e sensory channel t o which a stimulus is presented (ear, visual h a l f field, hand) a n d t h e side o f hemispace in which t h i s sensory channel is located. Consequently, attentional and sensory pathway effects cannot b e dissociated.

In a n i n i t i a l attempt t o separate t h e effects o f hemispace f r o m those o f sensory i n p u t channel, we gave a tactile l i n e bisection t a s k t o normal r i g h t - h a n d e d subjects (Bowers & Heilman, 1980). T h e y were asked t o bisect a horizontal r o d positioned in midline o r in l e f t or right hemispace. As expected on t h e basis o f o t h e r tactile studies, a l e f t - h a n d s u p e r i o r i t y was obtained. T h i s effect, however, was also dependent on t h e hemispace in which t h e t a s k was performed: T h e b e s t bisection performance was made by t h e l e f t hand i n l e f t hemispace, a n d t h e w o r s t performance by t h e right hand in right hemispace.

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F u r t h e r evidence f o r t h e notion t h a t each hemisphere may b e specialized t o a t t e n d a n d act on stimuli i n contralateral hemispace comes f r o m a v i s u a l choice R T s t u d y (Bowers, Heilman, & V a n Den Abell, 1981). Normal r i g h t - h a n d e d subjects responded t o lateralized lights, w h i l e d e v i a t i n g t h e i r eyes t o a f i x a t i o n point in l e f t hemispace, midline, o r right hemispace. When t h e v i s u a l half f i e l d in w h i c h a stimulus was' presented was aligned in a compatible side o f hemispace (e.g., right visual f i e l d right hemispace), R T s were f a s t e r t h a n when v i s u a l h a l f f i e l d a n d side o f hemispace were n o t aligned (e.g., l e f t visual h a l f f i e l d - r i g h t hemispace). These f i n d i n g s suggest t h a t t h e relation between each hemisphere a n d attentionaVintentiona1 processes in contralateral hemispace may b e an important determinant o f response speed t o lateralized stimuli. When a stimulus is presented in one side of space, t h i s activates t h e contralateral hemisphere a n d d i r e c t s a t t e n t i o n t o t h e c o r r e s p o n d i n g hemispace. Because t h e activated hemisphere also mediates responses i n contralateral hemispace, t h e h a n d located in t h a t hemispace w i l l b e favored, i r r e s p e c t i v e o f w h e t h e r it i s c o n t r o l l e d by t h e same o r opposite hemisphere. While t h e framework developed above suggests t h a t t h e attentional c o n t r o l o f each hemisphere can b e d e f i n e d according t o a s t r i c t spatial dichotomy, more r e c e n t evidence suggests t h a t t h i s may b e an oversimplification. A s already discussed, hemispace is a complex concept t h a t allows f o r a representation o f space a c c o r d i n g t o a number o f axes, n o t a l l of w h i c h a r e necessarily aligned. Additionally, it appears t h a t a t t e n t i o n a n d response p r e p a r a t i o n also have an important directional compon e n t (Kinsbourne, 1987). F o r example, when neglect p a t i e n t s a r e c u e d t o move t h e i r attention in t h e d i r e c t i o n contralateral t o t h e lesion, defects o c c u r i r r e s p e c t i v e of t h e side o f space in which t h e movement is p e r formed (Posner e t al., 1987). When t h e y bisect lines p r e s e n t e d in t h e "intact" hemispace, t h e y may s t i l l underestimate t h e ipsilesional side (Heilman & Valenstein, 19791, o r when t w o stimuli a r e presented in t h e same hemifield, e x t i n c t i o n may s t i l l b e p r e s e n t (Kinsbourne, 1977; Rapcsak, Watson, t Heilman, 1987). Patients w i t h hemispatial neglect may also have directional akinesia. When asked t o p e r f o r m directional R T s w i t h t h e i r normal limb, t h e y may b e slower i n i t i a t i n g a response toward contralateral hemispace t h a n toward ipsilateral hemispace, i r r e s p e c t i v e of t h e position o f t h e limb (Heilman, Bowers, Coslett, Whelan, & Watson, 1985). T a k e n together, these f i n d i n g s suggest t h a t when attentional resources a r e allocated t o one p a r t o f space, information processing in more lateral positions is mediated more by t h e contralateral hemisphere t h a n is information processing in more c e n t r a l positions. T h e p i c t u r e t h a t emerges, then, is a more dynamically o r i e n t e d one, in w h i c h t h e mediation of attentional processes is b o t h directional a n d relational in character. T h a t is, i n a f r e e - f i e l d situation, each hemis p h e r e is specialized a t d i r e c t i n g attention a n d i n t e n t i o n towards t h e contralateral side o f space. However, when stimuli or responses a r e conf i n e d t o one side o f space, t h e more lateral one w i l l b e f a v o r e d by t h e contralateral hemisphere. It is as if, temporarily, t h e functional midline o f space is shifted, a n d b o t h hemispheres r e - a l i g n attention a n d i n t e n t i o n t o t h a t side of space. A n e u r a l representation t h a t m i g h t u n d e r l i e such a f l e x i b l e mediat i o n o f attentional processes has r e c e n t l y been i d e n t i f i e d by Rizzolatti a n d his colleagues (Rizzolatti, Gentilucci, & Matelli, 1985; Rizzolatti, Scandolara, Matelli, & Gentilucci, 1981). These investigators f o u n d t h a t

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in t h e postarcuate c o r t e x o f monkeys, 208 o f t h e neurons have exclusively contralateral fields, 69% bilateral fields, a n d 2% ipsilateral fields. Bilateral f i e l d s a r e always centered a r o u n d t h e midline, whereas contralateral f i e l d s a r e f o u n d more a n d more e x c l u s i v e l y as one moves l a t e r a l l y . As a f u n c t i o n a l c o r r e l a t e o f t h i s t y p e o f neuronal d i s t r i b u t i o n , we suggest a g r a d i e n t o f resource allocation in which t h e extreme lateral side o f space i s mediated maximally by t h e contralateral hemisphere. As f o r t h e implications o f such a model f o r o u r u n d e r s t a n d i n g o f spatial compatibility effects, we suggest t h a t t h e f i n d i n g s of Nicoletti a n d h i s colleagues (Nicoletti e t al., 1982; Umilta & Nicoletti, 1985), discussed above, can easily b e i n t e r p r e t e d w i t h i n t h i s framework. Namely, when b o t h stimuli a n d b o t h response k e y s repeatedly o c c u r in t h e same side o f space, resources a r e d i v i d e d such t h a t t h e more lateral stimulus a n d response a r e mediated maximally by t h e contralateral hemisphere. Again, f a s t e r responses w i l l o c c u r when t h e same hemisphere mediates b o t h stimulus processing a n d response p r e p a r a t i o n . A t t h i s point, however, it remains unclear w h i c h component of attention is c r i t i c a l l y i n v o l v e d in t h e occurrence o f compatibility effects. Some f i n d i n g s addressing t h i s question w i l l b e discussed n e x t . Selective A t t e n t i o n v e r s u s I n t e n t i o n as a Determinant of Spatial C o m p a t i b i l i t y

In a recent s t u d y w i t h r i g h t - h a n d e d college students, we assessed t h e effects o f attentional manipulations on t h e o c c u r r e n c e o f S-R compatibility effects in a situation in w h i c h t h e location o f t h e stimulus was i r r e l e v a n t t o t h e t a s k ( i . e . , t h e Simon e f f e c t ) . T h i s s t u d y (Verfaellie, Bowers, & Heilman, 1988a) evolved f r o m a series o f experiments designed t o isolate t h e effects of attentional a n d intentional processes. T h i s was done in t h e c o n t e x t o f a choice R T t a s k in which subjects were asked t o respond as q u i c k l y as possible, w i t h t h e l e f t o r right hand, t o one o f t w o lateralized lights, one o f w h i c h was presented i n l e f t hemispace a n d t h e o t h e r in right hemispace. T h e l e f t h a n d was always positioned on t h e l e f t side o f t h e body, a n d t h e right h a n d on t h e right side. Each t a r g e t light c o u l d b e e i t h e r bright o r dim, w i t h one b r i g h t n e s s i n d i c a t i n g t h a t a l e f t - h a n d response was r e q u i r e d a n d t h e o t h e r b r i g h t n e s s t h a t a righthand response was r e q u i r e d . So, f o r instance, a bright light m i g h t indicate t h a t t h e l e f t h a n d should b e used, i r r e s p e c t i v e o f w h e t h e r t h e t a r g e t was p r e s e n t e d in l e f t or right hemispace, a n d a d i m light m i g h t indicate t h a t t h e right h a n d should b e used, i r r e s p e c t i v e o f w h e t h e r t h e light was presented in l e f t o r right hemispace. T h e assignment of each b r i g h t n e s s t o t h e l e f t o r right hands was counterbalanced across s u b jects. Subjects responded by releasing t h e a p p r o p r i a t e response k e y . attention a n d i n t e n t i o n were manipulated by intentional cues p r i o r t o onset o f t h e t a r g e t stimulus. Attentional cues consisted o f t e l l i n g t h e subject where in space t h e t a r g e t stimulus would occur. Intentional cues consisted o f t e l l i n g t h e subject w h i c h h a n d should b e used f o r responding. The w a r n i n g stimuli t h a t p r o v i d e d t h e attentional a n d intentional cues were presented in midline a n d consisted o f t w o sets of t h r e e v e r t i c a l l y aligned lights, w i t h one set positioned above t h e f i x a t i o n p o i n t a n d t h e o t h e r set below t h e f i x a t i o n p o i n t . Within t h i s paradigm,

giving selective attentional and/or

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T h e t o p a n d bottom light o f t h e a t t e n t i o n a l w a r n i n g stimulus set represented an a r r o w p o i n t i n g t o t h e l e f t o r right, i n d i c a t i n g where in space t h e t a r g e t w o u l d most l i k e l y o c c u r . So, f o r example, an a r r o w p o i n t i n g t o t h e l e f t indicated t h a t t h e t a r g e t light w o u l d occur, on t h e majority o f trials, t o t h e left o f fixation. T h e opposite was t r u e f o r t h e a r r o w p o i n t i n g t o t h e right. T h e middle light was a n e u t r a l stimulus, i n d i c a t i n g t h a t t h e n e x t t r i a l w o u l d occur, but giving n o selective attentional information. These cues were considered t o b e attentional in nature, because t h e information t h e y provided, if any, enabled subjects to s h i f t t h e i r attention t o t h e p r o b a b l e spatial location of t h e subsequent t a r g e t stimulus. T h e t o p a n d bottom light of t h e i n t e n t i o n a l w a r n i n g stimulus set represented a f i n g e r p o i n t i n g t o t h e l e f t o r right, i n d i c a t i n g w h i c h h a n d most l i k e l y w o u l d have t o b e used f o r responding. Thus, illumination o f t h e i n d e x f i n g e r p o i n t i n g t o t h e l e f t indicated that, i r r e s p e c t i v e of where t h e t a r g e t stimulus occurred, it would r e q u i r e a l e f t - h a n d response most o f t h e time. T h e opposite h e l d t r u e f o r t h e f i n g e r p o i n t i n g t o t h e right. Again, t h e middle light was a n e u t r a l stimulus, i n d i c a t i n g t h a t t h e n e x t t r i a l w o u l d o c c u r but giving n o selective p r e p a r a t o r y information. These cues were considered t o b e intentional i n nature, because t h e y p r o v i d e d information c o n c e r n i n g t h e h a n d t o use f o r r e s p o n d i n g t o t h e t a r g e t stimulus but n o t c o n c e r n i n g i t s location. T h e r e were f o u r cuing conditions, consisting o f e v e r y combination o f selective attentional a n d intentional information b e i n g e i t h e r p r e s e n t o r absent: (a) b o t h attentional a n d intentional information were given; (b) o n l y attentional information was given, t o g e t h e r w i t h t h e n e u t r a l stimulus o f t h e intentional set; (c) o n l y intentional information was given, t o g e t h e r w i t h t h e n e u t r a l stimulus o f t h e attentional set; a n d (d) no selective information was given, but t w o n e u t r a l w a r n i n g stimuli were presented. T h e information p r o v i d e d by t h e cues was v a l i d o r c o r r e c t o n 80% of t h e t r i a l s a n d i n v a l i d o r i n c o r r e c t on 2% o f t h e t r i a l s . O n i n v a l i d trials, t h e information p r o v i d e d by t h e attentional o r intentional c u e was i n c o r r e c t w i t h respect t o t h e subsequent t a r g e t stimulus. On invalid t r i a l s w i t h b o t h attentional a n d intentional information, b o t h cues g a v e i n c o r r e c t information. Each t r i a l began w i t h t h e simultaneous presentation o f t w o c e n t r a l w a r n i n g stimuli, one o f each set, f o r 500 ms. A f t e r v a r i a b l e i n t e r s t i m u l u s intervals, one o f t h e t a r g e t stimuli was presented f o r 500 ms, as well. T w o separate experiments were run t h a t d i f f e r e d o n l y w i t h respect t o t h e i n t e r s t i m u l u s i n t e r v a l s used between w a r n i n g stimulus a n d t a r g e t . In t h e f i r s t experiment, these r a n g e d between 2,500 a n d 3,400 ms, a n d in t h e Because, overall, t h e t w o e x p e r i second between 1,500 a n d 2,400 ms. ments y i e l d e d similar results, o n l y t h e r e s u l t s o f t h e f i r s t experiment w i l l b e discussed here. L e t us f i r s t consider t h e conditions in which v a l i d information was given. Response accuracy was h i g h e r t h a n 908 in a l l conditions, a n d none o f t h e f i n d i n g s could b e i n t e r p r e t e d in terms of a speed-accuracy trade-off. As f o r response speed, as expected, attentional cues reduced RTs (468 v s . 498 ms), a n d so did intentional cues (425 v s . 542 ms). O f more relevance f o r t h e p r e s e n t discussion, however, a r e t h e f i n d i n g s o f spatial compatibility. Across c u i n g conditions, a s i g n i f i c a n t interaction between h a n d a n d hemispace was obtained, i n d i c a t i n g t h a t t h e right h a n d was f a s t e r in r e s p o n d i n g t o stimuli o n t h e right t h a n t o stimuli on t h e

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l e f t (477 v s . 491 ms) a n d t h e l e f t h a n d was f a s t e r in r e s p o n d i n g t o stimuli on t h e l e f t t h a n t o stimuli o n t h e right (475 v s . 490 ms). However, h i g h e r o r d e r interactions indicated t h a t t h i s e f f e c t c r i t i c a l l y depended on t h e t y p e o f c u e t h a t was p r o v i d e d . As can b e seen in F i g u r e 1, compatibility effects were p r e s e n t o n l y on t r i a l s in w h i c h i n t e n In t h i s tional information was provided, but n o attentional information. condition, l e f t - h a n d responses were 35 ms f a s t e r t o stimuli on t h e l e f t t h a n t o stimuli o n t h e right, a n d r i g h t - h a n d responses were 33 ms f a s t e r In a l l o t h e r w a r n i n g t o stimuli on t h e right t h a n t o stimuli on t h e l e f t . conditions, however, compatibility effects were absent. These f i n d i n g s suggest t h a t compatibility effects may v a r y as a f u n c t i o n of attentional manipulations, a n d more importantly, t h a t an intentional mechanism may b e c r i t i c a l l y i n v o l v e d in t h e i r occurrence.

550

C: V

-1

-

550

500

-

500

450

-

450

0 u)

400 l-

a

I

LHS

RHS

Z

2

z

550

550

ATT=NO, INT=YES

500

500 450 400

ti

t ATT=NO, INT=NO

400

LHS

RHS

I Left Hand---

LHS Right Hand

-1

RHS

F i g u r e 1. Response latencies f o r t h e l e f t a n d right h a n d t o t a r g e t stimuli in l e f t a n d right hemispace, as a f u n c t i o n o f t h e presence o f attentional a n d intentional information ( f r o m Verfaellie e t al., 1988a). Additional s u p p o r t f o r t h i s hypothesis was obtained in an analysis o f t h e i n v a l i d c u i n g conditions. Response accuracy was h i g h e r in comp a t i b l e t h a n incompatible conditions, but again, t h i s was t r u e j u s t in t h e condition i n w h i c h o n l y i n v a l i d intentional cues were p r o v i d e d . In t h i s

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condition, subjects made more e r r o r s when t h e y h a d been i n c o r r e c t l y cued t o respond o n t h e same side as t h e stimulus t h a n when t h e y h a d been i n c o r r e c t l y c u e d t o respond o n t h e side opposite t o t h e stimulus. It i s as if once prepared, a response o n t h e same side as t h e stimulus i s much h a r d e r t o inhibit t h a n a response on t h e opposite side. Taken together, we i n t e r p r e t these f i n d i n g s as evidence f o r t h e notion t h a t response p r e p a r a t i o n may b e an important determinant of response speed t o lateralized stimuli. We suggest t h a t p r e p a r a t i o n o f a response primes t h e c o r r e s p o n d i n g hemisphere n o t o n l y f o r action but also f o r processing incoming information in t h e c o r r e s p o n d i n g side o f space. Once t h i s bias i s established, differences between compatible a n d incompatible conditions may obtain, r e f l e c t i n g t h e d i f f e r e n t i a l e f f i c i e n c y o f each hemisphere in mediating attentional processes in contralateral v e r s u s ipsilateral hemispace. It is i m p o r t a n t t o keep in m i n d t h a t t h e Simon e f f e c t has t r a d i t i o n a l l y been demonstrated in a paradigm i n w h i c h response p r e p a r a t i o n is n o t e x p l i c i t l y manipulated. Notwithstanding, we believe t h a t t h e absence o f compatibility effects in t h e conditions w i t h o u t intentional cues in t h e p r e s e n t s t u d y i s n o t an o d d finding. Rather, a closer consideration o f t h e differences in demand characteristics between b o t h t a s k s may p r o v i d e a c l u e f o r u n d e r s t a n d i n g how these effects a r e mediated. In t h e p r e s e n t s t u d y , t r i a l s w i t h d i f f e r e n t t y p e s of cues were randomly intermixed. Thus, on some t r i a l s , an e x p l i c i t intentional set was created, whereas on o t h e r t r i a l s , t h e absence o f intentional information may have r e s u l t e d in t h e absence o f an intentional s e t . It i s on those t r i a l s w i t h o u t intentional set t h a t t h e Simon e f f e c t disappeared. Traditionally, choice R T paradigms have n o t e x p l i c i t l y manipulated t h e presence o f intentional i n formation. However, an implicit, nonspecific readiness t o respond exists throughout t h e experiment. We believe t h a t it is t h i s readiness t o respond selectively t o a stimulus, w h e t h e r it b e p r e s e n t i m p l i c i t l y o r manipulated e x p l i c i t l y , t h a t primes each hemisphere f o r allocating a t t e n tional resources t o t h e contralateral side o f space.

T h e above argument leads t o t h e conclusion t h a t t h e Simon e f f e c t is p r i m a r i l y motor in n a t u r e . A similar mechanism may operate, we suggest, in those situations in w h i c h t h e location of t h e stimulus is d i r e c t l y r e l e v a n t t o t h e experimental t a s k . S u p p o r t f o r t h i s thesis comes f r o m recent studies by Bradshaw a n d h i s colleagues (Bradshaw, Nettleton, Pierson, Wilson, & Nathan, 1987), in which t h e stimulus location determined response selection. Measuring R T s t o lateralized v i b r o t a c t i l e stimuli, these i n v e s t i g a t o r s f o u n d compatibility effects u n d e r conditions o f response u n c e r t a i n t y b u t n o t u n d e r conditions o f stimulus u n c e r t a i n t y . T h e y similarly concluded t h a t t h e locus o f compatibility effects may reside in t h e p r e p a r a t i o n o f a response. It may n o t b e s u r p r i s i n g , therefore, t h a t compatibility effects a r e absent i n simple R T tasks, since i n t h i s paradigm, resources a r e d i r e c t e d p r i m a r i l y a t processing incoming information a n d n o t a t response p r e p a r a t i o n . Summary In t h i s chapter, we have attempted t o o u t l i n e t w o major accounts o f spatial compatibility effects. F u r t h e r , ' w e have proposed a model f o r how attention may b e mediated i n t h e b r a i n , a n d w i t h i n t h i s framework, we

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have p r o v i d e d some data t h a t f u r t h e r s p e c i f y t h e role o f attentional p r o cesses in t h e occurrence o f spatial compatibility effects. A n important aspect t h a t remains t o b e considered concerns t h e implications o f t h e data presented in t h i s c h a p t e r f o r a c o d i n g account of spatial compatibility. O u r f i n d i n g s suggest t h a t t h e spatial arrangement o f stimuli a n d responses is n o t a s u f f i c i e n t f a c t o r f o r compatibility effects t o o c c u r . As stated earlier, t h e attentional manipulations did n o t in a n y way a l t e r t h e spatial codes o f stimuli a n d responses, but t h e y did a f f e c t compatibility effects, i n d i c a t i n g t h a t o t h e r information-processing charact e r i s t i c s need t o b e considered as well. We suggest, however, t h a t t h e coding hypothesis a n d t h e attentional hypothesis need n o t b e m u t u a l l y exclusive. T h e experiment d e s c r i b e d was n o t a c r i t i c a l t e s t o f t h e coding hypothesis, because across all attentional manipulations, b o t h stimuli a n d responses c o u l d b e coded in terms o f t h e same l e f t / r i g h t f a c t o r . Therefore, o u r f i n d i n g s should n o t b e t a k e n as evidence against a spatial coding factor. Rather, we suggest t h a t an intentional mechanism may o p e r ate in conjunction w i t h a spatial coding f a c t o r . In o r d i n a r y circumstances, we may deal w i t h u n c e r t a i n t y by d i r e c t i n g a t t e n t i o n t o t h e response demands o f a t a s k . Thus, a readiness t o respond is p r e s e n t implicitly. B y manipulating t h i s f a c t o r e x p l i c i t l y , however, we have h i g h l i g h t e d i t s importance in t h e occurrence o f compatibility effects.

A f i n a l consideration is how attentional a n d spatial f a c t o r s may i n t e r a c t in c r e a t i n g spatial compatibility. Perhaps, when t h e hands a r e placed in a specific spatial position ( i . e . , o n t h e response keys), t h e subject develops a p r e p a r a t o r y set as a f u n c t i o n o f e i t h e r implicit o r e x p l i c i t experimental requirements. T h i s p r e p a r a t o r y set n o t o n l y activates a set o f premotor a n d motor neurons (Evarts, 1984) but also a c t i vates t h e spatial representation o f t h e location where t h e motor p l a n w i l l b e executed (Rizzolatti, Riggio, Dascola, & UmiltB, 1987; Rizzolatti & Camarda, 1987). Thus, each hemisphere may b e responsible f o r c r e a t i n g a three-dimensional spatial map o f i t s p o r t i o n o f t h e environment. Recent electrophysiological studies suggest t h a t t h i s spatial mapping may o c c u r a t an e a r l y stage o f processing ( H i l l y a r d , Munte, & Neville, 1987). When a p o t e n t i a l l y relevant e v e n t occurs, it is f i r s t localized in space w i t h r e f e r ence t o t h e subject's midline a n d t h e n undergoes f u r t h e r perceptual processing. T h e creation o f these spatial representations may c r i t i c a l l y depend on t h e i n t e g r i t y o f t h e parietal lobes (Hyvarinen, 1982). As r e g a r d s t h e occurrence o f compatibility effects, we suggest t h a t response p r e p a r a t i o n involves t h e specification o f t h e spatial coordinates o f t h e f i e l d i n which t h e response is t o b e executed. Since response p r e p a r a t i o n is coded i n terms o f l e f t / r i g h t , a similar map w i l l b e used t o code t h e stimuli (Nissen, 1987). Even if stimulus location is i r r e l e v a n t f o r t h e task, it is a salient f e a t u r e coded f o r t h e p u r p o s e o f t h e response. In conclusion, t h e p i c t u r e t h a t emerges is one in w h i c h a number o f f a c t o r s may j o i n t l y c o n t r i b u t e t o spatial compatibility effects. As f o r t h e t r a d i t i o n a l l e f t / r i g h t compatibility, we suggest t h a t t h e process by w h i c h we p r e p a r e motor responses in a spatial framework is an i m p o r t a n t f a c t o r in i t s occurrence.

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Simon, J. R., Sly, P. E., & Vilapakkam, S . (1981). Effects o f compatibility of S-R mapping on reactions toward t h e stimulus source. Acta Psychologica, 47, 63-81. Attention and coding effects i n S-R Umilt6, C . , & Nicoletti, R . (1985). compatibility due t o irrelevant spatial cues. In M . I. Posner & 0. S . Marin (Eds.), Attention and performance X I (pp. 457-471). Hillsdale, NJ: Erlbaum. Verfaellie, M . , Bowers, D., & Heilman, K. M. (1988a). Attentional factors in t h e occurrence of stimulus-response compatibility effects. Neuropsychologia, 26, 435-444. (1988b). Hemispheric Verfaellie, M., Bowers, D., & Heilman, K. M. asymmetries i n mediating intention, b u t not selective attention. Neuropsychologla, 26, 521-532. Wallace, R. J. (1971). S-R compatibility and t h e idea of a response code. Journal of Experimental Psychology, 8 8 , 354-360. Wallace, R. J. (1972). Spatial S-R compatibility effects involving kinesthetic cues. Journal of Experimental Psychology, 93, 163-168.

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PART IV MOTOR PERFORMANCE T h e f o u r chapters o f P a r t IV consider t h e r o l e o f compatibility effects in t h e s t u d y o f motor performance. C h a p t e r 11, by Zelaznik a n d Franz, examines t h e relation between S - R compatibility a n d motor p r o gramming processes, s t r e s s i n g t h e need t o c o n t r o l compatibility effects when motor programming is o f p r i m a r y concern. C h a p t e r 12, by Spijkers, a t t r i b u t e s t h e e f f e c t of average movement v e l o c i t y o n reaction time t o motor programming by showing t h a t b o t h spatial- a n d semanticcompatibility effects a r e independent f r o m t h e e f f e c t o f velocity. Chapter 13, by Heuer, extends t h e concept of compatibility to movement situations f o r w h i c h d i f f e r e n t responses a r e paired, i l l u s t r a t i n g t h e d i s t i n c t i o n between response-response compatibility a n d stimulus-response compatibility. Chapter 14, by Gordon, demonstrates compatibility effects in t h e perception a n d p r o d u c t i o n o f speech, p r o v i d i n g an i n t e r a c t i v e activation model t o account f o r t h e effects.

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STIMULUS-RESPONSE COMPATIBILITY

R. W. Proctor and KG. Reeve (Editors 0 Elsevier Science Publishers 6.V. (dorth-Holland). 1990

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STIMULUS-RESPONSE COMPATIBILITY AND THE PROGRAMMING OF MOTOR ACTIVITY: PITFALLS AND POSSIBLE NEW DIRECTIONS

HOWARD N. ZELAZNIK Motor Behavior Laboratory Department of Physical Education, Health and Recreation Studies and Department of Psychological Sciences Purdue University ELIZABETH FRANZ Motor Behavior Laboratory Department of Physical Education, Health and Recreation Studies Purdue University Over t h e past two decades, t h e s t u d y of motor behavior has been concerned w i t h the central control of skilled a c t i v i t y . One line of research has examined the notion of central control by attempting t o discredit t h e role of feedback i n the learning and control of skilled motor a c t i v i t y ( f o r reviews of these perspectives and t h e empirical research see Adams, 1971, 1977, 1987; Keele, 1981, 1986; Schmidt, 1975, 1976, 1988). A number of these studies supported t h e idea t h a t feedback was not crucial t o t h e control of skilled motor a c t i v i t y and, thus, t h a t t h e movement was governed by a central representation of t h e skilled activity. This central representation has been called a motor program (Keele, 1968; Schmidt, 1975). Theorists who accept t h e motor-programming construct assume t h a t some central representation of a skilled motor act resides i n memory. This central representation has t h e capability t o control a movement trajectory w i t h minimal involvement of other central processes a f t e r movement initiation. Another line of research has examined t h e time course of t h e motorprogramming processes. Because response selection and response execution are thought t o require time, t h e study of t h e variables t h a t influence t h e durations of these processes should result i n a better understanding about t h e contribution of central processes t o motor performance. Specifically, t h e focus of the present paper i s the use of reaction time ( R T ) as an index of t h e relative amount of programming time required by a process. For a discussion of t h e use of RT as a window t o mental events, t h e reader is r e f e r r e d t o an older, but excellent, exposition by Posner (1978). I n t h i s chapter, we: (a) review some of t h e notions and ideas t h a t underlie t h e motor-program construct, (b) discuss t h e implications of t h e motor-program construct f o r RT research, (c) discuss t h e role of stimulus-response (S-R) compatibility i n examining the motor-programming process, (d) discuss a prototypical experiment t h a t is necessary t o p r o vide some insights i n t o programming v i a an R T perspective, and (e) discuss the role of S-R compatibility i n automaticity. This paper shall focus primarily on the precuing methodology. Some of t h e issues t o be discussed have been published i n a review paper by Zelaznik and Larish

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Because theory and method are intertwined i n t h e R T analysis of (1986). programming, it is necessary t o cover some of t h i s ground again. While not everyone agrees on t h e explanatory value of t h e motorprogram construct (see e.g., Kelso, Holt, Rubin, & Kugler, 1981; Michaels & Carello, 19811, t h e present review does not enter i n t o t h a t controversy. Rather, we take a position similar t o t h a t of Rosenbaum (1983). in t h a t regardless of t h e construct involved i n skilled motor activity, t h e performer must decide upon certain characteristics of t h e tobe-performed action. Thus RT research, if done properly, can answer questions about t h e mental events involved i n response selection and motor programming. Because most of t h e research involving RT and motor control utilizes t h e motor program as t h e theoretical foundation (see Goodman and Kelso, 1980, f o r an exception), we now t u r n o u r attention t o a discussion of t h e extant literature concerned w i t h t h e motor program. T h e Motor-Program Construct Programming by Parts How t h e motor program is conceptualized will determine t h e t y p e of research t h a t i s conducted t o discover the properties o f motor program control. One conceptualization of t h e motor program is a feature-selection viewpoint (Rosenbaum, 1980, 1983). I n t h i s framework, a movement can be thought t o be comprised of d i f f e r e n t features o r components t h a t when p u t together and/or selected form t h e basis of a motor program. For example, Rosenbaum (1980) has attempted t o determine i n an aimed-hand movement whether t h e movement features of arm, direction, and extent of movement are selected i n a parallel o r serial fashion. I n t!is t y p e of research, t h e (I priori hypothesis is t h a t t h e motor system t h i n k s " i n If such is t h e case, then examining terms of arm, direction, and extent. t h e time course of these decisions i s a worthwhile endeavor. However, Goodman and Kelso (1980) reject t h i s feature-selection hypothesis and argue t h a t programming is a whole process. A t t h i s point, we do not want t o argue t h e merits of t h e i r position, b u t rather point o u t t h a t the working hypothesis concerning motor programming determines t h e nature of t h e manipulated variables and t h e t y p e of experimental questions asked (see Kerr, 1978). A corollary t o t h e feature-selection research comes from t h e work on complexity and RT conducted i n t h e past decade by Christina and colleagues (Anson, 1982; Christina, Fischman, Vercruyssen, & Anson, 1982; In these experiments, the number of movement compoFischman, 1984). nents was manipulated and RT was examined. If t h e r e is a monotonic relation between t h e number of components and RT, it i s assumed t h a t each additional component requires additional programming time. Thus, t h e feature-selection hypothesis would be supported, w i t h a feature corresponding t o a movement component. Programming via Generalized Motor Programs Another way t o examine t h e idea of motor programs i s via t h e generalized motor-program construct (Pew, 1974; Schmidt, 1975; 1988). Proponents of t h e generalized motor-program perspective propose t h a t the

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memory f o r a s k i l l e d action is abstracted f r o m all o f t h e instances o f t h a t class o f a c t i v i t y b e i n g performed. For example, a " t h r o w i n g " generalized motor p r o g r a m would contain t h e overall spatial-temporal f o r m of t h e action. However, t h e performer's p a s t h i s t o r y o f i n d i v i d u a l t h r o w s would n o t b e stored in memory. One o f t h e classic examples o f a generalized motor p r o g r a m comes f r o m Merton (19721, who demonstrated t h a t a p e r son's s i g n a t u r e o f various sizes p r o d u c e d an i n v a r i a n t spatial form.

In t h e generalized motor-program framework, a s k i l l e d motor action is comprised o f t w o sets o f characteristics, i n v a r i a n t characteristics a n d parameters. I n v a r i a n t characteristics a r e aspects o f t h e motor p r o g r a m t h a t make one generalized motor p r o g r a m d i f f e r e n t f r o m another. Hypothesized i n v a r i a n t characteristics a r e r e l a t i v e t i m i n g ( t h e time onsets of events d u r i n g t h e action; see Gentner, 1987, a n d Heuer, 1988, f o r t h o r o u g h discussions o f r e l a t i v e timing) a n d t h e spatial f o r m o f t h e movement. Thus, t h e spatial-temporal f o r m o f a movement distinguishes one generalized motor program f r o m another. However, because t h e motor-program memory is abstract, parameters must b e supplied t o t h e p r o g r a m t o g o v e r n a p a r t i c u l a r act. Variables such as speed, direction, limb, a n d size a r e t h o u g h t t o serve as parameters. These variables t a i l o r a generalized motor p r o g r a m t o c o n t r o l a p a r t i c u l a r a c t i v i t y . For a d i f f e r e n t perspective o n t h e n a t u r e o f i n v a r i a n t characteristics a n d parameters, see T u r v e y (1977) o r Kelso (1982). T h e generalized motor-program perspective could b e s t u d i e d in an R T paradigm by examining t h e natures of t h e program-selection a n d T h e a priori knowledge r e q u i r e d i s which parameter-selection processes. variables a r e programming variables (those t h a t determine which motor p r o g r a m t o employ) a n d which variables a r e parameter variables. While t h e r e have been some attempts t o d i s t i n g u i s h between programming a n d parameter variables (Megaw, 1972; Zelaznik, 1978, 1981), t h i s issue has been r e l a t i v e l y unexplored. T h e l a t t e r section o f t h i s c h a p t e r shall propose methods f o r examining t h e d i s t i n c t i o n between programming a n d parameter variables. Reaction Time a n d Motor Programming General Concerns R T as a window i n t o t h e motor-programming process has much i n t u i t i v e appeal. Motor programming should r e q u i r e time, a n d t h u s examining R T differences across various conditions should p r o v i d e a window t o examine t h e time course o f programming. T h e use of R T has an honored h i s t o r y in psychology (see Posner, 1978), and it i s n a t u r a l t o examine motor programming w i t h i n t h i s framework. However, consensus does not e x i s t concerning what i s b e i n g s t u d i e d v i a t h e R T techniques. T h i s lack of consensus exists because o f t h e intersection o f response-selection a n d motor-programming processes. Many, if n o t most, studies t h a t have used choice R T t o examine t h e motor-programming process also by necessity have i n v o l v e d t h e choice of a response, a n d t h u s a response-selection process. F o r example, in t h e mid 1970s, Klapp and colleagues ( K l a p p & Wyatt, 1976; Klapp, Wyatt, E Lingo, 1974) demonstrated t h a t choice R T was longer t o i n i t i a t e a Morse code "dah" response t h a n a Morse code "dit" response. T h i s finding was i n t e r p r e t e d as s u p p o r t f o r t h e idea

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t h a t it took more time t o program t h e "dah". However, Sternberg e t al. (1978) pointed o u t t h a t it also might have taken longer t o select t h e "dah" keypress. Thus, t h e observed RT difference might not be attributable t o t h e programming processes. While t h i s controversy has y e t t o be resolved (see Kerr, 1979), it highlights t h e problem of i n t e r p r e t i n g RT data in terms of motor-programming processes. In simple RT, t h e contribution of response-selection processes is minimized. However, other problems enter into t h e situation. I n simple RT, t h e issue concerns whether d i f f e r e n t movements possess d i f f e r e n t RT values. This research has i t s modern origins i n t h e complexity paradigm Differences i n simple RT t o developed by Henry and Rogers (1960). initiate d i f f e r e n t movements are thought t o reflect differences i n programming time, devoid of influences of response-selection processes. However, recent work indicates t h a t much of the RT effects could be explained by t h e biomechanical differences in t h e process of response initiation of d i f f e r e n t movements, not i n t h e i r time course of programming (see Anson, 1982).

Precuing Studies Rosenbaum (1980) generally has been credited w i t h developing the precuing paradigm. Although earlier studies utilized d i f f e r e n t variants of t h e precuing method (Kerr, 1976; Klapp, 1977; Kornblum, 1965; Megaw, 1972; Zelaznik, 1978a,b), Rosenbaum was t h e f i r s t t o formalize the method. I n t h e typical precuing experiment, a subject is placed i n a choice-RT situation i n which there are 2" possible response alternatives, w i t h 11 representing t h e number of b i n a r y dimensions manipulated. For example, suppose a movement could be produced in one of two directions - - l e f t o r r i g h t - - a n d one of two extents--near o r f a r . There are two dimensions of movement i n t h i s example, direction and extent, w i t h two values of each dimension. Therefore, there are f o u r (2") response alternatives. On each trial, p r i o r t o t h e presentation of an imperative stimulus, a precue is presented t h a t provides full, partial, o r no advance information of t h e upcoming movement. I n t h e above example, there would be f o u r possible precues: direction, extent, both direction and extent, and neither. By examining t h e pattern of RT data as a function of t h e number and t y p e of precued dimension(s1, inferences can be made concerning t h e o r d e r of specification of these dimensions. However, there are some assumptions underlying such an experiment t h a t warrant f u r t h e r examination. First, one must question whether the above experiment i s really s t u d y i n g t h e programming process, rather than t h e response-selection process. I n the precuing experiments, the precue provides information about t h e to-be-selected response. Thus, t h e observed savings i n RT might be t h e result of response-selection processes. Critics might argue t h a t we are making a semantic distinction, t h a t is, t h a t selection and p r o gramming are one and t h e same. However, selection is a much more cogn i t i v e process than i s motor programming.' I n a subsequent section, we

I T h i s is not t o imply t h a t cognition i s unimportant i n the learning and control of motor a c t i v i t y . Rather, t h e understanding of motor p r o gramming i n t h e situation i n which cognitive effects are small can provide much information about t h e motor-programming component of activity.

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a r g u e t h a t S - R compatibility effects reside in response selection a n d n o t in motor-programming processes a n d that, if one is i n t e r e s t e d in u n d e r s t a n d i n g t h e organization o f c e n t r a l motor processes, t h e selection v e r s u s programming d i s t i n c t i o n is an important one. Second, if one believes in t h e idea o f a n a b s t r a c t representation of movement, w h e t h e r it b e motor programs o r some o t h e r construct, t h e s t u d y of t h e selection of d i r e c t i o n a n d e x t e n t dimensions appears t o b e rooted in t h e s t u d y o f parameter selection. Let u s explain. Movements w i t h t h e same spatial-temporal form a r e t h o u g h t t o b e g o v e r n e d by t h e same motor p r o g r a m (Schmidt, 1975, 1988). I n t h e above example, these aimed hand movements o f d i f f e r e n t directions a n d e x t e n t s a r e j u s t parame t r i c variations of t h e same generalized motor p r o g r a m f o r aimed-hand movements. Thus, t h e process of motor-program selection, which i s important t o u n d e r s t a n d i n g t h e n a t u r e o f motor programs, i s bypassed in most o f t h e p r e c u i n g experiments.

Third, most o f t h e p r e c u i n g experiments s t u d y a set o f variables t h a t have been called nominal (Zelaznik 8 Hahn, 1985; Zelaznik & Larish, 1986). T h e p r e c u e d variable represents a name of some aspect o f a movement, such as " l e f t " o r "near"; how t h a t label becomes t r a n s l a t e d i n t o a language that t h e motor p r o g r a m utilizes t o p r o d u c e movement has y e t t o b e determined. These nominal variables, we believe, a r e many steps removed f r o m t h e programming process (see K e r r , 1978). In t h e p r e c u i n g literature, Rosenbaum's (1983) i n t e r p r e t a t i o n of t h e available evidence is t h a t programming is a f l e x i b l e - o r d e r process. This inference i s d e r i v e d f r o m t h e examination o f numerous p r e c u i n g studies t h a t manipulated variables such as arm, direction, a n d e x t e n t of movement in one set o f studies, o r f i n g e r a n d hand in another set. T h a t each set of studies p r o d u c e d n o general set o f programming o r d e r s suggested t o Rosenbaum t h a t programming i s flexible. However, t h e r e i s another, less positive way t o view t h i s state o f a f f a i r s - - t h a t t h e variables manipulated are so f a r removed f r o m t h e true motor-programming process t h a t one cannot expect a consistent o r d e r i n g t o emerge. One o f t h e major d i f f i c u l t i e s o f t h e p r e c u i n g paradigm as it p e r t a i n s t o discovering p r i n c i p l e s o f motor programming involves t h e role of S-R compatibility in determining t h e relations between precued movement dimensions. We t u r n o u r attention t o t h i s area n e x t . S-R Compatibility a n d Motor-Programming Processes

General Review As t h e chapters in t h i s book attest, t h e n a t u r e o f t h e relation between t h e stimulus set a n d t h e response set has a n important influence I n t h i s section, we on R T and, o f course, information-processing stages. review some o f t h e major f i n d i n g s t h a t highlight t h e role o f compatibility in t h e s t u d y o f motor-programming processes a n d t h e n discuss c r i t e r i a t h a t can b e applied t o ascertain whether t h e phenomena u n d e r investigation a r e t r u l y programming phenomena.

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The f i r s t of these findings comes from the work of Megaw (1972). His experiment was v e r y simple. Subjects moved a joystick t o align a pointer w i t h a c u r s o r t h a t moved t o the r i g h t o r left, t o a near o r f a r target. On some blocks of trials, t h e target always moved i n one direction, but the extent of t h e movement was uncertain. In other blocks of trials, t h e target moved either l e f t o r r i g h t , b u t t h e extent was t h e same in each direction. Thus, Megaw had a situation i n which t h e subject produced one of two equally l i k e l y responses t o one of two stimuli, but t h e critical difference between t h e two conditions was t h a t i n one condition t h e upcoming direction was certain, while t h e extent was unknown, and i n t h e other condition t h e extent was certain while t h e direction was unknown. Megaw (1972) found t h a t there was a 40-111s benefit t o RT when t h e direction was known b u t extent was unknown compared t o t h e case i n which direction was unknown but extent was known. One possible i n f e r ence from t h i s work is t h a t t h e direction of a movement is specified p r i o r t o extent. However, in t h e Megaw experiment, there was a problem w i t h t h e experimental arrangement t h a t produced a compatibility effect (Kerr, 1976). T o move t h e subject's cursor t o t h e right, t h e subject was required t o move t h e joystick t o t h e left. Thus, t h e S-R mapping was not compatible. When direction was certain, t h e subject could shorten t h e duration of t h e response-selection processes because t h e S-R compatibility problem was reduced. K e r r manipulated t h e S-R compatibility in Megaw's step-tracking task. She found t h a t when compatibility was low, t h e r e was a reduction i n RT if direction was known. However, t h i s effect was not observed when S-R compatibility was high. Compatibility effects also can be manifested when there is no appare n t discrepancy between t h e S - R mapping f o r d i f f e r e n t precuing conditions, as t h e r e was in t h e Megaw (1972) experiment. Work by Rosenbaum (1980) and Goodman and Kelso (1980) illustrates t h i s t y p e of compatibility effect. Rosenbaum conducted an experiment t h a t required subjects t o produce an aimed-hand movement w i t h one of two arms, i n one t o two directions (away o r toward t h e body), t o a near o r f a r target. The imperative stimuli consisted of color patches, with one particular mapping of colors t o movement utilized f o r all subjects. Precues were provided i n the form of capital letters, with "L" signifying l e f t arm, and so f o r t h . When no aspects of t h e upcoming m o v y e n t s were precued, there were eight possible response alternatives ( 2 1 . On any trial, a precue of three letters was presented. If a dimension was not t o be cued, an "X" would be presented. Thus, t h e uninformative precue would consist of "XXX. " Results of t h i s experiment suggested t h a t the specification times f o r arm, direction, and extent were diffet-ent. The time was greater f o r arm than f o r direction and f o r direction than f o r extent. However, no evidence was found f o r an o r d e r i n the specification of t h e dimensions. Rosenbaum concluded t h a t t h e three dimensions could be selected i n any o r d e r because, within a given level of number of precued dimensions, the precues f o r a l l types of values on dimensions facilitated RTs. Goodman and Kelso (1980) f u r t h e r examined Rosenbaum's (1980) f i n d i n g i n a series of experiments t h a t used a similar aimed-hand movement task. T o reduce the duration of the response-selection process, Experiments 2, 3, and 4 used a highly compatible arrangement of stimuli and precues based upon spatial locations. The results of these

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experiments showed t h a t o n l y t h e amount of information (the number of remaining choices) influences RT, not t h e nature of t h e information ( t y p e of precue). Goodman and Kelso concluded t h a t t h e precuing paradigm does not s t u d y motor programming, b u t non-motoric S-R translation processes (i.e., response selection). A similar point was made in w o r k by Larish (19851, which implicated t h e S-R translation process (Teichner & Krebs, 1974) as t h e locus of t h e major precuing effects. Issues in Precuing and S-R Compatibility According t o t h e precuing methodology, RT is comprised of nonmotoric and motoric processes (Rosenbaum, 1980; Zelaznik & Larish, 1985). The goal of t h e methodology is t o manipulate t h e durations o f t h e motoric processes while leaving t h e durations of t h e non-motoric processes unchanged. I n t h e usual manner by which these precuing experiments are conducted, when t h e number of movement dimensions being precued increases o r decreases, t h e number of S-R alternatives is concomitantly doubled o r halved, respectively. For example, comparing two precued dimensions versus one precued dimension involves a 1 bit change in t h e amount of S-R information (2 alternatives i n t h e former case is 1 bit of information, and 4 alternatives i n t h e latter case is 2 b i t s of information). T h e question is, what does t h i s change produce in terms of inferences about RT differences? Keeping i n mind t h a t t h e critical question i n precuing w o r k is whether a differential o r d e r i n g exists i n specification o f movement dimensions, Rosenbaum (1980) presented inequalities t h a t must be upheld t o argue t h a t parameters are specified serially. For example, t h e specification of arm and direction minus t h e specification time f o r direction should be greater than t h e difference between t h e specification times f o r direction and extent (see Rosenbaum, 1980, o r Zelaznik & Larish, 1985, f o r f u l l treatments of t h i s issue). Zelaznik and Larish pointed o u t t h a t t h i s inequality t e s t involves comparisons across d i f f e r e n t levels of S-R information. Hick (1952) and Hyman (1953) have shown t h a t t h e probability (a metric f o r information) of an S-R p a i r being presented is logarithmically related t o RT. I n choice-RT tasks, t h e probability of an S-R p a i r often is manipulated by v a r y i n g t h e number of S-R alternatives. As t h e number of S-R alternatives increases, t h e probability o f any p a i r being required decreases. RT increases as t h e probability decreases. Presumably, some of t h e increase i n RT as a function of t h e increased uncertainty is t h e result of an increase i n all t h r e e major processes: identification, selection, and programming. Under v e r y high S-R compatibility, RT is relatively unaffected by t h e manipulation o f S-R information (Leonard, 1959; Seibel, 1963). However, when compatibility is low t h e increase in R T as t h e amount of information increases i s large, w i t h t h i s increase presumably resulting from an increase in t h e duration of t h e response-selection translation process. T h e implications of these compatibility effects are important f o r i n t e r p r e t i n g t h e results of precuing studies. When S-R compatibility is low, t h e RT comparisons among d i f f e r e n t levels and types of precues will be dominated by t h e effects of precues on t h e response-selection process. When S-R compatibility is high, RT comparisons should reflect changes in motoric-processing time. We now examine the effects of changing

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information when precuing response dimensions, within t h e context of S-R compatibiIity

.

Zelaznik (1978; Zelaznik 8 Hahn, 1985; Zelaznik, Shapiro, E Carter, 1982) has stated t h a t t h e traditional precuing paradigm confounds t h e number of S-R alternatives (overall information) w i t h t h e durations of t h e motor-programming processes under investigation. T o circumvent t h i s problem, t h e precues should maintain a constant amount of S-R information. T o accomplish t h a t end, Zelaznik proposed t h a t t h e precues should not specify t h e dimension (such as arm o r direction) b u t instead should provide information about two responses. T h e common features across t h e two responses are considered t o be precued, and the non-common features s i g n i f y dimensions t h a t will be specified after t h e imperative stimulus. For example, f o r t h e movement parameters of arm, direction, and extent, responses could be t h e left-toward-near and t h e left-awayf a r . W i t h these response alternatives, arm is t h e common feature t h a t is precued. When no dimension is precued, the two responses could be t h e left-toward-near and right-away-far. Thus, we have a common response, left-toward-near, being produced in an arm-precue condition and i n a noprecue condition. T h e difference in RT between these identical responses across d i f f e r e n t levels of arm-precue information reflects t h e savings i n R T when t h e arm is precued. This method preserves t h e number of S-R alternatives across differences i n number of precued dimensions. What advantages does t h i s technique hold f o r s t u d y i n g motor-programming and By controlling t h e number of S-R S-R translation processes? alternatives, changes i n t h e duration of S-R translation processes should be minimized across changes in t h e number of precued dimensions. What remains t o be investigated concerns t h e effects of t h e number and t y p e of precued dimensions. Zelaznik and Larish (1986) have referred t o t h e Rosenbaum p r e cuing method as "variable," because t h e number of precued dimensions covaries w i t h t h e S-R information. T h e new technique described above, which was developed by Zelaznik e t al. (1982) and Zelaznik and Hahn (19851, is r e f e r r e d t o as "fixed," because t h e amount of S-R information is held constant across changes i n t h e number of precued dimensions. The question we now t u r n t o is whether t h e locus of precuing effects resides i n t h e S-R translation processes (response selection) o r i n motor-programming processes. T o answer such a question, additive-facIf it is assumed t h a t t h e t o r s logic (Sternberg, 1969) can be utilized. effects of precuing reside i n t h e S-R translation stage, and t h a t t h e effect of S-R uncertainty i s on t h a t stage, then if we manipulate precue information independently from S-R compatibility, we should see an i n t e r action between these two variables if precued variables are affecting the S-R translation process. On t h e other hand, if motor-programming effects due t o precues are o c c u r r i n g i n a d i f f e r e n t stage than t h e effects of S-R information, we should see the independence of t h e effects of t h e amount of information and t h e nature of t h e precued information. In a s t r i c t application of additive-factors methodology, t h e r e should be main However, effects of precue and of S-R compatibility b u t no interaction. *Of course, one choice.

when a l l dimensions are precued then RT is only f o r

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because t h e amount o f S-R information a n d t h e number o f p r e c u e d dimensions a r e confounded in a t r a d i t i o n a l p r e c u i n g experiment, t h e t e s t made in t h e a d d i t i v e f a c t o r s situation cannot b e made o n t h e R T scores. Rather, t h e t e s t is o n w h e t h e r t h e o r d e r i n g o f R T differences is maintained across d i f f e r e n t p r e c u i n g techniques. T o d r i v e home t h i s important point, one must u t i l i z e t h e Rosenbaum (1980) i n e q u a l i t y t e s t s t o determine t h e n a t u r e of, t h e relation between levels o f p r e c u e a n d levels of S-R compatibility. Pseudo-additivity" implies that, regardless o f t h e level of S-R compatibility, t h e i n e q u a l i t y t e s t s p r o d u c e t h e same p a t t e r n o f data. If t h e r e is an interaction between these t w o variables, t h e n t h e Rosenbaum a d d i t i v i t y t e s t s w i l l f a i l . Table 1 summarizes t h e numerical example discussed in t h e following text. In t h i s table, we p r e s e n t t w o cases. I n case 1, t h e processes responsible f o r s p e c i f y i n g A a n d B a r e o r d e r e d in a serial but f l e x i b l e In o t h e r words, dimensions A a n d B can b e specified in a n y fashion. order. We assume t h a t in simple RT, when b o t h A a n d B a r e certain, R T equals 200 u n i t s o f time. T h e times t o s p e c i f y dimensions A a n d B a r e 30 u n i t s a n d 20 units, respectively. We also assume a 40 unit o f time increase when t h e number of S-R alternatives i s doubled [i. e., t h e amount of information increases by 1 bit). Thus, in t h e f i x e d S-R method, R T increases by 70 u n i t s o f time (40 30) when A is u n c e r t a i n a n d by 60 u n i t s o f time (40 + 2 0 ) , when B is u n c e r t a i n . In t h e f i x e d method, t h e combined e f f e c t o f A a n d B b e i n g u n c e r t a i n is o n l y 90 u n i t s o f time more t h a n t h e simple R T case (40 30 20). Thus, according t o a d d i t i v e factors, t h i s u n d e r a d d i t i v e interaction, in w h i c h t h e combined e f f e c t o f making b o t h A a n d B u n c e r t a i n is less t h a n t h e i r i n d i v i d u a l effects, suggests processes t h a t o c c u r in parallel (Stanovich & Pachella, 1977), even t h o u g h o u r example forces these processes t o b e serial. +

+

+

Now, if we conduct t h i s same experiment w i t h a v a r i e d S-R paradigm, we see t h a t t h e p a t t e r n o f data y i e l d s a d d i t i v i t y . T h i s leads t o t h e conclusion t h a t t h e processes responsible f o r t h e selection o f A a n d B a r e independent a n d organized serially, but in f l e x i b l e o r d e r . However, in b o t h t h e f i x e d a n d v a r i e d methods, t h e Rosenbaum (1980) i n e q u a l i t y t e s t s w i l l r e s u l t i n s u p p o r t f o r a s e r i a l - o r d e r model. As long as S-R compatibility is independent f r o m t h e motoric processes, t h e n t h e Rosenbaum tests w i l l p r o d u c e t h e same conclusions f o r t h e f i x e d a n d v a r i e d methods (see Zelaznik & Larish, 1986, f o r some real data on t h i s issue). What happens if t h e processes responsible f o r selection o f A a n d B a r e ordered, such t h a t A must b e specified b e f o r e B? T h i s is an example o f a hierarchical organization a n d is i l l u s t r a t e d in case 2 , f o r w h i c h we assume t h a t specification o f B cannot proceed until A has been completed. t h e p a t t e r n of RT data Notice t h a t f o r the v a r i e d procedure, ( u n d e r a d d i t i v i t y ) in t h i s case is identical t o t h a t o f t h e serial case, although t h e numbers a r e somewhat d i f f e r e n t . For t h e f i x e d procedure, b o t h models p r o d u c e u n d e r a d d i t i v e interactions. However, t h e h i e r archical model produces t w o cells i n which t h e time t o specify t w o dimensions is no longer t h a n t h e time t o specify one dimension. T h i s is as opposed t o t h e v a r i e d case, in which u n d e r a d d i t i v i t y is o b s e r v e d o n l y when t w o processes a r e organized h i e r a r c h i c a l l y .

H.N. Zelaznik and E. Franz

288

Table 1 Numerical Example o f a Precuing Experiment Assuming Independence o f t h e S-R T r a n s l a t i o n Processes From Motor-Programming Processes U n d e r I n v e s t i g a t i o n

Assume:

1. T ( A ) - - t i m e t o s p e c i f y dimension A = 30 u n i t s o f time T ( B ) - - t i m e t o specify dimension 6 = 20 u n i t s o f time 3. R T when b o t h a r e c e r t a i n 200 u n i t s o f time 4. R T increases 40 u n i t s o f time f o r e v e r y bit increase in 2.

information. Case 1:

F i x e d S-R Information

V a r i e d S-R Information

A certain

A uncertain

certain

unvertain

certain

200

270

200

270

uncertain

260

290

260

330

B F i x e d S-R Information

Case 2:

V a r i e d S-R Information

A certain

A un c e r t a i n

certain

uncertain

certain

200

290

200

290

uncertain

260

290

260

330

6

Note.

In case 1 we assume t h a t dimension A a n d B a r e specified s e r i a l l y but in a n y o r d e r . I n case 2 we assume t h a t A must b e specified before 6, thus they are arranged hierarchically.

T h i s analysis o f t h e hypothetical situation p r e s e n t e d in Table 1 suggests t h a t situations in w h i c h u n d e r a d d i t i v i t y is o b s e r v e d w i t h t h e f i x e d method but a d d i t i v i t y is o b s e r v e d w i t h t h e v a r i e d method p r o v i d e evidence t h a t S-R t r a n s l a t i o n i s n o t mediating t h e R T effects. It also suggests t h a t t h e t w o processes u n d e r investigation a r e o r d e r e d s e r i a l l y in a f l e x i b l e fashion. T h e special u n d e r a d d i t i v i t y o b s e r v e d in case 2 would p r o v i d e similar evidence f o r independence o f S-R t r a n s l a t i o n f r o m motor-programming processes a n d s u p p o r t f o r a hierarchical model o f specification f o r t h e t w o dimensions u n d e r investigation.

If t h e o r d e r i n g o f t h e differences is n o t consistent across p r e c u i n g methodologies, t h e conclusion seems reasonable t h a t S-R t r a n s l a t i o n and motoric processes a r e b e i n g influenced by t h e p r e c u e d variables. In this situation, conclusions d r a w n about motoric p r e p a r a t i o n a r e tenuous a t best. A similar problem has been o u t l i n e d in a set o f experiments by Reeve a n d P r o c t o r (1984, 1985) t h a t examined t h e response-preparation e f f e c t o f M i l l e r (1982, 1985). Reeve a n d Proctor c l e a r l y showed t h a t t h e

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attribution of a same-hand advantage i n f i n g e r key-press responses could j u s t as l i k e l y reside in a response selection/decision stage as it could in a motoric, execution stage. Given t h e fact t h a t t h e S-R compatibility influence on RT is large and can severely cloud t h e inferences made from a motor-programming perspective, how can RT be utilized t o examine programming processes? The next section of o u r paper addresses t h a t issue. Critical Questions in Programming Research via R T Analysis

As we view t h e value of precuing experiments, t h e critical issue should be t o discover variables t h a t would be "in" t h e generalized motor In other words, if t h e program and variables t h a t would be parameters. generalized motor-program construct has v i a b i l i t y in t h e RT arena, then one should be able t o discover some variables t h a t determine t h e action (generalized motor program) and other variables t h a t are selected a f t e r t h e action has been determined. It is generally believed t h a t t h e spatialtemporal form of a movement determines t h e generalized motor program. Thus, drawing a circle would be governed by a d i f f e r e n t motor program than pressing a button. T h e prediction, then, i s t h a t selecting parameters f o r these actions would be dependent on knowledge of whether t h e action was a circle o r a button press. The experiment seems straightforward: Conduct a precuing experiment t h a t has a subject drawing a circle w i t h a movement duration o f either 200 o r 600 ms, o r producing a button-press response of either 200 o r 600 ms. Thus, t h e r e are f o u r possible responses (2 actions by 2 durations) and one, both, o r none of these dimensions can be precued. According t o the logic developed in t h e present paper, t h e programming of the response duration cannot be executed u n t i l t h e action (the generalized motor program) has been selected. Thus, precuing duration should not produce reductions i n RT compared t o precuing neither duration nor action. Only precuing action o r action and duration should p r o duce reductions i n RT. One could argue t h a t t h i s t y p e of experiment does not r u l e o u t S-R compatibility effects. However, we t h i n k that if a color-to-response mapping code was used, it would not produce a differential S-R compatibility problem. Since all of t h e mappings would need t o be learned, w i t h sufficient practice all color t o response codes should be of If there was a concern equivalent d i f f i c u l t y (see Rosenbaum, 1980). about S-R compatibility, the mapping of color t o response could be If t h e r e was no effect o f t h e introduced as an independent variable. t y p e of color t o movement mapping, then t h e implication t h a t t h e effects reside i n S-R translation processes would be questioned. By then manipulating precuing w i t h t h e f i x e d and variable methods, t h e locus of t h e R T effects in t h e S-R translation process could be examined. Furthermore, by using a d i f f e r e n t qualitative mapping of stimuli t o responses one can check on t h e role of S-R compatibility. I n t h e above example, t h e same t y p e of experiment could be conducted using an alphanumeric S-R code. If it s t i l l i s shown t h a t knowledge of t h e response duration o n l y can be used when t h e action i s known, support would be provided f o r t h e hierarchical notion of programming. Some early work t h a t attempted t o answer t h i s question was conducted by Klapp (1977). He argued t h a t selection o f a Morse code

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"dit" o r "dah" is a programming variable, a n d t h a t t h e programming o f t h i s action can proceed in parallel w i t h muscle selection (parameter decision). However, c a r e f u l analysis o f t h e p r o c e d u r e t h a t p r o d u c e d an u n d e r a d d i t i v e interaction between response t i m i n g (dit o r dah) a n d muscle selection suggests t h a t t h i s conclusion was n o t w a r r a n t e d (Zelaznik, 1978a). Furthermore, Zelaznik, Shapiro a n d C a r t e r (1982) showed t h a t t h e u n d e r a d d i t i v i t y o b s e r v e d by K l a p p is n o t a r o b u s t f i n d i n g . Recently, Lee, Elliott, a n d Carnahan (1987) attempted t o answer t h i s question by manipulating t h e d i r e c t i o n o f motion a n d t h e action t o b e performed a t t h e e n d o f t h e movement. T h e i r r e s u l t s indicated t h a t t h e d i r e c t i o n o f motion c o u l d b e specified w i t h o u t knowledge o f t h e action t o b e performed. T h i s r e s u l t c o n t r a d i c t s t h e position t a k e n in t h i s chapter. We believe t h a t Lee e t al.'s r e s u l t s can b e a t t r i b u t e d t o t h e f a c t t h a t t h e movements were n o t programmed as u n i t s . Rather, w h i l e t h e limb was moving t o t h e e n d location t o p e r f o r m t h e d i f f e r e n t actions, programming o f t h i s l a t t e r p o r t i o n o f t h e movement was o c c u r r i n g . Thus, R T did n o t r e f l e c t t h e t r u e time cost o f programming. T h e b e s t t e s t o f these ideas i s t o p e r f o r m v e r y r a p i d actions t h a t do n o t have p a r t s . If t h i s t e s t p r o v e s fruitful f o r t h e generalized motor-programming framework, extensions t o more complicated serial t a s k s m i g h t b e i n v o k e d . A u t o m a t i c i t y a n d S-R Compatibility E f f e c t s

In t h e motor b e h a v i o r domain it has been assumed t h a t w i t h e x tended p r a c t i c e t h e performance o f a s k i l l e d a c t i v i t y becomes automatic. Some motor-control t h e o r i s t s m i g h t a t t r i b u t e t h i s development o f autom a t i c i t y t o t h e development o f a motor p r o g r a m (Pew, 1974; Schmidt, 1975, 1988), w h i c h t h e n r u n s o f f automatically once selected. On the o t h e r hand, more c o g n i t i v e views o f automaticity stress t h a t consistent p r a c t i c e r e s u l t s in a stimulus automatically c a l l i n g up t h e p r o p e r response (Schneider & S h i f f r i n , 1977; S h i f f r i n & Schneider, 1977). Recent t h e o r i z i n g by Logan (1988) suggested t h a t automaticity can b e viewed as a memory r e t r i e v a l process. T h a t is, when performance becomes automatic it is n o t t h a t t h e p r e v i o u s l y used processes become f a s t e r a n d d o not demand attention, but t h a t the stimulus comes t o automatically a c t i v a t e t h e e n t i r e response sequence. T h i s l a t t e r notion o f automaticity r u n s c o u n t e r t o t h e o r i z i n g about motor programs. I n such theorizing, t h e subject selects t h e p r o g r a m a n d t h e n t h e parameters. Automaticity involves the speeding up of t h i s p r o cess a n d t h e lack o f capacity demands. However, in terms o f t h e Logan model o f automaticity, one m i g h t imagine t h a t t h e program-selection a n d parameter-selection processes a r e bypassed, a n d t h e subject j u s t r e t r i e v e s f r o m memory t h e exact details o f t h e upcoming response. T h i s kind o f model m i g h t v e r y well explain t h e process of parameter specification in most p r e c u i n g experiments, f o r w h i c h we a r g u e t h a t t h e same motor p r o g r a m is b e i n g used f o r all responses. L e t us re-examine t h e Rosenbaum (1980) a n d Goodman a n d Kelso (1980) precuing experiments. In those experiments, subjects produced-aimed h a n d movements t o t a r g e t s w i t h e i t h e r hand, towards or away f r o m t h e i r body, t o a near o r f a r t a r g e t . T h u s , t h e r e were e i g h t possible responses. Rosenbaum (1980) used a mapping of stimulus colors t o responses, whereas i n t h e i r Experiments 2 a n d 3, Goodman a n d Kelso used what t h e y called a " n a t u r a l " location code t o response mapping.

S-R Compatibilify and Motor Programming

29 1

B o t h Rosenbaum (1980) a n d Goodman a n d Kelso (1980) f o u n d t h a t R T decreased as t h e number o f p r e c u e d dimensions increased. This e f f e c t is n o t s u r p r i s i n g because S-R information c o v a r i e d w i t h t h e number o f precues (Zelaznik, 1978a). However, Rosenbaum f o u n d t h a t t h e specification time f o r arm was l o n g e r t h a n t h e specification time f o r d i r e c t i o n a n d t h a t t h e specification time f o r d i r e c t i o n was l o n g e r t h a n specification time f o r e x t e n t . Goodman a n d Kelso did n o t find a n y differences in t h e specification times f o r arm, direction, a n d e x t e n t . T h e y a r g u e d t h a t if t h e S-R mapping i s "natural" t h e n movements a r e organized as wholes. T h e differences in r e s u l t s between Rosenbaum (1980) a n d Goodman a n d Kelso (1980) c o u l d b e explained by a r g u i n g t h a t t h e movement o f an arm t o a t a r g e t i s automatic, in t h a t t h e movement i s generated v i a memory retrieval, n o t by a f e a t u r e - c o n s t r u c t i o n process. When t h e subject uses t h e same p r o g r a m f o r a l l o f t h e to-be-selected movements, t h e naturalness of t h e S-R mapping allows t h e subject t o operate in t h i s In t h e Rosenbaum procedure, because t h e color t o automatic mode. response mapping was n o t natural, t h e subject h a d t o determine w h i c h response t o produce. Thus, t h e b e n e f i t of t h e automatic r e t r i e v a l process was l o s t because S - R t r a n s l a t i o n was r e q u i r e d . We would l i k e t o speculate t h a t t h i s automaticity o f programming can o c c u r o n l y when t h e subject knows t h e action (i.e., the program t o execute). Thus, in situations in w h i c h t h e response choice i n v o l v e s o n l y selecting response parameters (such as arm, direction, a n d extent), t h e subject can u t i l i z e these automatic memory r e t r i e v a l strategies. B u t when t h e subject has t o choose t h e p r o p e r p r o g r a m t o use, because t h e action i s u n c e r t a i n until t h e imperative stimulus a r r i v e s , t h e n these automatic r e t r i e v a l processes cannot b e used. T h e p r e d i c t i o n i s t h a t when motor programs must b e selected t h e n t h e effects o f S-R compatibility should b e small, but when o n l y parameters a r e involved, highly S-R compatible situations should eliminate most o f t h e d i f f e r e n t i a l precuing e f f e c t s . Suggestions for F u t u r e Research T h e r e a r e t w o issues in t h e examination o f response p r e p a r a t i o n a n d programming v i a R T . One, t y p i f i e d by Reeve a n d Proctor (1984, 1985), is t o attempt t o u n d e r s t a n d t h e n a t u r e o f t h e S - R t r a n s l a t i o n process v i a t h e manipulation o f response p r e p a r a t i o n a n d S - R compatibility. T h a t research is c r i t i c a l l y important t o t h e u n d e r s t a n d i n g o f humaninformation processing. T h e second issue, t h e one t h a t is o f utmost concern t o us, is w h e t h e r R T techniques can illuminate t h e p r i n c i p l e s of c e n t r a l motor control, specifically t h e v i a b i l i t y o f t h e motor-program construct. For t h e l a t t e r issue, S - R compatibility is viewed as a problem t o b e controlled. T h e position in o u r c h a p t e r is t h a t t h e r e s u l t s o f t h e manipulation o f S - R compatibility, along w i t h v a r i e d a n d f i x e d methods o f precuing, p r o v i d e t h e operational t e s t o f w h e t h e r S - R t r a n s l a t i o n o r motor p r o gramming processes a r e dominating t h e p a t t e r n s o f R T data. From a narrow, motor-control perspective, t h e determination o f t h e motorprogramming effects a r e o f t h e g r e a t e s t concern.

It is also o u r position t h a t R T research used t o u n d e r s t a n d motor programming must have an a p r i o r i d e f i n i t i o n o f a motor program. We

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take th e generalized motor-programming perspective and suggest t h a t t h e crucial questions concern the relation between t h e selection of programming and parameter variables. This research should be attempted f i r s t i n t h e area of selecting spatial-temporal forms of movements compared t o variations i n size and speed. Th e benefit from t h i s t y p e of research strategy is that it borrows from a fast growing literature concerning invariances in motor performances (see Gentner, 1987; Heuer, 1988; Turvey, 1977). Acknowledgements H. N. Zelaznik was supported by a Fellowship from t h e L i t t l e C i t y Foundation, Center for Innovation and Research in Mental Retardation, during th e w r i t i n g of this paper. The revision was w r i t t e n while the f i r s t author was a Visiting Associate Professor in t h e Department of Physical Education and Dance, t h e University of Wisconsin-Madison. References Adams, J. A.

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STIMULUS-RESPONSE COMPATIBILITY R. W. Prodor and T.G. Reeve (Editors @ Elsevier Science Publishers 8.V. (dorth-Holland). 1990

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RESPONSE SELECTION AND MOTOR PROGRAMMING: EFFECTS OF COMPATIBILITY AND AVERAGE VELOCITY WILL A . C . SPIJKERS U n i v e r s i t y o f Technology Aachen, FRG T h e aim o f t h i s paper is t o p r o v i d e evidence f o r a motor p r o gramming i n t e r p r e t a t i o n o f t h e e f f e c t of average velocity on reaction time (RT). T h e general problem o f r e l a t i n g effects o f movement variables on R T t o motor programming is discussed. N e x t consideration is g i v e n t o how t h e effect o f average velocity on R T - - t h a t a f a s t movement i s initiated more r a p i d l y t h a n a slow movement--might b e explained by processes o t h e r t h a n motor programming. T h e way in which t h i s problem is experimentally approached i s followed hy t h e description o f t w o studies: one in which t h e spatial compatibility o f t h e stimulus-response relation was v a r i e d a n d another in which t h e semantic compatibility was v a r i e d . It is argued t h a t t h e f i n d i n g s o f these experiments corroborate t h e view that t h e e f f e c t o f average velocity on R T relates t o motor programming r a t h e r t h a n response selection processes. Motor Programming a n d t h e E f f e c t o f Average Velocity When t h e manipulation o f a movement variable (i.e., direction, arm, extent, velocity) affects t h e time t o i n i t i a t e a p a r t i c u l a r movement alternative, an explanation in terms o f g r e a t e r demands on motor programming processes i s o f t e n p u t f o r w a r d (e.g., H e n r y & Rogers, 1960; K e r r , 1978; Klapp, 1978). However, motor programming is n o t an unequivocally defined concept. It has been conceived in several ways: (a) as a set o f transformations necessary t o translate t h e selected response code i n t o a format suitable t o c o n t r o l movements (Klapp, Greim, & Marshburn, 1981); (b) as a two-stage process, one o f which computes motor c o n t r o l parameters and t h e o t h e r one o f which translates these parameter values i n t o a format appropriate f o r f o r c e p r o d u c t i o n ( K e r r , 1978); (c) as t h e selection and specification o f values o n d e f i n i n g dimensions o f a movement (Rosenbaum, 1980); and (d) as t h e r e t r i e v i n g o f an abstract code f r o m motor memory (Van Galen, Smyth, Meulenbroek, & Hylkema, 1987). From these descriptions alone it i s clear t h a t motor programming sometimes includes a response-selection characteristic, whereas in o t h e r cases it is r e s t r i c t e d t o t h e translation o f t h e already selected response code.

A consistent finding r e g a r d i n g t h e e f f e c t o f average movement velocity o n R T has been t h a t t h e time f o r initiating a slow movement is longer t h a n t h e time f o r i n i t i a t i n g a f a s t movement (Falkenberg & Newell, 1980; Klapp & Erwin, 1976; Spijkers, 1987; Spijkers & Steyvers, 1984; Spijkers & Walter, 1985). T h i s effect has been a t t r i b u t e d t o a more time consuming process t o p r o g r a m t h e time requirements o f slow movements ( e . g . , Klapp, 1978; Quinn, Schmidt, Zelaznik, Hawkins, & McFarquhar, 1980). Falkenberg a n d Newell (1980) p u t f o r w a r d a n inhibition-facilitation explanation o f t h i s effect. T h e y hypothesized t h a t t h r o u g h t h e i n h i b i t i o n o f more muscle fibers, slow movements r e q u i r e more c o n t r o l and,

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consequently, longer RTs t h a n f a s t movements. T h e processing times f o r fast movements a r e s h o r t d u e t o t h e facilitation o f t h e majority o f t h e muscle f i b e r s involved. T h e use o f R T as index o f motor programming may lead t o e r r o neous conclusions. T h a t is, a t t r i b u t i n g t h e e f f e c t o f a movement variable o n R T t o motor programming is questionable when p r o p e r controls a r e l a c k i n g (Klapp, 1978). T h e variable may have affected o t h e r informationprocessing stages, as well. In particular, t h e use o f a choice R T paradigm s u f f e r s f r o m t w o k i n d s o f compatibility effects (Sternberg, Monsell, Knoll & Wright, 1978). These are, f i r s t , t h e dependence o f t h e mapping o f a p a r t i c u l a r response to a p a r t i c u l a r stimulus o n t h e mappings o f t h e o t h e r S-R p a i r s and, second, t h e effects o f compatibility o f e n t i r e stimulus a n d response ensembles. Stimulus-Response Compatibility Stimulus-Response (S-R) compatibility is regarded as a major variable r e l a t i n g t o t h e speed o f t r a n s l a t i n g information. S-R compatibility r e f e r s t o t h e degree o f n a t u r a l association between members o f t h e S-R p a i r s in a choice t a s k . It is assumed t h a t f o r an incompatible S-R pair, c o g n i t i v e recoding processes o r additional translations a r e needed in o r d e r t o select t h e a p p r o p r i a t e response. Consequently, t h e R T f o r incompatible S-R relations is prolonged a n d e r r o r r a t e is usually h i g h e r . T h e compatibility o f a relation i s determined n o t o n l y by t h e p r o p e r t i e s o f isolated stimulus a n d response sets, but also by t h e S-R ensemble ( F i t t s & Deininger, 1954; F i t t s & Seeger, 1953). It is generally accepted t h a t S-R compatibility affects t h e processes t h a t a r e responsible f o r t h e selection o f t h e a p p r o p r i a t e response t o a stimulus (e.g., Broadbent, 1971; Hawkins, MacKay, Holley, Friedin, & Cohen, 1973; Sanders, 1980). However, t h e r e i s no clear u n d e r l y i n g continuum o f naturalness, which makes comparisons between studies o n S-R compatibility o f t e n d i f f i c u l t , because t h e operational meaning o f compatible a n d incompatible varies across experiments (Sanders, 1980; p. 339). Response Selection a n d Motor Programming T h e debate o v e r whether t h e observed R T p a t t e r n s a r i s i n g f r o m t h e variation o f c e r t a i n movement variables reflects motor programming i n t e n s i f i e d when Rosenbaum (1980) published h i s results obtained w i t h t h e movement-precuing technique. Goodman a n d Kelso (1980) s t r o n g l y doubted t h e motor-programming i n t e r p r e t a t i o n p r o v i d e d by Rosenbaum f o r t h e p r e c u i n g effects w i t h t h e movement variables o f extent, arm, and direction. Because Rosenbaum used colors as stimuli f o r spatial responses, Goodman a n d Kelso a r g u e d that t h e t a s k was u n n a t u r a l and t h a t S-R compatibility effects h a d caused t h e d i f f e r e n t i a l effects f o r t y p e of p r e c u e in Rosenbaum's experiments. L a r i s h (1986) explicitly investigated effects o f S - R compati biIit y w i t h t h e movement- precue met hod u s i n g similar movement variables t o those used by Rosenbaum. Larish showed t h a t incompatible S-R mappings could lead t o erroneous a t t r i b u t i o n s o f effects o f movement variables t o programming processes and concluded t h a t t h e precue method should b e used w i t h compatible spatial mapping among stimuli a n d responses. T h e question arises t h e n o f whether t h e e f f e c t o f average velocity reflects a programming effect o r emerges f r o m a change in d u r a t i o n o f

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another information-processing stage. T h a t is, w i t h a b a l l i s t i c movement, a f a s t R T may o c c u r because t h e response code is a t t h e t o p o f a s h o r t term, serially-scanned memory b u f f e r (Theios, 1977) o r because t h e t r a n s l a t i o n r u l e g o v e r n i n g t h e selection o f t h e movement is easily accessed (Duncan, 1977). T h i s would mean t h a t t h e response-selection stage, r a t h e r t h a n t h e motor-programming stage, i s affected by average velocity. A mixed o r i g i n may as well b e imaginable, in t h e sense t h a t one p a r t o f t h e e f f e c t i s elicited by response selection a n d a n o t h e r by motor programming. C o n t r i b u t i o n s o f o t h e r processing stages, such as signal encoding o r motor adjustment, t o t h e e f f e c t o f average v e l o c i t y a r e also possible, but n o t v e r y l i k e l y (e.g., Spijkers, 1987; S p i j k e r s & Walter, 1985). T h i s p o i n t will b e t a k e n up again in t h e discussion o f t h e second s t u d y . T h e neurophysiological explanation t h a t t h e r e c r u i t m e n t p a t t e r n o f motor u n i t s t o p r o d u c e slow movements i s t h e basis f o r t h e d i f f e r e n c e in R T between f a s t a n d slow movements is q u i t e u n l i k e l y f o r t w o reasons. F i r s t , t h e size o f t h e e f f e c t o f average v e l o c i t y o n R T amounts t o about 60 ms (e.g., Falkenberg & Newell, 1980; S p i j k e r s & Steyvers, 1984). Second, when t h e r e q u i r e d v e l o c i t y is precued, t h e d i f f e r e n c e in R T disappears ( e . g . , S p i j k e r s & Steyvers, 1984). However, a s t u d y by Haagh, Spijkers, v a n den Boogaart, a n d v a n Boxtel (1987) suggested t h a t neurophysiological aspects should n o t b e overlooked when effects of movements on R T a r e i n t e r p r e t e d . T h e movement variable o f average v e l o c i t y is t h e r e s u l t a n t o f t w o dimensions: time a n d space. Velocity's spatial component can b e easily manipulated experimentally w i t h compatible a n d incompatible spatial mappings. However, t h e compatibility o f t h e temporal component i s n o t so How compatible is t h e mapping o f a color o f an LED easy t o manipulate. (e.g., Klapp & Erwin, 1976; Spijkers, 1987), t h e number o f p i x e l s on a d i s p l a y (Zelaznik & Hahn, 1985), o r o f a v i s u a l l y p r e s e n t e d n u m b e r (Zelaznik & Hahn, 1985) t o t h e duratio; ( o r average velocity) o f a movement? O f course, when t h e signal f a s t " is presented, it w i l l b e clear f o r most people t h a t a f a s t movement is r e q u i r e d . However, i t s relation t o a c e r t a i n v e l o c i t y is less d i r e c t t h a n t h e way in w h i c h a spatially d e f i n e d stimulus corresponds t o a spatially d e f i n e d t a r g e t . Consequently, t h e r e is t h e potential t h a t response-selection a n d motorprogramming effects a r e confounded when average v e l o c i t y i s v a r i e d . One o f t h e ways t o c l a r i f y t h e a m b i g u i t y in r e l a t i n g effects of variables t o c e r t a i n information processes is t o v a r y b o t h variables factorially. T h e a d d i t i v e factors logic o f f e r s a s u i t a b l e method t o e x p e r i mentally e x p l o r e t h i s question (Sanders, 1980; Sternberg, 1969), a l t h o u g h i t s assumptions have been much debated. T h e basic logic i s well-known. If t w o variables have a main e f f e c t on RT, w h i l e t h e i r effects d o n o t interact, t w o d i f f e r e n t processing stages a r e l i k e l y t o b e i n v o l v e d . A l t e r n a t i v e l y , if t h e effects interact, t h e variables a r e l i k e l y t o a f f e c t a t least one common processing stage. Hence, when t h e e f f e c t o f average v e l o c i t y i s related t o response-selection processes, it should, a c c o r d i n g t o t h e a d d i t i v e f a c t o r s logic, i n t e r a c t w i t h o t h e r variables t h a t a f f e c t t h e process, such as S-R compatibility. T w o studies a r e d e s c r i b e d t h a t examined t h i s problem. In t h e f i r s t study, t h e spatial compatibility o f t h e location o f t h e imperative stimulus a n d t h e d i r e c t i o n o f movement were v a r i e d . In t h e second, t h e manipulation concerned t h e compatibility of a semantic relation between t h e imperative stimulus a n d t h e r e q u i r e d movement velocity.

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When t h e spatial relations between stimuli and responses are made less compatible, t h e R T is prolonged. The common explanation f o r t h i s increment i s t h a t additional S-R translations are needed. By v a r y i n g spatial S-R compatibility, together w i t h average velocity of t h e movement, one expects an interaction, if t h e effect of average velocity is related t o processes concerned w i t h the translation of t h e stimulus-code t o t h e velocity-code. Although t h i s question was addressed in a previous experiment (Spijkers E Sanders, 19841, t h e velocities used i n t h a t s t u d y could only be attained by a select g r o u p of subjects. Thus, replication w i t h less particular requirements on movement speed was needed. The data t h a t are reported here were obtained i n a study t h a t was aimed at t h e locus of effect of response-specificity, a variable which is defined i n terms of t h e commonality of t h e movement vector (Spijkers, 1987, 1988). By showing t h a t t h e effects o f foreperiod duration interacted w i t h t h e effects of response-specificity, Spijkers (1988) proposed t h a t t h e effect of response-specificity is related t o t h e motor-adjustment stage, a stage t h a t is assumed t o be concerned w i t h t h e setting of t h e level of readiness-torespond. I n t h e experiment, subjects performed a reaction task i n which t h e y carried o u t aimed sliding movements by moving a pencil from a s t a r t position t o a target. A t r i a l started w i t h a warning tone (loo0 Hz) one second a f t e r t h e pencil was positioned a t the s t a r t point. Four LEDs were used t o indicate t h e average velocity and direction of t h e movement. The arrangement o f t h e LEDs on t h e display corresponded spatially with t h a t of t h e targets on t h e response board. Each target was associated w i t h two LEDs of d i f f e r e n t colors; t h e color designated t h e average velocity of t h e movement, which could be 17.5 o r 70 cm/s. Spatial S-R compatibility was varied a t both levels of response specificity. Response specificity was defined as the angle of direction between t h e two targets. In t h e Specific condition, t h e angle between t h e two targets was 112.5 deg, which was realized by positioning one target a t "three o'clock'' and t h e other one a t "eleven o'clock''. I n the Aspecific condition, t h e "eleven o'clock'' target was shifted t o a "two o'clock'' position, which resulted in an angle of 25 deg. The s t a r t position was i n t h e middle of the circular arrangement, a t a distance of 7 cm from t h e targets (see Spijkers 1987, 1988, f o r f u r t h e r details). On each day, a session Subjects participated on two consecutive d a y s . was devoted t o one response-specificity condition. A session consisted of 12 series of 56 trials. Spatial compatibility of t h e S-R relation was varied between blocks of six series within a daily session. For the compatible assignment, t h e location of t h e stimulus corresponded directly w i t h t h e location of the target. For t h e incompatible assignment, the subjects had t o move t o t h e target a t t h e "two o'clock'' position when, i n t h e Aspecific condition, the stimulus l i g h t f o r t h e target at t h e "three o'clock'' position was illuminated; i n t h e Specific condition subjects had t o move t o t h e target a t t h e "eleven o'clock'' position i n t h i s case. A corresponding change of t h e S-R relation was performed f o r t h e other direction alternative. Prior t o t h e experiment, t h e eight subjects were trained t o execute the movements i n t h e prespecified times. They received t h r e e series f o r practicing t h e fast movement and two series f o r t h e slow movement. If

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necessary, t h i s p r a c t i c e p e r i o d was prolonged. T h e response-board contained an X - Y d i g i t i z e r t a b l e t ( K u r t a ; 0.025 cm spatial resolution) t h a t A computer (PDP 11-23) enabled m o n i t o r i n g t h e position of t h e pencil. controlled t h e experiment a n d s t o r e d t h e X a n d Y coordinates o f p e n c i l position, which were sampled w i t h a f r e q u e n c y o f 200 Hz. In t h e analysis, p r i n c i p a l i n t e r e s t was in t h e right movement (i.e., t h e movement t o t h e t a r g e t a t t h e " t h r e e o'clock'' position), because t h i s movement was t h e same f o r b o t h S p e c i f i c i t y conditions. In t h i s way, possible d i f f e r e n t i a i effects d u e t o biomechanical differences in t h e movements a r e excluded. A b o u t 2.2% o f t h e t r i a l s were d i s c a r d e d because o f an R T o f more t h a n 1,000 ms o r o f a movement i n t h e w r o n g d i r e c t i o n . T h i s l a t t e r e r r o r o c c u r r e d more o f t e n in t h e incompatible condition. Table 1 p r e s e n t s t h e mean R T as a f u n c t i o n o f average velocity, S-R compatibility, a n d response-specificity. Mean i n d i v i d u a l R T s p e r series were used as cell e n t r i e s f o r a 2 x 2 x 2 x 4 repeated-measurement analysis of variance (ANOVA), with response-specificity, S-R compatibility, average velocity, a n d series as f a c t o r s . Consistent w i t h p r e v i o u s studies (e.g., Spijkers, 1987), R T s p r e c e d i n g a f a s t movement were s h o r t e r by 101 ms t h a n those p r e c e d i n g a slow movement, F ( 1 , 7) = 39.44, p .001. T h e c o g n i t i v e recoding necessary f o r t h e incompatible spatial S-R relations prolonged t h e R T by 53 ms, F(1, 7) = 19.95, p < .003. Similar values were r e p o r t e d by L a r i s h (1986) f o r h i s d i r e c t i o n u n c e r t a i n conditions. A small p r a c t i c e e f f e c t o f 12 ms was revealed by a N e i t h e r responseseries main effect, F(3, 21) = 3.04, p = .052. s p e c i f i c i t y n o r a n y o f t h e interactions p r o v e d t o b e s i g n i f i c a n t . An ANOVA on t h e RTs f o r t h e o t h e r movements (i.e., t o t h e t a r g e t a t t h e " t w o o'clock'' a n d "eleven o'clock'' positions) y i e l d e d t h e same r e s u l t s as f o r t h e right movement. Table 1 Reaction Times a n d Movement Times ( i n ms) o f t h e R i g h t Movements A v e r a g e d O v e r Series a n d Subjects o f Experiment 1

Compatible Incompatible Average Velocity (cm/s) 70 17.5 70 17.5 Reaction Times Specific Aspecif i c

378 377

477 479

413 447

535 530

Movement Times Spec if ic As pec if ic

115 114

391 383

126 111

383 377

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T h e absence o f an i n t e r a c t i o n between spatial S-R compatibility a n d average v e l o c i t y replicates t h e f i n d i n g s o f S p i j k e r s a n d Sanders (1984). A c c o r d i n g t o t h e a d d i t i v e factors logic, t h e independence o f t h e effects o f average v e l o c i t y a n d S-R compatibility suggests t h a t t h e e f f e c t o f average v e l o c i t y is related t o a processing stage t h a t i s d i f f e r e n t f r o m t h a t affected by S-R compatibility. T h i s r e s u l t corroborates a motorprogramming i n t e r p r e t a t i o n o f average v e l o c i t y . In c o n t r a s t t o t h e data o f S p i j k e r s (1987), response-specificity did not affect RT. Even a negative e f f e c t o f response-specificity can b e o b s e r v e d in T a b l e 1 for t h e incompatible, f a s t movement condition. This p a r t i c u l a r condition also was accompanied by t h e h i g h e s t score o f movement-duration e r r o r s . A closer inspection o f t h e i n d i v i d u a l data revealed t h a t f o u r subjects h a d an unequal d i s t r i b u t i o n o f t h i s t y p e o f e r r o r s o v e r conditions. With t h e data f r o m these subjects excluded, t h e n e g a t i v e response-specificity e f f e c t disappeared ( f o r a more elaborate discussion o f response-specificity effects, see Spijkers, 1987, 1988). Semantic S-R C o m p a t i b i l i t y a n d A v e r a g e Velocity T h e above-mentioned r e s u l t s c l e a r l y show t h a t t h e spatial recoding needed in t h e incompatible S-R condition did n o t a f f e c t t h e e f f e c t o f average v e l o c i t y . Does t h i s mean t h a t a d e f i n i t i v e answer has been obtained o n t h e question o f w h e t h e r t h e e f f e c t o f average v e l o c i t y is related t o response-selection processes? B y v a r y i n g t h e compatibility o f t h e spatial mapping o f t h e stimulus o n t o t h e response, t h e coding o f t h e movement-direction v a r i a b l e is affected. However, it may b e t h a t f o r movement v e l o c i t y a n d movement direction, d i f f e r e n t t r a n s l a t i o n mechanisms a r e i n v o l v e d t h a t operate i n d e p e n d e n t l y f r o m each o t h e r . Manipulation o f average v e l o c i t y a n d o f i t s presumed r e c o d i n g process would b e a r more d i r e c t l y o n t h e question.

In a second s t u d y (see S p i j k e r s t Walter, 19851, t h e semantic relation o f t h e stimulus a n d response was manipulated. Imperative signals were D u t c h equivalents f o r t h e words s h o r t a n d long, i n d i c a t i n g s h o r t a n d l o n g d u r a t i o n movements, r e s p e c t i v e l y . T h e D u t c h equivalents f o r f a s t a n d slow c o u l d n o t b e employed because o f t h e i r v e r y d i s t i n g u i s h i n g phenomenological features. T h e incompatible condition was realized by a r e v e r s a l o f t h e semantic relation between t h e impet-zttive signal a n d movement d u r a t i o n . F o r example, when t h e stimulus s h o r t " appeared, t h e movement d u r a t i o n t o b e executed was long. It was hypothesized t h a t if t h e e f f e c t o f average v e l o c i t y on R T is d u e t o an S - R t r a n s l a t i o n process, a r e v e r s a l of t h e S - R assignment in t h e incompatible condition should a f f e c t t h e e f f e c t o f average v e l o c i t y . If t h e l o n g R T o f a slow movement is related t o a more complex t r a n s l a t i o n o f t h e stimulus code, t h e n t h e size o f t h e average v e l o c i t y e f f e c t w i l l b e l a r g e r in t h e incompatible condition. In a d d i t i o n t o t h e v a r i a b l e o f S - R compatibility, f o r e p e r i o d d u r a t i o n (i.e., t h e i n t e r v a l between t h e w a r n i n g signal a n d t h e imperative signal) was v a r i e d t o examine w h e t h e r t h e e f f e c t o f average v e l o c i t y is influenced by t h e level o f activation o f t h e muscular system. T h e variable o f f o r e p e r i o d d u r a t i o n is assumed t o b e related t o t h e motor-adjustment stage, w h i c h modulates t h e i n t e n s i t y o f p r e p a r a t i o n (e.g., Sanders, 1980). A c o n t r o l t a s k also was i n c l u d e d t o evaluate possible differences in time t o p e r c e i v e t h e t w o v e r b a l imperative signals a n d f o r differences

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in biomechanical f a c t o r s t h a t m i g h t p l a y a r o l e in t h e p r o d u c t i o n o f f a s t a n d slow movements.

In t h i s s t u d y , e i g h t subjects performed a d i s c r e t e two-choice s l i d i n g movement t a s k . T h e choice concerned t h e average v e l o c i t y o f t h e movement. Subjects moved a s t y l u s f r o m t h e s t a r t i n g position t o t h e right until a r e c t a n g u l a r t a r g e t was reached a t a distance o f 7 cm. T h e v i s u a l w a r n i n g signal, consisting o f f o u r crosses (XXXX), a n d t h e imperative signal were presented o n a c h a r a c t e r d i s p l a y located a t t h e t o p o f a sloping desk. T h e w a r n i n g stimulus was followed by a f o r e p e r i o d o f e i t h e r 2 or The f o u r character 7 s, whi-:h was constant w i t h i n a b l o c k o f t r i a l s . words, KORT" a n d "LANG", D u t c h equivalents f o r s h o r t a n d long, s e r v e d as imperative signals a n d appeared a t t h e same position as t h e w a r n i n g signal. In t h e compatible condition, subjects responded t o t h e signal "KORT" w i t h a s l i d i n g movement o f 100-ms d u r a t i o n a n d t o t h e signal "LANG" w i t h a 400-111s movement, r e s u l t i n g in average velocities of 70 cm/s a n d 17.5 cm/s, respectively, o v e r t h e f i x e d distance o f 7 cm. T h i s S-R relation was r e v e r s e d i n t h e incompatible condition. For both movement durations, a 30% deviation o f t h e goal d u r a t i o n was tolerated. T h e total experiment consisted o f t h r e e sessions, preceded by a t r a i n i n g phase. Each session consisted o f f o u r blocks o f 40 t r i a l s . S-R compatibility a n d f o r e p e r i o d d u r a t i o n were v a r i e d across blocks. T r a i n i n g a n d t h e f i r s t session were g i v e n on t h e f i r s t day, a n d t h e second a n d third sessions were r u n on another d a y .

I n t h e c o n t r o l condition, subjects performed a selective reaction t a s k . In t h i s task, o n l y one o f t h e t w o signal alternatives was responded t o w i t h a predetermined movement, a n d t h e o t h e r signal was t o b e ignored. Thus, although t h e r e was no u n c e r t a i n t y about t h e r e q u i r e d movement, t h e signal h a d t o b e i d e n t i f i e d in o r d e r t o decide w h e t h e r a response was a p p r o p r i a t e . T h e c o n t r o l t a s k h a d f o u r conditions, one f o r each combinination of signal a l t e r n a t i v e a n d compatibility. Assuming t h a t in a selective R T p r o c e d u r e subjects p r e s e t t h e r e l e v a n t information processes, t h e comparison o f t h e conditions w i t h identical average v e l o c i t y but d i f f e r e n t v e r b a l signals p r o v i d e s information r e g a r d i n g possible differences in perceptual-processing time f o r t h e signals. A comparison o f conditions w i t h d i f f e r e n t velocities but identical signals should reveal a n y d i f f e r e n t i a l e f f e c t between f a s t a n d slow movements d u e t o biomechanical f a c t o r s . F i g u r e 1 shows t h e mean c o r r e c t R T s averaged o v e r subjects a n d t h e t w o sessions o f t h e second d a y . A n ANOVA, w i t h subjects, average movement velocity, semantic compatibility, f o r e p e r i o d duration, and sessions as factors, was c a r r i e d o u t on t h e RTs o f t h e experimental task. Average movement velocity, F(1, 7) = 26.0, p < .005, semantic compatibility, F(1, 7) = 11.5, p < .05 a n d f o r e p e r i o d duration, F(1, 7) = 13.1, p < .01, h a d s i g n i f i c a n t effects on R T . No interactions between t h e effects o f t h e main factors were f o u n d . A separate analysis o f t h e third session alone revealed that, t h o u g h smaller, t h e e f f e c t of semantic compatibility was s t i l l significant, F(1, 7) = 6.7, p < .05. Thus, t h e manipulation o f t h e semantic relation between t h e v e r b a l signal a n d t h e imposed movement d u r a t i o n was successful.

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F i g u r e 1. Mean c o r r e c t R T o f f a s t (70 cm/s) a n d slow (17.5 cm/s) movements as a f u n c t i o n o f semantic S - R compatibility a n d f o r e p e r i o d d u r a t i o n in t h e experimental t a s k (circles) a n d as a f u n c t i o n o f semantic S-R c o m p a t i b i l i t y in t h e c o n t r o l t a s k ( t r i a n g l e s ) . P a r t i c u l a r l y i m p o r t a n t f o r t h e question b e i n g addressed i s t h a t t h e effects o f average v e l o c i t y a n d compatibility were a d d i t i v e . T h e f a c t t h a t t h e size o f t h e average v e l o c i t y e f f e c t does n o t depend o n t h e compatibility o f t h e semantic S - R relation suggests t h a t t h e response code t h a t is selected on t h e basis o f t h e stimulus code i s a b s t r a c t t o p a r t i c u l a r features o f t h e e n s u i n g movement. T h e specific motor features seem t o b e elaborated by motor programming, because t h e effects o f f o r e p e r i o d d u r a t i o n a n d average v e l o c i t y also were f o u n d t o b e additive. Foreperiod-duration effects a r e assumed t o b e related t o a motoradjustment stage t h a t involves t h e t y p e o f processes t h a t a f f e c t t h e general motor readiness t o respond (e.g., Sanders, 1983). T h e r e s u l t s of t h e c o n t r o l t a s k suggest t h a t n e i t h e r perceptual n o r biomechanical f a c t o r s c o n t r i b u t e d t o t h e average v e l o c i t y e f f e c t obtained

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in t h e experimental t a s k . R T s did n o t d i f f e r when movements were identical but t h e imperative signals d i f f e r e d , a n d v i c e versa. This s u p p o r t s t h e programming i n t e r p r e t a t i o n o f t h e v e l o c i t y e f f e c t ( f o r a more detailed discussion, see S p i j k e r s a n d Walter, 1985). General Discussion a n d Summary T h e r e s u l t s o f b o t h experiments c o r r o b o r a t e t h e view t h a t response selection a n d moror programming a r e s t r u c t u r a l l y a n d f u n c t i o n a l l y d i s t i n g u i s h a b l e stages. N e i t h e r t h e v a r i a t i o n o f t h e spatial compatibility n o r o f t h e semantic compatibility o f t h e S-R relation affected t h e size of t h e R T e f f e c t o f average movement velocity. O n t h e basis o f these results, t h e f o l l o w i n g p i c t u r e emerges c o n c e r n i n g t h e f u n c t i o n s of t h e response-selection a n d motor-programming stages in choice reactions. T h e t y p e o f processes related t o motor programming may b e d e s c r i b e d in terms o f elaboration o f specific parameters o f t h e f o r t h c o m i n g movement. These parameters a r e represented in t h e response code t h a t is selected o n t h e basis o f t h e stimulus code a n d t h a t is a b s t r a c t t o these p a r t i c u l a r motor features. Thus, t h e view o f response codes as b e i n g abstract, nonmotor representations o f t h e response (Keele, 1981; Kelso, 1981; K l a p p e t al., 1981; Sanders, 1970; Schmidt, 1988) i s f u r t h e r s u p p o r t e d by t h e p r e s e n t results. W i t h respect t o motor programming a n d t h e i n t e n s i t y o f motor preparation, t h e independent effects o f f o r e p e r i o d d u r a t i o n a n d o f average v e l o c i t y o b s e r v e d in t h e second experiment c o n f i r m t h e view t h a t t h e building o f a specific motor response i s n o t affected by v a r i a t i o n s in t h e level of general response readiness (e.g., Spijkers, 1988; S p i j k e r s & Steyvers, 1984; Sternberg, Wright, Knoll, & Monsell, 1980). A n i m p o r t a n t a t t r i b u t e o f motor programming appears t o b e t h a t movement variables can b e pre-programmed (e.g., Klapp, 1978). Preprogramming means t h a t t h e specification o f movement variables can t a k e place in advance o f t h e imperative signal. Programming need n o t b e an In a all-or-none process; it can b e c a r r i e d o u t i n a g r a d u a l fashion. s t u d y t h a t examined a selective-preparation i n t e r p r e t a t i o n o f t h e e f f e c t o f k e y - p r e s s d u r a t i o n on RT, K l a p p a n d Rodriguez (1982) f o u n d that, f o r low p r o b a b i l i t y , unanticipated movements, R T was l o n g e r p r i o r t o longd u r a t i o n responses t h a n t o s h o r t - d u r a t i o n responses. B u t R T was independent o f response d u r a t i o n f o r high p r o b a b i l i t y movements. With respect t o average velocity, S p i j k e r s (1987) replicated t h e finding t h a t increasing t h e p r o b a b i l i t y o f v e l o c i t y reduces t h e R T f o r i n i t i a t i n g a movement o f t h a t velocity. So, p r o b a b i l i t y appears t o a f f e c t t h e degree o f preprogramming, in t h a t a m o t o r - p r o g r a m v a r i a b l e w i t h a high p r o b a b i l i t y leads t o more p r e - p r o g r a m m i n g t h a n does a v a r i a b l e w i t h a low p r o b a b i l i t y . T h e fact t h a t R T f o r fast as well as slow movements varies i n v e r s e l y w i t h response p r o b a b i l i t y makes it u n l i k e l y t h a t t h e e f f e c t o f average v e l o c i t y is d u e t o selective p r e p a r a t i o n o f a f a s t movement. In t h e same s t u d y , S p i j k e r s showed t h a t t h e degree in w h i c h specification o f movement variables is p e r f o r m e d i n advance seems t o depend o n a t least t w o f a c t o r s : (a) t h e p r o b a b i l i t y o f t h e movement v a r i a b l e itself, a n d (b) t h e p r o b a b i l i t y o f o t h e r movement variables. F o r t h e variables o f movement d i r e c t i o n a n d average movement velocity, t h i s interdependence appeared t o b e facilitative, i n t h a t more c e r t a i n t y about t h e directional a t t r i b u t e enhanced t h e p r e - p r o g r a m m i n g o f t h e v e l o c i t y v a r i a b l e a n d v i c e versa.

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For t h e application of t h e movement-precuing technique, preprogramming is a decisive characteristic ( f o r a review on this technique, see Rosenbaum, 1983). This technique is based on t h e assumptions t h a t t h e relevant variables of t h e ensuing movements t h a t are presented in t h e precue are programmed and t h a t t h e RT reflects t h e programming of t h e remaining movement aspects indicated by the imperative signal. However, recent evidence indicates t h a t t h e precuing technique as a tool t o investigate motor programming is not without problems. The method of precuing (Zelaznik & Hahn, 1985), duration of t h e precue interval (Heuer, 1986; Reeve & Proctor, 19841, and t h e spatial compatibility of t h e precue t o t h e responses being precued (Hendrikx, 1986; Reeve & Proctor, 1984) all are factors t h a t may have considerable influence on t h e results obtained w i t h t h e precuing method. Evaluation of t h e proposed response-preparation model w i t h respect t o other studies examining t h e relation between S-R compatibility and movement variables is not a simple matter. A main reason f o r t h i s fact is t h a t t h e relevant studies were conducted w i t h the aim of examining t h e precuing technique. For example, Larish (1986, Experiment 2) examined t h e effects of spatial S-R compatibility and t h e movement variables of arm, direction, and extent, and found an interaction between the effects of S-R compatibility and number of uncertain (not precued) movement elements. Furthermore, his data suggest t h a t t h e size of t h e compatibility effect depended on t h e t y p e of movement element t h a t was uncertain. Although important w i t h respect t o t h e locus-of-effect of precuing, t h e results o n l y f u r t h e r confirm the known relation between t h e effects of S-R compatibility and t h e effects of number of alternatives and of response t y p e (e.g., Hendrikx, 1986; Sanders, 1980). With regard t o t h e effect of S-R compatibility and the effects of t h e individual values of t h e movement variables, which should be additive according t o t h e proposed model, Larish did not present any d a t a . References Broadbent, Press.

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Sternberg, S., Monsell, S., Knoll, R. L., & Wright, C. E. (1978). T h e latency a n d d u r a t i o n of r a p i d movement sequences: Comparisons of speech a n d t y p e w r i t i n g . I n G . E. Stelmach (Ed.), information processing In motor control and learning (pp. 117-152). New Y o r k : Academic Press. Sternberg, S., Wright, C . E., Knoll, R. L, & Monsell, S. (1980). In R. A. Cole Motor programs in r a p i d speech: Additional evidence. (Ed. ) , T h e perception and production of fluent speech (pp. 507-534). Hillsdale, NJ: Lawrence Erlbaum. Theios, J. (1977). Commentary on "Reaction time measurements in t h e s t u d y of memory processes: T h e o r y a n d data. I n G. Bower (Ed.), Human memory: Basic processes (pp. 243-251). New Y o r k : Academic Press. Van Galen, G. P., Smyth, M. M., Meulenbroek, R . G . J., & Hylkema, H. (1987, J u l y ) . T h e role of short-term memory and the motor buffer in handwriting under visual and non-visual guidance. Paper presented a t t h e third international symposium on H a n d w r i t i n g a n d Computer Applications, Montreal, Canada. Zelaznik, H . N, & Hahn, R . (1985). Reaction time methods in t h e s t u d y of motor programming: T h e p r e c u i n g of hand, digit, a n d d u r a t i o n . Journal of Motor Behavior, 1 7 , 190-218.

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STIMULUS-RESPONSE COMPATIBILITY

R.W. Proctor and T.G. Reeve (Editors 0 Elsevier Science Publishers 8.V. (drlh-HolIand), 1990

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RAPID RESPONSES WITH THE LEFT OR RIGHT HAND: RESPONSE-RESPONSE COMPATIBILITY EFFECTS DUE TO INTERMANUAL INTERACTIONS

HERBERT HEUER Fachbereich Psychologie d e r Philipps-Universit'aY Marbu r g , West Germany

Neisser (1967), in t h e f i r s t chapter o f h i s Cognitive Psychology, characterized t h e t a s k o f t h e cogFitive approach t o psychology as t h e s t u d y o f "the f a t e o f t h e stimulus. Consistent w i t h t h i s view, models o f information-processing in reaction-time ( R T ) experiments o f t e n consider presentation o f t h e stimulus as t h e f i r s t event. However, t h e r e is a d i f f e r e n t perspective (cf. Heuer E Prinz, 1987). Suppose we place a subject (e.g., a child) in f r o n t o f an R T apparatus w i t h o u t saying anything. F o r t h i s experiment one would :xpect r a t h e r unusual results. Obviously, t h e i n s t r u c t i o n is a necessary f i r s t event" which determines how t h e signals presented w i l l b e processed. T h e focus on t h e intentions of t h e subjects r a t h e r t h a n on t h e "fate" o f t h e stimulus has a n o l d t r a dition, which i s s t r o n g l y associated w i t h t h e names o f Narziss A c h a n d h i s students. According t o t h e "action-oriented perspective" f o r R T experiments, t h e stimuli impinge o n a subject who i s p r e p a r e d t o t a k e c e r t a i n courses o f action u n d e r certain conditions. He o r she could d o otherwise, b u t doesn't d o so. U s i n g another terminology, t h e signals represent i n p u t t o d i f f e r e n t k i n d s o f systems that can b e considered as t h e functional analogues o f t h e experience o f c e r t a i n intentions. T h e notion t h a t a subject in an R T experiment can b e represented as a special-purpose system, which has been assembled f o r t h e p a r t i c u l a r t a s k a t hand, b r i n g s i n t o focus t h e relation between t h e various response alternatives, because d i f f e r e n t sets o f response alternatives imply d i f f e r e n t systems. Some effects of t h e relation between responses on R T a r e well known. These effects a r e r e f e r r e d t o as response-response (RR) compatibility effects. For example, R T is influenced by whether responses a r e mechanically antagonistic o r n o t (Berlyne, 1957). Also, R T is influenced by whether responses have a f i r s t element in common o r n o t (Sanders, 1970); more generally, R T declines p r o g r e s s i v e l y when more and more i n i t i a l elements a r e common t o t h e a l t e r n a t i v e responses (Rosenbaum, H i n d o r f f , & Munro, 1987). Mechanical incompatibility and p a r t i a l response i d e n t i t y a r e r a t h e r More s u b t l e effects show up gross means t o v a r y R - R compatibility. when t h e d i f f e r e n t responses a r e performed w i t h d i f f e r e n t hands, so t h a t t h e responses are mechanically compatible a n d have u n i q u e identities. F o r t h i s situation, one m i g h t expect t h a t R-R compatibility effects could b e related t o intermanual interactions, which can b e observed when movements a r e performed w i t h b o t h hands simultaneously.

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One approach t o t h e s t u d y o f intermanual interactions is t h u s t o examine R - R compatibility effects. T h i s c h a p t e r gives an o v e r v i e w o f a series o f experiments t h a t h a d t h i s as an i n i t i a l purpose. However, it t u r n e d o u t t h a t t h e r e s u l t s h a d some implications also f o r t h e methodology used in t h e s t u d y o f motor programming, in p a r t i c u l a r f o r t h e assumpt i o n s t h a t u n d e r l i e t h e movement-precuing technique. These implications w i l l b e discussed f i r s t , a f t e r t h e basic phenomenon has been described. Thereafter, a l i n k w i l l b e established between processes during p r o g r a m m i n g a n d execution o f a movement, a n d f i n a l l y , t h e r e s u l t s obtained in R R compatibility experiments w i l l b e related t o r e s u l t s obtained in t h e s t u d y o f simultaneous movements o f t h e t w o hands. T h e Basic Phenomenon T h e experiments i n t h i s c h a p t e r have c e r t a i n common characteristics. T h e y were two-choice experiments in w h i c h t h e t w o responses were t o b e performed w i t h t h e l e f t o r right hand. In d i f f e r e n t conditions, same o r d i f f e r e n t movements were assigned t o t h e t w o hands. In o t h e r experiments ( o r o t h e r conditions), t h e r e s p o n d i n g f i n g e r s were same o r d i f f e r e n t r a t h e r t h a n t h e movements. Thus, t h e c o r e conditions o f t h e experiments d i f f e r e d w i t h respect t o t h e relation between t h e t w o a l t e r n a t i v e responses.

In most experiments, t h e response signals were l i g h t - e m i t t i n g diodes (LEDs) placed 2 cm t o t h e l e f t a n d t o t h e right o f a c e n t r a l w a r n i n g light. In some experiments, t h e signals were horizontal lines o f 2 cm l e n g t h presented t o t h e l e f t o r right o f t h e midline (imaginary) of an oscilloscope screen, a n d t h e w a r n i n g signal was a tone. In all experiments, subjects h a d t o respond w i t h t h e l e f t h a n d when t h e l e f t signal was presented a n d w i t h t h e right h a n d when t h e right signal was presented. In almost all experiments o f t h e series, t w o somewhat unusual movements were used. One was an up-and-down movement performed w i t h t h e index f i n g e r o r thumb, while t h e h a n d h e l d a h o r i z o n t a l l y o r i e n t e d handle. Basically t h i s was a movement as in a s t a n d a r d t a p p i n g task, a n d t h e r e f o r e it was called " t a p p i n g . " I n i t i a l l y t h e f i n g e r r e s t e d on a key, t h e n t h e f i n g e r h a d t o b e l i f t e d by an amount o f 3 cm o r more ( t h i s was c o n t r o l l e d by mechanical devices o r photo-electric makes-and-breaks), a n d t h e n t h e k e y h a d t o b e depressed again. T h e o t h e r movement was mainly a horizontal f i n g e r movement w i t h o n l y a small v e r t i c a l component. Again, t h e f i n g e r rested on a k e y i n i t i a l l y . I t h a d t o b e moved t o a second k e y t h a t was l a t e r a l l y displaced by about 3 cm, and. t h e n b a c k t o the f i r s t key. Between t h e k e y s t h e r e was a b a r r i e r o f about 1 cm h e i g h t . Since t h e ,keys h a d t o b e pressed alternately, t h e movement was ca I I e d "a It e r nat ing . In t h e f i r s t set of experiments (Heuer, 1982a,b), R T s f o r 16 conditions w e r e studied. These conditions were generated by assigning a l l possible p a i r i n g s o f t w o responses out of f o u r t o t h e t w o hands; t h e f o u r responses were t a p p i n g w i t h t h e index f i n g e r o r t h u m b a n d a l t e r n a t i n g w i t h t h e index f i n g e r o r t h u m b . Thus, i n d i f f e r e n t blocks o f t r i a l s , t h e responses assigned t o t h e t w o hands were e i t h e r t h e same (e.g., t a p p i n g index f i n g e r - - t a p p i n g index f i n g e r ) , o r d i f f e r e n t w i t h respect t o t h e movement ( e . g . , t a p p i n g index f i n g e r - - a l t e r n a t i n g index f i n g e r ) , w i t h respect t o t h e f i n g e r (e.g., t a p p i n g index f i n g e r - - t a p p i n g

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thumb), o r w i t h respect t o b o t h movement a n d f i n g e r (e.g., tapping i n d e x f i n g e r - - a l t e r n a t i n g thumb). F o r each of these f o u r d i f f e r e n t relations between responses, t h e r e were f o u r blocks o f t r i a l s in w h i c h d i f f e r e n t responses were assigned t o t h e t w o hands w h i c h shared t h e same relation. T h e f i r s t experiments revealed a t r i a d of effects t h a t were shown t o b e v e r y reliable in subsequent experiments. When t h e assignment t o t h e t w o hands i n v o l v e d different movements r a t h e r t h a n same movements, (a) mean R T was longer, (b) mean ( i n d i v i d u a l ) R T v a r i a b i l i t y was larger, a n d (c) f r e q u e n c y o f choice e r r o r s (responses w i t h t h e w r o n g hand) was smaller. However, in conditions w i t h different fingers assigned t o t h e t w o hands, none o f these effects c o u l d b e seen. T h e l a t t e r finding was in l i n e w i t h r e s u l t s r e p o r t e d by Rosenbaum a n d K o r n b l u m (1982).

S-R Compatibility In response t o Rosenbaum's (1980) seminal p a p e r on t h e movementp r e c u i n g methodology, Goodman a n d Kelso (1980) p o i n t e d o u t t h a t effects o f t h e relation between responses m i g h t b e highly sensitive t o stimulus conditions. T h e r e a r e in f a c t several data t h a t demonstrate t h e effects o f S-R compatibility on t h e r e s u l t s o f movement-precuing experiments (see Zelaznik 8 Larish, 1986, a n d C h a p t e r 11 by Zelaznik & Franz, in t h i s volume). Thus, t h e question h a d t o b e asked w h e t h e r t h e basic t r i a d o f effects m i g h t b e specific to t h e p a r t i c u l a r signals used. In an u n p u b l i s h e d experiment (see Heuer, 1984a), one g r o u p o f subjects was run u n d e r all 16 conditions w i t h a compatible S-R mapping, as in t h e f i r s t set o f experiments, while another g r o u p o f subjects was r u n w i t h an incompatible S-R mapping. In t h e incompatible conditions, t h e signals were a small a n d a l a r g e square t h a t were concentric, a n d t h u s h a d no spatial relation t o t h e responses. T h e r e s u l t s obtained u n d e r compatible a n d incompatible conditions were essentially t h e same. O n l y t h e increase o f mean R T when d i f f e r e n t movements were assigned t o t h e t w o hands was somewhat less u n d e r incompatible conditions (see Table 1 ) . T h e s i m i l a r i t y o f t h e r e s u l t s f o r t h e t w o conditions indicates t h a t t h e basic t r i a d o f effects is n o t d u e t o S-R compatibility. Speed - Acc ur a c y T r a d e o f f

A second concern about t h e basic t r i a d o f effects was t h e possibili t y t h a t it m i g h t b e d u e t o a speed-accuracy t r a d e o f f , because t h e increase o f mean R T i n conditions w i t h d i f f e r e n t movements assigned t o t h e t w o hands was accompanied by an increase i n accuracy. To examine t h i s potential explanation, an experiment was r u n w i t h f o u r conditions in w h i c h same o r d i f f e r e n t movements were assigned t o t h e t w o hands a n d a l l responses were performed w i t h t h e index f i n g e r s . Six subjects s e r v e d f o r about 20 sessions in which d i f f e r e n t R T deadlines were i n s t r u c t e d t o p r o v o k e d i f f e r e n t speed-accuracy c r i t e r i a (Heuer, 1983). Table 2 presents expected mean RTs f o r v a r i o u s levels o f accuracy in conditions w i t h same a n d d i f f e r e n t movements assigned t o t h e t w o hands. These data a r e means o f estimates f o r i n d i v i d u a l subjects t h a t were based on a logarithmic f u n c t i o n f i t t e d t o t h e o b s e r v e d data. For identical levels o f accuracy, t h e R T d i f f e r e n c e between conditions was

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r e d u c e d t o o n l y about 6 ms, but t h i s remaining d i f f e r e n c e was s i g n i f i cant. Similarly, f o r identical RTs, t h e accuracy levels were r e l i a b l y different. T h e conclusion f r o m these r e s u l t s was t h a t t h e d i f f e r e n c e between conditions w i t h same a n d d i f f e r e n t movements assigned t o t h e t w o hands i s o f a s t r u c t u r a l o r i g i n a n d n o t j u s t a r e s u l t o f d i f f e r e n t speedaccuracy c r i t e r i a . Table 1 T h e e f f e c t o f t h e relation between l e f t - h a n d a n d r i g h t - h a n d responses o n mean R T (m), mean R T v a r i a b i l i t y ( s ) , a n d choice accuracy (a) f o r comp a t i b l e a n d incompatible S-R relations ( a f t e r Heuer, 1984a).

Cornp a t ib l e finger

Note.

Incompat ib l e finger

Mean R T a n d R T v a r i a b i l i t y in ms.

Choice accuracy in p e r c e n t .

Table 2 Expected mean R T (in m s ) f o r c e r t a i n levels o f choice accuracy i n conditions w i t h same a n d d i f f e r e n t movements assigned t o t h e t w o hands (data f r o m Heuer, 1983).

Percentage of c o r r e c t choices 50

65

80

90

95

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T h e Advance-Specification Assumption a n d t h e Programming- Interactions Assumption How does t h e basic t r i a d o f effects come about? T h e experiments described i n t h e last section a r e highly similar t o movement-precuing experiments (Rosenbaum, 1980, 1983). In fact, t h e method i s essentially t h e same as t h e "new method o f p r e c u i n g " i n t r o d u c e d by Zelaznik, Shapiro, a n d C a r t e r (1982), a n d t h e logic o f t h e movement-precuing p r o c e d u r e p r o v i d e s t h e simplest i n t e r p r e t a t i o n f o r t h e results. T h e Advance-Specification Assumption Execution o f a movement i s preceded by p r e p a r a t o r y processes. When movements have t o b e performed in response to a signal, preparat o r y processes can b e observed a f t e r as well as b e f o r e t h e response signal (e.g., Brunia, 1987; Brunia, Haagh, & Scheirs, 1985). Generally speaking, preparation in advance o f t h e response signal appears t o cons i s t o f doing e v e r y t h i n g t h a t can b e done w i t h o u t t h e actual s t a r t o f t h e movement (see Naatanen & Merisalo, 1977; Niemi E Naatanen, 1981); if it goes one step too f a r , an a n t i c i p a t o r y response w i l l occur. A t least p a r t o f preparation i s specifically concerned w i t h t h e characteristics o f t h e forthcoming movement. T h i s p a r t generally i s r e f e r r e d t o as "programming." Depending on w h e t $ w it occurs b e f o r e o r after t h e response signal, "preprogramming" a n d programming" can b e distinguished. ( T h i s d i s t i n c t i o n i s t h e use o f t h e t w o terms in t h i s chapter, although sometimes t h e terms a r e used somewhat d i f f e r e n t l y . ) T h e movement-precuing technique is basically intended as a means t o vary t h e amount o f preprogramming. Programming o f t e n i s conceived as a series o f steps in which t h e d i f f e r e n t movement characteristics a r e specified. A fundamental assumpt i o n o f t h e movement-precuing technique i s t h a t characteristics common t o t h e response alternatives in a choice t a s k a r e specified in advance o f t h e response signal. O n l y characteristics t h a t a r e u n i q u e t o t h e signaled response have t o b e specified d u r i n g t h e R T i n t e r v a l . T h i s assumption w i l l b e called t h e advance-specification assumption. (More correctly, t h e assumption r e f e r s t o t h e dependency o f advance specification o n t h e relation between responses. ) According t o t h i s assumption, R T should b e longer t h e l a r g e r t h e number o f characteristics i s o n which t h e response alternatives d i f f e r . T h e advance-specification assumption easily explains w h y R T depends o n t h e relation between response alternatives. However, t h i s dependency is n o t always observed; a n example i s t h e lack o f a d i f f e r ence between conditions w i t h same a n d d i f f e r e n t f i n g e r s assigned t o t h e t w o hands. From t h i s r e s u l t it has been concluded t h a t multiple prepar a t i o n o f d i f f e r e n t responses m i g h t b e possible (Heuer, 1982b). Rosenbaum a n d Kornblum (1982) i n v o k e d t h e concept o f m u l t i p l e preparat i o n eyen t o explain a phenomenon t h a t is c o n t r a r y t o expectations based o n t h e advance-specification assumption: R T f o r choice between d i f f e r e n t f i n g e r s o f t h e same h a n d is longer t h a n f o r choice between d i f f e r e n t f i n g e r s o f d i f f e r e n t hands (Kornblum, 1965), a t least u n d e r c e r t a i n conditions (see Reeve & Proctor, 1988). Application o f t h e advance-specification assumption t o t h e t r i a d o f effects, which can b e observed when d i f f e r e n t movements a r e assigned t o

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t h e t w o hands as compared t o same movements, faces t w o d i f f i c u l t i e s . T h e f i r s t difficulty can b e overcome. T h e second, however, seems t o b e obstinate. These d i f f i c u l t i e s a r e related t o t h e differences between conditions w i t h respect to R T v a r i a b i l i t y a n d choice accuracy. Problems with t h e advance-specification assumption: R T v a r i a b i l i t y . A t f i r s t glance t h e increase o f R T v a r i a b i l i t y when d i f f e r e n t movements a r e assigned t o t h e t w o hands appears t r i v i a l . When more specifications have t o b e done during t h e R T interval, t h i s w i l l n o t o n l y increase mean RT, but, according t o elementary statistics, R T v a r i a b i l i t y as well, p r o v i d e d t h a t t h e times needed f o r d i f f e r e n t specifications a r e s u f f i c i e n t l y independent. However, t h e r e l a t i v e sizes of R T increase a n d v a r i a b i l i t y increase caused concern. F o r example, in one o f t h e e a r l y experiments (Heuer, 1982b) mean R T increased f r o m 306 ms in conditions w i t h same movements t o 332 ms in conditions w i t h d i f f e r e n t movements, while mean The distribution of the R T v a r i a b i l i t y increased f r o m 69 ms t o 83 ms. time needed f o r additional specifications should t h u s have a mean o f 26 ms a n d a s t a n d a r d deviation o f about 46 ms ( tU2 - 6g2 ), p r o v i d e d t h a t specification times a r e uncorrelated. One can t h i n k o f a d i s t r i b u t i o n w i t h moments l i k e this, f o r example, a gamma d i s t r i b u t i o n . However, it would b e abnormally skewed. Fortunately, t h e r e is a more plausible way t o make sense o f t h e comparatively l a r g e increase o f R T v a r i a b i l i t y .

I n a two-choice task, subjects t e n d t o expect one o r t h e o t h e r signal. When t h e y a r e asked t o indicate t h e i r expectation, t h e r e l a t i v e f r e q u e n c y o f t h e expectations closely matches t h e r e l a t i v e f r e q u e n c y o f t h e signals (Hinrichs, 1970). Suppose t h a t expectation of a signal i s accompanied by programming t h e c o r r e s p o n d i n g response, even in condiT h i s kind o f t i o n s w i t h d i f f e r e n t movemen!s assigned t o t h e hands. p r e p a r a t i o n has been called single p r e p a r a t i o n " a n d can b e contrasted w i t h "conjunctive" p r e p a r a t i o n (Heuer, 1982b; Rosenbaum, 1980; Semjen, Requin, & Fiori, 1978). In c o n j u n c t i v e preparation, advance specification is r e s t r i c t e d t o t h e common characteristics o f t h e responses, but in single p r e p a r a t i o n t h e one response is programmed completely. Therefore, t h e difference between conditions w i t h same a n d d i f f e r e n t movements assigned t o t h e t w o hands should show up o n l y f o r t r i a l s in w h i c h a non-expected signal i s presented. F o r these t r i a l s , t h e movement has t o b e r e - p r o grammed in conditions w i t h d i f f e r e n t movements but n o t in conditions w i t h same movements. It should b e noted t h a t t h e response-priming method i n t r o d u c e d by Rosenbaum a n d Kornblum (1982) r e s t s on t h e assumption o f single p r e p a r a t i o n .

It i s f a i r l y obvious t h a t t h e assumption of single p r e p a r a t i o n ( r a t h e r t h a n c o n j u n c t i v e preparation) explains t h e comparatively l a r g e increase of R T v a r i a b i l i t y in conditions w i t h d i f f e r e n t movements assigned t o t h e t w o hands. In these conditions responses t o expected signals should have R T d i s t r i b u t i o n s t h a t a r e about t h e same as t h e R T d i s t r i b u tions o f responses t o expected signals i n conditions w i t h same movements assigned t o t h e t w o hands. However, f o r responses t o non-expected signals, RTs should n o t o n l y b e more variable in conditions w i t h d i f f e r e n t movements, d u e t o t h e inclusion o f additional specification processes, b u t also l o n g e r on t h e average. T h e additional d i f f e r e n c e between mean RTs f o r responses t o expected a n d non-expected signals, o f course, produces an additional increase o f t h e v a r i a b i l i t y of t h e total R T d i s t r i b u t i o n .

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Whether o r n o t t h e assumption o f single p r e p a r a t i o n is v a l i d as an explanation f o r t h e p a r t i c u l a r l y l a r g e increase o f R T v a r i a b i l i t y was examined in a set o f experiments in w h i c h r e l a t i v e signal f r e q u e n c y was v a r i e d (Heuer, 1 9 8 2 ~ ) . Signals w i t h lower r e l a t i v e f r e q u e n c y a r e less o f t e n expected c o r r e c t l y t h a n signals w i t h a h i g h e r r e l a t i v e f r e q u e n c y . T h i s follows f r o m t h e f a c t t h a t (a) t h e r e l a t i v e f r e q u e n c y o f expectations matches t h e r e l a t i v e f r e q u e n c y o f t h e signals, a n d (b) t h e signals a r e independent o f t h e expectations. Thus, responses t o t h e less f r e q u e n t signal r e q u i r e re-programming more o f t e n t h a n responses t o t h e more f r e q u e n t signal, a n d t h e d i f f e r e n c e between conditions w i t h same a n d d i f f e r e n t movements assigned t o t h e t w o hands should b e l a r g e r f o r responses t o t h e less f r e q u e n t signal. T h i s expectation was confirmed, t h u s allaying t h e concerns about t h e l a r g e increase o f R T v a r i a b i l i t y . Problems with t h e advance-specification assumption: accuracy. The e f f e c t o n choice accuracy of t h e relation between responses appears particularly interesting. Generalizing b e y o n d t h e actual data, it can b e described as an enhancement o f t h e accuracy o f discrimination by way o f making t h e responses r a t h e r t h a n t h e signals more d i f f e r e n t . I am n o t aware o f systematic studies o f t h i s phenomenon. However, it is e x p l o i t e d in applied fields. F o r example, in remedial education, it i s beneficial t o use l a r g e g e s t u r e s o f t h e hands t o enhance discrimination o f t h e l e t t e r s o f t h e alphabet.

It appears plausible t h a t w i t h d i f f e r e n t movements assigned t o t h e t w o hands, choice should become more accurate. B u t there is no c o n v i n c i n g explanation f o r t h i s phenomenon. I n terms o f t h e movementp r e c u i n g rationale, choice between t h e l e f t a n d right h a n d i s a process t h a t i s d i s t i n c t f r o m specification o f t h e characteristics o f t h e movement t o b e performed. Therefore, it is h a r d t o see w h y t h e accuracy of choice between hands should depend on w h e t h e r characteristics o f t h e movements assigned t o t h e l e f t a n d right h a n d a r e preprogrammed o r n o t (but see Rosenbaum, Barnes, & Slotta, 1988). T h e r e was another finding o n e r r o r s t h a t k e p t t h e concerns about t h e v a l i d i t y o f t h e advance-specifFation i n t e r p r e t a t i o n alive. I n almost all experiments o f t h e series, execution e r r o r s " were recorded in addition t o choice e r r o r s . These e r r o r s were d e f i n e d as t a p p i n g responses o f i n s u f f i c i e n t h e i g h t a n d a l t e r n a t i n g responses in w h i c h t h e second k e y p r e s s was omitted. Mostly, execution e r r o r s were f a u l t y response executions. However, if t a p p i n g was p e r f o r m e d r a t h e r t h a n alternating, o r a l t e r n a t i n g r a t h e r t h a n tapping, t h i s was also recorded as an execution e r r o r . ( A l t e r n a t i n g instead of t a p p i n g may h a v e gone unnoticed in some cases when t h e f i n g e r reached a s u f f i c i e n t h e i g h t even w i t h an a l t e r n a t i n g response.) I n c o n t r a s t t o choice e r r o r frequency, t h e f r e q u e n c y o f execution e r r o r s was n o t consistently d i f f e r e n t between conditions w i t h same a n d d i f f e r e n t movements assigned t o t h e t w o hands. T h i s corresponded t o t h e informal observation t h a t almost n e v e r was t h e w r o n g h a n d used, but t h e c o r r e c t movement executed, o r t h e c o r r e c t h a n d used a n d t h e w r o n g movement executed--even when time p r e s s u r e a n d e r r o r rates were high, as i n t h e speed-accuracy t r a d e o f f experiment. T h e tight l i n k a g e between h a n d a n d movement seems t o b e a t variance w i t h t h e movement-precuing rationale, according t o w h i c h choice between hands a n d specification o f movement characteristics a r e d i f f e r e n t processes t h a t can b e separated in time. i f choice between hands a n d specification o f movement

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characteristics were in fact separate processes, they should also produce t h e i r separate errors.

The Programming- Interactions Assumption T h e t r i a d of effects t h a t is produced by a difference between t h e movements assigned t o the two hands cannot be interpreted in terms of t h e advance-specification assumption i n a satisfying way. Therefore, This search was guided by a alternative hypotheses were sought. fundamental presupposition: A response t h a t is not executed on a particular t r i a l can affect (pre-)programming of t h e other response o n l y when both responses are prepared simultaneously. From t h i s presupposition it follows that, within a serial-stages framework f o r t h e interpretation of R T s , t h e effect of t h e relation between responses has t o be ascribed t o processes i n advance of t h e response signal. A f t e r t h e response signal, programming begins again, but o n l y a f t e r signal identification. A t t h i s later point i n time, only t h e response ' t h a t is f i n a l l y executed is programmed. Thus, within the to the advance-specification serial-stages framework, alternatives assumption appear h a r d t o f i n d . This state of affairs changes as one t u r n s t o continuous models of According t o these information processing (e.g., McClelland, 1979). models, any information about signal i d e n t i t y is f e d i n t o subsequent stages immediately. Therefore, as long as information about signal i d e n t i t y is ambiguous, programming of both alternative responses is possible, and interactions between t h e simultaneous processes can occur. T h e assumption t h a t t h e phenomenon under s t u d y i s due t o such interactions will be called t h e programming-interactions assumption. From one perspective, t h e difference between t h e advancespecification assumption and t h e programming-interactions assumption is minor. According t o t h e f i r s t assumption, interactions occur d u r i n g preprogramming, and t h e y have t h e effect of limiting t h e level of preprogramming, o r advance-specification, according t o t h e differences between t h e alternative responses. Processing after presentation of t h e response signal then begins a t d i f f e r e n t levels of preparation, and t h i s causes t h e difference between conditions w i t h same and d i f f e r e n t movements assigned t o t h e two hands. According t o the second assumption, interactions occur d u r i n g programming, and they affect the time needed t o finish Viewed i n t h i s way, t h e programming of t h e response finally executed. two assumptions are n o t logical alternatives, because interactions could occur before as well as a f t e r t h e response signal. From t h e . other perspective, the difference between t h e advancespecification assumption and t h e programming-interactions assumption is major, because t h e assumptions are associated w i t h d i f f e r e n t general frameworks of information processing, t h a t is, t h e serial-stages framework and t h e continuous-processing framework, respectively. Strictly speaking, however, t h e continuous-processing framework is compatible It is o n l y the serial-stages framework t h a t is w i t h b o t h hypotheses. incompatible w i t h t h e programming-interactions assumption.

A formalization of t h e programming-interactions assumption. The programming-interactions assumption is attractive, because it explains easily t h e t r i a d o f differences between conditions w i t h same and d i f f e r e n t

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movements assigned t o t h e t w o hands. T o demonstrate t h i s point, t h e assumption has been formalized in a simple model (Heuer, 1987). This model is a s l i g h t modification o f an accumulator model s t u d i e d e x t e n s i v e l y by V i c k e r s (1979). F i g u r e 1 presents a flow diagram t h a t was used for Monte-Carlo simulations.

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F i g u r e 1. Flow diagram o f t h e modified accumulator model o f V i c k e r s (1979). T h e operations framed by b r o k e n lines a r e added ( a f t e r Heuer, 1987). A t each time interval, A t , an observation o f a normally d i s t r i b u t e d random variable, X, is d r a w n . T h e mean, A, o f t h e d i s t r i b u t i o n i s used t o model signal similarity; t h e variance o f t h e d i s t r i b u t i o n is set t o an 1.01, w h i c h amounts t o scaling t h e o t h e r a r b i t r a r y value (e.g., parameters o f t h e model. Each observed value x is compared w i t h a H i g h signal s i m i l a r i t y in an experiment i s mapped c r i t e r i o n value, X C . o n t o a smaller d i f f e r e n c e b e t w e e n r a n d x r . Whenever x i s l a r g e r t h a n XC, it is considered as evidence f o r t h e presentation o f one o f t h e t w o signals, a n d when it is smaller, as evidence f o r t h e o t h e r signal. It should b e noted t h a t t h e model so f a r corresponds t o t h e t h e o r y o f signal detection. T h e main d i f f e r e n c e is t h a t a series o f "observations" is taken, r a t h e r t h a n a single observation, b e f o r e a response is g i v e n .

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Depending on w h e t h e r x i s smaller o r l a r g e r t h a n xc, i t s absolute v a l u e is added t o one o r t h e o t h e r accumulator. As long as t h e threshold, k, is n o t reached, t h e c y c l e of observations continues. ( A l t h o u g h n o t shown, k may b e d i f f e r e n t f o r t h e t w o accumulators.) As soon as t h e t h r e s h o l d k is reached, t h e response i s executed.

So far, t h e model correspoyds t o t h e one described by V i c k e r s (1979). T h e addition t o V i c k e r s model is framed by b r o k e n lines in F i g u r e 1. T h e a d d i t i o n is a v e r y simple cross-coupling o f t h e t w o accumulators. Whenever x is added t o t h e one accumulator, it is s u b t r a c t e d f r o m t h e o t h e r a f t e r multiplication w i t h a constant c. The choice o f t h i s p a r t i c u l a r kind o f c o u p l i n g is based on formal considerations. T h e c o u p l i n g parameter, c, p r o v i d e s a continuous t r a n s i t i o n between accumulator models a n d random-walk models, which a r e t r e a t e d as d i s c r e t e alternatives otherwise (e.g., Heath, 1984). With c = 0, t h e model is an accumulator model, but w i t h c = 1, it is a random-walk model. T h e accumulators become m i r r o r images o f each o t h e r and, thus, c o r r e s p o n d t o a single accumulator f o r w h i c h a lower a n d a h i g h e r t h r e s h o l d a r e defined. T h e concept o f "accumulation" is f a i r l y a b s t r a c t a n d has been g i v e n different interpretations. T h e suggestion h e r e is t o t a k e t h e c u r r e n t state of t h e t w o accumulators as r e p r e s e n t i n g t h e amount o f programming done f o r each o f t h e t w o responses. Thus, it is supposed t h a t a n y e v i dence o n signal i d e n t i t y ( c o r r e s p o n d i n g t o t h e observations o f t h e random v a r i a b l e X ) has immediate consequences f o r programming. In o t h e r words, a v e r y tight l i n k a g e is assumed between perceptual discrimination a n d t h e p r e p a r a t i o n o f a p p r o p r i a t e actions. F i g u r e 2 presents t h e r e s u l t s o f Monte-Carlo simulations. T h e y a r e nicely in l i n e w i t h t h e phenomenon u n d e r discussion. With a l a r g e r coupling parameter, w h i c h corresponds t o conditions w i t h d i f f e r e n t movements assigned t o t h e t w o hands, t h e mean number o f cycles (n) is larger, a n d t h i s corresponds t o a longer mean R T . In addition, t h e s t a n d a r d deviation o f t h e number of cycles, s(n), is larger, which corresponds t o l a r g e r R T v a r i a b i l i t y , a n d choice accuracy is higher, as well. Thus, t h e concept o f programming interactions p r o v i d e s a simple a n d concise i n t e r p r e t a t i o n f o r t h e basic t r i a d o f effects. Contrasting predictions: signal similarity. The advancespecification assumption a n d t h e programming-interactions assumption can b e used t o d e r i v e a few c o n t r a s t i n g p r e d i c t i o n s . With a h i g h e r signal similarity, t h e programming-interactions assumption leads one t o expect a l a r g e r e f f e c t o f t h e relation between responses. T h e reason is that, w i t h a l o n g e r - l a s t i n g a m b i g u i t y w i t h respect t o signal i d e n t i t y , simultaneous programming o f b o t h responses should l a s t longer and/or programming should b e less focused on o n l y one response, so t h a t t h e r e is more room f o r programming interactions t o t a k e place. More detailed p r e d i c t i o n s a r e p r e s e n t e d in F i g u r e 2 , i n w h i c h h i g h e r signal s i m i l a r i t y corresponds t o smaller values of f i . T h e advance-specification assumption, in contrast; p r e d i c t s a d d i t i v e effects of signal s i m i l a r i t y a n d t h e relation between responses. T h e signals in t h e experiment designed t o t e s t t h e c o n t r a s t i n g p r e dictions (Heuer, 1987) were horizontal lines o f 2 cm l e n g t h . Signal simil a r i t y was v a r i e d by ineans o f v a r y i n g t h e distance o f t h e midpoints of

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F i g u r e 2. Predictions of t h e modified accumulator model f o r choice accur a c y (Pc), mean (n), a n d v a r i a b i l i t y ( s ( n ) ) of t h e number o f cycles. Signal s i m i l a r i t y is represented by M a n d relation between responses by c ( a f t e r Heuer, 1987). these lines f r o m t h e imaginary midline o f a 6 x 2 cm d i s p l a y p r o d u c e d by p l a c i n g a mask in f r o n t o f an oscilloscope screen. Signal s i m i l a r i t y was v a r i e d randomly w i t h i n blocks o f t r i a l s , while f o r each b l o c k t h e movements assigned t o t h e t w o hands were constant. T h e r e s u l t s a r e shown in F i g u r e 3 . With h i g h e r signal similarity, t h e e f f e c t o f t h e relation between responses on mean RT increased, as did t h e e f f e c t on choice accuracy. However, f o r R T v a r i a b i l i t y , t h e r e was no s i g n i f i c a n t interact i o n between t h e t w o factors o f i n t e r e s t . T h e reasons f o r t h i s f a i l u r e of t h e factors t o i n t e r a c t o n R T v a r i a b i l i t y a r e n o t fully clear. B u t it i s n o t too bothersome, because t h e t y p e o f model used t o generate t h e p r e d i c t i o n s o f F i g u r e 2 is somewhat u n r e a l i s t i c in i t s s i m p l i f y i n g assumptions, a n d t h e simplification should p r i m a r i l y a f f e c t t h e p r e d k t i o n s f o r R T v a r i a b i l i t y (see Pachella, 1974). in

Contrasting predictions: catch-trial frequency. A second c o n t r a s t t h e p r e d i c t i o n s o f t h e advance-specification assumption a n d t h e

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F i g u r e 3. T h e effects o f signal s i m i l a r i t y a n d relation between responses o n mean R T a n d mean R T v a r i a b i l i t y ( a f t e r Heuer, 1987). programming-interactions assumption r e f e r s t o a situation in w h i c h preprogramming i s impaired o r even completely l a c k i n g . Under t h e programming-interactions assumption, when preprogramming i s reduced a n d programming a f t e r presentation o f t h e response signal becomes more extensive, one would p r e d i c t an increased e f f e c t o f t h e relation between responses because, again, t h e r e is more room f o r programming interactions t o t a k e place. U n d e r t h e advance-specification assumption, in contrast, one would p r e d i c t a reduced e f f e c t o f t h e relation between responses. A t least in t h e extreme case, where preprogramming i s completely prevented, differences t h a t a r e d u e t o differences in t h e amount o f preprogramming should vanish. T w o experiments were conducted in w h i c h an attempt was made t o In t h e f i r s t , a secondary t a s k impair preprogramming (Heuer, 1986a). t h a t immediately preceded each response signal was unsuccessful. This r e s u l t i s in l i n e w i t h r e s u l t s o f Klapp, Greim, a n d M a r s h b u r n (19811, who also f a i l e d t o impair preprogramming by means o f a secondary t a s k . However, a second experimental manipulation a p p a r e n t l y was successful. T h e idea was t o impair preprogramming by means o f r e d u c i n g t h e overall level o f preparation, a n d t h i s impairment was achieved by t h e i n t r o d u c t i o n o f 30% a n d 70% c a t c h t r i a l s f o r w h i c h a w a r n i n g signal was p r e s e n t e d but no response signal. F i g u r e 4. p r e s e n t s t h e i n t e r a c t i v e effects o f c a t c h - t r i a l f r e q u e n c y a n d t h e relation between responses on mean R T a n d R T v a r i a b i l i t y . In addition, an i n t e r a c t i v e e f f e c t on c h o i c e - e r r o r f r e q u e n c y was f o u n d t h a t failed t o reach significance. T h e interactions again correspond t o t h e p r e d i c t i o n s based on t h e programming-interactions assumption. T h e r e is some independent evidence t h a t t h e i n t r o d u c t i o n o f catch t r i a l s indeed s e r v e d t o impair preprogramming. With more catch t r i a l s , T h i s outcome was p r o b a b l y t h e f r e q u e n c y of execution e r r o r s increased. d u e t o a reduced level o f preprogramming a n d c o r r e s p o n d i n g l y more

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extended programming a f t e r presentation o f t h e response signal. This programming h a d t o b e done u n d e r time p r e s s u r e a n d resulted in less accurate movement executions. In addition, in a second experiment r e p o r t e d below, t h e manipulation o f c a t c h - t r i a l f r e q u e n c y again led t o results t h a t were consistent w i t h t h e assumption t h a t a high c a t c h - t r i a l frequency serves t o impair preprogramming.

A T e n t a t i v e Evaluation of t h e Rival Hypotheses Considering t h e explanatory power o f t h e advance-specification assumption a n d t h e programming-interactions assumption w i t h respect t o t h e t r i a d of differences between choice tasks w i t h same a n d d i f f e r e n t movements assigned t o t h e t w o hands, one could b e inclined t o reject t h e advance-specification assumption a n d accept t h e programming-interactions i n t e r p r e t a t i o n as t h e c o r r e c t one. (Of course, programming-interactions do n o t necessarily have t h e simple format o f t h e model o f F i g u r e 2 ) . However, my presentation may have been biased, a n d t h e data may n o t b e s u f f i c i e n t t o decide between t h e rival hypotheses ( c f . Heuer 1988a; Rosenbaum, Barnes, & Slotta, 1988). In a n y case, t h e alternative hypotheses have been examined u s i n g a h i g h l y specific experimental t a s k . As w i l l b e shown below, n o t all differences between t h e movements assigned t o t h e l e f t a n d right hand p r o d u c e t h e t r i a d o f effects t h a t can b e explained so easily in terms o f t h e programming-interactions I am convinced t h a t n e i t h e r t h e advanceassumption. Therefore, specification assumption n o r t h e programming-interactions assumption is Rather, it is an experimental question how t h e effects v a l i d in general. o f t h e relation between t h e responses in a choice t a s k come about in each p a r t i c u l a r instance. T h e statement t h a t t h e mechanism o f t h e e f f e c t o f t h e relation between responses i s an experimental problem t h a t has t o b e studied f o r each specific instance may sound meaningless. However, t h e movementp r e c u i n g methodology rests on t h e advance-specification assumption as a general explanatory principle, a n d t h i s assumption goes untested (except f o r t h e alternatives o f multiple preparation and f i x e d o r d e r o f specificaIt i s f a i r l y tions, which a r e o n l y elaborations o f t h e basic assumption). obvious t h a t a research p r o g r a m t h a t rests on untested assumptions, which may b e right o r w r o n g a t d i f f e r e n t occasions, a n d t h a t draws general inferences t h a t presuppose t h e untested assumptions t o b e correct, w i l l e n d up w i t h a mess o f d a t a a n d c o n t r a d i c t o r y conclusions. T h i s seems t o b e t h e f a t e o f motor-programming research. T h e remedy could b e t o g o back t o t h e roots a n d t o examine t h e fundamental assumptions r a t h e r t h a n t o b u i l d complex s t r u c t u r e s on s h a k y g r o u n d . T h e C o n t i n u i t y Assumption So f a r , R - R compatibility effects in choices between l e f t - h a n d and right-hand responses have been i n t e r p r e t e d as b e i n g t h e r e s u l t o f programming interactions (or limited advance-specification), with interactions related t o t h e relation between t h e responses assigned t o t h e t w o hands. T h e n e x t step i s t o ask what is t h e source o f these interactions, which in t u r n generate t h e observed t r i a d o f effects. The t e n t a t i v e hypothesis t h a t g u i d e d t h e following experiments was t h a t t h e interactions u n d e r l y i n g R - R compatibility effects a r e basically t h e same as those observed when movements w i t h t h e l e f t a n d right hand a r e p e r formed simultaneously. A p r e r e q u i s i t e f o r t h e v a l i d i t y o f t h i s hypothesis is t h e validity o f t h e c o n t i n u i t y assumption, according t o which processes

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b e f o r e a n d a f t e r t h e s t a r t o f a movement a r e n o t q u a l i t a t i v e l y d i f f e r e n t (see K e r r , 1978; N l l t a n e n & Merisalo, 1977) Under the continuity assumption, one would expect that programming o f a c e r t a i n response is i n f l u e n c e d by t h e execution of a d i f f e r e n t response w i t h t h e o t h e r hand. T h i s expectation was examined in t w o experiments (Heuer, 1985). In t h e f i r s t experiment, same a n d d i f f e r e n t movements were assigned t o t h e t w o hands, a n d in t h e second experiment, same a n d d i f f e r e n t f i n g e r s w e r e used. In each t r i a l o f these experiments, b o t h response signals were p r e sented in sequence. T h e f i r s t signal was selected a t random, so t h a t t h e response t o it was a choice response. T h e r e s u l t s obtained f o r t h e f i r s t response replicated t h e t r i a d o f effects f o u n d before. T h a t is, when d i f f e r e n t , r a t h e r t h a n same, movements were assigned t o t h e t w o hands, mean R T was longer, R T v a r i a b i l i t y was l a r g e r , a n d choice accuracy was higher. In contrast, when d i f f e r e n t , r a t h e r t h a n same, f i n g e r s w e r e assigned t o t h e t w o hands, t h e r e was no d i f f e r e n c e between conditions. T h e second signal was p r e s e n t e d a f t e r t h e f i r s t one. I n response t o t h i s signal, t h e subjects h a d t o p e r f o r m a movement w i t h t h e h a n d n o t used f o r t h e f i r s t response. F o u r delays o f t h e second signal were used, t h e sequence o f w h i c h was random. T h e f i r s t t w o delays (Dl, D2) were t r i g g e r e d by t h e s t a r t o f t h e f i r s t movement, a n d t h e o t h e r t w o (D3, D4), by i t s end. D1 was 30 ms when t h e f i r s t response was t a p p i n g a n d 180 ms when it was alternating; D2 was 130 ms a n d 280 ms f o r t a p p i n g a n d a l t e r n a t i n g as t h e f i r s t response, respectively. B y using different delays f o r d i f f e r e n t f i r s t responses, t h e i n t e n t i o n was t o keep t h e temporal o v e r l a p between execution o f t h e f i r s t response a n d R T f o r t h e second response approximately constant across t h e d i f f e r e n t movement times f o r t a p p i n g a n d a l t e r n a t i n g . ( T h i s was n o t fully successful.) Delays D3 a n d D4 were 50 a n d 800 ms a f t e r t h e e n d o f t h e f i r s t movement. Table 3 p r e s e n t s mean R T s f o r t h e second response. R T was longer in conditions w i t h d i f f e r e n t movements assigned t o t h e t w o hands t h a n in conditions w i t h same movements, p r o v i d e d t h a t R T f o r t h e second response temporally overlapped t h e execution o f t h e f i r s t response. T h i s was t h e case i n almost 100% of t h e t r i a l s a t delay D1, but in less t h a n 40% a t D2; t h e r e was, o f course, no temporal o v e r l a p a t D3 a n d D4. In c o n t r a s t t o t h e r e s u l t s o f t h e f i r s t experiment, t h e r e was n o e f f e c t o f w h e t h e r t h e f i r s t a n d second response were p e r f o r m e d w i t h same o r d i f f e r e n t f i n g e r s o f t h e t w o hands. T h e f i n d i n g s show t h a t e x e c u t i n g a c e r t a i n movement i n t e r f e r e s w i t h programming a d i f f e r e n t movement f o r t h e o t h e r hand, b u t n o t w i t h programming a d i f f e r e n t f i n g e r . The f i n d i n g s c l e a r l y a r e evidence i n f a v o r of t h e c o n t i n u i t y hypothesis, a n d t h e y parallel t h e choice-RT r e s u l t s . Table 3 shows t h a t t h e e f f e c t o f t h e relation between t h e movements assigned t o t h e t w o hands declined f r o m D1 t o D2. There are two potential reasons f o r t h i s decline. T h e mean temporal o v e r l a p declined by 100 ms; in addition, a considerable p r o p o r t i o n o f t r i a l s a t 0 2 h a d no temporal o v e r l a p a t all. T h e decline o f temporal o v e r l a p should have been t h e r e l e v a n t f a c t o r if t h e R T increase was indeed d u e t o interactions between e x e c u t i n g one response a n d programming another. However, t h e decline o f temporal o v e r l a p was confounded w i t h t h e increase o f t h e time i n t e r v a l f r o m t h e s t a r t o f t h e f i r s t movement, a n d t h u s t h e delay since

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t h e e n d o f programming t h e f i r s t response. T o reject t h e hypothesis t h a t t h e e f f e c t o f t h e relation between responses is d u e to an a f t e r e f f e c t of programming t h e f i r s t response o n programming t h e second, r a t h e r t h a n t o i n t e r f e r e n c e between e x e c u t i n g t h e f i r s t response a n d programming t h e second, it h a d t o b e shown t h a t temporal overlap, r a t h e r t h a n delay since t h e s t a r t o f t h e movement, was t h e c r i t i c a l f a c t o r f o r t h e increase o f t h e R T f o r t h e second response. Table 3 T h e e f f e c t o f t h e relation between t w o successive responses w i t h t h e l e f t a n d right h a n d on R T o f t h e second response (data f r o m Heuer, 1985).

Delay D1

D2

D3

04

Same

255

228

229

206

Different

27 1

234

23 1

202

Movement .

Same

277

239

236

206

Different

278

242

238

205

Finger ___-.

D1: D2: D3: D4:

_-

-

30 ms a f t e r s t a r t o f tapping, 180 ms a f t e r s t a r t o f a l t e r n a t i n g 130 ms a f t e r s t a r t o f tapping, 280 ms a f t e r s t a r t o f a l t e r n a t i n g 50 ms a f t e r e n d of t h e f i r s t response 800 ms a f t e r e n d o f t h e f i r s t response

T o p r o v e t h a t temporal o v e r l a p was critical, f o r each subject a n d each condition ( t a p p i n g a n d a l t e r n a t i n g f i r s t x t a p p i n g a n d a l t e r n a t i n g second), t h e linear regression o f R T f o r t h e second response on temporal o v e r l a p w i t h t h e f i r s t response was computed f o r delay D1 (temporal o v e r l a p i s p r o p o r t i o n a l t o movement time o f t h e f i r s t response). For these trials, t h e delay since t h e s t a r t o f t h e f i r s t response was constant, a n d if t h i s were t h e c r i t i c a l factor, t h e regression coefficients should b e zero. T h e means were 0.54 f o r t h e two ,conditions w i t h same movements assigned t o t h e t w o hands a n d 0.66 f o r the t w o conditions w i t h d i f f e r e n t movements. Thus, t h e increase o f R T p e r millisecond o f temporal o v e r l a p was l a r g e r when d i f f e r e n t , r a t h e r t h a n same, movements were assigned t o t h e t w o hands .. Correlations t h a t a r e computed across a series of t r i a l s a r e s p u r i o u s l y i n f l a t e d by f l u c t u a t i o n s of t h e general performance level. These f l u c t u a t i o n s p r o d u c e common t r e n d s in t h e correlated variables. T o c o n t r o l f o r such t r e n d s , t h e regression of t h e second R T on movement time o f t h e f i r s t response was also con\pclted f o r D3, a t w h i c h t h e r e was no temporal o v e r l a p . Because d e l a y s . were randomized w i t h i n blocks of t r i a l s , common temporal t r e n d s should a f f e c t regression coefficients a t D1 a n d D3 in t h e same manner. B u t a t D1, t h e r e was t h e e f f e c t o f temporal

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overlap, in addition. A t D3, t h e regression coefficients were 0.33 a n d 0.28 f o r conditions w i t h same a n d d i f f e r e n t movements assigned t o t h e t w o hands, respectively. T h e d i f f e r e n c e between regression coefficients f o r conditions w i t h same a n d d i f f e r e n t movements was s i g n i f i c a n t l y l a r g e r w i t h Thus, temporal o v e r l a p (0.54 v s . 0.66) t h a n w i t h o u t (0.33 v s . 0.28). t h e increase o f t h e second R T when d i f f e r e n t movements w e r e assigned t o t h e t w o hands can b e a t t r i b u t e d t o t h e temporal o v e r l a p w i t h t h e execution o f t h e f i r s t response. Inte:-manual I n t e r a c t i o n s During Programming Given t h e hypothesis t h a t R-R compatibility effects in choices between l e f t - h a n d a n d r i g h t - h a n d responses a r e caused by t h e same intermanual interactions t h a t can b e evidenced in simultaneous movements o f t h e t w o hands, a n d g i v e n t h e s u p p o r t f o r t h e c o n t i n u i t y hypothesis, it seemed reasonable t o compare t h e R-R compatibility effects in more detail What was w i t h intermanual interactions f o u n d in o t h e r k i n d s o f t a s k s . expected in p r i n c i p l e were parallel results, a n d t h e rationale u n d e r l y i n g t h i s expectation was q u i t e simple. Suppose t h a t t h e movements assigned t o t h e t w o hands in t h e With respect to t h i s choice t a s k d i f f e r in a c e r t a i n c h a r a c t e r i s t i c . If t h e y d o characteristic, intermanual interactions may o r may n o t e x i s t . exist, simultaneous processes o f programming o r preprogramming t h e movements should i n t e r f e r e w i t h each other, as d o simultaneous executions. A t t h i s point, it is presupposed t h a t interactions a r e essentially t h e same during programming a n d execution. However, t h e question o f w h e t h e r i n t e r f e r e n c e is between simultaneous processes o f p r e - p r o g rammi ng (advance- s pecif ication assumption) or p r o g ramming (programming-interactions assumption) is n o t p a r t i c u l a r l y r e l e v a n t f o r t h e i n t e r p r e t a t i o n o f t h e choice-RT r e s u l t s w i t h respect t o intermanual interactions.

If t h e movements assigned t o t h e t w o hands d i f f e r in a c h a r a c t e r i s t i c t h a t is independent w i t h respect t o t h e hands, simultaneous programming o f b o t h responses should b e possible. Thus, whenever a d i f f e r e n c e between t h e movements assigned t o t h e t w o hands r e s u l t s in an increase o f RT, intermanual c o u p l i n g w i t h r e s p e c t t o t h i s c h a r a c t e r i s t i c can b e i n f e r r e d . B u t when no R T increase is observed, it can b e concluded t h a t t h e t w o hands a r e independent w i t h respect t o t h a t c h a r a c t e r i s t i c on w h i c h t h e t w o choice responses d i f f e r . A l t h o u g h t h e rationale t h a t underlies t h e i n t e r p r e t a t i o n o f t h e choice-RT experiments w i t h respect t o intermanual interactions i s f a i r l y simple, it is n o t necessarily c o r r e c t . T h e rationale can b e t e s t e d by comparing choice-RT r e s u l t s w i t h those f r o m studies o f simultaneous movements. However, p e r f e c t convergence cannot b e expected f o r f a i r l y obvious reasons. Intermanual interactions can, in principle, o r i g i n a t e a t various levels o f t h e c e n t r a l n e r v o u s system (see M a r t e n i u k & MacKenzie, 1980). F o r example, mutual i n h i b i t i o n between contralateral limbs ( T a u b & Berman, 1968) o r identical e f f e r e n t commands v i a crossed a n d uncrossed f i b r e s (Preilowski, 1975) m i g h t p l a y a role. Sometimes even mechanical interactions m i g h t arise. Such lower-level interactions a r e mainly d i s t u r b i n g f a c t o r s if t h e concern is w i t h interactions among c e n t r a l processes o f motor c o n t r o l . As f a r as R-R compatibility effects a r e concerned, o n l y h i g h e r - l e v e l interactions a r e r e l e v a n t . Therefore, in t h e s t u d y of simultaneous movements, intermanual i n t e r a c t i o n s m i g h t b e f o u n d

328

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t h a t have no c o u n t e r p a r t in choice-RT experiments. am not aware o f such instances.

So f a r , however, I

C o u p l i n g with Respect to Temporal Patterns T h e r e i s no need to d o an experiment t o p r o v e t h a t t a p p i n g and a l t e r n a t i n g a r e hard t o p e r f o r m simultaneously w i t h t h e t w o hands; t h e In d i f f i c u l t y i s obvious, a n d t h i s i s in l i n e w i t h t h e choice-RT results. addition t o these data, r e s u l t s obtained w i t h a "selective adaptation" p r o cedure proposed by Rosenbaum (1977) indicate t h a t t h e d i f f i c u l t y i s d u e The varit o high-level interactions r a t h e r than low-level interactions. a b i l i t y of t a p p i n g responses performed continuously f o r 30 s is enhanced a f t e r 30 s o f t a p p i n g w i t h t h e o t h e r h a n d as compared t o a preceding 30s p e r i o d o f a l t e r n a t i n g (Heuer, 1980, 1981). T h i s negative t r a n s f e r was taken t o indicate t h a t s t r u c t u r e s common t o t h e t w o hands a r e i n v o l v e d in t h e c o n t r o l o f t h e movements. T a p p i n g a n d a l t e r n a t i n g d i f f e r in a somewhat complex way, and it is n o t clear where, w i t h respect t o t h e characteristics o n which t h e y differ, intermanual interactions occur. Most obviously, t h e t w o movements have d i f f e r e n t trajectories and, when t h e y a r e performed as r a p i d l y as possible, d i f f e r e n t movement times. According t o Zelaznik e t al. (19821, a d i f f e r e n c e in d u r a t i o n o f k e y presses performed by t h e l e f t o r right h a n d i s s u f f i c i e n t t o increase R T . Thus, it could b e t h a t t h e d i f f e r e n t movement times o f t a p p i n g a n d a l t e r n a t i n g were t h e c r i t i c a l difference, while t h e d i f f e r e n c e w i t h respect t o trajectories would b e i r r e l e v a n t f o r t h e results. T h i s p o s s i b i l i t y was examined in a n experiment in which subjects were i n s t r u c t e d a n d t r a i n e d t o p e r f o r m t h e movements w i t h c e r t a i n d u r a tions (Heuer, 1984b). In one p a r t o f t h e experiment, t a p p i n g movements were used, a n d subjects were t r a i n e d t o p e r f o r m them in 200 ms a n d 400 ms. In d i f f e r e n t conditions, movements w i t h same o r d i f f e r e n t durations were assigned t o t h e t w o hands. It t u r n e d o u t t h a t .a difference in d u r a t i o n was s u f f i c i e n t t o produce t h e t r i a d o f effects, i.e., a longer mean RT, l a r g e r R T v a r i a b i l i t y , and h i g h e r choice accuracy. In t h e second p a r t o f t h e experiment, t a p p i n g and a l t e r n a t i n g were used as responses, a n d t h e subjects were t r a i n e d t o p e r f o r m these movements w i t h 400-ms d u r a t i o n . Here t h e d i f f e r e n c e in t h e t r a j e c t o r y o f t h e movements t u r n e d o u t t o b e s u f f i c i e n t t o produce t h e t r i a d o f effects, even when t h e d u r a t i o n o f t h e movements assigned t o t h e l e f t a n d right hand was t h e same. T h e f a c t t h a t differences w i t h respect t o d u r a t i o n and t r a j e c t o r y of t h e movements assigned t o t h e l e f t a n d right hands produced t h e same t r i a d o f effects suggested t h a t b o t h k i n d s o f differences might have something in common. A closer look a t t h e data revealed t h a t in b o t h p a r t s o f t h e experiment r e l a t i v e t i m i n g ( o r temporal p a t t e r n ) was same o r T h e r e is p l e n t y o f d i f f e r e n t in t h e d i f f e r e n t experimental conditions. evidence that, in general, d i f f e r e n t forms ( o r d i f f e r e n t spatial characteristics) o f movements a r e associated w i t h d i f f e r e n t temporal p a t t e r n s For t h e p a r t i c u l a r movements (e.g., Derwort, 1938; Viviani, 1986). used--tapping a n d a l t e r n a t i n g - - t h e d i f f e r e n t temporal p a t t e r n s are It is less obvious, however, t h a t t h e manipulation o f t h e obvious. d u r a t i o n of t a p p i n g response was also accompanied by a d i f f e r e n c e in r e l a t i v e timing. B u t it has often been observed t h a t a n increase o f total d u r a t i o n affects d i f f e r e n t segments o f a movement d i f f e r e n t i a l l y (see

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Roth, 1987). I n t h e p a r t i c u l a r experiment, an increase o f movement d u r a t i o n was achieved mainly by way o f i n c r e a s i n g t h e d u r a t i o n o f t h e "reversal time," t h a t is, t h e time t h e f i n g e r was i n t h e upmost position. T h e conclusion f r o m these considerations is t h a t h i g h - l e v e l i n t e r a c t i o n s e x i s t w i t h respect t o temporal p a t t e r n s ( o r r e l a t i v e t i m i n g ) . This conclusion seems t o b e consistent w i t h observations on simultaneous movements o f t h e t w o hands. The difficulties i n performing two different trajectories (e.g., a square a n d a c i r c l e ) simultaneously w i t h t h e t w o hands a r e obvious. Also, it is d i f f i c u l t t o p e r f o r m t w o d i f f e r e n t r h y t h m s simultaneously (e.g., Klapp, 1979). Less clear, however, is what exceptions t h e r e a r e t o t h i s rule, a n d w h a t makes these exceptions. For example, a 2 : l r h y t h m is easy, a l t h o u g h t h e r e l a t i v e t i m i n g i s o b v i o u s l y different. Certainly, intermanual interactions a r e more complicated t h a n expressed by t h e simple r u l e t h a t o n l y a common temporal p a t t e r n can b e applied t o simultaneous movements o f t h e t w o hands a n d o n l y one temporal p a t t e r n can b e programmed a t a time. However, t h i s simple r u l e represents a f i r s t approximation. Aimed Movements: Temporal C o u p l i n g a n d Force Independence T h e r e is some indication t h a t variation o f t h e t o t a l d u r a t i o n o f r a p i d aimed movements m i g h t n o t b e accompanied by so s t r o n g a v a r i a t i o n o f r e l a t i v e t i m i n g as was t h e v a r i a t i o n of t o t a l d u r a t i o n of t a p p i n g (e.g., Schmidtke, 1960), although r e l a t i v e t i m i n g is c e r t a i n l y n o t p e r f e c t l y i n v a r i a n t (see Schmidt, Sherwood, Zelaznik, & Leikind, 1985; Zelaznik, Schmidt & Gielen, 1986; a re-evaluation of c e r t a i n f i n d i n g s w h i c h a p p a r e n t l y a r e inconsistent w i t h i n v a r i a n t r e l a t i v e t i m i n g is p r e s e n t e d by Heuer, 1 9 8 8 ~ ) . I n a n y case, aimed movements o f v a r y i n g d u r a t i o n s a n d amplitudes appeared t o p r o v i d e b e t t e r o p p o r t u n i t i e s t o examine w h e t h e r t h e choice-RT paradigm a n d t h e simultaneous-movements paradigm i n f a c t lead t o c o n v e r g e n t conclusions. T h e r e a r e several data on simultaneous aimed movements w i t h t h e l e f t a n d right hands, but a p p a r e n t l y few have studied tapping and alternating. Studies o f simultaneous aimed movements w i t h t h e l e f t a n d right hands consistently indicate t h a t intermanual interactions do e x i s t w i t h respect t o movement duration, but n o t w i t h respect t o amplitude (which, f o r a g i v e n duration, depends on t h e accelerative a n d decelerative forces). F o r example, Kelso, Southard, a n d Goodman (1979a, b), a n d Kelso, Putnam, a n d Goodman (1983) f o u n d t h a t t h e d u r a t i o n s o f aimed movements w i t h d i f f e r e n t amplitudes a n d accuracy requirements became n e a r l y identical when t h e y were performed simultaneously. ( I n these experiments, t h e r e was no p a r t i c u l a r i n s t r u c t i o n t o keep d u r a t i o n s different.) A l t h o u g h movement times o f t h e t w o hands were n o t e x a c t l y t h e same (Corcos, 1984; M a r t e n i u k & MacKenzie, 1980; Marteniuk, MacKenzie & Baba, 1984), t h e remaining differences were small as compared t o t h e assimilation. Amplitudes ( a n d forces), o n t h e o t h e r hand, did n o t become similar; peak forces actually became more d i f f e r e n t . ( T h i s compensates t h e e f f e c t o f t h e assimilation o f d u r a t i o n s o n amplitude.) U s i n g a t a s k i n w h i c h subjects h a d t o p e r f o r m identical aimed movements w i t h t h e t w o hands, Schmidt, Zelaznik, Hawkins, Frank, a n d Quinn (1979) o b s e r v e d a h i g h correlation between t h e movement times o f t h e t w o hands but a negligible correlation between t h e amplitudes. Given the results on simultaneous aimed movements, the expectations f o r t h e choice t a s k a r e clearcut: A d i f f e r e n c e w i t h respect t o

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d u r a t i o n should increase RT, but a d i f f e r e n c e w i t h respect t o amplitude should not. T h i s hypothesis was examined in t w o experiments t h a t d i f f e r e d in o n l y minor ways (Heuer, 198613). In t h e f i r s t experiment, t a r g e t movement time was determined f o r each subject individually; f o r a g i v e n amplitude, t h e t a r g e t w i d t h was varied, a n d t h e movement time observed in a p r e t e s t was used as t h e target-movement time in t h e main p a r t of t h e experiment. In t h e second experiment, target-movement times were t h e same f o r all subjects. Table 4 presents t h e p r i m a r y f i n d i n g s o f t h e two experiments. When t h e movements assigned t o t h e t w o hands h a d d i f f e r e n t amplitudes, R T was increased by 6 ms a n d 3 ms in t h e t w o experiments. These differences were n o t significant. When t h e movements d i f f e r e d in movement time ( t h i s condition was used o n l y in t h e second experiment), R T increased by 9 ms, a n d t h i s was significant. Finally, when t h e movements d i f f e r e d w i t h respect t o b o t h d u r a t i o n a n d amplitude, R T increased by 10 ms a n d 7 ms in t h e t w o experiments, respectively. These d i f f e r ences, although small, were again significant. Thus, t h e results a r e consistent w i t h t h e expectations based on t h e results f r o m studies o f simultaneous movements.

Table 4 T h e e f f e c t o f t h e relation between amplitudes a n d durations movements on mean R T (in ms, a f t e r Heuer, 1986b).

Amplitude Same D i f f e r e n t Exp. 1

276

282

Exp. 2

263

266

Note.

Duration Same D i f f e r e n t

259

268

Ampl. Same

o f aimed

Durat. Different

+

262

272

259

266

When amplitude was varied, amplitudes were 25 a n d 50 mm; in E.xp. 1 mean t a r g e t d u r a t i o n was 303 ms, in E x p . 2 t a r g e t d u r a t i o n was 350 ms. When t a r g e t d u r a t i o n was varied, t a r g e t d u r a t i o n s were 350 and 230 ms, amplitude was 25 mm. When b o t h amplitudes a n d t a r g e t d u r a t i o n s were varied, amplitudes were 25 a n d 50 mm; in E x p . 1 mean t a r g e t durations were 226 a n d 369 ms, in E x p . 2 t a r g e t durations were 350 a n d 230 ms.

A l t h o u g h t h e r e s u l t s o f t h e t w o experiments w i t h aimed movements lead t o conclusions which converge w i t h conclusions based o n t h e s t u d y o f simultaneous movements, t h e r e a r e a t least t w o d i s t u r b i n g aspects. First, t h e R-R compatibility effects were v e r y small. Although the effects o f t h e relation between responses were small in all experiments o f t h e series, values below 10 ms were unusual.

Intermanual Interactions

33 1

So far, t h e rationale o f t h e choice-RT experiments has been discussed as if intermanual interactions were a matter o f existence vs. non-existence. T h i s is a conceptual simplification t h a t should b e considered as an a r b i t r a r y dichotomization o f a continuous variable. lntermanual interactions a r e a m a t t e r o f degree, a n d t h e size o f R-R compatibility e f f e c t s m i g h t well r e f l e c t t h e s t r e n g t h o f intermanual interactions. I n studies o f bimanual movements, assimilation o f d u r a t i o n s was less t h a n perfect; it appears n o t t o b e v e r y much h a r d e r t o p e r f o r m aimed movements o f d i f f e r e n t d u r a t i o n s simultaneously t h a n movements o f t h e same d u r a t i o n . Thus, t h e intermanual c o u p l i n g w i t h respect t o d u r a t i o n is p r o b a b l y weaker t h a n w i t h respect t o r e l a t i v e timing. Also, f o r bimanual movements o f d i f f e r e n t amplitudes, a small assimilation c o u l d b e o b s e r v e d (Marteniuk & MacKenzie, 1980; M a r t e n i u k e t al., 1984). Thus, a v e r y weak intermanual c o u p l i n g m i g h t also e x i s t w i t h respect t o t h i s movement c h a r a c t e r i s t i c . However, it is so weak t h a t t h e c o r r e s p o n d i n g R-R compatibility e f f e c t did n o t reach significance. T h e second d i s t u r b i n g aspect relates t o t h e r e s u l t s o n R T v a r i a b i l i t y a n d choice accuracy. I n c o n t r a s t t o t h e experiments in w h i c h t a p p i n g a n d a l t e r n a t i n g were used, t h e d i f f e r e n c e between conditions was limited t o mean RT, a n d t h e r e were n o consistent differences in R T v a r i a b i l i t y a n d choice accuracy. Thus, t h e r e s u l t s o f these experiments l e n d themselves n i c e l y t o an i n t e r p r e t a t i o n in terms o f t h e advancespecification assumption, but n o t t o an i n t e r p r e t a t i o n in terms o f t h e programming-interactions assumption. T h i s l a t t e r assumption p r e d i c t s t h e full t r i a d of effects. As mentioned above, it i s l i k e l y t h a t t h e question of w h e t h e r t h e one o r t h e o t h e r o f these t w o assumptions is c o r r e c t cannot b e answered in general. Rather t h e v a l i d i t y o f them depends on t h e p a r t i c u l a r characteristics o n w h i c h t h e choice responses d i f f e r . T h e r e may b e several reasons f o r t h e d i f f e r e n t outcomes o f choiceR T experiments w i t h r e g a r d t o R T v a r i a b i l i t y a n d choice accuracy. For example, t a p p i n g a n d a l t e r n a t i n g were f i n g e r movements, whereas t h e aimed movements were mainly extensions o f t h e w r i s t . One o f t h e more i n t e r e s t i n g differences between t h e experiments i s t h a t t a p p i n g a n d a l t e r n a t i n g appear q u a l i t a t i v e l y d i f f e r e n t a n d a r e p r o b a b l y c o n t r o l l e d by d i f f e r e n t generalized motor programs. However, aimed movements o f d i f f e r e n t d u r a t i o n a r e p r o b a b l y u n d e r c o n t r o l o f t h e same generalized motor p r o g r a m w i t h d i f f e r e n t parameter values. T h u s , differences with respect t o motor programs a n d differences w i t h respect t o parameter values may p r o d u c e R-R compatibility effects i n s l i g h t l y d i f f e r e n t ways. However, these considerations a r e n o t p a r t i c u l a r l y r e l e v a n t f o r t h e general convergence o f conclusions d e r i v e d f r o m d i f f e r e n t experimental paradigms. T h e Problem of Homologous Coupling T h e r e is one r e s u l t f r o m t h e choice-RT experiments t h a t appears n o t t o b e consistent w i t h o t h e r f i n d i n g s o n intermanual interactions. It has repeatedly been f o u n d t h a t a d i f f e r e n c e between t h e f i n g e r s assigned t o t h e t w o hands does n o t p r o d u c e an R T increase (Heuer, 1982a, b, c; Rosenbaum & ! b r n b l u m , 1982). In contrast, o t h e r evidence indicates consistently a homologous c o u p l i n g . ” Rabbitt, Vyas a n d Fearnley (1975) f o u n d f a s t e r R T s f o r simultaneous responses w i t h homologous f i n g e r s o f t h e t w o hands t h a n f o r responses w i t h non-homologous f i n g e r s . Wakelin (19761, in an experiment o n t h e psychological r e f r a c t o r y period, f o u n d f a s t e r R T s when t h e t w o successive responses (choices between index a n d

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middle finger of the l e f t and right hand, respectively) were performed w i t h homologous fingers. MacKay and Soderberg (1971), i n a task f o r which subjects had t o tap sequentially with t h e fingers of both hands i n a sequence f r o m l e f t t o right o r r i g h t t o left, found simultaneous responses w i t h homologous fingers t o be t h e most frequent e r r o r s . Rabbitt and Vyas (1970) reported t h a t erroneous responses i n a serial RT task were more frequently performed w i t h a f i n g e r t h a t was homologous t o t h e one used f o r t h e preceding correct response than w i t h a nonhomologous f i n g e r . Finally, negative intermanual transfer could be observed f o r homologous fingers as compared t o non-homologous ones, using Rosenbaum's (1977) method of selective adaption (Heuer, 1980,

1981). Why is it t h a t the conclusions from choice-RT experiments and various other kinds of experiments do not appear t o converge as f a r as homologous coupling is concerned? There seems t o be a simple reason t h a t becomes most obvious when a t a s k employed by Cohen (1971) is examined. The task is simply t o oscillate w i t h both hands around the wrist. Cohen (1971) f o u n d higher cross-correlations between symmetric oscillations of the two hands than between asymmetric oscillations. In addition, he observed t h a t subjects sometimes shifted from asymmetric It is movements t o symmetric ones, but never i n t h e reverse direction. easy t o demonstrate t h a t asymmetric movements do not pose any problem as long as they are performed slowly. However, when t h e y are speeded According t o up, t h e s h i f t t o symmetric movements i s h a r d t o avoid. This Kelso (1984), t h e s h i f t occurs at a particular frequency. phenomenon has received extensive modeling i n terms of coupled oscillators (e.g., Haken, Kelso, & Bunz, 1985; Kay, Kelso, Saltzman, & Schoner, 1987; Schoner, Haken, & Kelso, 1986; a more elementary treatment is given by Kelso & Kay, 1987). T h e f i n d i n g t h a t homologous coupling shows u p w i t h "high-speed movements" b u t not w i t h slow movements can be taken t o suggest t h a t it i s a transient phenomenon. More specifically, it appears t o exist only d u r i n g f i n g e r selection (or selection of a certain muscle group), b u t no longer a f t e r t h i s has been accomplished. Therefore, homologous coupling will be hidden from observation when the muscles are activated a sufficient time a f t e r selection has taken place. B u t it will become visible when activation occurs while selection i s not y e t finished. T h i s i s the I n choice-RT case in rapid movement sequences b u t not i n slow ones. experiments, i n which subjects know well i n advance of the response signal which fingers t o use f o r left-hand and right-hand responses, there is thus no reason t o expect any indication of intermanual interactions.

If t h e hypothesis i s correct t h a t homologous coupling i s a transient phenomenon t h a t is bound t o f i n g e r specification, it should be possible t o modify t h e design of choice-RT experiments i n a way t h a t RT becomes longer when d i f f e r e n t fingers are assigned to the two hands rather than In principle, t h i s could be achieved t h r o u g h conditions same fingers. where f i n g e r specification is not y e t finished when t h e response signal is presented. Two attempts were made t o establish such conditions (Heuer, 1986~). T h e most obvious way t o present a response signal when f i n g e r selection is not y e t finished is t o limit t h e time available f o r finger specification. This was done i n a n experiment i n which d i f f e r e n t precuing intervals were used. The method was h i g h l y similar t o t h e procedure

Intermanual Interactions

333

i n t r o d u c e d by Miller (1982). I n d e x a n d middle f i n g e r s o f b o t h hands rested on response k e y s . T h e p r e c u e consisted o f t w o LEDs t h a t were switched o n 500 ms a f t e r t h e e n d o f an a u d i t o r y w a r n i n g signal. The response signal was an LED located d i r e c t l y beneath one o r t h e o t h e r o f t h e t w o p r e c u i n g lights, a n d t h e p r e c u i n g i n t e r v a l s ( t h e delays between t h e onsets o f p r e c u e a n d response signal) were 0, 250, 800, a n d 2,000 rns in d i f f e r e n t blocks o f t r i a l s . With a v e r y b r i e f p r e c u i n g i n t e r v a l , t h e t a s k is essentially a f o u r choice t a s k . T h e r e should b e no e f f e c t o f w h e t h e r t h e precues d e f i n e a choice between same o r d i f f e r e n t f i n g e r s o f t h e t w o hands. Similarly, w i t h a long p r e c u i n g i n t e r v a l , t h e r e should b e p l e n t y o f time t o f i n i s h f i n g e r specification b e f o r e presentation o f t h e response signal. Again, no e f f e c t o f t h e relation between responses is expected. However, f o r some intermediate p r e c u i n g intervals, where f i n g e r specification has begun, but is n o t y e t finished, one would p r e d i c t longer R T when d i f f e r e n t f i n g e r s a r e assigned t o t h e t w o hands r a t h e r t h a n same f i n g e r s . Mean R T a n d R T v a r i a b i l i t y a r e p r e s e n t e d i n F i g u r e 5. (Mean R T is also shown f o r choices between f i n g e r s o f t h e same hand; these were slower t h a n choices between d i f f e r e n t hands, c o n t r a r y t o t h e r e s u l t s obtained in similar experiments by Miller, 1982, a n d Reeve a n d Proctor, 1984). T h e interaction between p r e c u i n g i n t e r v a l a n d conditions w i t h same a n d d i f f e r e n t f i n g e r s assigned t o t h e t w o hands was s i g n i f i c a n t f o r b o t h mean R T a n d R T v a r i a b i l i t y . Post hoc tests indicated t h a t a t t h e delay o f 250 ms, R T was longer a n d v a r i a b i l i t y was l a r g e r in d i f f e r e n t f i n g e r conditions, but n o t a t t h e o t h e r delays. Thus, a t t h i s delay R T appears t o r e f l e c t intermanual interactions w i t h respect t o t h e n o t y e t f i n i s h e d f i n g e r specification. ( T h e l a r g e r e v e r s e d d i f f e r e n c e a t t h e delay o f 800 ms f o r R T v a r i a b i l i t y is mainly d u e t o a single subject who h a d extreme v a r i a b i l i t y in one o f t h e choice c o n d i t i o n s . )

A second a n d less obvious way t o p r e s e n t a response signal when f i n g e r specification is n o t y e t completed is again t o use h i g h c a t c h - t r i a l frequencies. These should impair preprogramming, as in t h e experiment r e p o r t e d above. I n c o n t r a s t t o t h e experiment w i t h variable p r e c u i n g delays, subjects in t h i s experiment knew well in advance o f t h e response signal which f i n g e r of each h a n d t o use, because t h e f i n g e r s assigned t o t h e t w o hands were constant f o r each block o f t r i a l s . However, a high c a t c h - t r i a l f r e q u e n c y should discourage them f r o m u s i n g t h e i r knowledge f o r t h e a p p r o p r i a t e motor p r e p a r a t i o n . T h e r e s u l t s o f t h i s experiment a r e shown in F i g u r e 6 . Corresponding t o expectations, t h e d i f f e r e n c e between conditions w i t h same a n d d i f f e r e n t f i n g e r s assigned t o t h e t w o hands increased w i t h a h i g h e r catch-trial frequency. T h i s was t r u e f o r mean R T as well as f o r R T variability. T h e r e was also a weak t e n d e n c y o f t h e d i f f e r e n c e in choicee r r o r f r e q u e n c y t o increase when more catch t r i a l s were presented. T h e o n l y d i s t u r b i n g finding of t h i s experiment was t h a t t h e d i f f e r ence between conditions was also o b s e r v e d w i t h 0% catch t r i a l s , w h i c h was c o n t r a r y t o t h e r e s u l t s o f p r e v i o u s experiments. T h e r e a r e t w o potential reasons f o r t h i s . F i r s t , t h e effects o f catch t r i a l s c o u l d have spread across t h e whole experiment; all subjects h a d experienced t h e 70% catch trials d u r i n g t h e practice period. Second, advance specification o f t h e

334

H. Heuer

mean RT 370

w w i t h i n hands M same

fingers

&-a d i f f e r e n t f i n g e r s

0

250

800

2000

Precuing i n t e r v a l Imsl

F i g u r e 5. T h e e f f e c t o f t h e relation between responses (same a n d d i f f e r e n t f i n g e r s ) o n mean R T .and mean R T v a r i a b i l i t y f o r d i f f e r e n t p r e c u i n g i n t e r v a l s ( a f t e r Heuer, 1 9 8 6 ~ ) . f i n g e r s may have been less p e r f e c t t h a n in t h e o t h e r experiments because h e r e b o t h f i n g e r s r e s t e d on k e y s a n d could b e used, while i n t h e e a r l i e r experiments it was o n l y one f i n g e r o f each h a n d . Thus, f i n g e r specification was in some sense enforced by t h e experimental setup. No matter w h i c h o f these t w o reasons is correct, t h e y a r e b o t h consistent w i t h t h e view t h a t homologous c o u p l i n g can b e evidenced f r o m choice-RT experiments as l o n g as advance specification o f f i n g e r s assigned t o t h e t w o hands i s less t h a n p e r f e c t a t t h e time t h e response signal i s p r e sented. Conclusions T h e p u r p o s e o f t h e series o f experiments r e p o r t e d i n t h i s c h a p t e r was t o examine s u b t l e R-R compatibility effects t h a t can b e observed in t a s k s in w h i c h r a p i d responses a r e t o b e performed w i t h e i t h e r t h e l e f t o r right hand. These effects come about t h r o u g h intermanual interactions during programming o r preprogramming, i.e., t h r o u g h intermanual i n t e r actions t h a t a r e p r e s e n t in t h e p r e p a r a t o r y p e r i o d . T h i s view is supported, f i r s t , by s u p p o r t i v e evidence f o r t h e c o n t i n u i t y assumption, a n d second, by a general convergence o f R-R compatibility effects w i t h r e s u l t s o n intermanual interactions obtained w i t h o t h e r k i n d s o f tasks.

Intermanual Interactions

335

mean RT

360

350 3 LO

t

M

P

same f i n g e r s

o--a d i f f e r e n t

fingers /

/’

/

/

300 S.D. ( R T 1

29

0%

30%

70%

R e l a t i v e catch- trial f r e q u e n c y

F i g u r e 6. T h e e f f e c t of t h e relation between responses (same a n d d i f f e r e n t f i n g e r s ) on mean R T a n d mean RT v a r i a b i l i t y f o r d i f f e r e n t c a t c h - t r i a l frequencies ( a f t e r Heuer, 1 9 8 6 ~ )

Compatibility effects o f t e n have o n l y l i t t l e theoretical foundation, but can b e p r e d i c t e d w i t h r a t h e r high accuracy by i n t u i t i v e reasoning. However, t h e effects r e p o r t e d h e r e would n o t have been p r e d i c t e d on an i n t u i t i v e basis, but t h e y can b e p r e d i c t e d on a more t h e o r e t i c a l l y - o r i e n t e d basis, namely as a r e s u l t o f t h e s t r u c t u r a l limitations imposed on t h e independent use o f o u r t w o hands. I n c o n t r a s t t o many o t h e r coinpatibility effects, t h e ones r e p o r t e d in t h i s c h a p t e r a r e f a i r l y small. T h e r e fore, t h e i r p r a c t i c a l importance, f o r example, in d u a l - t a s k situations, is limited. However, t h e y may b e of importance as a means to study h i g h e r - l e v e l intermanual interactions a n d as a methodological a d d i t i o n t o this field of inquiry.

336

H. Heuer

T h e s u p p o r t o f t h e view t h a t R-R compatibility effects in choice between l e f t - h a n d a n d r i g h t - h a n d responses have t h e same o r i g i n as intermanual interactions during simultaneous responses seems t o j u s t i f y t w o r a t h e r general conclusions. F i r s t , intermanual interactions o r i g i n a t e a t a c e n t r a l level o f c o n t r o l a n d a r e n o t mediated by c r o s s t a l k o n t h e spinal level, by ipsilateral f i b r e s o f t h e pyramidal t r a c t , o r by o t h e r lowlevel sources. T h i s generalization has t o b e q u a l i f i e d somewhat. It is n o t clear how f a r it can b e extended t o phenomena o f intermanual c o u p l i n g t h a t have n o t been s t u d i e d by means o f t h e choice-RT method. F u r t h e r , if one i s s t r i c t , t h e conclusion cannot b e t h a t low-level i n t e r a c tions a r e b e h a v i o r a l l y i r r e l e v a n t . T h e demonstration o f intermanual i n t e r actions w i t h o u t low-level c o n t r i b u t i o n s j u s t i f i e s o n l y t h e conclusion t h a t t h e r e a r e c e n t r a l sources, but n o t t h a t t h e r e is no c o n t r i b u t i o n o f lowlevel interactions a t all when movements a r e performed simultaneously. T h e second conclusion is t h a t intermanual interactions a r e n o t b o u n d t o processes t h a t a r e i n v o l v e d in t h e c e n t r a l c o n t r o l o f t h e ongoing movement. Rather t h e y can b e o b s e r v e d already d u r i n g p r e p a r a t i o n o f a movement. From this, it follows t h a t processes d u r i n g p r e p a r a t i o n a n d zxecution o f a movement a r e continuous a t least w i t h respect t o t h e i n t e r f e r e n c e criterion," i .e., w i t h respect t o t h e p a t t e r n s o f i n t e r f e r e n c e t h a t t h e y p r o d u c e if combined w i t h o t h e r simultaneous processes. The p h y s i c a l s t a r t o f t h e movement, therefore, is n o t a h i g h l y d i s t i n g u i s h e d p o i n t in time as f a r as motor c o n t r o l is concerned. Rather, it is d e f i n e d o n l y by r a t h e r a r b i t r a r y c r i t e r i a ( e . g . , thresholds f o r electromyographic a c t i v i t y , force, o r displacement). T h e convergence between f i n d i n g s f r o m choice-RT a n d o t h e r k i n d s o f experiments m i g h t also b e important f o r r a t h e r formal reasons t h a t have n o t h i n g t o d o w i t h t h e p a r t i c u l a r problem studied. Motorprogramming research sometimes appears to five in something l i k e a scientific n i c h e w h i c h is d e f i n e d by R T experiments. T h e danger that arises when problems a r e s t u d i e d u s i n g o n l y a single experimental paradigm is t h a t t h e answers obtained m i g h t b e i r r e l e v a n t f o r e v e r y t h i n g except t h e paradigm i t s e l f . T h e human c e n t r a l n e r v o u s system has t h e c a p a b i l i t y t o f u n c t i o n as d i f f e r e n t systems, depending on t h e t a s k a t hand. T h e r e is n o t h i n g l i k e " t h e information-processing system"; r a t h e r d i f f e r e n t such systems can b e assembled. Therefore, artificial experimental t a s k s in p r i n c i p l e can have t h e e f f e c t of i n d u c i n g a r t i f i c i a l systems (see Heuer, 1988b). The purpose of experimental tasks, however, is to tap characteristics o f c e n t r a l n e r v o u s system a c t i v i t y t h a t a r e important f o r o t h e r t a s k s as well. Therefore, convergence o f conclusions d e r i v e d f r o m d i f f e r e n t experimental tasks is of c r i t i c a l importance t o ascertain t h a t more has been gained t h a n some i n s i g h t i n t o how a c e r t a i n a r t i f i c i a l t a s k is accomplished. With respect t o motor-programming research, t h i s means t h a t R T experiments should no longer b e k e p t separate f r o m physiological w o r k on motor p r e p a r a t i o n o r o t h e r behavioral data. One such attempt has been made w i t h t h e series of experiments presented i n t h i s c h a p t e r . T h e r a t h e r consistent p i c t u r e t h a t emerged can g i v e some confidence t h a t t h e choice-RT experiments in fact tapped some characteristics o f motor c o n t r o l t h a t were n o t p a r t i c u l a r t o t h e h i g h l y a r t i f i c i a l t a s k o f choosing as q u i c k l y as possible between l e f t - h a n d a n d r i g h t - h a n d movements.

337

Intermanual Interactions

Summary I n t h i s chapter, it has been a r g u e d t h a t R - R compatibility effects t h a t can b e o b s e r v e d in r a p i d choice between l e f t - h a n d a n d r i g h t - h a n d responses a r e related t o intermanual interactions f o u n d i n o t h e r k i n d s o f tasks. T h i s argument proceeded in t h r e e steps: (a) T h e e f f e c t o f t h e relation between responses assigned t o t h e t w o hands was i n t e r p r e t e d as r e s u l t i n g f r o m interactions during simultaneous processes o f programming ( o r preprogramming) d i f f e r e n t responses; (b) T h e c o n t i n u i t y assumption, according t o w h i c h processes during p r e p a r a t i o n a n d execution o f a movement a r e a t least p a r t l y t h e same, was s u p p o r t e d by showing t h a t e x e c u t i n g a response w i t h one h a n d affects programming of another response f o r t h e o t h e r hand; (c) T h e general convergence o f R-R comp a t i b i l i t y effects w i t h f i n d i n g s f r o m studies o f simultaneous movements o f t h e t w o hands was shown. Acknowledgements T h i s c h a p t e r is herein was s u p p o r t e d 1187/1, He 1187/2-1, h e l p f u l comments on an

dedicated t o Anna. T h e research summarized by Deutsche Forschungsgemeinschaft ( g r a n t s He He 1187/3-1). T h a n k s g o t o Will S p i j k e r s f o r e a r l i e r version.

References Berlyne, D. E. (1957). C o n f l i c t a n d choice time. Psychology. 48. 106-1 18.

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PERCEPTUAL-MOTOR PROCESSING I N SPEECH PETER C. GORDON Department o f Psychology Ha r v a rd U n i v e r s i t y N a t u r a l language consists o f p a t t e r n s a t many levels o f analysis (sentences, phrases, words, syllables, phonemes, e t c . ) t h a t can b e analyzed w i t h o u t d i s t i n g u i s h i n g between language p e r c e p t i o n a n d p r o d u c tion. In fact, most l i n g u i s t i c analyses a r e n e u t r a l in t h i s r e g a r d . The compelling n a t u r e o f these analyses has l e d many psychologists t o accept, as a plausible i n i t i a l assumption, t h a t t h e processes of language percept i o n a n d p r o d u c t i o n employ representations o f t h e u n i t s t h a t l i n g u i s t s p o s i t a t t h e d i f f e r e n t levels. B u t because psychologists d o d i s t i n g u i s h between t h e processes o f perception a n d production, t h e y must c o n f r o n t t h e question of how t h e u n i t s used by t h e t w o systems a r e related. One p o s s i b i l i t y is t h a t perception a n d p r o d u c t i o n i n v o l v e parallel, but comp l e t e l y separate, representations o f l i n g u i s t i c u n i t s . However, t h i s conception seems o v e r l y r e s t r i c t i v e , a n d o v e r t h e years many intriguing explanations of speech phenomena have been o f f e r e d in terms o f percept i o n a n d p r o d u c t i o n i n v o l v i n g i n t e r a c t i n g a n d o v e r l a p p i n g representations.

It i s n o t s u r p r i s i n g t h a t t h e o r i s t s m i g h t propose t h a t perception ( o r perhaps comprehension) a n d p r o d u c t i o n i n t e r a c t a t t h e higher, more abstract, levels of language. A f t e r all, t h e goal o f comprehension is t o discover meanings in a signal, whereas t h e goal o f p r o d u c t i o n is t o encode meanings i n t o a signal. T h u s , a t t h e h i g h e s t level, language perception a n d p r o d u c t i o n must i n t e r a c t w i t h common semantic representations t h a t a r e presumably l i n k e d t o e x t r a - l i n g u i s t i c sources o f meaning. It is more s u r p r i s i n g t h a t t h e o r i s t s have a r g u e d t h a t perception a n d p r o d u c t i o n processes i n t e r a c t a t t h e level o f t h e lower u n i t s o f speech. Here, perception m u s t analyze acoustic patterns, whereas p r o d u c t i o n must t r a n s l a t e l i n g u i s t i c u n i t s i n t o c h a n g i n g shapes o f t h e vocal t r a c t . While these t w o domains, acoustics a n d v o c a l - t r a c t shape, a r e l i n k e d by a f a i r l y t i g h t , reasonably well understood set o f r u l e s (Fant, 1970; Wakita, 19761, t h e problems of a u d i t o r y p a t t e r n recognition a n d a r t i c u l a t o r y movement c o n t r o l a r e s u f f i c i e n t l y d i f f e r e n t that, a t f i r s t glance, it is h a r d t o see how o r w h y t h e y m i g h t i n t e r a c t . T h e Motor T h e o r y o f Speech Perception Proposals t h a t speech perception a n d p r o d u c t i o n processes a r e tightly l i n k e d have come in many forms (Ladefoged, Lindau, Papcun, & DeClerk, 1972; Liberman, Cooper, Shankweiler, & Studdert-Kennedy, 1967; Liberman & Mattingly, 1985; Stevens & Halle, 1967). Though the f o r m of these proposals has varied, t h e u n d e r l y i n g motivation b e h i n d them has been v e r y similar. T h i s motivation has been t h a t t h e complexity o f one o f t h e systems (perception o r p r o d u c t i o n ) c o u l d b e u n d e r s t o o d i n terms of t h e a p p a r e n t l y g r e a t e r simplicity of t h e o t h e r system. So, for example, t h e motor t h e o r y o f speech perception (e.g., Liberman e t al., 1967; Liberman & M a t t i n g l y , 1985) has been based on t h e idea t h a t o u r

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perception o f speech is more closely related t o salient aspects o f a r t i c u l a t o r y g e s t u r e s t h a n t o salient aspects o f t h e acoustic stimulus. T h a t is, according t o motor theory, an apparent absence o f i n v a r i a n t acoustic cues t o o u r perceptual categories can b e explained by t h e presumed presence o f i n v a r i a n t a r t i c u l a t o r y motor commands. Conversely, t h e r e is a t least one proposal t h a t t h e problem o f motor equivalence in speech p r o d u c t i o n can b e accounted f o r in terms o f t h e perceptual equivalence o f t h e o u t comes (Ladefoged e t al., 1972). While t h e r e have been many motor theories, t h e one t h a t has been developed a t Haskins Laboratories (Liberman e t al., 1967; Liberman & Mattingly, 1985) i s t h e most prominent.' T h e basic thesis has been t h a t t h e phenomena o f phonetic perception ( t h a t is, t h e way t h a t we p e r c e i v e l i n g u i s t i c a l l y r e l e v a n t information) o n l y make sense if t h e process of p e r ception i s t i e d d i r e c t l y t o o u r i m p l i c i t knowledge o f a r t i c u l a t o r y motor c o n t r o l . T h i s thesis has been s u p p o r t e d by a v a r i e t y o f experiments t h a t seemed t o demonstrate t h a t phonetic perception was v e r y d i f f e r e n t f r o m general a u d i t o r y perception, in ways t h a t a r e p o t e n t i a l l y understandable if t h e n a t u r e of a r t i c u l a t i o n is t a k e n i n t o account. T h e o r i g i n a l 'list o f phenomena p r o m i n e n t l y f e a t u r e d categorical perception a n d acousticphonetic noninvariance (Liberman e t al., 1967). T h e uniqueness o f these phenomena t o speech, a n d t h e i r b e a r i n g on t h e motor t h e o r y , has been c r i t i c a l l y analyzed a t l e n g t h ( f o r r e p r e s e n t a t i v e discussions, see Gordon & Meyer, 1984; MacNeilage, 1978; Repp, 1982). More r e c e n t proposals have focused on t h e t e n d e n c y f o r phonetic perception t o preempt non-phonetic perception, t h e i n t e g r a t i o n o f acoustic a n d v i s u a l information about a r t i c ulation, a n d t h e existence o f m u l t i p l e acoustic cues t o phonetic i d e n t i t y (Liberman 8 Mattingly, 1985; Repp, 1982). T h e motor theory has become tightly b o u n d up w i t h t h e belief t h a t phonetic perception i s a separate mental f a c u l t y , o r module, in t h e sense proposed by Fodor (1983). Liberman a n d M a t t i n g l y (1985; M a t t i n g l y & Liberman, in press) took t h i s view a n d proposed t h a t t h e same module serves b o t h perception a n d production, t h o u g h i t s basis is in u n i q u e characteristics o f a r t i c u l a t o r y motor control, n o t in perception (hence t h e T h e y f u r t h e r a r g u e d t h a t t h i s module is an innate p r o d motor t h e o r y ) . u c t o f evolution t h a t addresses t h e biologically coherent problem o f l i n g u i s t i c communication. A c c o r d i n g t o Liberman a n d M a t t i n g l y (19851, t h e i n n a t e basis o f t h i s module spares it t h e v e r y d i f f i c u l t l e a r n i n g t a s k o f tying t o g e t h e r t h e m u l t i p l e acoustic correlates o f articulation. T h e motor t h e o r y has h a d appeal f o r psychologists n o t d i r e c t l y concerned w i t h speech p e r c e p t i o n . It is a p a r t i c u l a r l y i n t r i g u i n g example of a rationalist/constructivist t h e o r y o f perception, in w h i c h a specialized source o f knowledge about t h e stimulus i s used t o g u i d e perception. T h e embodiment, w i t h i n t h e motor t h e o r y , of t h e idea t h a t speech perception is d i s t i n c t f r o m o t h e r k i n d s o f a u d i t o r y perception appeals t o those who believe t h a t language is a u n i q u e l y human a b i l i t y t h a t is d i s t i n c t f r o m o t h e r mental processes. 'Liberman a n d M a t t i F g l y (1985) pointed o u t ,:hat while t h e i r t h e o r y i s o n l y "a motor t h e o r y , t h e y r e f e r t o it as the motor t h e o r y " f o r e x p o s i t o r y case. I expect t h a t t h e i r use of t h e d e f i n i t e a r t i c l e is in no way misleading, a n d I wit1 follow it.

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O n t h e o t h e r hand, c r i t i c s o f t h e motor t h e o r y have p o i n t e d o u t t h a t no detailed specification o f t h e w o r k i n g s o f t h e phonetic module have been developed. I n addition, t h e empirical evidence implicating reliance o f phonetic perception on t h e motor system has, f o r t h e most p a r t , h a d a v e r y indirect quality. T h e argument begins w i t h t h e d i s c o v e r y o f some c u r i o u s aspect o f phonetic perception, a p p a r e n t l y inexplicable in terms o f a u d i t o r y processing, w h i c h is presented as eyidence f o r t h e motor t h e o r y ( o r f o r t h e related notion t h a t speech is special"; Liberman, 1982). Opponents o f t h e motor t h e o r y t h e n p r o d u c e an analog o f t h e phenomenon w i t h non-speech stimuli a n d claim t h a t t h e phenomenon is consistent w i t h general p r i n c i p l e s o f a u d i t o r y perception. A debate t h e n ensues about t h e degree t o w h i c h t h e non-speech phenomenon matches t h e speech phenomenon, a n d about t h e e x t e n t t o w h i c h t h i s match indicates- t h a t t h e same mechanism underlies t h e two. Even if t h e perceptual phenomenon i s accepted as u n i q u e t o speech, i t s basis in t h e motor system i s n o t d i r e c t l y demonstrated. T h e indirectness o f t h i s kind o f argument has l e d some researchers t o e x p l o r e t h e relation between t h e perceptual a n d motor systems by s t u d y i n g t h e influence o f speech perception on speech p r o d u c t i o n a n d v i c e versa. One approach has made use o f selective adaptation t e c h niques (Cooper, 1975; see Gordon & Meyer, 1984, f o r a discussion). A second approach, which is t h e p r e s e n t focus, has examined stimulusresponse (S-R) compatibility u s i n g speech stimuli a n d responses. Stimulus-Response Compatibility Research with Speech Beyond t h e arena o f speech, S-R compatibility research has h a d b o t h p r a c t i c a l a n d theoretical motivations. T h e p r i n c i p l e p r a c t i c a l conc e r n has been t o design displays t h a t human operators can i n t e r a c t w i t h i n a n a t u r a l a n d e f f i c i e n t way. T h i s p r a c t i c a l concern does n o t e x t e n d t o n a t u r a l speech communication. It appears t h a t people q u i t e n a t u r a l l y modify t h e i r articulations in ways t h a t a r e a p p r o p r i a t e t o t h e situation, t a k i n g i n t o account, f o r example, t h e amount o f ambient noise (Draegert, 1951), t h e h e a r i n g capabilities o f t h e i r i n t e r l o c u t o r (Picheny, Durlach, & Braida, 1983), a n d t h e r e d u n d a n c y o f t h e message (Lieberman, 1963). Thus, it appears t h a t w i t h r e g a r d t o d e s i g n i n g t ir articulations, people have implicit knowledge about e f f e c t i v e strategies.

f!

Theoretical goals i n s t u d y i n g S-R compatibility have been diverse, but i n one way o r another have usually attempted t o elucidate some aspect o f stimulus encoding, response formation, a n d t h e interaction of perceptual a n d motor systems. T h e interaction between perceptual a n d motor systems has been s t u d i e d b o t h w i t h r e g a r d t o t h e processing o f e x t e r n a l l y generated stimuli, such as b u t t o n - p r e s s i n g i n response t o 'This implicit knowledge about how t o t a i l o r one's articulations t o t h e communicative environment does n o t t r a n s f e r t o scientists' a b i l i t y t o synthesize speech. S y n t h e t i c speech is less r e s i s t a n t t h a n n a t u r a l speech t o environmental degradation (Pisoni, Nusbaum, & Greene, 1985). In addition, while t h e focus h e r e is on speech, t h e p o i n t concerning speakers' a b i l i t y t o accommodate listeners' needs extends reasonably well t o h i g h e r - l e v e l aspects o f language. Grice's (1975) analysis o f conversat i o n according t o t h e "cooperativeness p r i n c i p l e " r e s t s in p a r t o n speakers b e i n g able t o accurately gauge listeners' knowledge.

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l i g h t s ( F i t t s & Seeger, 19531, a n d w i t h r e g a r d t o t h e processing o f i n t e r n a l l y generated stimuli, such as in w o r k on efference c o p y ( v o n Holst, 1954). In a c u r i o u s way, research o n S-R compatibility w i t h speech combines t h e concerns of b o t h t y p e s o f studies. It examines t h e processing o f e x t e r n a l l y generated stimuli, but these stimuli a r e ones f o r w h i c h t h e p e r c e i v e r has intimate knowledge of t h e manner o f i n t e r n a l generation in t h e f o r m o f h i s o r h e r own a r t i c u l a t o r y system. One a p p a r e n t consequence o f t h i s knowledge o f generation is a v e r y

high degree o f compatibility between p e r c e i v i n g a n d p r o d u c i n g speech, as In t h e well-known has been revealed by shadowing experiments. shadowing task, a subject must repeat a spoken message as h e o r she hears it. O f t e n t h i s involves simultaneously p e r c e i v i n g a n d p r o d u c i n g speech in a continuous fashion. O n t h e face o f it, t h i s t a s k seems r a t h e r d i f f i c u l t , but a p p a r e n t l y it does n o t pose too g r e a t a problem f o r s u b jects. In fact, some subjects a r e able t o shadow remarkably well. Marslen-Wilson (1973) f o u n d t h a t approximately 10% o f subjects were "close shadowers," able t o r e p r o d u c e speech a t average latencies o f approximately 270 ms, w h i c h is n o t much longer t h a n t h e average d u r a t i o n o f a syllable. A l t h o u g h Marslen-Wilson's p r i n c i p l e theoretical conc e r n s l a y elsewhere, his f i n d i n g s demonstrate v e r y high compatibility between speech perception a n d p r o d u c t i o n . Kozhevnikov a n d C h i s t o v i c h (1965) conducted a d i s c r e t e shadowing t a s k in w h i c h t h e y were d i r e c t l y concerned w i t h t h e relation between t h e perceptual a n d motor systems f o r speech. Subjects were r e q u i r e d t o repeat vowel-consonant-vowel ( V C V ) stimuli as soon as t h e y h e a r d them. Latencies t o i n i t i a t e some aspects o f t h e consonants were as s h o r t as 100 ms. T h i s latency, in a choice reaction-time ( R T ) task, is considerably s h o r t e r t h a n t h e value o f 140 ms t h a t is t r a d i t i o n a l l y c i t e d as a minimum f o r latencies in a simple a u d i t o r y R T t a s k (Woodworth, 1938). Kozhevnikov a n d C h i s t o v i c h reasoned t h a t these v e r y s h o r t latencies could o n l y b e obtained if t h e r e were d i r e c t connections between t h e speech perception a n d p r o d u c t i o n systems. One possible problem w i t h t h i s i n t e r p r e t a t i o n is t h a t Kozhevnikov a n d C h i s t o v i c h (1965) may have underestimated t h e e x t e n t t o which t h e i n i t i a l vowel in a V C V sequence contains c o a r t i c u l a t o r y information about t h e following consonant. More recent experiments (e.g., Martin & Bunnel, 1982) have f o u n d t h a t listeners can use c o a r t i c u l a t o r y information t o anticipate t h e i d e n t i t y o f an up-coming segment. Therefore, Kozhevnikov a n d Chistovich's estimates o f t h e latencies may have been too low. A more general problem w i t h Kozhevnikov a n d Chistovich's w o r k is i t s reliance on t h e comparison o f absolute values o f RTs across d i f f e r e n t stimuli, responses, a n d t a s k demands. More d i r e c t inferences about t h e relation between ' p e r c e p t i o n a n d p r o d u c t i o n can b e made if t h e relevant controls a r e built d i r e c t l y i n t o t h e experiment. Greenwald (1970), i n s t u d y i n g "ideomotor" compatibility, performed experiments t h a t d o p r o v i d e some evidence f o r r e l a t i v e l y p r i v i l e g e d l i n k s 3While M a r t i n a n d Bunnel (1982) s t u d i e d t h e perceptual effects o f c o a r t i c u l a t o r y information about vowels, t h e r e i s also c o a r t i c u l a t o r y i n f o r mation available c o n c e r n i n g consonants. It is l i k e l y t h a t listeners c o u l d e x p l o i t t h a t information as well.

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between speech perception a n d p r o d u c t i o n systems. T h e experiments i n v o l v e d l e t t e r s presented a u d i t o r i l y o r v i s u a l l y as stimuli, a n d p r o d u c e d vocally o r in w r i t i n g as responses. H e f o u n d t h a t modality o f presentat i o n interacted w i t h manner o f response, such t h a t w r i t t e n responses t o v i s u a l stimuli a n d vocal responses t o a u d i t o r y stimuli were f a s t e r t h a n w r i t t e n responses t o a u d i t o r y stimuli a n d vocal responses t o v i s u a l stimuli. A basic t e n e t of Greenwald's ideomotor t h e o r y i s t h a t response selection is g u i d e d by images o f t h e feedback expected f r o m t h e response. H e i n t e r p r e t e d t h e r e s u l t s as i n d i c a t i n g t h a t presentation o f t h e stimulus in t h e same modality as t h e dominant f o r m o f feedback f r o m a response facilitates t h e c o n s t r u c t i o n of t h a t image.

From t h e p r e s e n t perspective, Greenwald's (1970) r e s u l t s dernons t r a t e high compatibility between perception o f speech stimuli a n d p r o d u c t i o n o f vocal responses, in a situation t h a t controls f o r t h e n a t u r e o f t h e task, t h e ease o f encoding t h e stimuli, a n d t h e ease o f i n i t i a t i n g t h e responses. T h e experiments d o not, however, enlighten us as t o t h e level o f processing a t w h i c h t h e compatibility between speech perception a n d p r o d u c t i o n takes place. It i s impossible t o t e l l w h e t h e r t h e facilitat i o n is solely d u e t o a u d i t o r y perception o r w h e t h e r it has something t o d o w i t h speech perception p e r se, a n d if it does d e r i v e f r o m speech p e r ception, o n what level o f speech processing it depends. Perceptual-Motor Processing of Phonetic Features in Speech

In attempting t o f u r t h e r u n d e r s t a n d t h e relation between speech perception a n d p r o d u c t i o n processes, D a v i d Meyer a n d I p e r f o r m e d a series o f S-R compatibility experiments t h a t allowed us t o b e t t e r specify t h e level o f perceptual-motor i n t e r a c t i o n (Gordon & Meyer, 1984). In these experiments, subjects were presented w i t h an a u d i t o r y consonantvowel (CV) syllable (e.g.. PUH), a n d h a d t o respond by s a y i n g another syllable (eg., PUH, BUH, TUH, o r DUH) as r a p i d l y as possible. T h e R T t o i n i t i a t e t h e vocal response was s t u d i e d as a f u n c t i o n o f t h e relation o f i t s i n i t i a l consonants, in phonetic features, t o t h e i n i t i a l consonants o f t h e a u d i t o r y stimulus syllable. T h e rationale o f t h i s perceptual-motor priming procedure is If a close relation e x i s t s between t h e perceptual a n d straightforward. motor systems f o r processing phonetic features, t h e n p e r c e i v i n g a speech sound o u g h t t o influence t h e motor processes t h a t p r o d u c e t h e features in t h a t sound, a n d t h i s influence i n t u r n should b e reflected in t h e R T t o i n i t i a t e an u t t e r a n c e t h a t contains those f e a t u r e s . B y v a r y i n g t h e modality of t h e stimulus syllables ( u s i n g v i s u a l r a t h e r t h a n a u d i t o r y stimuli), it i s possible t o c o n t r o l f o r o t h e r factors t h a t m i g h t a f f e c t RTs. U s i n g t h e p r i m i n g procedure, we f o u n d t h a t t h e perceptual a n d motor systems o f speech i n t e r a c t i n processing t h e v o i c i n g features of When t h e consovarious t y p e s of consonants (Gordon & Meyer, 1984). nants of t h e stimulus a n d response syllables h a d matched v o i c i n g features ( e . g . , h e a r i n g PUH a n d s a y i n g TUH), t h e R T t o i n i t i a t e t h e response was faster t h a n when t h e v o i c i n g features d i f f e r e d ( e . g . , h e a r i n g PUH a n d s a y i n g DUH). A matched place of a r t i c u l a t i o n f e a t u r e (e.g., h e a r i n g PUH a n d s a y i n g BUH) h a d n o e f f e c t on t h e R T t o i n i t i a t e t h e response. T h i s p a t t e r n of r e s u l t s h e l d f o r s y n t h e t i c a n d n a t u r a l l y p r o d u c e d stop consonants, f o r fricatives, a n d f o r stimulus a n d response syllables t h a t did n o t contain t h e same vowel. When t h e stimulus syllables were p r e sented v i s u a l l y instead of a u d i t o r i l y , t h e r e were no effects o f a n y

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matched phonetic features. T h e lack o f effects w i t h v i s u a l presentation indicates t h a t t h e r e s u l t s obtained w i t h a u d i t o r y stimulus presentation stem from an i n t e r a c t i o n between t h e perceptual a n d motor systems o f speech, r a t h e r t h a n f r o m some o t h e r a n c i l l a r y aspect o f t h e task, such as ease o f remembering t h e S-R p a i r s . T h e finding o f a d i f f e r e n c e between t h e v o i c i n g a n d place features i s s u r p r i s i n g . V o i c i n g a n d place show similar characteristics in a v a r i e t y of speech-perception tasks, such as categorical perception, selective adaptation, a n d d i c h o t i c l i s t e n i n g (see Gordon & Meyer, 1984, f o r a d i s cussion of these similarities). Moreover, f o r speech production, Meyer a n d Gordon (1985) have f o u n d t h a t b o t h t h e v o i c i n g a n d place features p l a y s i g n i f i c a n t roles in t h e motor programming o f utterances. Therefore, t h e d i f f e r e n c e o b s e r v e d in perceptual-motor processing o f v o i c i n g a n d place o f a r t i c u l a t i o n seems t o o f f e r an important c o n s t r a i n t on o u r conceptualization o f perceptual-motor processing in speech.

In p r e v i o u s work, we (Gordon & Meyer, 1984) presented a number o f hypotheses t h a t seek t o e x p l a i n t h e voicing/place d i f f e r e n c e in terms o f low-level mechanisms t h a t map characteristics o f t h e stimulus o n t o t h e response. These mechanisms would p r o d u c e facilitation f o r shared v o i c i n g but n o t f o r shared place, because t h e p h y s i c a l characteristics o f v o i c i n g in t h e acoustic a n d a r t i c u l a t o r y realms seem more similar t h a n those f o r place. F o r example, a major component o f t h e v o i c i n g d i s t i n c t i o n is voice-onset time (VOT) . 4 A c c o r d i n g t o one hypothesis we presented, facilitation d u e t o shared v o i c i n g r e s u l t s f r o m t h e operation o f a t i m i n g mechanism t h a t processes t h e temporal component o f voicing, b o t h in t h e perceptual a n d motor systems. O n t h e perceptual side, t h e t i m i n g mechanism would determine w h e t h e r t h e stimulus h a d a s h o r t o r long VOT. If t h e a p p r o p r i a t e response h a d t h e same v o i c i n g status, t h e n t h e t i m i n g mechanism w o u l d n o t need t o b e reset, r e s u l t i n g in s h o r t e r RTs as compared t o S-R p a i r s t h a t d i f f e r in voicing. In comparison t o voicing, t h e acoustic a n d a r t i c u l a t o r y correlates o f place o f a r t i c u l a t i o n have no such simple s h a r e d characteristics t h a t m i g h t b e processed by a common mechanism. T h e acoustic cues t o place a r e spectral in nature, consisting o f t h e d i r e c t i o n o f formant t r a n s i t i o n s (Delattre, Liberman, & Cooper, 1955) a n d t h e spectral p a t t e r n o f t h e i n i t i a l b u r s t (Stevens i% Blumstein, 1978). T h e a r t i c u l a t o r y correlates of place of a r t i c u l a t i o n (as t h e name implies) consist o f t h e location in t h e vocal t r a c t w h e r e t h e closure, o r constriction, is made. We could t h i n k of no obvious mechanism, analogous t o t h e t i m i n g mechanism f o r voicing, t h a t c o u l d process b o t h spectral acoustic cues a n d spatial location in t h e vocal t r a c t .

A less d i r e c t mapping between acoustic a n d a r t i c u l a t o r y characteristics o f place as compared t o v o i c i n g was t h e r e f o r e o f f e r e d as a possible explanation f o r t h e finding o f facilitation d u e t o shared voicing, but n o t shared place (Gordon i% Meyer, 1984). While t h i s hypothesis was i n i t i a l l y 4VOT r e f e r s t o t h e time between t h e release o f t h e consonant a n d t h e onset o f v i b r a t i o n in t h e vocal folds ( i . e . , v o i c i n g ) . Voiced consonants (e.g., /b/ a n d /d/) have s h o r t VOTs of r o u g h l y 0 t o 10 ms, while voiceless consonants (e.g.,/p/ a n d /t/) have longer V O T s of r o u g h l y 50 t o 70 ms ( L i s k e r & Abramson, 1964).

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appealing t o us, t h e r e a r e a number o f reasons f o r d o u b t i n g i t s plausibility. As noted earlier, t h e r e a r e numerous acoustic cues t o most phonetic distinctions. V O T i s o n l y one o f t h e acoustic correlates t o v o i c i n g in i n i t i a l stop consonants (others i n c l u d e t h e amplitude o f t h e i n i t i a l b u r s t a n d t h e amount o f aspiration), a n d it is by no means c e r t a i n t h a t t h e temporal component o f v o i c i n g i s t h e most c r i t i c a l . Even if it is, V O T could p e r h a p s b e processed by a t h r e s h o l d mechanism f o r processing simultaneity (Miller, Wier, Pastore, Kelly, & Dooling, 1976), in w h i c h time would n o t b e e x p l i c i t l y represented. Similarly, on t h e p r o d u c t i o n side, it is n o t clear t h a t c o n t r o l o v e r t h e temporal relation between t h e consonant release a n d v o i c i n g onset is controlled by a mechanism t h a t e x p l i c i t l y r e p resents t i m i n g information. While it i s by n o means clear t h a t t h e p e r ception a n d p r o d u c t i o n o f v o i c i n g d o n o t i n v o l v e a t i m i n g mechanism, it i s by no means clear t h a t t h e y do. Even if t h e y do, it is n o t clear t h a t t h e same kind o f mechanism c o u l d process t h i s temporal component on b o t h t h e perceptual a n d motor sides. While t h e above arguments d o n o t r u l e o u t t h e shared t i m i n g mechanism as an explanation o f t h e r e s u l t s o f Gordon a n d Meyer (1984), t h e y lessen i t s a l l u r e a n d have l e d t o t h e search f o r a l t e r n a t i v e explanations o f t h e d i f f e r e n t i a l p a r t i c i p a t i o n o f v o i c i n g a n d place o f a r t i c u l a t i o n in t h e perceptual-motor p r i m i n g t a s k . Below, 1 p r e s e n t arguments a n d some data in s u p p o r t o f one such a l t e r n a t i v e explanation Perceptual Salience a n d Perceptual-Motor P r i m i n g T h e perceptual-motor p r i m i n g t a s k reveals t h e speed w i t h w h i c h t h e stimulus can b e encoded a n d t h e response selected a n d produced. Therefore, one p o s s i b i l i t y f o r t h e observation o f facilitation f o r shared voicing, but n o t shared place o f articulation, is t h a t t h e temporal c h a r acteristics f o r processing t h e v o i c i n g f e a t u r e a r e d i f f e r e n t f r o m those f o r t h e place f e a t u r e . In p a r t i c u l a r , I w i l l e n t e r t a i n t h e p o s s i b i l i t y t h a t v o i c i n g is more p e r c e p t u a l l y salient t h a n place a n d t h a t t h i s f a c t o r allows f o r more r a p i d processing o f v o i c i n g in a way t h a t d r i v e s t h e perceptualmotor system so as t o p r o d u c e t h e facilitation o b s e r v e d f o r S-R p a i r s w i t h shared voicing. Some evidence t o s u p p o r t t h e assumption t h a t v o i c i n g a n d place o f a r t i c u l a t i o n do n o t have equal perceptual salience comes f r o m a wellknown s t u d y by M i l l e r a n d Nicely (1955). T h e y h a d subjects p e r f o r m absolute identifications o f a u d i t o r y C V syllables presented in noise. More confusions o c c u r r e d between syllables whose consonants h a d d i f f e r e n t places o f a r t i c u l a t i o n t h a n between ones whose consonants h a d d i f f e r e n t voicing. For example t h e consonant /p/ t e n d e d t o b e i d e n t i f i e d more o f t e n as /t/ t h a n as /b/. T h i s suggests t h a t t h e place features were p e r c e p t u a l l y less salient t h a n t h e v o i c i n g features. While t h e r e s u l t s o f t h e Miller a n d Nicely (1955) s t u d y p r o v i d e s t r o n g s u p p o r t t h a t v o i c i n g is, in some sense, more p e r c e p t u a l l y salient t h a n place, o t h e r evidence suggests t h a t t h e situation is n o t e n t i r e l y clear c u t . Place-of-articulation changes a r e easier t o detect in a mispronunciation-detection t a s k (Cole, Jakimik, & Cooper, 1978), a n d in phoneme-monitoring tasks, subjects make more false alarms t o phonemes t h a t share place w i t h t h e t a r g e t t h a n t o phonemes t h a t share v o i c i n g (Newman & Dell, 1978). Furthermore, Gordon a n d Meyer (1984) f o u n d t h a t v o i c i n g a n d place cues appeared equally salient in a same-different task .

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O f these results, those o f Miller a n d Nicely (1955) seem most relev a n t t o t h e i n t e r p r e t a t i o n o f t h e f i n d i n g s f r o m t h e perceptual-motor p r i m i n g t a s k because t h e i r identification t a s k i n v o l v e d presentation o f d i s c r e t e syllables, as does t h e p r i m i n g t a s k . I n contrast, continuous speech is used in t h e phoneme-monitoring t a s k (Newman & Dell, 1978). T h e continuous speech may p r o v i d e more advance c o a r t i c u l a t o r y informat i o n about place t h a n about voicing, t h u s leading t o t h e more accurate judgments in t h e phoneme-monitoring t a s k f o r place o f articulation. A c c u r a t e performance in t h e mispronunciation t a s k (Cole, Jakimik, & Cooper, 1978) r e q u i r e s a r a t h e r h i g h - l e v e l judgment (i.e., it must b e based o n lexical representations), a n d t h u s may r e f l e c t f a c t o r s o t h e r t h a n t h e e a r l y perceptual identification of phonetic features. Similarly, t h e r e i s evidence t h a t R T s in same-different t a s k s may depend on r e l a t i v e l y late comparison a n d decision processes t h a t o c c u r a f t e r i n i t i a l perceptual processing has been completed (Santee & Egeth, 1982). F o r these reasons, it seems l i k e l y t h a t t h e f i n d i n g s o f M i l l e r a n d Nicely a r e t h e most r e l e v a n t f o r judging t h e salience o f phonetic features in t h e p e r ceptual-motor p r i m i n g t a s k . Below, I b r i e f l y p r e s e n t t w o experiments t h a t examine w h e t h e r p e r ceptual salience, a t least as revealed by confusions u n d e r noise, p r o v i d e s an indication o f w h i c h phonetic features e x h i b i t perceptual-motor p r i m i n g . T h e experiments examine t w o manner-of-articulation distinctions, stop v e r s u s nasal a n d f r i c a t i v e v e r s u s nasal, t h a t have been shown t o b e p e r c e p t u a l l y salient (Miller & Nicely, 1955; Shepard, 1972). T h e r e s u l t s lead t o t h e development o f a model p o s i t i n g a close connection between t h e perceptual a n d motor systems a t t h e f e a t u r a l level, in w h i c h p e r c e p t u a l l y salient phonetic information is automatically recoded i n t o a r t i c u l a t o r y form. Stop/Nasal D i s t i n c t i o n Shepard (1972) performed a multidimensional scaling o f t h e conf u s i o n data obtained by M i l l e r a n d Nicely (1955). T h e r e s u l t s (Shepard, 1972, F i g u r e 4.1, p. 76) indicate t h a t 99.4% o f t h e variance in t h e data c o u l d b e accounted f o r by t w o dimensions c o r r e s p o n d i n g r o u g h l y t o v o i c i n g a n d nasality. T h i s suggests t h a t t h e manner-of-articulation d i s t i n c t i o n between stop consonants a n d nasal (e.g., BUH v s . MUH) o u g h t t o b e as p e r c e p t u a l l y salient as t h e v o i c i n g d i s t i n c t i o n (e.g., BUH v s . PUH). If facilitation in t h e perceptual-motor p r i m i n g t a s k is d u e t o a shared v a l u e o n a p e r c e p t u a l l y salient dimension, t h e n we would expect t h a t S-R p a i r s w i t h a shared manner f e a t u r e (e.g., BUH-DUH a n d MUHNUH) would p r o d u c e f a s t e r RTs t h a n p a i r s w i t h o u t a shared manner feature (e.g., BUH-NUH a n d MUH-DUH). T h e experiment r e p o r t e d below tests t h i s p r e d i c t i o n . T h e p r o c e d u r e was v e r y similar t o t h e one used by Gordon a n d Briefly, Meyer (1984), a n d t h a t a r t i c l e should b e consulted f o r details.” t h e stimuli a n d responses consisted of t h e syllables, BUH, MUH, DUH, a n d NUH. T h e i n i t i a l consonants o f t h i s set v a r y along t w o dimensions; manner o f a r t i c u l a t i o n ( s t o p v s . nasal) a n d place o f a r t i c u l a t i o n (labial 5A more complete d e s c r i p t i o n o f t h e methods a n d r e s u l t s o f t h e specific experiments r e p o r t e d h e r e is available f r o m t h e a u t h o r on request.

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35 1

v s . alveolar).6 Each subject was t a u g h t f o u r S-R syllable p a i r s t h a t e x h i b i t e d t h e f o u r possible f e a t u r a l relations between t h e consonants o f t h e stimuli a n d those o f t h e responses; f o r example: MUH-MUH shares b o t h features, BUH-DUH shares manner but n o t place o f articulation, DUH-NUH shares place but n o t manner, a n d NUH-BUH shares n e i t h e r feature. Across subjects, t h e assignment of specific syllables t o S-R p a i r s was v a r i e d so t h a t each syllable p a r t i c i p a t e d in each o f t h e phonetic f e a t u r e relations, b o t h as a stimulus a n d a response. A f t e r l e a r n i n g t h e S-R pairs, t h e subjects p e r f o r m e d an R T t a s k in w h i c h t h e stimulus syllable was p r e s e n t e d a u d i t o r i l y , a n d t h e subject h a d t o make t h e a p p r o p r i a t e vocal response as r a p i d l y as possible. T h e time between t h e onset o f t h e stimulus syllable a n d t h e onset o f t h e vocal response was measured. E i g h t subjects p a r t i c i p a t e d in a t o t a l o f 240 R T t r i a l s each. T h e p r i n c i p l e r e s u l t s o f t h e experiment appear in T a b l e 1, w h i c h shows t h e mean RTs o f c o r r e c t responses a n d e r r o r rates f o r t h e various t y p e s o f syllable p a i r s . T h e overall e f f e c t o f t h e p h o n e t i c - f e a t u r e relations w i t h i n t h e p a i r s was significant, F(3, 21) = 3.31, p < .05. A series o f planned comparisons was performed on t h e differences between t h e S-R p a i r s w i t h matched features a n d those w i t h n o matched features. When t h e stimulus a n d response syllables were identical, R T s were 75 ms Table 1 Results From Response-Priming T a s k With Manner (Stop v s . Nasal) a n d Place-of-Articulation D i s t i n c t i o n s

Matched phonetic features

Mean R T (ms)

Errors

manner a n d place (e.g., BUH-BUH)

555

1 .o

manner (e.g., MUH-NUH)

579

2.3

place

62 1

3.5

630

1.3

(e.g.,

NUH-DUH)

none (e.g.,

DUH-MUH)

(8)

%ith r e g a r d t o manner o f articulation, t h e consonants/b/ a n d /d/ a r e called stop consonants because t h e y i n v o l v e a complete closure o f t h e vocal t r a c t , w h i c h stops t h e a i r f l o w . Nasal consonants, such as /m/ a n d /n/ i n v o l v e a closure o f t h e o r a l c a v i t y , but t h e nasal c a v i t y is open. With r e g a r d t o place o f articulation, labial consonants, l i k e /b/ a n d /m/, have t h e i r p o i n t o f maximum v o c a l - t r a c t c o n s t r i c t i o n a t t h e l i p s . Alveolar consonants a r e c o n s t r i c t e d by t h e tip of t h e t o n g u e t o u c h i n g t h e alveolar ridge.

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f a s t e r t h a n when t h e r e were no matched features, t(21) = 2.7, p < .05. A matched manner f e a t u r e y i e l d e d a s i g n i f i c a n t f a c i l i t a t i v e e f f e c t o f 50 ms, t(21) = 1.86, p <.05 [one-tailed]. O n t h e o t h e r hand, a matched place f e a t u r e y i e l d e d a n o n s i g n i f i c a n t e f f e c t o f 9 ms, t ( 2 1 ) = .34, p >

.25. While e r r o r rates were generally low, t h e y e x h i b i t a p o t e n t i a l l y troubling pattern. Fewer e r r o r s o c c u r r e d f o r t h e S-R p a i r s w i t h no matched features, t h a n f o r t h e p a i r s w i t h one matched f e a t u r e . This raises t h e p o s s i b i l i t y t h a t t h e f a c i l i t a t i v e e f f e c t o b s e r v e d on R T f o r t h e p a i r s w i t h matched manner may b e d u e t o a speed-accuracy t r a d e o f f . However, examination o f i n d i v i d u a l subjects' data revealed t h a t a l l o f t h e e r r o r s i n v o l v i n g t h e matched manner p a i r s were committed by a single subject. Thus, a g r e a t e r p r o p e n s i t y t o commit e r r o r s cannot account f o r t h e s h o r t e r RTs o b s e r v e d across subjects when t h e stimuli a n d responses h a d t h e same manner o f a r t i c u l a t i o n .

As in p r e v i o u s studies (Gordon & Meyer, 1984). t h i s experiment p r o v i d e d no evidence o f perceptual-motor interaction f o r place o f a r t i c u lation. However, it did p r o v i d e evidence o f perceptual-motor i n t e r a c t i o n f o r t h e manner d i s t i n c t i o n o f s t o p v e r s u s nasal, demonstrating t h a t v o i c i n g is n o t t h e o n l y f e a t u r e f o r which interaction occurs. This interaction has o n l y been o b s e r v e d f o r features t h a t have been shown t o b e p e r c e p t u a l l y salient (i.e., voiced/voiceless a n d stop/nasal) by Shepard's (1972) multidimensional scaling o f t h e Miller a n d Nicely (1955) a u d i t o r y confusion matrices. T h e n e x t experiment p r o v i d e s another t e s t o f w h e t h e r high perceptual salience of a f e a t u r e is a good p r e d i c t o r o f p e r ceptual-motor i n t e r a c t i o n in t h e p r i m i n g t a s k . Nasal/Fricative D i s t i n c t i o n T h i s experiment employed t h e syllables MAH, VAH, THA, a n d NAH, w h i c h i s a set composed o f nasal consonants (i.e., /m/ a n d /n/) a n d voiced f r i c a t i v e s (i.e., /v/ a n d / a / ) . These syllables were used t o f o r m S-R p a i r s t h a t h a d e i t h e r a matched manner f e a t u r e (e.g., mah-nah) o r unmatched manner features (e.g., vah-mah). T h e r e a r e v e r y few c o n f u sions between voiced f r i c a t i v e s a n d nasals i n t h e Miller a n d Nicely (1955) confusion data, i n d i c a t i n g t h a t t h e d i f f e r e n c e between them is p e r c e p t u a l l y q u i t e salient. T h e experiment was similar t o t h e one above except t h a t it o n l y e x amined t w o p h o n e t i c - f e a t u r e relations: matched manner (fricative/nasal), o r n o matched features. T w e l v e subjects p a r t i c i p a t e d in a t o t a l o f 240 t r i a l s each. Table 2 shows t h e mean RTs o f c o r r e c t vocal responses a n d e r r o r rates f o r t h e d i f f e r e n t p h o n e t i c - f e a t u r e relations between t h e p a i r e d s y l lables. A s i g n i f i c a n t e f f e c t o f a matched manner f e a t u r e (nasal/fricative) was obtained. Responses were, on average, 38 ms f a s t e r w i t h a matched f e a t u r e t h a n without, t ( l 1 ) = 2.36, p < .05. E r r o r rates increased w i t h mean R T s . A c c o r d i n g t o t h e logic o f t h e perceptual-motor p r i m i n g procedure, these results demonstrate perceptual-motor interaction for the nasal/fricative d i s t i n c t i o n . L i k e t h e v o i c i n g d i s t i n c t i o n a n d t h e stop/nasal distinction, t h e nasal/fricative d i s t i n c t i o n is v e r y salient p e r c e p t u a l l y

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(Miller & Nicely, 1955; Shepard, 1972). Perceptual salience w i l l p l a y a k e y role i n t h e model o f perceptual-motor interaction o f f e r e d below. Table 2 Results From Response-Priming T a s k With Manner (Voiced F r i c a t i v e v s . Nasal) D i s t i n c t i o n

Matched phonetic features

Mean R T (ms)

E r r o r s (%)

manner (e.g., MAH-NAH)

65 1

2.1

none (e.g.,

689

5.3

NAH-VAH)

Relevant Phenomena A viable model o f perceptual-motor processing in speech needs t o account f o r several phenomena: First, it must e x p l a i n t h e f a c i l i t a t i v e effects on RTs obtained w i t h t h e p r e s e n t perceptual-motor p r i m i n g p r o c e d u r e u s i n g p e r c e p t u a l l y salient phonetic features, such as v o i c i n g (Gordon & Meyer, 1984), t h e stop/nasal manner distinction, a n d t h e n a s a V f r i c a t i v e manner d i s t i n c t i o n . T h e model must likewise e x p l a i n t h e apparent absence o f an e f f e c t f o r t h e less salient place-of-articulation d i s t i n c t i o n . T h i s should b e done w i t h o u t d i s c a r d i n g place o f articulation as p a r t o f speech processing, because t h e r e is ample evidence t h a t t h e place f e a t u r e p l a y s a s i g n i f i c a n t r o l e in b o t h speech perception a n d p r o d u c t i o n (see Gordon & Meyer, 1984, a n d Meyer & Gordon, 1985, f o r discussions o f t h i s evidence). Second, t h e model must handle some i n h i b i t o r y effects o n RTs t h a t we obtained in a complementary v e r s i o n o f t h e perceptual-motor p r i m i n g p r o c e d u r e (Meyer & Gordon, 1984). Subjects i n t h i s t a s k p r e p a r e d t o p r o d u c e a prespecified vocal response syllable, but were occasionally r e q u i r e d t o switch a n d i d e n t i f y an a u d i t o r y t e s t syllable as r a p i d l y as possible w i t h o u t making t h e o r i g i n a l l y p r e p a r e d response. R T s t o identify t h e t e s t syllable were slower when it h a d a v o i c i n g f e a t u r e matched w i t h t h e p r e p a r e d response t h a n when it h a d n o matched features. The model should account f o r how t h e manifestation o f perceptual-motor i n t e r action depends on t a s k differences.

Third, t h e model should b e consistent w i t h o t h e r f i n d i n g s about t h e motor programming o f phonetic features d u r i n g speech p r o d u c t i o n . Meyer a n d Gordon (1985) r e p o r t e d i n h i b i t o r y effects o f b o t h matched v o i c i n g features a n d matched place-of-articulation features employed as p a r t o f a nonperceptual response-priming p r o c e d u r e . In t h a t s t u d y , subjects p r e p a r e d t o p r o d u c e a prespecified vocal response syllable. After the

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p r e p a r a t i o n i n t e r v a l , t h e y sometimes h a d t o switch a n d vocally p r o d u c e another related response syllable instead. Performance depended on b o t h t h e phonetic features o f v o i c i n g a n d place o f a r t i c u l a t i o n . Longer RTs o c c u r r e d when t h e p r e p a r e d a n d actual vocal responses h a d e i t h e r matched v o i c i n g o r matched place o f a r t i c u l a t i o n as opposed t o no matched features. T h i s outcome shows that, u n d e r c e r t a i n circumstances, a matched place f e a t u r e can i n d u c e s i g n i f i c a n t effects even t h o u g h it fails t o d o so in t h e perceptual-motor p r i m i n g procedure. I n t e r a c t i v e - A c t i v a t i o n Model G i v e n these phenomena, we propose an interactive-activation model o f speech processing t o characterize perceptual-motor interaction. The model specifies a n e t w o r k o f nodes a t various levels of abstraction t h a t communicate w i t h each o t h e r by sending e x c i t a t o r y a n d i n h i b i t o r y signals across l i n k s between t h e nodes. Processing is parallel in t h a t a c t i v i t y proceeds .simultaneously t h r o u g h o u t t h e n e t w o r k . Models such as t h i s have become v e r y p o p u l a r as accounts o f v i s u a l l e t t e r a n d w o r d recognit i o n (McClelland & Rumelhart, 19811, t y p i n g (Rumelhart & Norman, 19811, speech perception (Elman & McClelland, 1984; McClelland & Elman, 19861, a n d speech p r o d u c t i o n (Dell, 1984, 1986; Dell & Reich, 1980; MacKay, 1982; Meyer & Gordon, 1985).’

A schematic diagram o f t h e model appears in F i g u r e 1. It contains separate p h o n e t i c - f e a t u r e nodes a n d phoneme nodes f o r t h e perceptual a n d motor systems. To simplify t h e c u r r e n t presentation, t h e f i g u r e o n l y shows nodes f o r t h e v o i c i n g (voiced a n d voiceless) a n d place-of-articulation, (labial a n d alveolar) features, a n d t h e i r connections t o stop consonants. A more complete v e r s i o n o f t h e model would also i n c l u d e o t h e r phonemes a n d f e a t u r e s . Basic Assumptions O n t h e perceptual side, f e a t u r e nodes a r e assumed t o b e activated by r e l e v a n t sensory input. In turn, t h e y send e x c i t a t o r y signals t o t h e phoneme nodes to w h i c h t h e y a r e connected. A s a phoneme node becomes more a n d more excited, it sends lateral i n h i b i t o r y signals t o o t h e r related phoneme nodes (i.e., ones w i t h some of t h e same f e a t u r e s ) . A phoneme i s p e r c e i v e d when i t s node exceeds a t h r e s h o l d activation level a n d t h e r e b y wins a competition w i t h t h e o t h e r phoneme nodes. The purpose o f t h e i n h i b i t i o n is t o p r e v e n t spurious misperceptions o f phonemes t h a t 7With t h e publication of t h e t w o volume collection on parallel d i s t r i b u t e d processing (Rumelhart & McClelland, 1986), t h i s f o r m o f t h e o r i z ing has been presented as an a l t e r n a t i v e paradigm f o r c o g n i t i v e p s y c h o l ogy, t h a t has aroused considerable c o n t r o v e r s y (Minsky E Papert, 1988; P i n k e r & Prince, 1988). T h e p r e s e n t model is o f f e r e d w i t h o u t a n y commitment t o t h e b r o a d e r claims of connectionism concerning neurobiological plausibility, o r ultimate computational benefits o f parallel processing. T h e choice o f formalism was made because i s seems clear t h a t i n t e r a c t i v e activation models p r o v i d e a n a t u r a l mode o f expression f o r explanations of phenomena t h a t i n v o l v e t h e interaction of d i f f e r e n t k i n d s o f information (such as t h e perceptual a n d motor interaction described in t h e p r e s e n t paper).

Perceptual-Motor Processing in Speech s h a r e some features competition between (1981) t o account McClelland (1984) t o

355

with t h e actual input. Similar assumptions about nodes have been made by McClelland a n d Rumelhart f o r v i s u a l w o r d recognition a n d by Elman a n d account f o r a u d i t o r y w o r d recognition.

ACOUSTIC INPUT

MOTOR OUTPUT

F i g u r e 1. A n i n t e r a c t i v e - a c t i v a t i o n model o f perceptual-motor processing i n speech. T h e model assumes t h a t a similar arrangement e x i s t s on t h e motor side, o n l y in a top-down fashion. There, phoneme nodes receive excitat o r y signals f r o m h i g h e r - l e v e l (e.g., lexical a n d syllabic) o u t p u t processes, a n d t h e y send e x c i t a t o r y signals t o c o r r e s p o n d i n g f e a t u r e nodes. Also, t h e phoneme nodes send lateral i n h i b i t o r y signals t o o t h e r related phoneme nodes, p r e v e n t i n g t h e motor system f r o m m i s f i r i n g a n d p r o d u c i n g a similar, but incorrect, phoneme (Meyer & Gordon, 1985). In addition t o t h e connections w i t h i n t h e perceptual system a n d w i t h i n t h e motor system, t h e model assumes t h a t t h e r e a r e b i d i r e c t i o n a l e x c i t a t o r y cross l i n k s between t h e f e a t u r e nodes f o r speech perception and production. These perceptual-motor l i n k s a r e d r a w n w i t h dashed lines because t h e y a r e weaker t h a n t h e p u r e l y perceptual a n d p u r e l y motor l i n k s . Information about t h e place f e a t u r e as well as t h e v o i c i n g f e a t u r e is t r a n s m i t t e d v i a t h e perceptual-motor links, even t h o u g h no i n t e r a c t i o n has y e t been o b s e r v e d between t h e p e r c e p t u a l a n d motor systems f o r place o f a r t i c u l a t i o n .

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Finally, t h e model incorporates an assumption about t h e n a t u r e o f perceptual salience. T h i s assumption is t h a t differences in t h e p e r c e p t u a l salience o f phonetic features, as measured by t h e c o n f u s a b i l i t y o f audit o r y syllables presented in noise (Miller & Nicely, 1955; Shepard, 1972), stem from t h e r a t e a t w h i c h a u d i t o r y input activates t h e a p p r o p r i a t e p h o n e t i c - f e a t u r e nodes during speech perception. Accordingly, salient features such as v o i c i n g a n d manner would have more r a p i d l y activated nodes t h a n w o u l d nonsalient features such as place o f articulation. One p o s s i b i l i t y i s t h a t t h e d i f f e r e n t i a l r a t e o f activation arises because a u d i t o r y cues t o t h e place f e a t u r e ( p a r t i c u l a r l y t h e second formant transit i o n ) depend h e a v i l y o n t h e i d e n t i t y o f t h e following vowel. Thus, place discriminations may n o t o c c u r until t h a t vowel has been identified, whereas v o i c i n g a n d manner discriminations m i g h t b e completed b e f o r e t h e vowel occurs.

Account of Relevant Phenomena G i v e n these assumptions, it is easy f o r t h e model t o explain t h e p a t t e r n o f r e s u l t s obtained during o u r studies o f speech performance (Gordon & Meyer, 1984; Meyer & Gordon, 1985). In t h e perceptual-motor p r i m i n g procedure, t h e subject hears a syllable a n d m u s t p r o d u c e another response syllable. F o r example, suppose t h e stimulus syllable is PUH. T h e f i r s t e v e n t to happen is t h a t activation s t a r t s increasing i n t h e voiceless f e a t u r e node o f t h e perceptual system. As t h e activation rises, t h e voiceless node begins t o send s t r o n g e x c i t a t o r y signals t o relevant phoneme nodes, s u c h as /p/ a n d /t/. Concurrently, t h e voiceless node o f t h e perceptual system also begins t o e x c i t e t h e voiceless node o f t h e motor system, t h r o u g h t h e cross-links, causing it t o become activated. Meanwhile, acoustic information about t h e place f e a t u r e begins t o a r r i v e more slowly a n d t o a c t i v a t e t h e labial node o f t h e perceptual system. T h i s node in t u r n sends excitation t o a p p r o p r i a t e phoneme nodes, but by t h e time it can s t a r t t r a n s m i t t i n g excitation across t h e perceptual-motor cross-links, t h e r e is already s u f f i c i e n t information f o r phonemic i d e n t i f i cation. As a r e s u l t of t h e phoneme identification, l o n g - t e r m memory connections, l a i d down t h r o u g h l e a r n i n g t h e S-R syllable p a i r i n g s , a r e activ a t e d a n d send excitation t o t h e a p p r o p r i a t e response phoneme. If t h e response phoneme shares t h e v o i c i n g f e a t u r e w i t h t h e stimulus phoneme, t h e n it has t o send less activation down t o t h e f e a t u r e level f o r i n i t i a t i n g t h e response, because t h e o u t p u t node o f t h e v o i c i n g f e a t u r e has already been a c t i v a t e d across t h e perceptual-motor l i n k s . T h i s yields t h e s h o r t e r R T s o b s e r v e d when t h e stimulus a n d response syllables have salient matched phonetic features, such as v o i c i n g o r manner o f articulation. However, when t h e stimulus a n d response syllables have a matched place of articulation, w h i c h is p e r c e p t u a l l y less salient, t h e r e is l i t t l e o r n o r e d u c t i o n in R T . T h e late a r r i v i n g place-of-articulation information does n o t have time t o cross t h e r e l a t i v e l y weak perceptual-motor l i n k s b e f o r e t h e p u r e l y perceptual a n d p u r e l y motor l i n k s fulfill t h e i r f u n c t i o n s . T h e model also can account f o r the results o b s e r v e d f o r t h e proced u r e in w h i c h subjects p r e p a r e d t o p r o d u c e a vocal response syllable, but occasionally were r e q u i r e d t o switch a n d instead i d e n t i f y an a u d i t o r y R T s t o i d e n t i f y t h e stimulus stimulus syllable (Meyer & Gordon, 1983). syllable increased when it h a d a v o i c i n g f e a t u r e matched w i t h t h e p r e p a r e d u t t e r a n c e . No effect o f a matched place-of-articulation f e a t u r e was f o u n d . T h i s outcome follows d i r e c t l y from t h e p r e v i o u s assumptions.

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When a subject p r e p a r e s t o p r o d u c e a phoneme, i t s o u t p u t node i s activated a n d sends e x c i t a t o r y signals t o t h e a p p r o p r i a t e p h o n e t i c - f e a t u r e nodes o f t h e motor system. T h e f e a t u r e nodes o f t h e motor system in turn send activation across t h e perceptual-motor l i n k s t o t h e associated perceptual nodes f o r those features. Cross signals would b e equally s t r o n g f o r t h e place a n d v o i c i n g features. T h e f e a t u r e nodes o f t h e p e r ceptual system would n e x t send signals u p w a r d t o t h e phoneme nodes w i t h which t h e y a r e connected. T h e phoneme node o f t h e perceptual system t h a t is identical t o t h e i n t e n d e d u t t e r a n c e would b e most activated. O t h e r phoneme nodes t h a t share o n l y one feature, v o i c i n g o r place, would b e p a r t i a l l y activated. As a result, t h e c u r r e n t state o f t h e system would influence t h e identification o f a subsequent a u d i t o r y stimu l u s syllable. T h e most r a p i d l y available acoustic information a b o u t t h e a u d i t o r y stimulus syllable involves i t s v o i c i n g feature. If t h e v o i c i n g f e a t u r e i s t h e same as t h a t o f t h e p r e v i o u s l y p r e p a r e d vocal response, t h e n t h e c o r r e c t phoneme node o f t h e perceptual system begins t o receive activation. However, i t s g r e a t e s t competitor, t h e perceptual phoneme node identical t o t h e p r e p a r e d response, w i l l also receive excitation based on acoustic information. T h e competing phoneme node w i l l b e in a h i g h e r state o f activation t h a n t h e p e r c e p t u a l l y c o r r e c t phoneme node, because t h e competitor has received more activation f r o m t h e motor system. Thus, it w i l l t a k e a r e l a t i v e l y long time f o r t h e c o r r e c t phoneme node o f t h e perceptual system t o w i n t h e competition when it has a v o i c i n g f e a t u r e matched w i t h t h e p r e p a r e d response. In contrast, l i t t l e o r no i n h i b i t i o n w i l l o c c u r when t h e a u d i t o r y stimulus syllable has a place f e a t u r e matched w i t h t h e p r e p a r e d vocal response. T h e r a p i d l y available a u d i t o r y v o i c i n g information w i l l allow t h e c o r r e c t perceptual phoneme node t o q u i c k l y dominate t h e competition against i t s already activated neighbors, because t h e y have d i f f e r e n t v o i c i n g features f r o m it.

Furthermore, t h e model can accommodate t h e r e s u l t s f r o m t h e nonperceptual response-priming p r o c e d u r e (Meyer & Gordon, 1985). In t h a t procedure, subjects p r e p a r e d t o p r o d u c e a specified vocal-response syllab l e r a p i d l y , b u t were sometimes r e q u i r e d t o s w i t c h a n d instead u t t e r another related o r u n r e l a t e d response syllable. Slower RTs t o p r o d u c e t h e a l t e r n a t i v e vocal-response syllables o c c u r r e d when t h e y h a d e i t h e r t h e same v o i c i n g o r t h e same place o f a r t i c u l a t i o n as t h e p r e p a r e d prim a r y response t h a n when t h e y h a d no matched features. The inhibitory e f f e c t o f matched place features presumably r e s u l t e d f r o m t h e t a s k b e i n g accomplished essentially w i t h i n t h e confines o f t h e a r t i c u l a t o r y motor system, where we assume t h a t t h e v e r t i c a l (top-down) l i n k s f o r t h e place a n d v o i c i n g features have equal status. Functions of t h e Perceptual-Motor L i n k s From t h e p r e s e n t theoretical perspective, perceptual-motor i n t e r action does n o t d i r e c t l y p e r f o r m t h e f u n c t i o n s t h a t it does in t h e motort h e o r y o f speech perception (Liberman, e t al., 1967; Liberman & Mattingly, 1985). T h e model does n o t use motor codes t o overcome t h e lack o f i n v a r i a n c e i n t h e acoustic speech signal. N o r does it d i r e c t l y p r o v i d e a rationalization f o r t h e d i v e r s e acoustic correlates o f phonetic percepts. T h e r e are, however, several i m p o r t a n t f u n c t i o n s t h a t t h i s interaction m i g h t serve.

P.C. Gordon L i n k s between t h e f e a t u r e nodes o f t h e perceptual a n d motor s y s tems c o u l d have substantial developmental u t i l i t y . T h e y would allow p e r ceptual input t o stimulate t h e motor system, w h i c h m i g h t facilitate i t s maturation. In addition, such l i n k s would allow t h e motor system t o easily o b t a i n information a b o u t t h e acoustic consequences o f i t s o u t p u t . T h i s information would b e useful f o r increasing t h e system's c o n t r o l o v e r t h e accuracy o f t h e a r t i c u l a t o r s . (See Nottebohm, 1970, f o r similar notions on t h e development of b i r d s o n g . ) T h e c r o s s - l i n k s m i g h t also p r o v i d e a basis f o r l e a r n i n g t h e d i v e r s e acoustic correlates o f a r t i c u l a t o r y categories, since t h e y would p r o v i d e a basis f o r c o r r e l a t i n g articulation w i t h acoustics.

In adults, perceptual-motor l i n k s c o u l d also help t h e motor system p r o d u c e speech. T h e l i n k s w o u l d p r o v i d e a mechanism f o r computing t h e expected consequences o f an a r t i c u l a t o r y plan. Knowledge o f these consequences m i g h t b e u s e f u l in m o n i t o r i n g w h e t h e r t h e p l a n is b e i n g successfully executed (Lindblom e t al., 1979). Adequate i n p u t t o t h e p e r ceptual system may b e i m p o r t a n t f o r successful execution o f a r t i c u l a t o r y plans, as revealed by t h e d i s r u p t i v e effects o f delayed a u d i t o r y feedb a c k . T h e perceptual-motor l i n k s proposed h e r e would allow t h e a r t i c u l a t o r y motor system t o p r i m e t h e perceptual system f o r t h e input expected as a r e s u l t o f i t s a c t i v i t y . By A c t i v e perceptual-motor tin k s could also a i d speech perception. mapping an a u d i t o r y stimulus o n t o a r t i c u l a t o r y features, t h e l i n k s would p r o v i d e an easy way o f g e t t i n g perceptual information t o t h e " a r t i c u l a t o r y loop" of s h o r t - t e r m memory, w h i c h has been postulated as an important component o f language comprehension (Baddeley, 1986). A t a more basic level, t h e l i n k s f r o m t h e motor system t o t h e perceptual system may cons t i t u t e a mechanism w h e r e b y top-down information is used t o p r i m e t h e perceptual system. Priming of t h i s s o r t p l a y s an important p a r t i n numerous models o f speech perception, i n c l u d i n g ones t h a t d o n o t have an e x p l i c i t motor component (Elman & McClelland, 1984; McClelland & Elman, 1986). It is likewise consistent w i t h t h e analysis-by-synthesis model o f speech perception (Steven & Halle, 19671, which assumes t h a t t h e c o n s t r u c t i v e capabilities of t h e a r t i c u l a t o r y motor system a r e used i n g e n e r a t i n g perceptual hypotheses. Thus, perceptual-motor interaction, as characterized here, complements a n d extends t h e more t r a d i t i o n a l conceptualizations o f a r t i c u l a t o r y p a r t i c i p a t i o n in speech perception. Summary S - R compatibility research u s i n g speech stimuli a n d responses should b e understood against t h e b a c k d r o p of l o n g - s t a n d i n g i n t e r e s t s i n possible i n t e r r e l a t i o n between t h e perceptual a n d motor systems f o r speech. Speech researchers have been i n t e r e s t e d i n perceptual-motor relations because many c u r i o u s aspects o f t h e relation between acoustic stimuli a n d phonetic perception make sense when t h e n a t u r e o f a r t i c u l a t i o n is t a k e n i n t o account. T h i s has led t o t h e development o f t h e motor theory o f speech p e r c e p t i o n . S - R compatibility t a s k s p r o v i d e a r e l a t i v e l y d i r e c t w a y o f s t u d y i n g t h e influence o f speech perception on speech p r o duction. Research u s i n g these t a s k s has shown t h a t a v e r y close relation exists between t h e perception a n d p r o d u c t i o n o f some phonetic features. T w o experiments were r e p o r t e d t h a t examine perceptual-motor interaction f o r t h e nasal/stop consonant d i s t i n c t i o n a n d f o r t h e n a s a V f r i c a t i v e d i s tinction. Perceptual-motor interaction i s shown f o r b o t h features. I t is

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a r g u e d t h a t S-R compatibility t a s k s o n l y reveal perceptual-motor i n t e r action f o r features t h a t a r e p e r c e p t u a l l y salient. An interactive-activat i o n model, c o n t a i n i n g perceptual-motor l i n k s a t t h e level o f phonetic features, was presented t o account f o r S - R compatibility effects in speech. T h e possible f u n c t i o n s o f these l i n k s in development, speech production, a n d speech perception were discussed. Acknowledgements Preparation o f t h i s c h a p t e r was s u p p o r t e d by g r a n t #87-305 f r o m I would l i k e t o t h a n k K a y Bock t h e AFOSR L i f e Sciences Directorate. a n d Sandy Waxman f o r h e l p f u l comments on an e a r l i e r v e r s i o n o f t h e manuscript. References Baddeley, A. D. s i t y Press.

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Experimental psychology.

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PART V APPLICATIONS T O HUMAN FACTORS T h e t h r e e chapters of P a r t V discuss applications o f t h e knowledge r e g a r d i n g stimulus-response compatibility to human-factors issues. C h a p t e r 15, by Kantowitz, T r i g g s , a n d Barnes, i l l u s t r a t e s a v a r i e t y o f compatibility effects t h a t o c c u r i n real-world s e t t i n g s . T h e y discriminate such effects on t h e basis o f w h e t h e r t h e effects a r i s e f r o m frames, rules, o r response tendencies. C h a p t e r 16, by E b e r t s a n d Posey, presents a framework, based on t h e notion of mental models, t h a t has been developed t h r o u g h applied research. T h e framework stresses t h e importance of d e s i g n i n g environments t o b e consistent w i t h good mental models, r a t h e r t h a n w i t h those o f novices. C h a p t e r 17, by J o h n a n d Newell, presents an e n g i n e e r i n g model o f stimulus-response Compatibility a n d p r o vides examples i l l u s t r a t i n g t h e p r e d i c t i v e power o f t h e model.

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STIMULUS-RESPONSE COMPATIB I LlTY AND HUMAN FACTORS BARRY H. KANTOWITZ, THOMAS J. TRIGGS, a n d VALERIE E. BARNES Battelle Human A f f a i r s Research Centers Seattle, Washington Human f a c t o r s i s a pragmatic discipline; i t s goal is t o solve problems. Human factors has been defined as " t h e discipline t h a t t r i e s t o optimize t h e relationship between technology a n d t h e human" (Kantowitz c Sorkin, 1983, p. 4). In practice, however, systems involving people a r e so complex t h a t human-factors e x p e r t s a r e q u i t e h a p p y o n l y t o improve t h e relationships, even if that improvement is less t h a n optimal. In o r d e r f o r human f a c t o r s t o p r o v i d e an e f f e c t i v e solution, a p p r o p r i a t e problems must f i r s t b e recognized. A n important goal o f t h i s c h a p t e r is t o recognize t h a t problems associated w i t h stimulus-response (S-R) compatibility a r e more t h a n j u s t i n t e r e s t i n g phenomena t h a t fascinate experimental psychologists. T h i s c h a p t e r w i l l a r g u e t h a t systems t h a t d i s r e g a r d S-R compatibility s u f f e r diminished success because o f such neglect. We w i l l o f f e r several real-world examples t o make t h i s p o i n t salient. Recognizing a problem, while a c r i t i c a l f i r s t step, i s n o t sufficient for s o l v i n g t h e problem. E f f e c t i v e human factors r e q u i r e s t w o essential elements (Kantowitz, 1987). First, one must have practical knowledge of t h e system u n d e r s t u d y . Without such f i r s t - h a n d experience, research degenerates i n t o academic exercises that w i l l n o t solve problems efficiently. Second, one must have a t h o r o u g h u n d e r s t a n d i n g o f t h e o r y a n d methodology t o b e applied t o t h e problem. Without t h i s academic knowledge, solutions can b e technically unsound even t h o u g h t h e y may a t f i r s t create t h e illusion o f craftsmanship. Accordingly, t h i s c h a p t e r w i l l examine b o t h p r a c t i c e a n d t h e o r y . A l t h o u g h an academic audience need n o t b e i n s t r u c t e d in t h e v i r t u e s o f theory, it i s o u r hope t h a t t h i s c h a p t e r w i l l also b e read by p r a c t i t i o n e r s desirous o f s o l v i n g problems. Thus, it is w o r t h w h i l e b r i e f l y t o recapitulate arguments f o r t h e premise t h a t t h e o r y is t h e b e s t practical tool available (Kantowitz, 1988; Kantowitz & Casper, 1988). T h e o r y f i l l s in where human-factors data are lacking. No handbook w i l l e v e r contain all t h e answers. Where some data a r e available, t h e o r y i s r e q u i r e d f o r T h e o r y yields precise predictions accurate and sensible interpolation. w i t h o u t h a v i n g t o f i r s t build t h e system t o s t u d y a situation empirically. By t h e n it i s o f t e n too late t o f i x problems. T h e o r y p r e v e n t s us f r o m u n w i t t i n g l y r e i n v e n t i n g t h e wheel w i t h each new problem by allowing us t o recognize similarities among problems. If an a p p r o p r i a t e t h e o r y is available, it can b e used as a design tool t o avoid potential problems.

In t h e remainder o f t h e chapter, t h e f i r s t t o p i c discussed involves theoretical aspects o f S-R compatibility problems. T h i s discussion i s followed by a review of S-R compatibility in actual practice. T h i s descript i o n i s followed by a more theoretical perspective t h a t postulates sources o f S-R compatibility in terms o f a nested h i e r a r c h y o f frames, rules, a n d

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response tendencies. Finally, we conclude by comparing pragmatic a n d theoretical aspects o f S-R compatibility f r o m a human f a c t o r s perspective. Theoretical Aspects o f S-R C o m p a t i b i l i t y

In t h i s chapter, we l i m i t discussion o f theoretical explanations o f S R compatibility t o o n l y t h r e e aspects. F i r s t , how is S-R compatibility defined? Second, how i s S-R compatibility measured? Third, how v a l i d is S-R compatibility as a psychological c o n s t r u c t a n d as a human f a c t o r s construct? O u r i n t e n t is n o t t o establish o r j u s t i f y a n y p a r t i c u l a r theoretical explanation but instead t o consider t h e global pre-theoretical issues t h a t m u s t b e elucidated b e f o r e a n y c o n s t r u c t can b e applied effectively. D e f i n i n g S-R Compatibility In i t s simplest form, S-R compatibility is a p r o p e r t y o f t h e spatial mapping between a set o f stimuli a n d a c o r r e s p o n d i n g set o f responses [ F i t t s & Seeger, 1953). Spatial compatibility has been t h e most common f o r m o f S-R compatibility s t u d i e d in t h e l a b o r a t o r y . However, in human f a c t o r s practice, S-R compatibility problems o f t e n a r i s e d u e t o t h e conceptual mapping between stimulus a n d response. F o r b o t h spatial a n d conceptual mappings, t h e problem in p r e c i s e l y d e f i n i n g S-R compatibility centers a b o u t w h e t h e r some f u n c t i o n computed d i r e c t l y on t h e stimulus a n d response sets (e.g., M o r i n E Grant, 1955) is a s u f f i c i e n t i n d e x . Often, it i s necessary t o b e less d i r e c t a n d d e f i n e S-R compatibility as a f u n c t i o n o f people's b e h a v i o r when faced w i t h t h e t w o sets (Kantowitz & Sorkin, 1983, p. 147). B o t h approaches have been used a n d b o t h have been successful. Since one cannot d i s t i n g u i s h among competing d e f i n i t i o n s by a s k i n g w h i c h d e f i n i t i o n is t r u e (all d e f i n i t i o n s a r e t r u e by definition), one i s f o r c e d t o ask w h i c h d e f i n i t i o n is most useful. The d e f i n i t i o n t h a t is most useful, in turn, depends o n t h e goals o f t h e persons f o r m u l a t i n g t h e d e f i n i t i o n .

A n experimental psychologist i n t e r e s t e d in u n d e r s t a n d i n g human information processing would b e b e t t e r o f f w i t h a d e f i n i t i o n t h a t c o u l d b e d i r e c t l y calculated f r o m o n l y the stimulus a n d response sets. A direct d e f i n i t i o n avoids t h e dangers o f c i r c u l a r i t y t h a t a r i s e when S-R compatibility is d e f i n e d as t h a t spatial arrangement t h a t (a) people r a t e as most compatible o r (b) produces some d e s i r e d b e h a v i o r such as minimizing reaction time ( R T ) : T h e most compatible o f N arrangements is t h e one t o w h i c h subjects respond most q u i c k l y because it is t h e most compatible arrangement. However, t h e d e f i n i t i o n a l purity of t h e d i r e c t determination when achieved is o f t e n a t t h e cost o f c r e a t i n g problems i n measuring S - R compatibility, w h i c h t h e following section w i l l address. I n d i r e c t definitions, a l t h o u g h u s u a l l y c i r c u l a r , have been p r o v i d e d a n d used (even by experimental psychologists) more f r e q u e n t l y t h a n d i r e c t definitions. T h e use o f i n d i r e c t measures is less embarrassing f o r t h e human f a c t o r s specialist, whose goal is to create e i t h e r an optimal, o r a t least a satisfactory, arrangement t h a t w i l l p e r m i t people a n d systems t o interact efficiently. It is o f t e n possible t o solve a problem s a t i s f a c t o r i l y w i t h less t h a n complete u n d e r s t a n d i n g o f t h e theoretical p r i n c i p l e s involved, a l t h o u g h most scientists w o u l d agree t h a t such u n d e r s t a n d i n g (if available) should in p r i n c i p l e lead t o b e t t e r problem solutions. So,

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most human f a c t o r s specialists have been c o n t e n t w i t h i n d i r e c t d e f i n i t i o n s based o n e i t h e r r a t i n g s o r system o u t p u t s . Measuring S - R Compati biIity It is a dictum in psychblogy t h a t a n y t h i n g t h a t e x i s t s m u s t e x i s t in some q u a n t i t y a n d so should b e amenable t o measurement. Alas, t h e r e is less t h a n u n i v e r s a l agreement about t h e u n i t o f S-R compatibility. The most common way t o measure S - R compatibility, w i d e l y used in human factors, is t o t a k e a vote; t h a t is, several arrangements a r e p o r t r a y e d a n d users a r e asked t o select t h e S-R mapping t h e y find most,,congenial. This democratic measurement technique produces a population stereotype." A s l i g h t l y more scientific method, albeit logically equivalent, r e q u i r e s a computer t o conduct an experiment. RTs a n d e r r o r rates a r e recorded, a n d t h e fastest a n d most accurate mapping i s deemed t h e most compatible. Scientists who a r e suspicious a b o u t t h e subjective element in r a t i n g schemes a r e o f t e n more contented w i t h o b j e c t i v e latencies a n d e r r o r rates. O f course, b o t h o f these i n d i r e c t methods s u f f e r f r o m t h e c i r c u l a r i t y discussed earlier, f o r example, t h e most compatible mapping is t h e fastest a n d t h e fastest is t h e most cornp a t ible . One extremely c l e v e r solution t o t h e measurement problem was f o r mulated by M o r i n a n d G r a n t (1955) who i n v e n t e d a d i r e c t q u a n t i t a t i v e estimate o f S-R compatibility. Stimuli were a horizontal a r r a y o f 10 lamps, and responses were a r o w o f t e n k e y s . Elements in b o t h a r r a y s c o u l d b e numbered f r o m 1 t o 10. Kendall's tau, a non-parametric c o r r e lation coeffecient, was used t o measure S-R compatibility a p r i o r i . For example, a t a u of '1.0 indicated t h a t lamp 1 was controlled by k e y 1, lamp 2 by k e y 2 , , . . . , lamp 10 by k e y 10. A t a u o f -1.0 indicated t h a t lamp 1 was controlled by k e y 10, lamp 2 by k e y 9, a n d so on. Results of t h e i r experiment a r e shown i n F i g u r e 1. R T was an i n v e r t e d

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U-shaped f u n c t i o n of tau, w i t h f a s t e r R T f o r t a u = '1 v e r s u s t a u = -1. T h i s outcome agrees pleasingly w i t h o u r i n t u i t i v e feel f o r w h a t is meant by S-R compatibility. B u t as elegant as t h i s approach is, it i s limited t o those cases w h e r e numbers can b e meaningfully applied t o stimulus a n d response a r r a y s in t h e same way. F o r example, t h i s approach cannot b e used when a c i r c u l a r stimulus a r r a y i s mapped t o a l i n e a r response array. E x c e p t f o r a small subset o f a p p r o p r i a t e l y geometered S-R mappings (cf. M o r i n & Grant, 1955), it is d i f f i c u l t t o d i r e c t l y measure S-R compatibility. T h i s i s especially t r u e when t h e S-R compatibility concept is expanded t o i n c l u d e t h e o t h e r t y p e s o f S-R ambiguities (Fitts, 1951) discussed l a t e r in t h i s c h a p t e r . Hence, puzzlements about i n d i r e c t measurement o f S-R compatibility c e n t e r about lack o f c o n v e r g i n g operations r a t h e r t h a n a b o u t t h e i n h e r e n t c i r c u l a r i t y o f i n d i r e c t It i s q u i t e possible t h a t t w o d i f f e r e n t i n d i r e c t measures o f S measures. R compatibility may y i e l d d i f f e r e n t outcomes. F o r example, i n addition t o an R T measure o f performance, M o r i n a n d G r a n t (1955) also asked subjects t o d r a w t h e mappings between stimuli a n d responses. It seems reasonable t o expect t h a t less compatible mappings (taus nearer t o zero) m i g h t b e h a r d e r t o learn. However, s u f f i c i e n t p r a c t i c e was g i v e n so t h a t a l l subjects were able t o d r a w t h e mappings c o r r e c t l y . Hence, a measure o f S-R compatibility based on recall indicated no differences, even t h o u g h To consider t h e one based on R T showed dramatic differences. misleading question o f w h i c h measure (time o r recall) is correct, we now turn to a b r i e f discussion o f v a l i d i t y .

Construct V a l i d i t y T h e r e a r e many k i n d s o f v a l i d i t y t h a t experimental psychologists should consider (Elmes, Kantowitz, & Roediger, 1989; Nunnally, 1967). Here we discuss o n l y c o n s t r u c t v a l i d i t y . " T o t h e e x t e n t t h a t a variable is a b s t r a c t r a t h e r t h a n concrete, we speak of it as b e i n g a c o n s t r u c t . Such a v a r i a b l e i s l i t e r a l l y a c o n s t r u c t in t h a t it is something t h e scientist put t o g e t h e r f r o m h i s own imagination, something t h a t does not, e x i s t as an isolated, observable dimension o f behavior" (Nunnally, 1967, p . 85). T h e goal o f s c i e n t i f i c research is t o establish a c o n s t r u c t (Nunnally, 1967, p. 88): (a) t h a t is well d e f i n e d in terms o f a v a r i e t y o f observables; (b) f o r w h i c h t h e r e a r e one o r several variables t h a t well r e p r e s e n t t h e domain o f observables; a n d ( c ) t h a t e v e n t u a l l y p r o v e s t o relate s t r o n g l y w i t h o t h e r c o n s t r u c t s of i n t e r e s t . F o r now, we merely note t h a t v a l i d i t y depends in p a r t on t h e goals of t h e researcher. While t h e basic researcher is concerned w i t h an a b s t r a c t construct, t h e applied researcher cares n o t o n l y t h a t t h e a b s t r a c t c o n s t r u c t e x i s t s but also t h a t it w o r k s in some p a r t i c u l a r application. S-R compatibility is a c o n s t r u c t . I t s existence cannot b e observed but can o n l y b e i n f e r r e d f r o m changes i n b e h a v i o r . When t h e S-R compatibility c o n s t r u c t is used i n basic research, one hopes t h a t t h e c o n s t r u c t i s independent o f a n y p a r t i c u l a r measuring i n s t r u m e n t o r measurement technique. B u t when used t o solve an applied problem in human factors, f o r example, t o design a c o n t r o l - d i s p l a y panel, t h e c o n s t r u c t must b e validated by t h e specific measurement i n s t r u m e n t used; t h i s makes it similar t o a psychometric i n d i c a t o r (Campbell, 1976, p. 203).

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T h e basic researcher who uses t h e measure o f R T has made a wise choice, because R T has an extremely small domain o f related observable variables (Nunnally, 1967, pp. 84-86). A l t e r n a t i v e methods f o r measuring R T a r e v e r y closely related. B u t t h i s does n o t necessarily imply t h a t R T is t h e b e s t measure o f S-R compatibility. T h e applied researcher, who may already believe in S-R compatibility because of basic R T research, s t i l l has no guarantee that t h e c o n s t r u c t w i l l p r o v e v a l i d in h i s application. If something o t h e r t h a n R T m u s t b e used t o index S-R compatibility, t h e researcher must ensure t h a t t h e observable index Even then, it i s an empirical remains in t h e domain o f t h e c o n s t r u c t . issue t o determine if t h e S-R compatibility c o n s t r u c t f u n c t i o n s in some manner t o influence t o t a l system o u t p u t (Chapanis, 1988). For example, t a k e a system (such as a chemical process plant) where t h e r e is a long time delay a n d t h e operator has several minutes t o respond t o displayed If poor S-R compatibility o n l y influences t h e system by information. increasing R T by 500 ms, it may n o t m e r i t a n y consideration in system design. B u t if it also increases operator e r r o r rates by 258, no responsible system designer would f a i l t o improve S-R compatibility w i t h i n t h e system. T h e reader should now realize why t h e question posed earlier in t h i s section--Should S-R compatibility b e measured by time o r by e r r o r ? - is misleading. T h e r e is n o u n i v e r s a l m e t r i c f o r S-R compatibility t h a t is suitable f o r all situations. Choice o f a m e t r i c i s intimately related t o one's scientific o r pragmatic goals. Indeed, f o r many o f t h e humanfactors examples t h a t a r e t o follow, t h e choice o f a metric i s implicit because data o f t e n have n o t been systematically collected. Instead, based on anecdotal evidence o r quasi-systematic observation o f a small number of operators, inferences a r e d r a w n t h a t a g i v e n S-R compatibility relation w i t h i n a human-technology system i s inadequate. T h e implicit metric i s usually e r r o r , b u t l i t t l e consideration is devoted t o how such e r r o r can b e measured or, even if it was actually measured, what implications t h i s has f o r c o n s t r u c t v a l i d i t y . These limitations should b e k e p t in mind as t h e following section is read. Varieties of S-R Compatibility Observations o v e r a wide r a n g e o f operational settings have i d e n t i f i e d a number o f sources o f incompatibility in generating a p p r o p r i a t e responses t o p a r t i c u l a r stimulus configurations. Here we r e f e r t o incomp a t i b i l i t y as a general lack o f agreement across t h e population of potential t h e linkages between i n d i v i d u a l stimuli a n d users concerning (a) responses o r (b) t h e movement relation between a d i s p l a y and i t s associated control. Failure t o find good response u n i f o r m i t y across users may r e s u l t f r o m situational ambiguity, where people a r e generally confused about t h e c o r r e c t S-R relation. Alternatively, i n d i v i d u a l s may respond w i t h a h i g h degree of confidence, b u t because o f d i f f e r e n t ways o f p e r c e i v i n g t h e situation o r because o f d i f f e r i n g experiences, t h e r e i s not a s t r o n g u n i f o r m i t y in responses across users. I n a n i n f l u e n t i a l publication, F i t t s (1951) classified t h e t y p e s of incompatibilities f o u n d in stimulus-response relationships. Th: f o u r basic classes w i l l b e discussed here, s u p p o r t e d by some o f F i t t s examples, along w i t h some additional categories t h a t we have added in o r d e r t o broaden t h e o r i g i n a l classifications.

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Basic Classification Figure-ground. I n some classes of displays, a potential source of It may not always be possambiguity is t h e figure-ground relationship. ible t o separate t h e principal element (or "figure") from t h e background information (or "ground"). An example of the classical confusion between An everyday example of t h e f i g u r e and g r o u n d i s shown i n Figure 2 . figure-ground ambiguity is t h e confusion often experienced a t a r a i l station when two trains located on adjacent tracks. A passenger i n one of t h e trains often has d i f f i c u l t y in initially determining whether it is his o r h e r t r a i n that is moving o r t h e adjacent one. Figure-ground ambiguity is a major factor i n t h e long-standing debate o f whether orientation displays i n aviation and space should be When aircraft-referenced o r horizon-referenced (Fitts & Jones, 1947). viewing such an orientation display, users can on occasion be uncertain whether t h e element t h a t moves is the symbol f o r the a i r c r a f t o r the horizon. T h e traditional artificial horizon display shown i n Figure 3 evolved f o r t h e development of t h e original S p e r r y b l i n d - f l y i n g instrument i n t h e 1930s. This display is organized so t h a t it simulates t h e view seen by a pilot looking ahead o u t of t h e cockpit. I n other words, the element t h a t "moves" is t h e horizon. However, it is not clear t h a t a significant proportion of pilots naturally i n t e r p r e t t h e display in this manner. Certainly, many inexperienced pilots will respond as though t h e element t h a t moves is t h e symbol representing the aircraft. While t h e r e are other factors of importance i n this situation (Roscoe, 1968, 19871, such as the temporal characteristics of t h e display, t h e figure-ground confusion has been a major contributor t o response e r r o r s associated w i t h t h i s particular instrument and many associated aviation incidents and accidents. Another example of figure-ground ambiguity i s t h e function of t h e "scroll" key on a visual display terminal (VDT). The scroll key allows t h e V D T user t o change the information t h a t is displayed on the screen. The scroll key can be designed so t h a t t h e data displayed are "the f i g u r e " t h a t moves behind t h e screen o r "the ground" across which the screen moves t o reveal information i n the data file. Bury, Boyle, Evey, and Neal (1982) conducted a series of studies t o determine which function B y allowing users t o of the scroll key is p r e f e r r e d by computer users. define t h e function of the scroll key, B u r y et al. found a strong p r e f e r ence f o r t h e scroll key t o act EI,S though it,,moves the screen across the data, a function they termed windowing. When t h e scroll key was defined by t h e experimenters t o move t h e data behind the screen (i.e., t o scroll), users performed t h e experimental tasks more slowly and Because required a larger number of keystrokes t o complete t h e tasks. VDTs v a r y i n how t h e scroll keys function, users are faced w i t h this ambiguity when switching among systems. E r r o r versus command. T h e second basic,, t y p e of ambiguity identiThe confusion t h a t can fied by Fitts i s t h a t of " e r r o r versus command. arise i n some types of display o r communication is whether the information presented is specifying t h a t an e r r o r exists o r whether t h e operator i s being issued a command o r directive. For example, statements exchanged between operators will often be in shorthand form and as a consequence omit t h e v e r b (e.g., a f i r e department hoseman may simply s a y "pressure

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Figure 2. Classical example of figure-ground ambiguity: . picture of a vase o r of two people facing one another?

Figure 3 . Artifical horizon display for an aircraft cockpit. element should move, the horizon or t h e aircraft?

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high" o r an i n s t r u c t o r p i l o t may say " l e f t - w i n g low").' Visual displays in i n d u s t r i a l s e t t i n g s a r e sometimes set u p t o indicate a command, b u t have been i n t e r p r e t e d as i n d i c a t i n g an e r r o r . Associated w i t h t h i s t y p e o f ambiguity a r e v e r b a l expressions that, because o f t h e i r specific use o f words, a r e m i s i n t e r p r e t e d as statements o f command r a t h e r t h a n as an indication o f system s t a t u s . A well-known example is t h e a i r c r a f t captain r e d u c i n g power when accelerating down t h e a i r p o r t r u n w a y when h e hears t h e co-pilot say "Take-off power." In a t least one incident, t h e captain did n o t c o r r e c t l y i d e n t i f y t h e statement as meaning t h a t t h e c o r r e c t power level h a d been reached. T h i s ambiguity can also b e f o u n d i n some i n d u s t r i a l situations. Statements i n t e n d e d t o describe t h e p u r p o s e o r goal o f an a c t i v i t y can b e m i s i n t e r p r e t e d as commands. F o r example, emergency o p e r a t i n g proced u r e s a t many nuclear power p l a n t s p r e s e n t i n s t r u c t i o n s t o mitigate t h e t r a n s i e n t as u p p e r - l e v e l steps t h a t summarize a t a s k accompanied by a series of lower-level steps t h a t explain i n more detail how t o p e r f o r m t h e t a s k (see Table 1). If an o p e r a t o r performs t h e u p p e r - l e v e l step a n d t h e n performs each o f t h e lower-level steps, h e w i l l have performed t h e action twice. In t h e example in Table 1, t a k e n f r o m an actual nuclear power p l a n t procedure, an o p e r a t o r following w h a t appears t o b e i n s t r u c t i o n s in b o t h t h e ACTlONS column a n d t h e DETAlLS column would t r a n s f e r c o n t r o l t o t h e remote shutdown station twice.

Table 1 Passage From a Nuclear Power Plant Emergency O p e r a t i n g Procedure in Which a Section Heading Appears t o b e an Action Step DETAILS

ACTIONS WHEN t h e Remote Shutdown is manned, THEN t r a n s f e r c o n t r o l t o t h e Remote Shutdown Station.

I

Establish communications the Control Room, if

2

"6" Notify SSOD to transfer controls a n d v e r i f y "6" t r a n s f e r status light is lit.

3.

N o t i f y SSOD t o t r a n s f e r " A " cont r o l s i n t h e A ES 4 160V room, a n d v e r i f y "A" t r a n s f e r status l i g h t i s lit.

4.

N o t i f y SSOD t o proceed t o t h e Remote Shutdown Station. " A 6" a n d " N0N - S A FE T Y " T r a n sf e r controls.

5.

6.

N o t i f y C o n t r o l Room t h a t is complete, if possible.

with the possible.

transfer

'These examples were developed by one of the authors (Triggs) w i t h P. M. F i t t s in 1963.

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I n t e r v e n i n g variables. A t h i r d source o f incompatibility between t h e indication on a d i s p l a y a n d t h e response of an o p e r a t o r is t h e r o l e o f i n t e r v e n i n g variables. A n y o n e who has endeavored t o b a c k a t r a i l e r attached t o t h e r e a r o f an automobile i n t o a n a r r o w space f o r t h e f i r s t time w i l l recognize how an i n t e r v e n i n g v a r i a b l e (namely t h e l i n k a g e between t h e t w o vehicles) can influence t h e difficulty o f selecting t h e a p p r o p r i a t e response t o p r o d u c e a d e s i r e d p a t h a n d position o f t h e trailer . Consider t h a t a d i s p l a y is i n d i c a t i n g t h a t an i n d u s t r i a l process i s o p e r a t i n g a t above-average temperature a n d t h a t t h e p l a n t o p e r a t o r can manipulate a c o n t r o l t o a l t e r t h i s temperature. Does t h e o p e r a t o r t u r n t h e c o n t r o l t o reduce t h e temperature d i r e c t l y , o r t o increase t h e flow o f coolant water t h r o u g h t h e system? T h i s choice is l i k e l y t o depend v e r y much o n t h e operator's knowledge of t h e p l a n t operations, h i s knowledge o f how t h e c o n t r o l room is set up a n d operates, a n d t h e specific labeling associated w i t h t h e d i s p l a y a n d t h e c o n t r o l . In some countries, in recent years, some automobiles were equipped w i t h f u e l economy displays, shown in F i g u r e 4, w i t h a single p o i n t e r moving f r o m low t o high. A number of anecdotal r e p o r t s showed t h a t t h i s d i s p l a y caused confusion because was open t o t w o i n t e r p r e t a t i y s - a high reading c o u l d b e i n d i c a t i n g high r a t e o f f u e l use" o r high economy." T h e response one would make t o t h i s t y p e o f d i s p l a y would depend l a r g e l y on w h e t h e r t h e dial indicated t h e o v e r a l l v a r i a b l e o f concern (economy) o r t h e i n t e r v e n i n g v a r i a b l e o f r a t e o f f u e l use.

\;

F i g u r e 4. Fuel economy d i s p l a y : Does a low r e a d i n g indicate "low f u e l economy" o r "low r a t e o f f u e l use"?

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Polar vs. r e c t i l i n e a r coordinates. T h e f o u r t h t y p e o f confusion is t h a t represented by t h e c o n f l i c t between p o l a r a n d r e c t i l i n e a r coordinates. When operators a r e asked t o c o r r e c t e r r o r s u s i n g c i r c u l a r d i s plays, it has been found t h a t t h e speed a n d accuracy o f response depend on t h e sector o f t h e c i r c u l a r d i s p l a y in w h i c h t h e p o i n t e r a n d "null" p o i n t a r e positioned. T h e null p o i n t i s t h e position on t h e d i s p l a y r e p r e s e n t i n g t h e r e q u i r e d v a l u e t h a t t h e moving p o i n t e r must b e set against. F i g u r e 5 shows f o u r d i f f e r e n t null positions t o which t h e p o i n t e r needs t o b e aligned t o cancel t h e d i s p l a y e r r o r . T h e 9 o'clock a n d t h e 12 o'clock null positions y i e l d t h e f a s t e r a n d more accurate performance compared w i t h t h e 6 o'clock a n d 3 o'clock positions. T h i s b e t t e r performance is because, w i t h c i r c u l a r displays, t h e relation between t h e d i s p l a y i n d i c a t o r a n d t h e associated response is g o v e r n e d by t w o tendencies. A deviation f r o m t h e null is r e g a r d e d in terms o f w h e t h e r it is a clockwise o r counterclockwise displacement ( c i r c u l a r coordinates). T h e r e is a t e n d e n c y t o r e g a r d a clockwise rotation f r o m t h e null as an increase, r e q u i r i n g a response t h a t reduces t h e value (Loveless, 1963). In addition, t h e r e is also a tendency in t h e v i c i n i t y o f t h e null p o i n t t o consider a displacement o f t h e p o i n t e r above o r t o t h e right o f t h e n u l l p o i n t ( r e c t i l i n e a r coordinates) as r e p r e s e n t i n g a value g r e a t e r t h a n null. When these t w o tendencies a r e i n congruence, as in t h e case o f t h e 9 o'clock a n d 12 o'clock locations, b e t t e r performance obtains t h a n when t h e y a r e in t h e c o n f l i c t situations o f t h e 3 o'clock a n d 6 o'clock positions.

I t should b e noted t h a t t h i s ambiguity o f coordinates can also p l a y An a role in situations where t h e c o n t r o l is, i n fact, also t h e d i s p l a y . example o f t h i s i s t h e automobile s t a l k c o n t r o l . T h e s t a l k is mounted o u t f r o m t h e s t e e r i n g wheel column e i t h e r t o t h e l e f t o r right. One mode o f operation is p u s h i n g up o r down, w h i c h corresponds t o clockwise o r counterclockwise motion. B e t t e r performance in t h i s mode would b e p r e d i c t e d f o r s t a l k controls mounted on t h e l e f t side o f t h e s t e e r i n g wheel (in t h e 9 o'clock position).

12 O'clock

3 O'clock

Null

Null

6 O'clock Null

9 O'clock

Null

F i g u r e 5. Which o f these f o u r possible n u l l positions y i e l d f a s t e r a n d more accurate responses when t h e t a s k i s t o manipulate some c o n t r o l t o move t h e p o i n t e r t o t h e n u l l position?

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Additional Types of Incompatibility Some f u r t h e r incompatibilities, which were not considered by Fitts, will now be presented. These are intended t o show how subtle some of t h e factors t h a t influence S-R compatibility and t h e associated strength of population stereotypes can be. The examples were selected t o provide a basis f o r identifying some sources of incompatibility i n S-R relations i n addition t o t h e f o u r basic types defined by Fitts. Multiple sub-tasks. Many display-control environments involve disp l a y s and controls w i t h multiple elements. Combinations of displays introduce the possibility of display elements interacting i n ways not intended f o r t h e task by t h e system designer. An illustration of this, shown i n Figure 6, was found i n a U.S. a i r p o r t . 2 T h e issue here is t h a t of association between t h e gate numbers and t h e direction arrows. Respondents confronted with the problem of what direction t o turn t o reach Gate 4, f o r example, are confused by t h e display. This d i f f i c u l t y is easily solved by the introduction of an appropriate line (either vertical o r horizontal, depending on whether Gates 1-5 are t o t h e l e f t o r t h e right) between t h e display elements. However, t h i s example is representative of a class of multi-element guidance signs i n public areas t h a t can be confusing.

GATE

6-10

Figure 6.

I n which direction should one t u r n t o reach Gate 4?

Another example of t h i s type, involving a commercially available a i r c r a f t radio receiver, has been provided by Hawkins (1976). The general configuration of t h i s device is shown i n Figure 7. The task required of t h e pilot is t o set a f o u r - d i g i t radio frequency using t h e f o u r r o t a r y controls provided i n two ganged pairs. The general problem here is t o determine which knob controls the counter readout at each of t h e f o u r positions. The solution chosen by t h e designer was t o associate t h e smaller l e f t knob with t h e f i r s t counter position, t h e larger l e f t knob w i t h t h e second counter, t h e larger r i g h t knob w i t h t h e third counter, and t h e smaller r i g h t knob w i t h t h e f i n a l position. This set of display-control linkages is v e r y d i f f i c u l t f o r pilots t o learn, p a r t i c u l a r l y as the problem is compounded by a mix i n t h e direction of movement of t h e counters 2This example was pointed o u t t o one of t h e authors by R. S. Nickerson.

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themselves. A major c o n t r i b u t o r t o t h e problem w i t h t h i s t y p e o f device is t h e need t o make t h e set o f displays a n d controls compact. Nevertheless, it should b e recognized t h a t an unclear correspondence can i n t r o d u c e considerable a m b i g u i t y in S - R relations.

A i r c r a f t r a d i o receiver: F i g u r e 7. readout a t each o f t h e f o r positions?

Which

k n o b controls t h e c o u n t e r

T h i s issue o f compatibility in m u l t i p l e s u b - t a s k situations is also i l l u s t r a t e d by an example o b s e r v e d a t an electric utility sub-station. The station consisted o f f o u r l a r g e but separate t r a n s f o r m e r systems. A small c o n t r o l room a t t h e sub-station h a d d i s p l a y panels associated w i t h each o f t h e t r a n s f o r m e r s a r r a n g e d f r o m l e f t t o right, as shown schematically in F i g u r e 8. T h e panels a r e numbered 1, 2, 3, a n d 4. T h e engineers responsible f o r t h e system h a d a p p a r e n t l y chosen w h i c h o f t h e actual t r a n s f o r m e r s were associated w i t h each u n i t on t h e control-room d i s p l a y o n t h e basis o f d r a w i n g s o f t h e p l a n t t h a t were o r i e n t e d w i t h n o r t h u p . T h i s a p p a r e n t l y rational method was, however, dangerous because t h e

1 F i g u r e 8.

2

3

4

Schematic d i s p l a y c o n t r o l room panel.

operators e n t e r e d t h e p l a n t a t t h e n o r t h e n d a n d viewed t h e f o u r t r a n s f o r m e r systems in a s o u t h - f a c i n g d i r e c t i o n . T h e t r a n s f o r m e r shown as number 4 in t h e c o n t r o l room was on his f a r left, w i t h t h e t r a n s f o r m e r shown as number 1 on his f a r right, a s indicated in F i g u r e 9. The o p p o r t u n i t y f o r confusion is e v i d e n t . In t h e i n c i d e n t reported, t h e

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operator, a f t e r s h u t t i n g down one o f t h e t r a n s f o r m e r s f o r maintenance, did n o t c a r r y o u t t h e necessary mental rotation when h e e n t e r e d t h e p l a n t . H e proceeded t o w o r k o n a "live" t r a n s f o r m e r system in t h e p l a n t itself. Fortunately, h e was n o t seriously i n j u r e d but t h e r e s u l t i n g elect r i c a l "explosion" d i s r u p t e d power t o t h e local community, as well as damaging t h e p l a n t . Again, t h i s t y p e o f S - R compatibility problem can b e r e a d i l y solved, but it i l l u s t r a t e s t h e problem w i t h m u l t i p l e d i s p l a y c o n t r o l systems.

Figure 9

Plan view o f substation

A n o t h e r example o f S - R a m b i g u i t y in a m u l t i p l e s u b - t a s k situation, i n t h i s case compounded by r e v e r s a l o f a population stereotype, is p r o v i d e d by t h e use o f r e d a n d g r e e n l i g h t s on c o n t r o l room panels in nuclear power p l a n t s . These l i g h t s indicate t h e status o f t h e f l o w o f e l e c t r i c i t y t h r o u g h b r e a k e r s a n d t h e flow o f coolant t h r o u g h valves. In c o n t r a s t t o what one would expect f r o m t h e use o f r e d a n d g r e e n l i g h t s t o g o v e r n t h e f l o w o f t r a f f i c , r e d l i g h t s h e r e a r e used t o indicate t h a t e l e c t r i c i t y a n d coolant a r e flowing, whereas g r e e n l i g h t s indicate t h a t t h e flow is stopped. Operators' responses t o these i n d i c a t o r l i g h t s a r e made more complex by t y p i c a l o p e r a t i n g procedures t h a t focus on t h e state o f t h e valves o r b r e a k e r s r a t h e r t h a n on t h e flows t h a t t h e y g o v e r n . When d i r e c t e d t o close a v a l v e in a procedure, operators must look f o r a g r e e n light t o determine t h a t t h e i r actions have succeeded i n s t o p p i n g coolant flow. When d i r e c t e d t o close a b r e a k e r , however, t h e o p e r a t o r m u s t look f o r a r e d light t o indicate t h a t h e has succeeded in i n i t i a t i n g t h e flow of electricity . F i g u r e 10 shows an arrangement o f controls o n a c o n t r o l room panel t h a t was p o i n t e d o u t by an o p e r a t o r t o one o f t h e a u t h o r s d u r i n g a v i s i t lp a nuclear power p l a n t in t h e southeastern U.S. T h e "A" t r a i n a n d B" t r a i n controls were installed on t h e panels when t h e p l a n t was f i r s t built. T h e "C" a n d "D" controls were installed l a t e r during a p l a n t modification t o a d d r e d u n d a n t systems f o r improved safely. T h e engineers who placed t h e controls in t h e positions shown in t h e f i g u r e were conc e r n e d w i t h s a v i n g space on t h e already-crowded panel a n d w i t h e n s u r i n g t h a t t h e new controls were contiguous w i t h those already in place.

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Fortunately, operators were not required t o interact w i t h t h e controls sequentially f o r any tasks. The operator who pointed O U ; ~ t h i s control arrangement, however, stated t h a t he had had several near misses'' when reaching f o r a particular control among t h e four.

C

D

Multi-Element Displays

Figure 10. Electrical t r a i n i n g controls on a control nuclear power plant.

room panel i n a

These examples, and others t o be presented later provide support f o r t h e following: Factors contributing to S-R incompatibility will frequently exist in multi-element displays or combined display-response systems because of the potential for competing response tendencies or inconsistency in association across sub-tasks. I n general, it is probably better t o have consistent, i n f e r i o r S - R associations across sub-tasks than a mix of good and bad rules. T h e use of arrows on displays. Arrows are frequently used i n a v a r i e t y of display types. However, t h e y do not necessarily convey t h e same t y p e of information i n t h e d i f f e r e n t displays. Arrows frequently denote direction and required t y p e of movement. However, t h e y can also be used t o i d e n t i f y o r highlight a location (e.g., t h e arrow pointer w i t h t h e words "you are here" on a map of a particular industrial site, f a i r ground, o r college campus). Sometimes, confusion can arise over which of these two uses is intended. An example of t h i s situation is found i n t h e case of small t u b s of butter, honey, and j e l l y provided in restaurants o r f o r meals on aircraft, as shown i n Figure 11. The opening "instructions" p r i n t e d on t h e tear-away seal consist of t h e words "tear here" and a set of arrows. Such a use of arrows causes confusion as t o whether t h e seal should be t o r n away i n t h e direction of t h e arrows, o r whether t h e arrows are pointing t o t h e location a t which t h e seal removal should s t a r t .

379

tear here

-

tear here

tear h e r e

-b++-b-b+-b-b

F i g u r e 11. T o open t h i s t u b of jelly, should one tear from t h e bottom l e f t corner, toward t h e r i g h t (direction), o r from t h e bottom right corner (location)? Arrows have often been used on moving-scale displays attached t o control knobs, such as those used on thermostats in houses o r refrigeraHere t h e An example of such a display,,is shown i n ,Figure 12. tors. arrows associated w i t h t h e words High" and Low" can have two interpretations. One o f these i s t h a t t u r n i n g t h e knob i n t h e direction of t h e pointer associated w i t h t h e word "High" increases t h e temperature. T h e other interpretation i s t h a t t h e arrow i s indicating t h e locotlon on This l a t t e r meaning would t h e scale of t h e higher temperature settings. require t h e user t o turn t h e knob so as t o set a h i g h e r temperature value against t h e f i x e d index a t t h e top.

Figure 12. I n which direction should t h i s knob be t u r n e d t o lower t h e temperature i n t h e refrigerator? Highway signs indicating t h e existence of a median o r center-strip have sometimes been of t h e arrow type, as shown in Figure 13. While t h e exact meaning of t h e sign is usually clear because of t h e context, t h e highway configuration, and t h e roadway delineation used, as well as drivers' responses being h i g h l y overlearned, t h e fact t h a t children will occasionally question t h e i r parents about t h e sign points o u t an ambiguity. Does t h e sign w i t h t h e arrow and "KEEP RIGHT" indicate t h e direction in which t h e vehicle should move, o r does it suggest t h a t t h e d r i v e r should position his vehicle w i t h t h e sign located on t h e right of t h e vehicle?

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I

RIGHT

Figure 13. Does t h i s sign t e l l a d r i v e r t o move r i g h t o r t o position his vehicle t o t h e l e f t of t h e sign? These examples provide specific instances of t h e following ization: The use of arrow pointers on displays hos the potential confuslon concerning the appropriate response because arrows used to present either movement-direction information or information.

generalto cause can be location

Conflicting environments. In some countries, lights are t u r n e d on by pulling t h e switch down, which is opposite t o t h e standard i n North America. For most international travelers, t h i s probably poses no more than a minor i r r i t a t i o n when entering a d a r k hotel room. However, it has potential significance i n airplane cockpits. Many cockpit designs conform t o t h e "sweep-on" p r i n c i p l e f o r operating toggle switches (Hawkins, 1976). When t h e switch is mounted low i n t h e cockpit on a horizontal panel, it is switched on by a hand movement sweeping forward. When positioned on a vertical panel i n f r o n t of t h e pilot, an upward movement o f the hand t u r n s t h e switch on. For t h e switch mounted above t h e crew position, a sweep of t h e hand from f r o n t t o back is t h e appropriate "switching-on" movement. All these movements conform t o a single "sweep-on" tendency. However, if a pilot from overseas is used t o turning switches on by p u l l i n g them down i n other situations, a switch mounted i n t h e cockpit on a vertical panel w i l l operate i n a manner opposite t h a t t o which he is accustomed. This r u l e violation is likely t o d i s r u p t t h e strength of t h e response stereotype associated with t h e sweep-on design principle. Such an observation leads t o t h e conclusion t h a t the efficacy of simple rules governing S -R situations can be reduced by the prior or continuing experience of some respondents in other sltuattions.

A specific case of t h i s t y p e exists i n several two-unit nuclear power plant sites (i.e., a power plant w i t h two reactors) a t which t h e controls and displays f o r one u n i t are configured as a mirror-image of t h e controls and displays f o r t h e other unit, in t h e same control room. Thus, instruments located t o t h e operator's l e f t f o r one u n i t are located Further, at some plants, t h e operation of t h e t o his right f o r t h e other. controls is also mirror-image between units, so t h a t switches t h a t are thrown l e f t in one unit, f o r example, are thrown r i g h t i n the other t o obtain t h e same effect. T h e potential f o r significant operator e r r o r s represented by t h i s control room configuration is exacerbated by t r a i n i n g operators in control room simulators t h a t only simulate t h e panels f o r one

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unit a n d by t h e common practice o f assigning operators t o o n l y one unit f o r a p a r t i c u l a r day o r week a t a time. One operator described a method h e uses t o overcome t h e potential f o r e r r o r created by t h e mirror-image panels a t h i s site. When w o r k i n g on t h e right-hand u n i t , he keeps h i s l e f t hand in h i s pocket a n d o n l y He t h e n switches hands when manipulates controls w i t h h i s right hand. w o r k i n g on t h e o t h e r u n i t . A l t h o u g h t h i s technique a n d o t h e r s t h a t operators have developed are useful u n d e r normal conditions, t h e response habits developed d u r i n g t r a i n i n g o r l o n g exposure t o one unit may reappear a n d cause e r r o r s u n d e r t h e stress associated w i t h an emergency event. Incomplete d i s p l a y information. Finally, a n example w i l l b e g i v e n t o i l l u s t r a t e how ambiguity can r e s u l t when information is obscured. Some radios come equipped w i t h a horizontal slide switch, as shown in F i g u r e 14, f o r s w i t c h i n g between AM a n d FM. T h e design is sometimes such that one o f these t w o modes i s hidden f r o m view by t h e slide i t s e l f . Does t h e mode t h a t is showing represent t h e c u r r e n t selected value (FM in t h e f i g u r e ) o r should t h e slide switch be moved towards t h e mode indicator? T h i s is a basic question o f "status v e r s u s command," b u t t h e o b s c u r i n g o f t h e o t h e r mode indicator (AM in t h e example) appears t o c o n t r i b u t e t o t h e confusion. T h i s example suggests t h a t partial obscuring of some display elements in a particular mode of operation can augment incompatibilities in S- R relations.

AM

0

600

FM aoo

1200

1000

VOLUME F i g u r e 14.

0 TUNING

I s t h i s radio receiving an AM or an FM station?

A second example o f t h i s incompatibility category is t h a t o f one t y p e o f automobile rear-vision m i r r o r . T h e m i r r o r is designed f o r b o t h daytime a n d nighttime operation. T h e appearance o f t h e t w o modes i s shown in F i g u r e 15. T o change mode, one presses t h e v i s i b l e indicator upwards. T h e question is does t h e indication on t h e knob indicate t h e c u r r e n t setting, o r t h e requirement t o p u s h t h e indicator u p t o select t h e

..... , ;Q%y I

I _.:.:

NIGHT

I

- - - - I

F i g u r e 15.

I s t h i s rear-view m i r r o r set f o r daytime o r nightime use?

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indicated mode? Again, t h i s is a case o f t h e "status-versus-command" dilemma, but comments by users suggest t h a t t h e presentation a t a n y time o f o n l y one mode indicator increases t h e u n c e r t a i n t y . Furthermore, t h e s e t t i n g o f c o u n t e r displays is also made more d i f f i c u l t because one usually does n o t have a view o f t h e position o f t h e adjacent d i g i t s . I s the h i g h e r digit value positioned above o r below t h e one c u r r e n t l y displayed?

Some General Observations These categories o f S - R incompatibilities f o r m a classification t h a t almost c e r t a i n l y can b e extended. A d e s c r i p t i v e taxonomy of t h i s t y p e can h e l p t h e design o f a system t o some e x t e n t by p r o v i d i n g a reference l i s t o f potential d i f f i c u l t i e s . However, even more importantly, it can p r o v i d e a framework t h a t has t h e potential t o assist f u t u r e theoretical development. Sources of S-R Compatibility: Frames, Rules, a n d Response Tendencies A l t h o u g h a taxonomy o f S-R incompatibilities can help a system designer avoid some design errors, a n y such d e s c r i p t i v e l i s t is l i k e l y t o b e incomplete. For example, t h e authors were able t o i d e n t i f y f o u r additional t y p e s o f incompatibilities t h a t could n o t b e easily subsumed u n d e r F i t t s ' o r i g i n a l f o u r categories. It is l i k e l y t h a t t h e reader may b e able t o i d e n t i f y additional categories. A l t h o u g h a taxonomy t h a t is n o t exhaustive can p r e s e n t a challenge t o t h e researcher, an incomplete classification may leave t h e system designer w i t h S - R compatibility problems t h a t h e did n o t expect, may n o t b e able t o explain, a n d so may have d i f f i c u l t y in resolving. A model t h a t systematically organizes t h e t y p e s o f S-R incompatibilities described above in terms o f theoretical concepts o f S-R compatibility i s l i k e l y t o b e o f g r e a t e r value t o t h e designer t h a n a simple l i s t . T h e concepts o f frames, rules, a n d response tendencies have t h e potential t o p r o v i d e such an organizing scheme. Information processing t h e o r y suggests that when faced w i t h a g i v e n stimulus array, whether it i s an experimenter's l i g h t s a n d b u t t o n s o r t h e c o n t r o l panels o f a nuclear power plant, a n d t h e knowledge t h a t some response i s expected, an i n d i v i d u a l w i l l search f o r an i n t e r p r e t a t i o n o f t h e stimuli t o g u i d e o r constrain t h e search f o r a n appropriate response. T h e i n t e r p r e t a t i o n t h a t t h e i n d i v i d u a l attaches t o t h e stimuli and, hence, t o t h e response t h a t is selected may b e d e r i v e d f r o m information p r o v i d e d by an experimenter (e.g., t h e r e d light means " p u s h b u t t o n A"), f r o m information p r o v i d e d by t h e stimuli (e.g., ON/OFF labeling above a n d below a toggle switch), f r o m information t h a t t h e i n d i v i d u a l b r i n g s t o t h e situation (e.g., a yellow light on t h e f u e l display in a 1980 Honda Prelude indicates a low f u e l supply), o r f r o m some combination o f these sources. T h e internalized representation o f t h e information t h a t t h e i n d i v i d u a l uses t o i n t e r p r e t a n d respond t o a g i v e n stimulus situation can b e defined as a frame, a rule, o r a response tendency, depending on t h e e x t e n t t o which t h e information represented i s elaborated and i s l i n k e d t o o t h e r knowledge, expectations, and habits (overlearned behaviors) o f t h e i n d i v i d u a l . T h e t e r m "frame" ( B a r r & Feigenbaum, 1981; Minsky, 1975) has been used t o describe how knowledge is organized in memory. Frames

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are thought t o assist individuals t o i n t e r p r e t t h e i r experience, t o d i r e c t t h e i r attention, t o provide t h e information needed f o r making inferences, and t o facilitate recall of information. Frames are comprised of knowledge about objects, events, and t h e i r interrelations, and may include action instructions. Knowledge is obtained and organized i n t o a frame t h r o u g h repeated experience w i t h a particular stimulus configuration. When activated, t h e appropriate frame can assist t h e individual t o determine which aspect of a display is intended t o serve as t h e f i g u r e versus t h e g r o u n d o r whether a communication is intended as an e r r o r r e p o r t rather than as a command. Because of t h e i r structure, frames reduce t h e number o f possible responses considered and so facilitate t h e speed w i t h which a response is made. For t h e purposes of t h i s chapter, rules can be thought of as t h e internal representation of relatively simple S - R relations t h a t may be created, f o r example, t h r o u g h repeated exposure t o some stimulus situation o r artificially t h r o u g h instructions from a system designer (or an experimenter). Although a particular rule, o r condition-action pair, usually will not be limited i n i t s applicability t o a single instance, t h e conditions t h a t activate a r u l e will be significantly narrower than those For example, t h e expectation among U.S. citizens t h a t activate a frame. t h a t moving a l i g h t switch u p will turn room lights on and moving it down will t u r n them o f f can be defined as a rule. Because rules can be embedded within frames, however, an individual's access t o an S-R r u l e may depend on activation of t h e appropriate frame. Thus, when faced with a r o t a r y control knob, most individuals know t h a t t o increase volume on a radio, t h e knob typically should be rotated i n a clockwise direction, whereas increasing t h e flow t h r o u g h a water faucet w i t h such a knob i s typically accomplished by rotating t h e knob i n a counterclockwise direction. The information required t o determine i n which direction t h e knob should be t u r n e d (i.e., whether t h e knob controls an electric component o r fluid flow) can be considered as constituting t h e frame, while t h e direction i n which the knob is t u r n e d (i.e., clockwise o r counterclockwise) constitutes t h e rule. The concept of a response tendency can be traced t o learning theory and implicit responses. Because a tendency, by definition, is an intervening variable t h a t cannot be d i r e c t l y observed, i t s existence is established indirectly. Concepts l i k e response tendencies t h a t are derived from classic learning theories have not been popular w i t h cognit i v e psychologists i n recent times. Indeed, the response-tendency cons t r u c t is less cognitive than either frames o r rules. Some more-direct evidence f o r t h e presence and utility of response tendencies in human information processing tasks was provided by Kantowitr (19731, who recorded continuous response force i n a double-stimulation paradigm. Changes i n response force i n a condition where no o v e r t response was required were systematically related t o important independent variables. While response tendencies can be evoked by complex cognitive structures, such as frames o r rules, t h e y also can be learned by less cognitive mechanisms, such as repetition and association. This is especially t r u e f o r a r b i t r a r y S - R pairings t h a t are often present in laboratory experiments on compatibility. This conceptualization of S-R compatibility i n terms of frames, rules, and response tendencies, then, suggests t h a t compatibility is a function of how individuals process information: High S - R compatibility can be expected when t h e response arising from t h e interpretation t h a t

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t h e individual imposes on t h e stimulus matches the response required f o r t h e task. Incompatibilities result either from t h e application of an incorrect frame, rule, o r response tendency, o r from difficulties i n imposing an interpretation on t h e stimulus configuration. T h e concepts of frames, rules, and response tendencies do not cons t i t u t e mutually exclusive categories: One individual may i n t e r p r e t an S - R configuration i n terms of a frame, while t h e same configuration evokes a rule-based response in another person. For example, consider again a r o t a r y control knob that, in t h i s case, controls a row of lamps such t h a t turning t h e knob clockwise lights them i n series from t h e right t o t h e left. Whereas most individuals might perform taslc? involving t h i s configuration simply on t h e basis of t h e simple r u k if t h e knob is rotated clockwise, t h e lamps are lit from right t o left, others w i t h an engineering background (such as one of t h e authors who developed t h i s example) might impose a frame derived from t h e i r experience w i t h d i f f e r e n t types of controls and displays t o conceive of a gear operating between t h e knob and t h e lamps t h a t explains t h e r i g h t - t o - l e f t activation (see Figure 16). Therefore, individual differences i n knowledge and experience can result i n t h e same S-R configuration being h i g h l y compatible f o r one person and not a t all compatible f o r another.

w Gear- .image

Knob

Figure 16. Introduction of a frame-derived gear-image t o explain t h e right t o l e f t activation of t h e lights. Although we have divided sources of S-R compatibility i n t o three components, much as Caesar had Gaul i n t o t h r e e p a r t s divided, o u r parsing of these components is nested rather than linear. The parsimony of t h i s t r i p a r t i t e conceptualization gains considerable power due t o t h e nested hierarchical arrangement shown in Figure 17. Actions (responses) always require response tendencies before t h e y can be initiated. A stimulus d i r e c t l y linked t o a response tendency is sufficient t o generate an action. A r u l e generates actions indirectly by innervating response tendencies. A frame generates actions i n d i r e c t l y by exciting a r u l e t h a t i n t u r n innervates a response tendency. Thus, frames and rules control

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Frames

Rules

Response

Tendencies

F i g u r e 17. Nested hierarchical relations among frames, response tendencies as sources o f S-R compatibilities.

rules,

and

actions by selecting subsets o f response tendencies. T h e appropriateness of these subsets is in p a r t g o v e r n e d by S-R compatibility.

S-R compatibility when developing some Thus, to ensure human/machine interface, t h e system designer must f i r s t i d e n t i f y t h e context t h a t w i l l encourage t h e desired response. If it is possible t o evoke a dominant frame f r o m another s e t t i n g a n d if t h e S-R relation associated w i t h t h a t frame can b e e x a c t l y replicated in t h e new situation, t h e designer can b e assured o f a h i g h degree o f S-R compatibility. In t h e absence of a dominant frame, however, t h e designer would w o r k down t h r o u g h possible r u l e s a n d t h e n evaluate response tendencies t o find an S - R match. O n l y in t h e case t h a t e x i s t i n g frames, rules, or response tendencies cannot b e a p p r o p r i a t e l y evoked i n t h e new system should t h e designer consider r e q u i r i n g users t o develop new frames, rules, o r response tendencies, because o f t h e likelihood t h a t t h e y w i l l c o n f l i c t w i t h those already available t o t h e system users. In summary, t h i s model suggests t h a t t h e theory o f human information processing can p r o v i d e useful methods f o r avoiding t h e t y p e s o f S - R incompatibilities described in t h e p r e v i o u s section a n d observed in actual practice. Conclusion

Writing t h i s chapter has compelled u s t o consider t h e differences, if any, between t h e approach t o u n d e r s t a n d i n g S - R compatibility taken by t h e experimental psychologist a n d t h e human-factors scientist. One o f us (Kantowitz, 19821, in r e g a r d i n g t h e relationship between information p r o cessing a n d human factors, was incautious enough t o dwell on S-R comp a t i b i l i t y as a nexus between t h e t w o disciplines. T h e conclusion reached

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a t t h a t time was t h a t S - R compatibility, although w i d e l y used as an e f f e c t i v e human-factors c o n s t r u c t , was s t i l l n o t understood adequately f o r a n y e x t a n t theoretical models t o b e o f g r e a t assistance t o t h e humanfactors practitioner. However, t h e hope was implied t h a t f u t u r e e f f o r t s by experimental psychologists a n d human-factors e x p e r t s would lead t o mutual gains in t h e u n d e r s t a n d i n g a n d application o f S - R compatibility. T h e p r e s e n t volume is a step t o w a r d i n t e g r a t i n g t h e science a n d t h e human f a c t o r s o f S-R compatibility, but complete i n t e g r a t i o n remains t o b e accomplished. Perhaps o u r analysis o f frames, rules, a n d response tendencies bears o n t h i s p o i n t . Most l a b o r a t o r y studies have focused on situations w h e r e response tendencies, simple rules, o r b o t h a r e dominant (e.g., Duncan, 1977; H e n d r i k x , 1986; Shulman & McConkie, 1973). In contrast, most human-factors problems concern situations where behavior is cont r o l l e d by frames o r r u l e s . T h i s may explain w h y t h e r e s u l t s a n d models a r i s i n g f r o m l a b o r a t o r y investigation o f S - R compatibility have as y e t h a d l i t t l e impact o n human f a c t o r s . Models a n d experiments t h a t r e v o l v e a r o u n d simpler e x p l a n a t o r y concepts t h a t a r e a t t h e bottom o f o u r a n a l y t i c h i e r a r c h y a r e n o t l i k e l y t o p r o d u c e answers u s e f u l t o a system designer whose operators a r e controlled by frames. A higher unders t a n d i n g o f S-R compatibility is needed b e f o r e models can h e l p t h e p r a c titioner. T h i s observation is n o t intended t o d e n i g r a t e t h e c u r r e n t activities of theorists. T h e level o f data in human f a c t o r s i s q u i t e elementary. Results a r e evaluated, a n d hypotheses formulated, on t h e basis o f anecdotal evidence a n d observations t h a t d o n o t p e r m i t statements o f causality. B u t human-factors e x p e r t s d o n o t have t h e l u x u r y o f time t o formulate sophisticated theories. Human f a c t o r s as a d i s c i p l i n e satisfices r a t h e r t h a n optimizes (Simon, 1964). A n incomplete solution now is b e t t e r t h a n w a i t i n g f o r a t h e o r y a t t h e e n d of t h e rainbow. B o t h humanf a c t o r s scientists a n d experimental psychologists t e n d t o use i n d i r e c t measures o f compatibility instead o f p r e f e r a b l e d i r e c t measures. While we s t i l l hope t h a t human f a c t o r s a n d c o g n i t i v e psychology can u n i t e t o explain S-R compatibility, t h i s union may n o t develop r a p i d l y . A l t h o u g h t h e l i s t o f e i g h t varieties o f S-R compatibility discussed p r e v i o u s l y is q u i t e u s e f u l f o r human-factors design, it has a t least one serious shortcoming f r o m a s c i e n t i f i c viewpoint. A s a l r e a d y noted, t h e l i s t is n o t exhaustive. J u s t as o u r taxonomy adds categories t o t h e o r i g i n a l l i s t proposed by Fitts, so w i l l f u t u r e human-factors w o r k a d d t o o u r expanded l i s t . T h i s problem arises when a n y proposed taxonomy i s based solely o n observation a n d d e s c r i p t i o n . T h e more theoretical taxonomy based o n frames, rules, a n d response tendencies appears, a t least f o r t h e moment, t o b e exhaustive. It seems l i k e l y t h a t f u t u r e exemplars o f S-R compatibility can b e accommodated by a t least one of these t h r e e categories. Such g r e a t e r scope is an i m p o r t a n t advantage o f a more theoretical p e r s p e c t i v e on S-R compatibility. It is clear t h a t f u r t h e r theoretical development o f these ideas is necessary, in addition t o empirical assessment o f t h e i r w o r t h . Even in i t s c u r r e n t form, however, t h e value o f t h e model d e s c r i b e d in t h i s c h a p t e r lies in t h e guidance it p r o v i d e s t o a system designer who must b e concerned w i t h S-R compatibilities when developing some h u m a d m a c h i n e interface. Rather t h a n p r o v i d i n g him o r h e r w i t h a c h e c k l i s t of S-R

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incompatibilities t o avoid, as derived from a taxonomy such as t h a t p r e sented earlier, introduction of t h e concepts of frames, rules, and response tendencies will ensure t h a t t h e designer n o t o n l y considers t h e features of t h e stimulus array, b u t also t h e interaction of those features w i t h t h e characteristics of t h e environment i n which t h e a r r a y will be used, as well as w i t h t h e characteristics of t h e system users. References Barr, A., & Feigenbaum, E. (1981). The handbook of artificial inteliigence: Voi, 1 . Stanford, CA: Heuristech Press. Bury, K., Boyle, J., Evey, R. J., & Neal, A. versus scrolling on a visual display terminal. 385-394.

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STIMULUS-RESPONSE COMPATISILITY R. W. Proctor and T.G. Reeve (Editors

fBElsevmr Science Publishers 8.V. (do&- Holland).

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THE MENTAL MODEL I N STIMULUS-RESPONSE COMPATIBILITY RAY E. EBERTS a n d J A C K W. POSEY School o f I n d u s t r i a l E n g i n e e r i n g Purdue University Complex human-machine interaction tasks r e q u i r e extensive cogn i t i v e processing by t h e operator. T h i s processing o f information by t h e operator goes t h r o u g h many intermediate c o g n i t i v e steps between t h e An important i n t e r m e d i a r y s t r u c t u r e in t h i s stimulus a n d t h e response. processing i s t h e operator's model o f t h e system o r t h e task; t h i s model is o f t e n called t h e mental model. T h e effectiveness o f t h e t o t a l humanmachine system i s dependent on, especially, t h e compatibility between t h e stimulus ( t h e displayed information) a n d t h e c e n t r a l processing ( t h e operator's mental model). Incompatibility w i l l r e s u l t in mental transformations (which t a k e time a n d can cause e r r o r s ) , o r it w i l l r e s u l t in confusion a n d mistakes. Stimulus-Response (S-R) compatibility theories t h a t p r o v i d e useful applications to these complex human-machine interaction t a s k s m u s t specify t h e role o f these c o g n i t i v e mediators in information processing. T h e role o f these intermediary steps between stimulus a n d response has been recognized a n d s t u d i e d in t h e past. In particular, Wickens, Sandry, a n d V i d u l i c h (1983) examined t h e role of Stimulus-Central Processing-Response (S-C-R) compatibility in complex human-machine interaction tasks. In such tasks, operators a r e r e q u i r e d t o r e t a i n a stimulus input and incorporate t h e input in t h e i r mental models f o r a substantial p e r i o d o f time b e f o r e p e r f o r m i n g a response. T h e code o f r e p r e sentation o f t h e mental model, which is an essential p a r t o f c e n t r a l p r o cessing, is important t o determine so t h a t t h e d i s p l a y o f information a n d forms o f response can b e designed t o b e compatible w i t h c e n t r a l processing. Whereas S-R compatibility theories emphasize t h e compatibilities between t h e stimulus a n d response, S-C-R compatibility emphasizes t h e compatibilities between t h e stimulus and c e n t r a l processing a n d between c e n t r a l processing a n d response. I n t h i s chapter, t h e S-C-R compatibility t h e o r y o f Wickens e t al. (1983) is reviewed. T h i s t h e o r y has been useful t o explain some results in dual-task experiments, b u t many unanswered questions remain. By examining some new concepts i n t h i s area and i n t e g r a t i n g theories t h a t p r e v i o u s l y have been considered separately, t h e S-C-R compatibility t h e o r y can b e r e f i n e d a n d extended. Previously r e p o r t e d research is reviewed in t h e c o n t e x t o f t h i s new conceptualization. Finally, research t h a t has been performed r e c e n t l y in o u r l a b o r a t o r y a n d is as y e t unpublished also w i l l b e examined in terms o f S-C-R compatibility t h e o r y .

390

R.E. Eberts and J.W. Posey Conceptual Framework

In building a conceptual framework f o r an S-C-R compatibility theory, t h r e e areas w i l l b e reviewed: p a s t considerations o f S-C-R comp a t i b i l i t y , research o n t h e mental model, a n d theories about automatic/controlled processing. Initially, these t h r e e areas may appear t o b e unrelated, b u t t h e following sections explain how t h e areas can b e i n t e grated. T h e S-C-R compatibility t h e o r y o f Wickens e t al. (1983) postulates a c e n t r a l role f o r t h e mental model b u t tends t o t a k e a r a t h e r n a r r o w view o f t h i s concept by examining o n l y t h e code o f representation o f t h e mental model. O t h e r aspects of mental models could b e important t o t h e compatibility t h e o r y . A n important p a r t o f mental model research i s t h e relation between t h e information received by t h e operator t h r o u g h t h e d i s p l a y media a n d t h e development o f a mental model o v e r time. A related concept i s t h e specification o f t h a t which constitutes a good and a b a d mental model. To address these t w o issues, t h e l i t e r a t u r e o n automatic/controlled processing w i l l b e reviewed, w i t h a special emphasis on t h e role of consistency i n t h e development o f s k i l l e d behavior. Consist e n c y could also b e used in t h e definitions o f good/bad mental models.

S-C - R Compatibi Iity Wickens a n d h i s associates have long been interested in t h e s t r u c t u r e of attentional processing resources. T h e y have used t h i s s t r u c t u r e in a theoretical framework t o account f o r t h e allocation o f attention a n d t h e processing of information in complex tasks, such as p i l o t i n g an a i r craft, in which t h e operator w o r k s in a high-workload environment and has d i f f i c u l t y processing a l l t h e possible information (see Wickens, 1980, 1984; Wickens, Sandry, & Vidulich, 1983). A n important p a r t o f t h i s multiple resource theory is t h a t d i f f e r e n t tasks have d i f f e r e n t represent a t i o n codes associated w i t h them. In terms o f attentional resources, Wickens (1980) determined t h a t tasks w i t h v e r b a l codes a n d those w i t h spatial codes p u l l e d attentional resources f r o m d i f f e r e n t "pools," so t h a t these t w o k i n d s o f tasks could b e b e t t e r time-shared t h a n if t h e tasks p u l l e d resources f r o m t h e same "pool ." Because a p o s i t i v e relation exists between t h e q u a l i t y o f performance a n d t h e amount o f resources devoted t o a task, performance in these high-workload situations was dependent on t h e s t r u c t u r e s o f t h e tasks a n d on t h e representation codes o f t h e component tasks. T h e multiple resource t h e o r y has shown t h a t t h e code o f representation o f a t a s k i s important f o r theoretical explanations o f human p e r f o r mance in complex situations. O t h e r research, examined elsewhere in t h i s book (e.g., see Chapter 1 by Alluisi and Warm), shows t h a t certain stimulus/response p a i r s a r e more compatible t h a n o t h e r p a i r s . If a t a s k has a c e r t a i n code associated w i t h it, t h e n t h i s code may have an effect o n t h e t o t a l compatibility, f r o m t h e stimulus t h r o u g h t h e c e n t r a l processing o f t h e t a s k t o t h e response. Evidence f o r t h i s S-C-R compatibility has been examined most extensively by Wickens and his associates. According t o t h e S-C-R compatibility t h e o r y o f Wickens e t a l . (1983). t h e compatibility hinges on t h e t a s k t y p e t h a t t h e n can b e matched w i t h t h e representation code. T h e t a s k t y p e s a r e v e r b a l and spatial. A v e r b a l t a s k is defined t o b e one " . . . t h a t requires t h e use o f language o r some a r b i t r a r y symbolic coding f o r i t s completion. " A spatial

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t a s k is one t h a t r e q u i r e s ' I . . .a judgment o r i n t e g r a t i o n c o n c e r n i n g t h e t h r e e axes o f t r a n s l a t i o n o r orientation." These researchers n o t e d t h a t some t a s k s a r e represented as p u r e l y spatial o r v e r b a l but t h a t many t a s k s r e q u i r e a combination o f t h e two. T h e y suggested t h a t t h e t w o classifications s e r v e as endpoints o f a continuum a n d t h a t research should b e designed t o r e f i n e a n d validate t h i s continuum. Once t h e representation code o f a t a s k is determined, t h e n t h e t a s k environment can b e designed so t h a t S-C compatibility a n d C - R compatibility a r e high. F o r S-C compatibility, t h e t y p e o f input must b e conside r e d . Wickens (19M)d e s c r i b e d f o u r t y p e s o f input as d e f i n e d by a 2 x 2 m a t r i x formed by t h e a u d i t o r y a n d v i s u a l modalities a n d t h e v e r b a l a n d spatial p r i m a r y perceptual codes. T h e f o u r t y p e s a r e (a) t e x t (visual modality, v e r b a l code); (b) speech ( a u d i t o r y modality, v e r b a l code); (c) sound localization a n d p i t c h ( a u d i t o r y modality, spatial code); a n d (d) analog p i c t u r e (visual modality, spatial code). For tasks with a v e r b a l representation code, Wickens e t al. (1983) f o u n d t h a t t h e most F o r t a s k s w i t h a spatial representation compatible input was speech. code, t h e most compatible i n p u t was v i s u a l analog. Thus, by k n o w i n g t h e representation code used in c e n t r a l processing, t h e compatible stimulus input can b e specified. F o r C-R compatibility, Wickens e t al. (1983) examined o n l y t w o t y p e s o f response o u t p u t : manual responses [ s u c h as a k e y p r e s s o r a j o y s t i c k input) a n d speech. T h e y stated t h a t matching v e r b a l processing w i t h speech o u t p u t a n d matching spatial processing w i t h manual o u t p u t r e s u l t s in t h e h i g h e s t compatibility. T h i s was based on t h e assumption t h a t most manual responses were spatial in n a t u r e a n d t h a t most speech responses were v e r b a l in n a t u r e . T h e usefulness o f S-C-R compatibility in terms o f specifications f o r system design hinges upon t h e a b i l i t y t o s p e c i f y t h e representation code of t h e task. If t h e t a s k can b e specified as a v e r b a l task, t h e n t h e information should b e displayed u s i n g speech a n d t h e response should also b e a speech response. If t h e t a s k can b e specified as a spatial task, t h e n t h e information should b e displayed u s i n g analog p i c t u r e a n d t h e response should b e manual. I f t h e t a s k i s a combination o f v e r b a l a n d spatial, t h e n p r e s e n t l y t h e t h e o r y loses i t s p r e d i c t a b i l i t y . Experimental evidence. T w o experiments t o s t u d y S-C-R compatibility a r e r e p o r t e d in Wickens e t al. (1983). T h e f i r s t experiment was designed t o t e s t mainly t h e m u l t i p l e resource t h e o r y , but evidence f o r SC - R compatibility was f o u n d in one of t h e t a s k s o f t h i s d u a l - t a s k e x p e r i ment. One t a s k was a S t e r n b e r g memory search t a s k t h a t r e q u i r e s v e r b a l processing. This t a s k was performed f o r a l l f o u r i n p u t / o u t p u t (I/O) combinations o f a u d i t o r y / v i s u a l i n p u t s a n d speech/manual o u t p u t s . Evidence f o r S-C-R compatibility was f o u n d f o r t h i s memory t a s k by examining t h e e r r o r data f o r all t h e 1/0 combinations. A s p r e d i c t e d by t h e t h e o r y , t h e combination o f a u d i t o r y i n p u t a n d speech o u t p u t f o r t h i s v e r b a l t a s k h a d less e r r o r s t h a n a n y o f t h e o t h e r 1/0combinations. T h e second experiment used t h r e e tasks. One t a s k was a flight t a s k t h a t was o n l y performed w i t h visual input a n d manual o u t p u t . The task, d e f i n e d as a spatial task, consisted o f f l y i n g an a i r c r a f t simulator t h r o u g h a three-dimensional t u n n e l . T h e o t h e r t w o tasks, one r e q u i r i n g spatial processing a n d one v e r b a l processing, were p e r f o r m e d o v e r a l l f o u r combinations of input a n d o u t p u t . T h e spatial t a s k was a t a r g e t

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localization task, a n d t h e v e r b a l t a s k was a communication, navigation, a n d identification (CNI) t a s k . These t h r e e tasks were performed as single tasks a n d as dual-tasks in which t h e flight t a s k was p a i r e d w i t h one of t h e o t h e r t w o tasks. T h e analysis o f t h e data f r o m t h e t a r g e t localization and C N I single tasks showed significant interactions between t y p e o f t a s k a n d input and between t y p e o f t a s k a n d o u t p u t . Both o f these interactions showed s u p p o r t f o r S-C-R compatibility, because a u d i t o r y l s p a t i a l is t h e most compatible I/O combination f o r v e r b a l tasks a n d verbaVmanual i s t h e most compatible 1/0 combination f o r spatial t a s k s . Analysis o f t h e dual-task data gave f u r t h e r evidence o f t h e concept o f S-C-R compatibility. T h e response-time data f o r t h e t a r g e t localization t a s k a n d t h e C N I t a s k showed significant interactions between t y p e o f t a s k and i n p u t a n d between t y p e o f t a s k and o u t p u t t h a t also s u p p o r t e d S-C-R compatibility. These data also showed t h a t f o r t h e t a r g e t localization task, which was a spatial task, C-R compatibility dominated t h e comp e t i t i o n f o r o u t p u t resources. Thus, compatibility was important t o dualt a s k efficiency f o r tasks u s i n g t h e same c e n t r a l processing resources. These researchers also stated t h a t interactions between t a s k a n d modality f o r all d u a l tasks showed t h a t compatibility and competition f o r processing resources were related: T h e competition f o r processing resources, as shown by time s h a r i n g efficiency, decreased as compatibility increased, and a t a r a t e d i f f e r e n t f r o m t h a t which would b e p r e d i c t e d by resource use a n d compatibility effects f r o m a single t a s k . Performance data f o r F l i g h t t a s k performt h e flight t a s k also s u p p o r t e d S-C-R compatibility. ance f o r t h e flight task/CNI t a s k p a i r was highest f o r auditory/speech 1/0 o n t h e CNI task; f o r t h e flight t a s k / t a r g e t localization t a s k pair, performance was h i g h e s t f o r visuaVmanual 1/0 in t h e t a r g e t localization t a s k . T h e t a s k response times f o r t h e CNI t a s k and t h e t a r g e t localization t a s k showed t h a t S-C-R compatibility was responsible f o r opposite changes in performance o f t h e secondary t a s k when t h e d i f f i c u l t y o f t h e p r i m a r y flight t a s k was increased. For t h e least compatible 1/0 pairings, performance o f each side t a s k declined; f o r t h e most compatible, performance o f each t a s k improved. Data showing t h e e f f e c t o f flight t a s k diffic u l t y o n response times f o r t h e o t h e r t w o task! showed that compatibility effects a n d resource competition effects were amplified" by increases in workload. For low compatibility conditions, performance declined; b u t f o r conditions o f high compatibility, performance improved. F u r t h e r evidence f o r S-C-R compatibility was f o u n d by Robinson T h e experiment examined a n d Eberts (1987) in a c o c k p i t environment. t h e possibilities o f i n c o r p o r a t i n g synthesized speech in cockpits f o r communicating emergency information t o t h e p i l o t . Earlier w o r k (Wicker, 1980) f o u n d t h a t p i l o t s f e l t t h a t emergency systems o u g h t n o t t o b e actuated by speech i n p u t and should b e actuated by manual responses. Because t h e response should b e manual in t h i s situation, S-C-R compatibility can b e used t o define t h e code and modality o f t h e o t h e r p a r t s o f t h e t a s k . From t h e t h e o r y , f o r compatibility w i t h a manual response, t h e code should be spatial a n d t h e display should b e analog p i c t u r e . This condition was compared t o a synthesized speech d i s p l a y t h a t should n o t b e compatible w i t h t h e response a n d possibly n o t w i t h t h e code e i t h e r . As p r e d i c t e d by S-C-R compatibility, subjects could respond faster t o t h e analog p i c t u r e display t h a n t o t h e synthesized speech d i s p l a y .

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Importance of S-C-R compatibility. Based o n t h e above experiment a l evidence, S-C-R compatibility t h e o r y has been i n c o r p o r a t e d successfully t o e x p l a i n a n d p r e d i c t experimental results. T h e t h e o r y also offers a method f o r designing systems based on t h e p r e d i c t e d compatibilities. In most cases, t h e code of t h e c e n t r a l processing i s t h e k e y t o determining t h e system design. If t h e code is determined, t h e n t h e most compatible stimulus a n d t h e most compatible response a r e completely specified. Once again, Wickens (1984) has been i n v e s t i g a t i n g methods t o determine these codes by u s i n g his m u l t i p l e resource t h e o r y . This t h e o r y states t h a t if t w o t a s k s have t h e same code t h e n t h e y share t h e If these t a s k s a r e p e r f o r m e d c o n c u r same attentional resource "pool. " r e n t l y , i n t e r f e r e n c e should o c c u r . If t h e y d o n o t h a v e t h e same code, t h e n n o i n t e r f e r e n c e should o c c u r . B y p e r f o r m i n g several -dual-task experiments, t h e codes c o u l d conceivably b e determined. T h i s t h e o r y is also i m p o r t a n t in t h a t more complex t a s k s a r e considered t h a n a r e u s u a l l y investigated i n S-R compatibility research. These a r e t h e k i n d s of t a s k s t h a t a r e most o f t e n s t u d i e d in human factors: human-machine interactions, process c o n t r o l tasks, a n d t r a c k i n g tasks. Most o f these t a s k s r e q u i r e e x t e n s i v e c o g n i t i v e processing, so t h a t many intermediaries e x i s t between t h e stimulus a n d t h e response. T o explain these intermediaries, Wickens e t al. (1983) a n d Robinson a n d E b e r t s (1987) b r i n g in t h e concept o f t h e mental model in c e n t r a l p r o cessing. T h i s nicely combines research on attention a n d information processing w i t h t h a t on t h e mental model. Problems with t h e S-C-R c o m p a t i b i l i t y approach. As Wickens e t al. S-C-R compatibility i s now b e s t applied t o t a s k s t h a t a r e e i t h e r p u r e l y v e r b a l o r p u r e l y spatial. Mixed v e r b a V s p a t i a l t a s k s may b e a problem because n o good techniques, such as dual-task experiments, e x i s t t o determine p r e c i s e l y t h e p r o p o r t i o n o f v e r b a l a n d spatial coding. Even if t h i s p r o p o r t i o n c o u l d b e determined f o r t h e code, a manual/speech m i x t u r e f o r t h e response, t o b e compatible w i t h t h e verbal/spatial mix f o r t h e code, would b e d i f f i c u l t t o determine a n d design. A t t h e stimulus e n d o f t h e processing, m i x i n g a u d i t o r y a n d visual displays would b e possible, but experimental evidence is needed t o determine if t h i s kind of mixed display i s u s e f u l f o r n o n r e d u n d a n t i n f o r mation.

(1983) admit,

A n o t h e r problem w i t h t h e t h e o r y is t h a t in t h e p r e s e n t f o r m it is static a n d has l i t t l e o r no p r o v i s i o n f o r t h e representation code of a t a s k t o change o v e r time. Wickens e t al. (1983) imply t h a t all t a s k s can b e assigned a code on some verbal/spatial continuum. Evidence e x i s t s t h a t t h e code (a) can change during learning, (b) may depend on how t h e t a s k was encoded initially, a n d (c) can b e d i f f e r e n t f o r e x p e r t s a n d novices. Even a t a s k such as computer programming, w h i c h appears to b e v e r b a l based upon t h e use of commands, keywords, a n d lines o f comp u t e r code, has been f o u n d t o b e a spatial t a s k f o r e x p e r t programmers (Molzberger, 1983). A method i s needed t o specify how t h e code of representation f o r t a s k s changes o v e r time w i t h l e a r n i n g . A f i n a l problem w i t h S-C-R compatibility t h e o r y is t h e r o l e o f t h e mental model. What is t h e relation between t h e mental model a n d t h e task? Does t h e mental model s e r v e a n y o t h e r p u r p o s e besides h a v i n g a representation code associated w i t h i t ? How a r e mental models developed? What constitutes a good o r b a d mental model? How can an i n d i v i d u a l

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mental model b e e x t r a c t e d f r o m t h e operator? compatibility must b e specified b e t t e r .

T h e mental model in S-C-R

T h e Mental Model A mental model is c o n s t r u c t e d when an o p e r a t o r attempts t o i n t e r nalize how t h e system w o r k s a n d t h e parameters t h a t c o n t r o l t h e system. A mental model can b e d e f i n e d as t h e conceptual representations o r models formed by t h e o p e r a t o r t h r o u g h interaction w i t h t h e system a n d system documentation. T h i s model, o r m u l t i p l e models, is t h e n used t o c o n t r o l b e h a v i o r a n d assist decision-making a b o u t t h e system dynamics o r determine possible causes f o r malfunctions. Researchers say t h a t s u b jects f o r m a mental model o f a t a s k t h r o u g h t h e application o f analogies o r metaphors (Mayer, 19751, t h r o u g h t h e imposition o f a spatial model (Eberts E Schneider, 1985; Hollan, 19841, o r t h r o u g h t h e i r goals a n d plans (Moran, 1981). T h e t e r m mental model can mean many d i f f e r e n t t h i n g s t o d i f f e r e n t researchers. A mental model i s based upon t h e operator's p r e v i o u s b a c k g r o u n d a n d experiences w i t h t h e p a r t i c u l a r problem a n d o t h e r problems l i k e it. A mental model, t h u s , can r a n g e f r o m a b a d model t o a good model. Norman (1983) has made some observations on t h e k i n d s o f mental models users u t i l i z e when i n t e r a c t i n g w i t h machines. Norman states t h a t t h e models are incomplete, unstable, unscientific, a n d parsimonious. Furthermore, h e f o u n d t h a t t h e abilities t o "run" them a r e severely limited, a n d no f i r m boundaries e x i s t between t h e separate mental models. In observations t h a t h e made o f calculator users, Norman f o u n d t h a t users always t o o k e x t r a steps o r declined t o t a k e advantage of features o f t h e calculator. As an example, h e f o u n d t h a t users w r o t e down p a r t i a l r e s u l t s when t h e y c o u l d have been s t o r e d in memory, h i t t h e clear k e y several times t o clear t h e calculator b e f o r e each problem, a n d would not use memory f o r o f t e n - o c c u r r i n g constants. Generalizing these aspects o f mental models t o all mental models i s n o t possible; e x p e r t calculator users would most c e r t a i n l y have b e t t e r models a n d would n o t make these mistakes. These observations a r e important, however, i n t h a t t h e y indicate t h a t j u s t because a person has a mental model o f a system does n o t mean t h a t it is a good model. Code o f representation a n d s t r u c t u r e o f mental models. Especially important t o t h i s discussion o f t h e mental model i s i t s code o f representat i o n . Many problems e x i s t w i t h t h e i n t e r p r e t a t i o n o f t h e use o f d i f f e r e n t codes o f representation in t h e d i f f e r e n t mental models developed. These problems have l o n g been recognized i n c o g n i t i v e psychology, where a debate has been r a g i n g between those who believe t h a t images a r e a viable code (Paivio, 1971), those who believe t h a t a l l knowledge is nonspatial a n d is represented in a propositional (i.e., verbal) form (Pylyshyn, 1973), a n d those who postulate the existence o f b o t h codes o r t h e i n t e g r a t i o n o f b o t h codes i n t o one o v e r a l l code (Anderson, 1978). Anderson also a r g u e d t h a t experimental r e s u l t s t h a t a r e explained by one code can also b e explained by t h e o t h e r code. S p e c i f y i n g completely t h e code o f representation f o r a mental model is difficult. Related t o t h e code o f representation is t h e s t r u c t u r e o f t h e mental model. Several s t r u c t u r e s have been considered: image-based spatial, frame-based, p r o d u c t i o n system, a n d goal h i e r a r c h y . Different kinds of tasks seem t o have d i f f e r e n t s t r u c t u r e s associated w i t h them. To determine compatibilities, then, v e r b a l a n d spatial codes alone may n o t b e

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s u f f i c i e n t in t h e c o n t e x t o f t h e mental model; these o t h e r s t r u c t u r e s m u s t also b e considered, a n d methods t o p r e s e n t stimulus information t h a t i s compatible w i t h these s t r u c t u r e s m u s t b e determined. Each o f t h e s t r u c t u r e s a r e discussed below.

In an image-based spatial mental model, t h e o p e r a t o r c o n s t r u c t s an image o f t h e system t h a t is used t o solve problems a n d make decisions about t h e p r o p e r c o n t r o l i n p u t s . Hollan, Stevens, a n d Williams (1980) described t h e image-based spatial mental models o f e x p e r t operators o f s h i p steam ?/ants. T h e experimenters r e p o r t e d t h a t these e x p e r t operat o r s r a n a simulation in t h e i r head" o f t h e system operations. As an example, if t h e y wanted t o k n o w w h a t would happen t o t h e system if t h e y closed a c e r t a i n valve, t h e y would visualize t h e closing o f t h e v a l v e a n d t h e e f f e c t it would have o n o t h e r p a r t s o f t h e system. T o b e compatible w i t h t h i s e x p e r t mental model, Hollan e t al. (1980) designed t h e i n t e r f a c e f o r a t r a i n i n g simulator t o use g r a p h i c s a n d animation. In a frame-based mental model, t h e o p e r a t o r has a collection o f facts s t o r e d w i t h v a r i o u s s t e r e o t y p e d situations. According t o Minsky (1975), a frame can b e t h o u g h t o f as a n e t w o r k o f nodes a n d relations where t h e t o p levels o f t h e frames a r e fixed, r e p r e s e n t i n g t h i n g s t h a t a r e always t r u e about t h e situation, a n d t h e lower levels have many slots t h a t must b e f i l l e d w i t h specific instances. Each o f these slots, o r terminals, can have conditions specified w i t h them t o determine if t h e incoming information meets those conditions. Frames have become an i m p o r t a n t conc e p t in a r t i f i c i a l intelligence a n d seem t o b e a v i a b l e c o n s t r u c t f o r human mental models also. A p r o d u c t i o n system is a collection o f p r o d u c t i o n s o r rules, each o f w h i c h is composed o f a condition a n d an action. A condition is a g r o u p of objects a n d c o r r e s p o n d i n g a t t r i b u t e values o f t h e objects. A n action is a g r o u p o f operations t o b e performed on objects in t h e environment when t h e object a t t r i b u t e values o f t h e c o r r e s p o n d i n g condition a r e satisf i e d by t h e c u r r e n t state o f t h e environment. T h e environment does n o t necessarily have to b e r e s t r i c t e d t o e x t e r n a l objects; it can encompass objects i n t e r n a l a n d e x t e r n a l t o t h e human. A n example o f a p r o d u c t i o n system is Anderson's (1976) A C T . Anderson, when d i s c u s s i n g A C T a n d o t h e r p r o d u c t i o n systems, l i s t s several a t t r a c t i v e features o f a p r o d u c t i o n system: (a) t h e " d a t a - d r i v e n " c h a r a c t e r i s t i c t h a t enables a p r o d u c t i o n system t o respond immediately t o changes in t h e environment; (b) t h e modular n a t u r e wherein t h e addition, revision, o r removal o f a p r o d u c t i o n w i l l n o t have a tremendous e f f e c t on t h e performance o f t h e p r o d u c t i o n system; and, (c) because o f t h e small number o f p r i m i t i v e components o f a p r o d u c t i o n system, t h e minimization o f t h e e f f e c t o f production-system components on modeled cognition. Solving problems in complex systems is an important f u n c t i o n o f mental models. Newell a n d Simon (1972) use t h e concept o f goals a n d a goal stack t o d e s c r i b e how humans solve problems. T h e i r model o f p r o b lem s o l v i n g consists o f decomposing a p r i m a r y goal i n t o a hierarchical t r e e o f subgoals w i t h branches o f l e n g t h s t h a t depend on t h e degree o f subgoal decomposition. A t t h e e n d nodes o f t h e t r e e a r e subgoals t o w h i c h elementary information processes can b e applied. Newell a n d Simon d e f i n e elementary information processes t o b e elemental Components o f a problem s o l v i n g system f r o m w h i c h all problem s o l v i n g methods a r e constructed. T h e y use a stack t o st:re t h e subgoal t r e e . T w o stack operations, "pushing" a n d "popping, c o n t r o l goal decomposition a n d application o f

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elementary information processes. T h e t r e e is decomposed in a d e p t h f i r s t fashion, t h u s o n l y p a r t s o f t h e t r e e a r e on t h e stack a t one time. T h e stack lengthens when a goal o r subgoal i s s p l i t a n d shortens when elementary information processes a r e performed. If e v e r y subgoal a t e v e r y level can b e s p l i t i n t o o t h e r subgoals o r elementary information processes, t h e n e v e n t u a l l y t h e stack w i l l become empty, a t w h i c h time t h e attainment o f t h e p r i m a r y goal is achieved. In a l l cases except f o r t h e image-based spatial model, these potent i a l mental-model s t r u c t u r e s have been used t o model human behavior on t h e computer. Whether o r n o t these computer models a r e v a l i d psychologically in terms o f some i n t e r n a l mental model is difficult t o determine experimentally. We feel these models p r o v i d e a good s t a r t i n g p o i n t t o look for c e r t a i n k i n d s o f s t r u c t u r e s in people's mental models.

Problems w i t h t h e mental model concept. T h e concept o f a mental model has been used as a catch-all t e r m so t h a t it has l o s t some of i t s meaning. As seen from t h e above discussion o n t h e code o f representation o f t h e mental model, d e t e r m i n i n g a code o r even an o v e r a l l s t r u c t u r e is difficult. In i t s broadest sense, a mental model can b e s t r u c t u r e d as an image-based spatial model, as a p r o d u c t i o n system, as a goal stack, o r as a frame-based model. T h i s set is most p r o b a b l y incomplete. The p a r t i c u l a r kind of s t r u c t u r e used depends on t h e t a s k a n d possibly on t h e experience o f t h e o p e r a t o r . Research has n o t y e t been performed t h a t relates t h e t a s k s t r u c t u r e a n d t h e e x p e r t i s e o f t h e o p e r a t o r t o t h e s t r u c t u r e o f t h e mental model. A n o t h e r problem w i t h t h e mental model concept is t h e difficulty o f e x t r a c t i n g knowledge f r o m t h e o p e r a t o r about t h e f o r m a n d content o f a mental model. Several techniques have been used, but t h e y a l l can b e questioned in terms o f accuracy a n d completeness. F o r a goal stack model, t h e most-used e x t r a c t i o n t e c h n i q u e is v e r b a l protocols: T h e o p e r a t o r i s g i v e n a problem t o solve a n d verbalizes t h e processes as h e o r she solves t h a t problem (see Ericsson & Simon, 1980, f o r a discussion o f t h i s technique). Protocols can also b e used f o r determining spatial models, but o t h e r techniques have also been used. One o f t h e most obvious methods t o assess t h i s kind o f model i s t o have t h e o p e r a t o r d r a w p i c t u r e s . McCloskey (1983) a n d McCloskey, Caramazza, a n d Green (1980) used t h i s t e c h n i q u e t o determine t h e ways t h a t subjects i n t e r n a l l y r e p r e sent movement characteristics, a n d E b e r t s a n d Schneider (1985) used it t o determine subjects' spatial internalization o f system dynamics. For a p r o d u c t i o n system o r rule-based model, subjects can b e g i v e n an i n p u t t o t h e system ( t h e "if" p a r t o f t h e r u l e ) a n d b e asked t o generate t h e o u t put of t h e system ( t h e "then" p a r t o f t h e r u l e ) . T h e set o f r u l e s can t h e n b e determined (Eberts, 1988). No good methods e x i s t f o r determining t h e k i n d s o f frames used by an operator. A f i n a l problem is d e f i n i n g t h e specifications o f a good o r b a d mental model. One obvious method would b e t o determine if t h e model can b e used t o solve problems, make decisions, or c o n t r o l a system. T e s t i n g an o p e r a t o r on t h e whole set of possible problems, decisions, o r c o n t r o l situations is, however, impossible. Some method is needed so t h a t t h e evolution o f forms of mental models, f r o m b a d t o good, can b e specified.

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Consistency T h e concept o f consistency comes f r o m t h e research done o n automatic a n d controlled processing (see Schneider & S h i f f r i n , 1977; S h i f f r i n & Schneider, 1977; S h r i f f r i n , Dumais, E Schneider, 1981). In t h i s research, humans a r e assumed t o use one o r a combination o f automatic a n d c o n t r o l l e d processing in t h e processing o f information. Automatic processing is fast, can b e done i n parallel, a n d r e q u i r e s l i t t l e o r n o attentional resource consumption. C o n t r o l l e d processing, o n t h e o t h e r hand, i s slow, p e r f o r m e d i n serial, a n d i s resource consumptive. Basic research has established t h a t automatic processing w i l l n o t develop unless t h e mapping between t h e stimulus a n d response is consistent (Schneider & S h i f f r i n , 1977). As an example, f o r a S t e r n b e r g memorysearch task, consistent mapping o c c u r s when t h e memory-search items a r e always t a r g e t s a n d n e v e r d i s t r a c t o r s during t r a i n i n g ; inconsistent ( o r v a r i e d ) mapping occurs when a p a r t i c u l a r memory-search item can b e a t a r g e t o n one t r i a l a n d a d i s t r a c t o r on o t h e r s . F o r more complex t a s k s t h a n those t y p i c a l l y s t u d i e d in l a b o r a t o r y settings, t h e important aspect o f s k i l l e d b e h a v i o r i s n o t necessarily w h e t h e r t h e information is processed automatically o r controlled. Rather, t h e important aspect is w h e t h e r o r n o t t h e o p e r a t o r can p e r c e i v e t h e consistencies between t h e system input a n d t h e system o u t p u t . In some cases f o r complex tasks, t h e consistencies a r e n o t salient t o t h e operator, a n d t h e o p e r a t o r may r e l y upon t h e w r o n g information. In o t h e r cases f o r complex tasks, t h e o p e r a t o r must have a b s t r a c t c o g n i t i v e intermediaries in o r d e r t o i n t e r p r e t t h e information a n d make t h e t a s k components consistent. T h e r o l e consistency plays in t h e development of s k i l l e d b e h a v i o r could have a place in mental model research. With p r a c t i c e a n d e x p e r i ence, operators c o u l d p o s s i b l y t r y t o achieve consistency in t h e i r mental models. Thus, an analysis o f consistency would p r o v i d e a t h e o r y on t h e development o f mental models a n d a possible m e t r i c f o r a good/bad model. F o r S-C-R compatibility, consistency o f a mental model c o u l d also possibly b e used t o d e f i n e t h e kind o f d i s p l a y needed a n d t h e response r e q u i r e d . I n t e g r a t i o n o f S-C-R Compatibility, Mental Models, a n d Consistency T h e research on mental models a n d consistency can b e i n t e g r a t e d t o In t h e Wickens e t al. (1983) r e f i n e a n d redefine S-C-R compatibility. t h e o r y , t h e code o f representation o f c e n t r a l processing was important, a n d t h i s code was s t a t i c a n d dependent on t h e t a s k . In t h e new d e f i nition, t h e mental model aspect o f t h e c e n t r a l processing assumes an important role, a n d t h i s mental model can change w i t h p r a c t i c e . T h e mental model must b e used by t h e o p e r a t o r t o d e f i n e a consist e n t mapping between a stimulus a n d a response. T h e o p e r a t o r should b e able t o perceive a n d encode a stimulus o r class o f stimuli a n d use t h e mental model t o consistently map t h a t input t o a response. To b e consistent, a one-to-one mapping should e x i s t between t h e stimulus a n d t h e response. T h e response c o u l d b e a mental response instead o f a physical response, s u c h as a manual b u t t o n p r e s s o r speech o u t p u t . Consistency is t h e i m p o r t a n t concept instead o f t h e code o f t h e mental model. F o r S-C-R compatibility, t h e information d i s p l a y should b e compatib l e w i t h t h e mental model, so t h a t t h e mental model can b e used t o

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specify some k i n d of response. We may not want t o design t h e information display so t h a t a compatibility exists with t h e c u r r e n t mental model but w i t h t h e mental model t h a t will develop a f t e r extensive practice w i t h t h e system. T o do this, we need t o define the characteristics of t h i s latter mental model. A new definition states t h a t a task is S-C-R compatible if t h e operator has a mental model t h a t shows how t h e stimuli are consistently mapped t o responses. Some tasks are so complex t h a t t h e consistent mappings will require t h e development of complex mental models. These mental models may require t h e extensive use of context and the ability of t h e operator t o classify events and complex patterns. Differences between experts and novices will exist f o r these complex situations. To close the gap between the experts and t h e novices, special techniques, such as t h e use o f metaphors and computer graphics, could be used d u r i n g t r a i n i n g . T o define good and bad mental models, a consistency task analysis can be performed t o determine t h e form of t h e mental model t h a t must be developed. Examples of a consistency task analysis are i n t h e next section. This consistent mental model can then be used as a standard w i t h which t o judge t h e mental models of operators. Supporting Research Some research has already been performed t o support the above characterization o f t h e role o f t h e mental model in S-C-R compatibility. Much of t h i s research, which has been performed i n o u r lab at Purdue University, i s recent and has not y e t been published. Several p a r t s must be pieced together: a task analysis based upon input/output consistency; experimental evidence on t h e usefulness of consistent mental models and t h e development of mental models over time; and some method t o extract information about an operator's mental model. Consistency Task Analysis As operators become better at tasks, they t r y to develop mental models t h a t consistently connect t h e system i n p u t t o t h e system output. T o determine the kind of mental model t h a t will be developed, a task analysis based upon consistency must be performed. Methods t o display information t o make these consistencies more salient t o t h e operator can be developed a f t e r t h e consistencies have been identified.

The best way t o describe a consistency task analysis is t o provide The example task is second-order tracking. an example of one. I n this task, t h e operator moves a joystick t o control a simulated vehicle along a c u r v i n g t r a c k . The control dynamics can be described by a second o r d e r equation, such t h a t each position of t h e joystick provides an acceleration t o t h e system. Controlling a second-order system i s difficult; it i s a couple of magnitudes more d i f f i c u l t than controlling a zero-order position system o r a f i r s t - o r d e r velocity system. Some explanations of t h i s d i f f i c u l t y have been hypothesized. First, as t h e o r d e r increases, t h e number of control movements needed t o move the system also increases (Poulton, 1974). T h e number of control movements i s always one more than t h e o r d e r of the system. A second explanation i s t h a t the lead required f o r the

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optimal c o n t r o l o f a system increases as t h e o r d e r o f c o n t r o l increases. For a second-order system, therefore, t h e o p e r a t o r must b e able t o p r e d i c t f u r t h e r in advance t h e p a t h o f t h e system t h a n if t h e system were zero- o r f i r s t - o r d e r . T h e difficulty o f t h i s t a s k can also b e explained t h r o u g h consistencies of t h e t a s k . F o r zero- a n d f i r s t - o r d e r systems, t h e t w o easiest systems t o control, t h e system movement i s f a i r l y consistent w i t h t h e j o y s t i c k position. I n a z e r o - o r d e r system, t h e system w i l l move in t h e same d i r e c t i o n a n d in exact p r o p o r t i o n a l amounts t o t h e movement o f t h e joystick. I n a f i r s t - o r d e r system, a j o y s t i c k position t o t h e right of c e n t e r w i l l cause t h e system t o move t o t h e right w i t h a c e r t a i n velocity. However, t h e consistency between j o y s t i c k position a n d system movement b r e a k s down f o r systems o f second o r d e r a n d above. F i g u r e 1 compares t h e consistent relation between j o y s t i c k position a n d system movement f o r f i r s t - a n d second-order systems. T h e second column p r e s e n t s t h e f u t u r e p a t h o f t h e system f o r v a r i o u s levels o f c o n t r o l input f o r a f i r s t - o r d e r system. T h e third column depicts f u t u r e p a t h s f o r a second-order system w i t h i n t h e limited time window t h a t an o p e r a t o r would normally have available. F o r t h e f i r s t - o r d e r system, t h e d i r e c t i o n o f system movement is dependent on t h e j o y s t i c k position. However, f o r t h e second-order system, t h e d i r e c t i o n o f t h e system movement depends o n b o t h s t i c k position a n d t h e c u r r e n t v e l o c i t y o f t h e system. As can b e seen, t h e same c o n t r o l input can r e s u l t in t o t a l l y opposite system movements as t h e c u r r e n t v e l o c i t y goes f r o m one extreme t o t h e o t h e r .

Also, f o r a second-order system, a p a r t i c u l a r amount of j o y s t i c k movement i s n o t always consistent w i t h t h e amount o f system movement. f'igure 2 compares f i r s t - a n d second-order systems again. For a f i r s t o r d e r system ( t h e second a n d third columns), d o u b l i n g t h e j o y s t i c k position r e s u l t s in a p e r c e p t i b l e increase in t h e v e l o c i t y . For the second-order system ( t h e f o u r t h a n d fifth columns), d o u b l i n g t h e j o y s t i c k position does n o t always r e s u l t in a p e r c e p t i b l e change in system movement. In terms of S - R compatibility, l i t t l e compatibility e x i s t s f o r a second-order system, whereas much compatibility e x i s t s f o r t h e f i r s t o r d e r system. In t h e second-order system, t h e stimulus ( t h e d i s p l a y representation of t h e movement o f t h e system) appears t o have l i t t l e relat i o n t o t h e response made by t h e o p e r a t o r u s i n g t h e j o y s t i c k . T o make t h e t a s k S-C-R compatible, t h e consistency analysis must b e used t o determine t h e consistencies between t h e system input a n d t h e system o u t put f o r t h e second-order system. T h e o p e r a t o r m u s t develop a mental model t h a t relates these two. T h e consistent relation between system input a n d system o u t p u t f o r t h e second-order t r a c k i n g t a s k is t h a t each position o f t h e j o y s t i c k p r o v i d e s a u n i q u e acceleration input f o r t h e system. Acceleration can b e d e s c r i b e d geometrically by a parabola. Spatially a n d graphically, t h e r e fore, each j o y s t i c k position w i l l have a u n i q u e parabola associated w i t h it. Such a parabola w i l l t r a c e o u t t h e f u t u r e a n d p a s t p a t h s o f t h e system. Also, a t time equal t o zero, t h e c u r r e n t v e l o c i t y is t h e t a n g e n t l i n e t o t h e acceleration parabola. T o p e r f o r m t h e second-order t r a c k i n g task, t h e o p e r a t o r must b e able t o perceive t h e d e s i r e d f u t u r e p a t h ( t h e stimulus input), t h e j o y s t i c k position t h a t w i l l cause t h a t p a t h t o o c c u r ( t h e response), a n d t h e

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Current Velocity 10

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Figure 1. T h e f i r s t column of numbers represents d i f f e r e n t c u r r e n t velocities at t h e time of a control i n p u t . The second column represents the path t h a t a f i r s t order, o r velocity control, system would take with a constant control i n p u t of 10 units per second a t each of t h e c u r r e n t velocities. T h e t h i r d column represents the path t h a t a second order, o r acceleration control, system would take w i t h a constant i n p u t of 0.4 units p e r second squared a t each of t h e c u r r e n t velocities.

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Figure 2. This shows t h e effect of doubling t h e control input. The f i r s t column of numbers represents different c u r r e n t velocities a t t h e time of a control input. The second and t h i r d columns show t h e effect of doubling the control input, from 5 t o 10 units p e r second, f o r a velocity control system a t each of the c u r r e n t velocities. The f o u r t h and f i f t h columns show t h e effect of doubling t h e control input, from 0.4 t o 0.8 units p e r second squared, f o r an acceleration control system a t each of t h e c u r r e n t velocities.

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mental model t h a t w i l l relate t h e stimulus t o t h e response. If t h e operat o r can image t h e parabola t h a t would fit t h e f u t u r e p a t h a n d relate t h e parabola t o a j o y s t i c k input, t h e n t h e t a s k c o u l d b e performed easily a n d consistently. S-C-R compatibility would o c c u r in t h e following steps: (1) T h e o p e r a t o r perceives t h e f u t u r e p a t h o f t h e system on t h e d i s p l a y along w i t h a representation o f t h e c u r r e n t v e l o c i t y in t h e f o r m o f a l i n e v e c t o r a t time equal t o zero; (2) t h e o p e r a t o r images a parabola t h a t would fit t h e c u r v e o f t h e f u t u r e system p a t h a n d t h a t also has t h e v e l o c i t y v e c t o r as a t a n g e n t line; (3) t h e o p e r a t o r relates t h e kind o f parabola t o t h e j o y s t i c k position t h a t w o u l d p r o d u c e t h e parabola; a n d (4) t h e o p e r a t o r moves t h e j o y s t i c k t o t h e a p p r o p r i a t e position. Steps 1 a n d 4 r e p r e s e n t t h e stimulus a n d response aspects o f t h e t a s k a n d steps 2 a n d 3 r e p r e s e n t t h e mental model t h a t allows t h e t a s k t o b e S-C-R compatible. Without steps 2 a n d 3, t h e o p e r a t o r would n o t b e able t o see t h e consistencies in t h e t a s k . F o r S-C-R compatibility t o exist, t h e mental model must b e developed. Merely c h a n g i n g t h e d i s p l a y a n d response configurations f o r t h i s complex t a s k w i l l b e inadequate. T h e o p e r a t o r should b e trained, in steps 2 a n d 3 above, t o fit a parabola t o a c u r v e a n d a t a n g e n t l i n e a n d t o b e able t o associate a u n i q u e j o y s t i c k position t o a p a r t i c u l a r parabola. T h e consistency t a s k analysis specifies t h e k i n d s o f displays a n d methods t h a t c o u l d b e i n c o r p o r a t e d during a t r a i n i n g p r o g r a m . F o r t a s k s besides t h e second-order t r a c k i n g considered above, a t a s k analysis m u s t b e p e r f o r m e d t o determine t h e d i f f i c u l t i e s in t h e t a s k a n d t h e consistencies t h a t e x i s t . One o f t h e problems w i t h such a t a s k analysis is t h a t each t a s k i s d i f f e r e n t a n d each has d i f f e r e n t consistencies. T h e t a s k analyzer may have t o become an e x p e r t a t t h e t a s k in o r d e r t o p e r f o r m such an analysis. A s an a l t e r n a t i v e method, o r possibly as an a i d t o t h i s process, knowledge e x t r a c t i o n o f t h e mental models o f e x p e r t s c o u l d b e performed t o determine t h e k i n d s o f consistencies t h e e x p e r t s use. T h i s knowledge-extraction method a n d r e s u l t s f r o m it a r e considered in a l a t e r section. Experimental Evidence Experimental evidence has been collected t o s u p p o r t t h e role o f t h e mental model in S-C-R compatibility. If a t a s k is S-C-R compatible d u e t o a good mental model t h a t allows a one-to-one mapping between stimulus a n d response, several r e s u l t s could b e expected. F i r s t , subjects who have developed a good mental model should b e able t o p e r f o r m t h e t a s k b e t t e r t h a n subjects who have n o t developed a mental model o r who have developed a p o o r mental model. Second, subjects who e x h i b i t S-C-R compatibility d u e t o a good mental model should b e able t o process i n f o r mation more e f f i c i e n t l y t h a n subjects w i t h n o mental model o r a poor mental model. Finally, t a s k characteristics used by subjects should change as t h e mental model develops o v e r time, so t h a t displays compatible w i t h a developed mental model w i l l n o t necessarily b e compatible w i t h t h e mental model e a r l y i n i t s development stages. I n t h e experiment, t h e consistency t a s k analysis was used t o determine t h e k i n d s o f augmenting cues t h a t c o u l d b e used so t h a t t h e operators would b e able t o develop a mental model t h a t c o n t r i b u t e d t o S C-R compatibility. F o r a consistent model, subjects were g i v e n a parabola augmenting c u e d u r i n g t r a i n i n g t h a t f u l f i l l e d t h e requirements o f steps 2 a n d 3 in t h e above section. Specifically, as t h e subject moved

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t h e joystick, t h e parabola w o u l d change accordingly, so t h a t each j o y s t i c k position h a d a u n i q u e parabola associated w i t h it, a n d each parabola h a d t h e c u r r e n t v e l o c i t y as i t s t a n g e n t l i n e a t time equal t o zero. T h e subject would p e r c e i v e t h e f u t u r e p a t h of t h e system a n d t r y t o match a parabola t o t h a t p a t h . Subjects received t h i s augmenting c u e o n h a l f t h e t r i a l s during s i x 1 - h o u r t r a i n i n g sessions, so t h a t t h e y w o u l d n o t become dependent o n t h e c u e a n d would t r y t o develop a mental model o f t h e c u e o n those t r i a l s when it was n o t p r e s e n t . T h i s consistent model g r o u p was compared t o a c o n t r o l g r o u p t h a t received no augmenting c u e during training. A third g r o u p o f subjects, t h e inconsistent model group, received an augmenting c u e t h a t was shown in a t a s k analysis t o p r o v i d e inconsistent information t o t h e subjects (see E b e r t s & Schneider, 1980). D u r i n g t h e t r a i n i n g sessions in w h i c h all t h r e e g r o u p s t r a c k e d t h e second-order system, performance (in terms o f t r a c k i n g e r r o r ) w i t h n o augmenting cues p r e s e n t was about t h e same f o r a l l thre:, groups. However, in another experiment, t h e g r o u p s were g i v e n a stopped action" situation in w h i c h t h e y were p r o v i d e d w i t h a c u r r e n t v e l o c i t y a n d a p o i n t t o h i t in t h e f u t u r e , a n d t h e y were g i v e n as much time as needed t o make t h e response. In t h i s experiment, t h e consistent g r o u p p e r f o r m e d b e t t e r t h a n t h e c o n t r o l group, w i t h t h e inconsistent group e x h i b i t i n g t h e w o r s t performance (see Experiment 3 o f E b e r t s & Schneider, 1985). T h i s experiment showed c l e a r l y t h e beneficial effects t o performance o f a mental model t h a t allows t h e subjects t o know t h e consistent mappings between stimuli a n d responses. A n o t h e r experiment examined t h e e f f i c i e n c y o f information processing u n d e r consistent a n d inconsistent models. In t h i s experiment, t h e same t h r e e g r o u p s o f subjects p e r f o r m e d t h e t r a c k i n g t a s k u n d e r high workload conditions (see Eberts, 1987). Subjects in t h e inconsistent model g r o u p h a d a much h i g h e r decrement i n performance t h a n did t h e consistent model g r o u p when t h e y h a d t o p e r f o r m t h e second-order t r a c k i n g t a s k in a high workload situation. A p p a r e n t l y , t h e S-C-R compatibility allowed t h e consistent model g r o u p t o process t h e information more e f f i c i e n t l y t h a n did t h e inconsistent model g r o u p .

A f i n a l experiment examined t h e development o f t h e mental model o v e r time a n d i t s e f f e c t o n S - C - R compatibility. T h e Wickens S-C-R compatibility t h e o r y (Wickens e t al., 1983) w o u l d state t h a t a t a s k has a code associated w i t h it based on t h e t a s k s t r u c t u r e , so t h a t compatibility would n o t change. T h e r e v i s i o n o f t h e t h e o r y states t h a t t h e mental model is developed o v e r time, so t h a t compatibility can change. The experiment (see Eberts, 1983) was a "stopped action" experiment similar t o t h a t mentioned above. Performance o f s i x g r o u p s was analyzed: t h r e e no-mental-model g r o u p s (consistent, control, a n d inconsistent) a n d t h r e e mental-model g r o u p s (consistent, control, a n d inconsistent). T h e nomental-model g r o u p s were g i v e n minimal p r a c t i c e on t h e same augmenting cues as d e s c r i b e d p r e v i o u s l y f o r t h e t h r e e mental model g r o u p s . The r e s u l t s were t o t a l l y d i f f e r e n t depending on w h e t h e r a mental model h a d time t o develop. If t h e mental model was developed, t h e n performance was b e s t f o r t h e g r o u p t r a i n e d w i t h t h e parabola augmenting c u e a n d w o r s t f o r t h e inconsistent model g r o u p . F o r t h e no-mental-model groups, performance was b e s t in t h e c o n t r o l condition a n d w o r s t in t h e consistent rnodel (parabola) condition. T h e Wickens S-C-R compatibility theory would have a d i f f i c u l t time i n c o r p o r a t i n g these results, because t h e code o f t h e t a s k does n o t change but t h e compatibility a p p a r e n t l y does; t h u s some o t h e r mechanism w o u l d b e needed. B y p l a c i n g more o f t h e emphasis

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in t h e theory o n t h e mental model aspect instead o f t h e code o f r e p r e sentation, these r e s u l t s can b e handled b e t t e r . T h e r e s u l t s described in t h i s section p r o v i d e d good s u p p o r t i n g evidence f o r t h e r o l e o f t h e mental model in S-C-R compatibility. If subjects w e r e t r a i n e d t o develop a mental model based o n t h e consistency t a s k analysis, t h e n performance was b e t t e r a n d t h e processing o f information was more e f f i c i e n t compared t o those who did n o t develop a good mental model. I n addition, if t h e o p e r a t o r did n o t have time t o develop a mental model, t h e n c h a n g i n g t h e d i s p l a y characteristics t o i n c o r p o r a t e t h e augmenting cues indicated by t h e consistency t a s k analysis h a d n o e f f e c t o n performance. A mental model i s always c h a n g i n g a n d developing t o become a good mental model where "good" is d e f i n e d as c o n s i s t e n t l y showing t h e relation between t h e system input a n d t h e system o u t p u t . S u p p o r t i n g evidence was f o u n d f o r t h i s o n t h e one t a s k in a system c o n t r o l environment where t h e consistencies c o u l d b e stated s p a t i a l l y in terms o f t h e parabola. Development o f t h e operator's mental model c o u l d t h e n b e assisted t h r o u g h t h e use o f computer g r a p h i c s on t h e display. These concepts c o u l d b e t r a n s f e r a b l e t o o t h e r tasks, such as decision making a n d problem solving, t h a t d o n o t have t h e same possibilities f o r spatial representations a n d f o r w h i c h t h e consistency t a s k analysis may b e complex f o r a n o n - e x p e r t in t h e f i e l d . F o r these k i n d s o f tasks, t h e e x t r a c t i o n o f knowledge about t h e operator's mental model assumes an i m p o r t a n t r o l e in t h e specification of S-C-R compatibility. E x t r a c t i o n of Knowledge T h e i m p o r t a n t r o l e o f d e t e r m i n i n g t h e f o r m o f t h e mental model f o r t h e specification of information displays has been discussed in t h e p r e v i ous sections. A c c o r d i n g t o t h e Wickens e t al. (1983) theory, t h e import a n t knowledge t o e x t r a c t is t h e code o f representation o f t h e mental model f o r t h e t a s k . A c c o r d i n g t o t h e r e d e f i n i t i o n o f t h e t h e o r y stated in t h e p r e v i o u s sections, t h e knowledge t h a t must b e e x t r a c t e d is t h e consistencies between t h e system input a n d t h e system response. T h i s kind o f knowledge may b e e x t r a c t e d in a t a s k analysis if t h e researcher is familiar w i t h t h e task, b u t in complex tasks, e x p e c t i n g t h e researcher t o b e a t a s k e x p e r t is u n r e a l i s t i c . Being able t o e x t r a c t t h a t knowledge f r o m subject-matter e x p e r t s is essential in these cases. Once the c o n sistencies a r e determined, methods t o make these consistencies salient t o t h e operators can b e devised. F o u r knowledge e x t r a c t i o n techniques t e s t e d in o u r l a b a t P u r d u e U n i v e r s i t y a r e considered: situational analysis t o determine possible r u l e s used in various situations; s i m i l a r i t y judgments u s i n g multi-dimensional scaling techniques, in w h i c h subjective similarities a r e c o n v e r t e d t o distances a n d a dimensional s t r u c t u r e o f t h e knowledge is constructed; v e r b a l protocols in w h i c h t h e operators' p r o b lem s o l v i n g methods a r e verbalized d u r i n g a problem s o l v i n g task; a n d pause analysis, which i s also used t o c o n s t r u c t t h e goal hierarchies. A situational analysis determines t h e k i n d s o f Situational analysis. r u l e s used by an o p e r a t o r n o t by a s k i n g him o r h e r t o verbalize t h e r u l e s but by p r o v i d i n g a situation ( t h e I F p a r t o f a r u l e ) a n d a s k i n g t h e o p e r a t o r t o show t h e action ( t h e THEN p a r t o f a r u l e ) . Such an analysis was p e r f o r m e d f o r subjects in t h e t h r e e g r o u p s o f t h e second-order t r a c k i n g experiment. T h e subjects in t h i s experiment were allowed time t o generate a mental model by i n t e r a c t i n g w i t h a second-order c o n t r o l system in which

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t h e d i s p l a y was augmented by t h e consistent augmenting c u e o r t h e inconsistent augmenting cue. Each o f these g r o u p s o f subjects was comp a r e d t o a g r o u p o f subjects s e r v i n g in a c o n t r o l condition w i t h no augmenting cue. A f t e r t r a i n i n g a n d t h e development o f t h e models, s u b jects were g i v e n a booklet t h a t depicted various situations o f t h e system. As an example, a subject c o u l d see t h e position o f t h e t r a c k , t h e c u r r e n t v e l o c i t y vector, a n d t h e position o f t h e j o y s t i c k . F o r each situation, a subject was t o l d t o d r a w t h e p a t h o f t h e system. B y c o n s t r u c t i n g a l l t h e important situations a n d analyzing t h e d r a w n paths, i f - t h e n r u l e s can b e formulated a n d t h e set o f r u l e s can b e d e f i n e d as t h e subject's mental model f o r t h i s t a s k . A t a s k analysis (Eberts, 1988) showed t h a t t h e t a s k c o u l d b e described by a set o f s i x r u l e s (see T a b l e 1). Table 1 C o r r e c t Rules f o r C h a r a c t e r i z i n g t h e I n p u t / O u t p u t Relationships f o r a Second-Order System ~~

Condition

Action

1. j o y s t i c k input

o u t p u t moves in parabolic p a t h

2.

joystick i n p u t

o u t p u t moves i n i t i a l l y parallel t o c u r r e n t velocity

3.

joystick u p r i g h t

o u t p u t moves i n s t r a i g h t l i n e

4. j o y s t i c k n o t upright

output path curved

5.

output curves r i g h t

joystick left

6. j o y s t i c k right

output curves left

Table 2 contains examples o f some o f t h e r u l e s generated f o r t h r e e subjects, w h i c h a r e f a i r l y r e p r e s e n t a t i v e o f t h e o t h e r s in each o f t h e g r o u p s . Subject 4 f r o m t h e c o n t r o l g r o u p generated f o u r r u l e s but a l l o f them were o v e r - s i m p l i f i e d a n d all were i n c o r r e c t . O n l y one o f t h e f o u r subjects in t h a t g r o u p generated a s i g n i f i c a n t number of r u l e s t h a t were correct. Subject 12 f r o m t h e inconsistent g r o u p generated t h r e e rules, a n d all were i n c o r r e c t . In t h e inconsistent group, o n l y one o f t h e subjects generated a n y c o r r e c t r u l e s a t all. Finally, subject 19 generated t h r e e r u l e s t h a t were a l l c o r r e c t . O n l y one o f t h e e i g h t subjects f r o m t h e consistent g r o u p generated a s i g n i f i c a n t number o f i n c o r r e c t rules. Overall, 808 o f t h e r u l e s generated w e r e c o r r e c t f o r t h e consistent g r o u p , 30% o f t h e r u l e s generated were c o r r e c t f o r t h e inconsistent g r o u p , a n d 26% o f t h e r u l e s generated were c o r r e c t f o r t h e c o n t r o l group. T h i s r e s u l t was s t a t i s t i c a l l y s i g n i f i c a n t . Also, t h e consistent g r o u p generated more r u l e s t h a n a n y o f t h e o t h e r g r o u p s . T h e r e s u l t s f r o m t h i s situational analysis correspond t o t h e r e s u l t s f r o m t h e o t h e r experimentation on t h e second-order t r a c k i n g t a s k : The consistent g r o u p h a d t h e most accurate a n d most complete mental model.

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Table 2 Examples o f Rules Generated f o r Selected Subjects Condition

Action

Subject 4: joystick joystick joystick joystick

input upright

right left

output output output output

moves moves moves moves

in s t r a i g h t l i n e s t r a i g h t down

right left

Subject 12 : j o y s t i c k upright joystick and velocity point in opposite d i r e c t i o n s joystick and velocity vector p o i n t in same d i r e c t i o n

o u t p u t c u r v e s down o u t p u t moves in s t r a i g h t l i n e output curves back

Subject 19: j o y s t i c k input j o y s t i c k upright j o y s t i c k n o t upright

o u t p u t moves i n parabolic p a t h o u t p u t moves i n i t i a l l y parallel t o v e l o c i t y vector output path curved

T h i s i s t h e kind o f information t h a t should have been f o u n d when u s i n g a knowledge e x t r a c t i o n technique f o r t h e mental model. T h i s kind o f t e c h n i q u e is v e r y good a t c h a r a c t e r i z i n g a n d q u a n t i f y i n g t h e differences between t h e mental models, but t h e r e s u l t s d o n o t imply t h a t t h e subjects a c t u a l l y coded t h e information in terms o f t h e rules. T h e o n l y kind o f information e x t r a c t e d was t h e action t h a t t h e subjects would t a k e in a g i v e n situation, a n d t h i s information was c o n v e r t e d i n t o rules. Rules have been used p r e v i o u s l y t o characterize knowledge. F i t t s (1964) concluded t h a t people use r u l e s when p e r f o r m i n g motor tasks, such as t h e t r a c k i n g t a s k in t h e above experiment. Dulany, Carlson, a n d Dewey (1984) concluded t h a t subjects used r u l e s when deciding if s t r i n g s c o n s t r u c t e d f r o m f i n i t e - s t a t e grammars e i t h e r followed t h e grammatical r u l e s o r did n o t follow t h e rules. O t h e r evidence, however, indicates t h a t a set o f unorganized r u l e s d o n o t c o n s t i t u t e a mental model. A s an example, t h e e x p e r t system MYCIN has been developed t o p e r f o r m medical diagnoses. Clancey (1983) t h e n t u r n e d t h e e x p e r t system a r o u n d t o use it t o t r y t o teach diagnosis t o medical students. T h i s method did n o t w o r k because t h e s t u d e n t s h a d d i f f i c u l t y remembering all t h e rules, a n d t h e r u l e s were discrete, w i t h l i t t l e organization a n d relation between them. Some kind o f o r g a n i z i n g s t r u c t u r e must b e p r e s e n t in a good mental model. T h e situational analysis can e x t r a c t r u l e s but cannot

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e x t r a c t t h e s t r u c t u r e o f t h e mental model. I n terms o f S-C-R compatibility, t h i s e x t r a c t i o n technique p r o v i d e s l i t t l e information on t h e d i s p l a y methods t h a t make t h e stimulus compatible w i t h t h e mental model. S i m i l a r i t y judgments. E x t r a c t i o n o f knowledge about t h e mental model u s i n g s i m i l a r i t y judgments is u s e f u l f o r d e t e r m i n i n g possible s t r u c t u r e s o f t h e mental model. In a s i m i l a r i t y judgment analysis, subjects a r e g i v e n a questionnaire t h a t includes all possible p a i r s of items f r o m some domain. T h e subjects a r e asked t o rate, t y p i c a l l y u s i n g a 1 t o 7 scale, t h e s i m i l a r i t y o f each p a i r o f items. These s i m i l a r i t y r a t i n g s can t h e n b e used t o impose a s t r u c t u r e on t h e items t h a t were paired. T w o general k i n d s o f s t r u c t u r e s can b e determined: a dimensional s t r u c t u r e a n d a tree structure. For a dimensional s t r u c t u r e , t h e similarities a r e c o n v e r t e d i n t o distances. A statistical t e c h n i q u e called multidimensional scaling is used t o place those distances i n t o an n-dimensional space. Techniques a r e available t o determine t h e number o f dimensions t h a t b e s t fit t h e data; t y p i c a l l y t w o o r t h r e e dimensions a r e used. T h e dimensions must t h e n b e i n t e r p r e t e d t o p r o v i d e some meaning t o t h e s t r u c t u r e . F o r a t r e e s t r u c t u r e , c l u s t e r analysis is used t o c o n v e r t t h e simil a r i t y r a t i n g s i n t o distances w h i c h a r e t h e n c o n v e r t e d i n t o c l u s t e r s . Those items t h a t were r a t e d h i g h l y similar w o u l d b e c l u s t e r e d together, a n d those items r a t e d d i f f e r e n t would b e placed in d i f f e r e n t c l u s t e r s . In actuality, t h e similarities a r e again c o n v e r t e d t o distances. T h e distances a r e depicted t h r o u g h a s t r u c t u r e o f nodes a n d branches, so t h a t t h e d i s tance between a n y t w o items corresponds t o t h e distance one must t r a v e l along t h e branches a n d nodes t o g e t f r o m one item t o another. As an example, t w o items r a t e d h i g h l y similar would b e connected by a b r a n c h from one item t o a h i g h e r level node a n d t h e n down a b r a n c h t o t h e o t h e r item. T o find t h e connection between t w o h i g h l y dissimilar items would T h e nodes would r e q u i r e t h a t several branches a n d nodes b e t r a v e r s e d . t h e n b e i n t e r p r e t e d t o p r o v i d e some meaning t o t h e data.

A s t u d y u s i n g these techniques t o e x t r a c t knowledge was conducted In t h i s study, e x p e r t a t o u r P u r d u e U n i v e r s i t y l a b by Caldwell (1987). a n d novice programmers r a t e d all possible p a i r s o f 16 keywords used in t h e BASIC programming language. T h e multidimensional scaling t e c h n i q u e was applied u s i n g t h e K Y S T statistical package ( K r u s k a l & Wish, 1977). A three-dimensional space was determined t o fit t h e data best. Table 3 contains t h e aggregated dimensions f o r t h e novice subjects, a n d Table 4 contains t h e aggregated dimensions f o r t h e e x p e r t s . Dimensional spaces f o r i n d i v i d u a l subjects c o u l d also b e calculated. T h e subjects were shown these dimensions a n d were asked t o h e l p i n t e r p r e t t h e meaning o f t h e dimensions. A c l u s t e r analysis u s i n g t h e SAS package (SAS, 1985) was also performed f o r each i n d i v i d u a l subject a n d f o r t h e aggregate data. T h e t r e e s t r u c t u r e f o r t h e e x p e r t s is shown in F i g u r e 3 a n d t h e t r e e s t r u c t u r e f o r t h e novices is shown in F i g u r e 4. T h e subjects w e r e shown t h e i r i n d i v i d u a l t r e e s t r u c t u r e a n d were asked t o t r y t o label t h e nodes. T h e r e s u l t s f r o m t h i s s t u d y indicated t h a t these techniques showed l i t t l e differences between t h e s t r u c t u r e s o f t h e data f o r t h e e x p e r t s a n d novices. T h i s outcome can b e explained in several ways. First, t h e mental model s t r u c t u r e s have few differences f o r e x p e r t s a n d novices. Second, t h e novices a n d e x p e r t s used i n t h i s s t u d y were n o t t h a t much d i f f e r e n t a n d so would n o t have d i f f e r e n t mental models. Third, these

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Table 3 Results of t h e Multidimensional Scaling Procedure f o r Novice Subjects Dimension

I END STOP GOTO RETURN ELSE GOSUB THEN NEXT PRINT IF READ DATA FOR DIM INPUT LET

Dimension

Dimension

Ill

II IF THEN ELSE FOR GOTO GOSUB NEXT LET RETURN READ DIM INPUT DATA PRINT STOP END

PRINT DIM ELSE IF GOSUB STOP LET THEN GOTO INPUT FOR END RETURN READ DATA NEXT

Table 4 Results o f t h e Multidimensional Scaling Procedure f o r E x p e r t Subjects Dimension I

Dimens ion II

Dimension Ill

INPUT DATA DIM READ LET PRINT FOR NEXT IF ELSE END THEN STOP GOSUB RETURN GOTO

END STOP DATA RETURN DIM PRINT NEXT READ GOTO INPUT THEN LET GOSUB FOR ELSE IF

PRINT IF STOP THEN INPUT ELSE READ DATA END RETURN LET GOTO NEXT FOR GOSUB

DIM

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IF THEN ELSE FOR NEXT GOSUB

GOT0 RETURN

END STOP INPUT READ LET DATA

PRINT DIM

Figure 3.

Cluster analysis t r e e diagram f o r expert subjects.

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END STOP

PRINT

Figure 4 .

Cluster analysis t r e e diagram f o r novice subjects.

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41 1

techniques d o n o t r e a l l y e x t r a c t t h e s t r u c t u r e o f t h e subjects' mental models. T h e main d i f f e r e n c e between t h e e x p e r t s a n d novices was in t h e labels t h a t t h e y p r o v i d e d f o r t h e dimensions a n d t h e t r e e s t r u c t u r e s . E x p e r t s were more l i k e l y t o b e a b s t r a c t in t h e i r i n t e r p r e t a t i o n o f t h e t r e e nodes a n d dimensions. We believe t h a t t h i s a b i l i t y to b e a b s t r a c t r e f l e c t s some u n d e r l y i n g method t o e x t r a c t t h e consistent relation between t h e items. Because t h e novices were more l i k e l y t o think of an item in conc r e t e terms in a specific context, t h e y were n o t able t o extrapolate a consistent meaning f o r t h e item t h a t would b e consistent in a l l contexts. We believe t h a t t h e e x p e r t s w e r e able t o d o this, as shown by t h e v e r b a l labeling o f t h e dimensions a n d t r e e nodes. What a r e t h e implications o f S-C-R compatibility f r o m these results? C u r r e n t l y , we a r e examining d i s p l a y techniques in w h i c h t h e d i s p l a y methods a r e compatible w i t h some kind of u n d e r l y i n g mental model, as d e f i n e d f r o m t h e multidimensional scaling a n d t h e t r e e s t r u c t u r e s . A new d i s p l a y c o u l d b e designed in w h i c h d i f f e r e n t windows o n t h e d i s p l a y correspond t o t h e abstractions verbalized by t h e subjects. As an example, t h e d i s p l a y c o u l d have d i f f e r e n t locations f o r memory, operations, a n d o u t p u t . T h e p a r t i c u l a r k i n d of movement between t h e locations would depend on t h e c l u s t e r s a n d t h e dimensions f r o m t h e statistical techniques. For t h i s d i s p l a y technique, t h e c l u s t e r s a n d multidimensional scaling o u t p u t would map o n t o t h e d i s p l a y movements, a n d t h e v e r b a l r e p o r t s o f t h e abstractions o f t h e e x p e r t s would map o n t o d i s p l a y locations. We believe t h a t u s i n g a n d r e f i n i n g t h i s d i s p l a y t e c h n i q u e c o u l d r e s u l t in high compatibility between t h e d i s p l a y a n d t h e mental model of t h e programming task. Verbal protocols. V e r b a l protocols a r e used in problem-solving t a s k s t o determine t h e k i n d s o f goals a n d subgoals a subject uses in s o l v i n g a problem. F o r t h i s methodology, a subject is asked t o solve a problem v e r b a l i z i n g t h e goals a n d t h e methods f o r a t t a i n i n g those goals. A researcher w i l l r e c o r d these protocols a n d t h e n t r y t o c o n s t r u c t a goal h i e r a r c h y f r o m them. T h e goal h i e r a r c h y w i l l i n c l u d e t h e goals a n d t h e decomposition o f t h e goals i n t o subgoals. T h i s h i e r a r c h y p r o v i d e s a road map o f t h e problem solving s t r u c t u r e . In research a t P u r d u e U n i v e r s i t y , Koubek (1987) collected v e r b a l protocols a n d c o n s t r u c t e d goal hierarchies f o r e x p e r t s a n d s u p e r - e x p e r t s in computer programming. T h e e x p e r t s a n d s u p e r - e x p e r t s were f r o m i n d u s t r y , a n d t h e e x p e r t s were r a t e d by t h e i r peers as b e i n g good programmers, whereas t h e s u p e r - e x p e r t s were r a t e d as b e i n g t h e b e s t programmer a t t h e site. T h e subjects were g i v e n a 350-line database program, w r i t t e n in FORTRAN, t h a t p r o v i d e d t h e u s e r w i t h CREATE, UPDATE, SHOW, a n d DELETE f u n c t i o n s . T h e subject was r e q u i r e d t o a d d a new command, RESTORE, t h a t would allow r e c o v e r y o f a deleted r e c o r d during t h e same session. V e r b a l protocols were collected f r o m t h e subjects during t h e i r problem-solving t a s k . Koubek recorded t h e p r o t o cols a n d c o n s t r u c t e d goal hierarchies f r o m them.

The results of this from t h e Caldwell (1987) programmers. T h e main e x p e r t s was in t h e goal super-experts inserted a

s t u d y were similar, i n some ways, t o t h e r e s u l t s s t u d y on e x p e r t / n o v i c e differences i n computer differences between t h e e x p e r t s a n d t h e s u p e r h i e r a r c h y level j u s t below t h e main goal. The level o f a b s t r a c t subgoals in w h i c h t h e y t r i e d t o

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understand t h e whole program b e f o r e deciding o n a solution t o t h e p r o b lem. T h e experts, o n t h e o t h e r hand, did n o t t r y t o acquire an overall u n d e r s t a n d i n g of t h e p r o g r a m a n d were v e r y d i r e c t e d in t h e i r search. Again, we believe t h a t t h e super-experts had t h i s e x t r a abstract level t o make t h e i r mental model o f t h e program more consistent. T h e y could add code t o t h e program, a n d t h e response o f t h e program would b e consist e n t w i t h t h e model t h a t t h e y h a d o f t h e response. On t h e o t h e r hand, t h e e x p e r t s would t r y a c e r t a i n solution in t h e c o n t e x t o f p a r t o f t h e program, a n d t h e n t h e response o f t h e program would n o t b e consistent t h r o u g h o u t t h e code. Consequently, t h e y were o f t e n f o r c e d t o abandon t h e i r i n i t i a l solution a n d develop a new one. T h e implications o f u s i n g goal s t r u c t u r e s f o r d i s p l a y i n g information as defined by these goal structures, is unclear a t t h i s time. F u r t h e r research is A p o s s i b i l i t y is t o p r o v i d e t h e programmer w i t h some kind o f needed. assistance o n t h e task, based o n t h e goal s t r u c t u r e used by a superexpert. T h i s goal s t r u c t u r e allows t h e programmer t o see t h e consistencies in t h e t a s k . For t h i s kind o f display, t h e programmer would b e r e q u i r e d t o c o n s t r u c t c e r t a i n goals and subgoals t o p e r f o r m t h e task, especially t h e goal level o f u n d e r s t a n d i n g a b s t r a c t l y t h e whole program. In addition, e r r o r feedback could b e p r o v i d e d t o t h e programmer based on t h e goal s t r u c t u r e instead o f t h e s y n t a x . T h i s again would b e comp a t i b l e w i t h t h e mental model. so t h a t t h e information is compatible w i t h t h e mental model,

Pause analysis. T h e last knowledge e x t r a c t i o n method t o b e considered is pause analysis. T h e purpose o f pause analysis is t o e x t r a c t goal hierarchies w i t h o u t such an o b t r u s i v e a n d subjective method as t h e v e r b a l protocols discussed above. For t h e v e r b a l protocols, t h e q u a l i t y o f t h e data depends o n many factors: t h e a b i l i t y o f t h e subject t o verbalize t h e problem-solving strategies, t h e assumption that t h e subject is v e r b a l i z i n g t h e methods always used, and t h e assumption t h a t all p a r t s o f t h e problem solving are open t o conscious introspection. T h e process o f verbalization may change t h e n a t u r e o f t h e t a s k . In addition, t h e experimenter must collect t h e data properly, t h e data must b e i n t e r preted, and t h e s t r u c t u r e of t h e goal h i e r a r c h y must b e imposed on t h e data by t h e experimenter. C o n s t r u c t i n g goal hierarchies by a pause analysis can avoid some of these problems. Pause analysis i s based on t w o assumptions. The f i r s t assumption is t h a t completion o f a goal o r subgoal requires t h r e e p a r t s : (sub)goal initiation, (sub)goal execution, a n d (sub)goal evaluation. Observable actions, such as keystrokes o r mouse movements, w i l l occur during t h e execution. For t h e initiation a n d evaluation parts, however, n o observable actions w i l l occur a n d a pause w i l l b e present. T h e second assumption is t h a t t h e pauses f o r a l l (sub)goal initiations a n d ( s u b l g o a l evaluations w i l l b e approximately equal. Using t h i s last assumption, pause analysis p r o v i d e s a means t o objectively determine t h e s t r u c t u r e o f t h e goal hierarchies. Generally, long pauses mean t h a t i n t e r - l e v e l t r a n sitions o c c u r a n d s h o r t pauses mean t h a t i n t r a - l e v e l transitions occur. For i n t r a - l e v e l transitions, o r transitions between t w o goals o r subgoals a t t h e same level in t h e hierarchy, a pause w i l l consist o f an evaluation For i n t e r of t h e last (sub)goal a n d t h e initiation of t h e n e x t (sub)goal. level transitions, a pause w i l l consist o f t h e following f o u r p a r t s : subgoal evaluation, goal evaluation, goal initiation, a n d subgoal initiation. T h i s kind o f i n t e r - l e v e l t r a n s i t i o n should b e approximately twice as long

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as t h e i n t r a - l e v e l t r a n s i t i o n . I n t e r - l e v e l t r a n s i t i o n s between t h r e e levels should r e q u i r e six p a r t s a n d t a k e even longer. Pause analysis has been used in t h e p a s t t o c o n s t r u c t goal h i e r a r chies. Robertson a n d Black (1983) collected t h e pauses between keystrokes in an e d i t i n g t a s k a n d f o u n d t h a t t h e pauses were meaningful; thus, a goal h i e r a r c h y o f t h i s simple t a s k could b e constructed. Card, specified a model o f human-computer Moran, a n d Newel1 (1983) interaction, called GOMS, t h a t was specified in terms o f goal stacks a n d A version of t h e GOMS model, called t h e keystrokegoal hierarchies. level model, emphasized t h e importance o f collecting t h e pauses t o gain an understanding o f t h e u n i t s o r c h u n k i n g used by t h e subject.

In o u r lab a t Purdue U n i v e r s i t y , we have been trying t o e x t e n d pause analysis b e y o n d these t w o e f f o r t s . F i r s t , we a r e attempting t o e x t e n d pause analysis beyond t h e simple tasks s t u d i e d by Robertson a n d Black (1983) t o include a whole session o f a t a s k where t h e subject has l a t i t u d e i n t h e k i n d s o f strategies used. Whereas pauses in t h e Robertson a n d Black s t u d y were measured in milliseconds, t h e pauses collected in o u r l a b a r e o f t e n meaningful when measured in t h e seconds. Second, C a r d e t al. (1983) collected v e r b a l protocols a t t h e same time i n t e r - k e y s t r o k e times were collected. T h e s t r u c t u r e o f t h e goals was based mostly on t h e protocols instead o f t h e pause times. We would l i k e t o c o n s t r u c t t h e goal h i e r a r c h y w i t h o u t collecting v e r b a l protocols, so t h a t t h e hierarchies a r e n o t so dependent o n i n t e r p r e t a t i o n . In addition, b o t h previous studies o n l y used e x p e r t subjects t o minimize pauses d u e t o o t h e r causes. When p e r f o r m i n g pause analysis, one must b e c a r e f u l t o d i f f e r e n t i a t e pauses d u e t o o t h e r causes, such as reading o r errors, f r o m t h e goal evaluation a n d initiation pauses. We a r e trying t o e x t e n d pause analysis t o novices also. Pause analysis was used by Eberts, Lang, a n d Gabel (1987) t o analyze t h e differences between e x p e r t s and novices in a computer-aided design (CAD) problem. T h e novices were undergraduates who had completed t w o one-semester courses on C A D w i t h one o f t h e courses devoted t o t h e C A D system tested in t h e s t u d y . One o f t h e e x p e r t s had o v e r 20 years experience in engineering design b u t l i t t l e experience on t h e CAD system used. T h e o t h e r expert, besides b e i n g an e x p e r t designer, was experienced o n t h e system. T h e C A D system u t i l i z e d three-dimensional wire-frame modeling t o d i s p l a y t h e object. T h e subjects were g i v e n a description o f t h e object t o design in t h r e e dimensions. For t w o o f t h e novices and t h e t w o experts, t h e sessions were videotaped so t h a t a pause analysis could b e performed o n t h e t a s k . Table 5 shows t h e goal s t r u c t u r e f o r one o f t h e experts, a n d Table 6 shows t h e goal s t r u c t u r e f o r one of t h e novices. T h e numbers f o r t h e pauses indicate t h e time in seconds when t r a n s i t i o n i n g between t h e goal boxes. A general requirement f o r determining t h i s s t r u c t u r e was t h a t t h e pauses f o r t h e h i g h e r level goals should always b e l o n g e r t h a n t h e In some cases, such as t h e f i r s t goal pauses f o r t h e lower level goals. in Table 5 a n d t h e third goal in Table 6, some o f t h e lower level pauses a r e s h o r t e r t h a n t h e h i g h e r level pauses. These adjustments o c c u r r e d because, w i t h o u t them, clearly unrelated subgoals would have been placed u n d e r one common goal s t r u c t u r e . Some adjustment a n d i n t e r p r e t a t i o n of t h e pauses is needed.

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Table 5 Goal S t r u c t u r e o f Design E x p e r t Goa I/S ubgoa I 1.

D r a w circles a n d t a n g e n t lines between circles 1.1 1.2

1.3 1.4 1.5 1.6 2.

Draw Draw Draw Draw Connect circles Connect circles

V e r t i c a l lines T o p horizontal lines Bottom horizontal lines

L a r g e circles Keyway Keyway

Draw Draw Draw

Left p a r t Right p a r t

s s s s s

_5.5 s 6.5 s 7.0 s

Clean connections Trim first part T r i m second p a r t

Project 4.1 4.2

-_ 12.1 13.4 12.1 5.1 10.1

31.0 s

Clean a n d t r i m 3.1 3.2 3.3

4.

Small l e f t c i r c l e Large left circle A r c right Small right c i r c l e T a n g e n t lines t o p T a n g e n t lines bottom

Pause 16.7 s

Draw keyway 2.1 2.2 2.3

3.

Description

__ 7.0 s 10.7 s 9.0 s

Circles a n d k e y w a y Circles

5.1 s 8.6 s

Clear differences in t h e s t r u c t u r e s o f t h e strategies used by t h e e x p e r t s a n d novices were seen in t h e r e s u l t s f r o m t h e pause analysis. T h e e x p e r t s t r u c t u r e was organized a r o u n d t h e f u n c t i o n s o f t h e commands, w i t h t h e p a r t i c u l a r p a r t s o f t h e designed object a r r a n g e d as subgoals. T h e novice s t r u c t u r e was organized a r o u n d t h e p a r t s o f t h e object, w i t h t h e p a r t i c u l a r command f u n c t i o n s a r r a n g e d as subgoals. E b e r t s e t al. . (1987) termed t h e e x p e r t s t r u c t u r e a f u n c t i o n - r e l a t e d s t r a t e g y a n d t h e novice s t r u c t u r e an object-related s t r a t e g y . The funct i o n - r e l a t e d s t r a t e g y was highly s t r u c t u r e d , w h e r e b y t h e e x p e r t f i r s t d r e w t h e major p a r t s o f t h e object, d r e w t h e connections between these p a r t s , trimmed t h e design, a n d projected t h e 2 D image t o a 3 D image. T h e object-related s t r a t e g y was n o t as h i g h l y s t r u c t u r e d , in t h a t subjects drew, connected, trimmed, a n d projected i n one area o f t h e design, a n d t h e n did t h e same p r o c e d u r e in another area o f t h e design. This s t r a t e g y i n v o l v e d b a c k t r a c k i n g t o t h e various subtasks.

41 5

The Mental Model in S-R Compatibility Table 6 Goal S t r u c t u r e o f Novice Subject ~~

Goal/S ubgoal

Pause

1 . Draw l e f t p a r t o f object

9.3 s

1.1 Small l e f t c i r c l e 1.2 L a r g e l e f t c i r c l e 1.3 Project b o t h circles

9.2 s 11.2 s

2. D r a w right p a r t o f object 2.1 Small right c i r c l e 2.2 Project small c i r c l e o n l y 2.3 D r a w a r c right a n d p r o j e c t 2.3.1 Project a n d d r a w t o p t w o t a n g e n t lines 2.3.2 D r a w bottom t a n g e n t l i n e 2.3.3 D r a w second bottom t a n g e n t l i n e 3. Draw keyway 3.1 D r a w horizontal lines 3.1.1 I n s e r t f i r s t l i n e 3.1.2 I n s e r t second l i n e 3.1.3 I n s e r t t h i r d l i n e 3.2 D r a w v e r t i c a l lines 3.3 T r i m lines a n d c i r c l e connections 3.3.1 T r i m f i r s t l i n e 3.3.2 T r i m second l i n e 3.3.3 T r i m third l i n e 3.3.4 T r i m f o u r t h l i n e 3.3.5 Clean f i r s t c i r c l e connections on keyway 3.3.6 Clean second c i r c l e connections o n k e y w a y 3.3.7 Clean third c i r c l e connections on k e y w a y

--

17.7

s

--

17.5 s 16.0 s

_-

7.3 s 8.4 s 26.2 s

_-4.3 3.4 15.6 7.9

_-

s s s s

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If t h e goal s t r u c t u r e s c o n s t r u c t e d f r o m t h e pause analysis truly reflect a subject’s mental model, t h e n we should expect differences between t h e mental models of e x p e r t s a n d novices, as was shown in t h e experiment. Also, t h e mental models should d i f f e r , based on perceived consistencies in t h e t a s k . T o t e s t t h i s l a t t e r notion, a t r a n s i t i o n analysis was performed on t h e data (see F i g u r e 5 f o r t h e t r a n s i t i o n s f o r a novice a n d F i g u r e 6 f o r t h e t r a n s i t i o n s o f an e x p e r t ) . F o r a t r a n s i t i o n analysis, t h e t a s k is b r o k e n down i n t o subtasks t h a t must b e accomplished t o successfully p e r f o r m t h e total t a s k . I n t h e figures, t h e subtasks a r e r e p r e sented by t h e boxes. T h e t r a n s i t i o n s of a p a r t i c u l a r subject a r e indicated by t h e arcs between t h e boxes; t h e a r c s on t h e right indicate t h e needed t r a n s i t i o n s a n d t h e a r c s on t h e l e f t indicate unneeded, e x t r a t r a n s i t i o n s d u e t o p o o r s t r u c t u r i n g o f t h e t a s k . A n y k i n d s o f e r r o r s by t h e subjects were n o t i n c l u d e d i n t h e transitions, so t h a t t h i s analysis

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o n l y examined s t r u c t u r i n g o f t h e t a s k independent o f t h e number of errors. Also, if t h e subject perceived t h e consistencies in t h e t a s k and how subgoals relate t o each o t h e r a n d could b e organized together, t h e n t h e minimal number o f t r a n s i t i o n s would b e needed. Thus, t h e number o f t r a n s i t i o n s represent a m e t r i c f o r t h e consistency in t h e mental model o f a subject: Fewer t r a n s i t i o n s mean a h i g h e r degree o f consistency in t h e model. As Figures 5 a n d 6 g r a p h i c a l l y illustrate, t h e e x p e r t ' s model is In t h i s study, t h e novices h a d more consistent t h a n t h e novice's model. about twice as many t r a n s i t i o n s as t h e e x p e r t s . Bringelson (1988) has f u r t h e r s t u d i e d pause analysis, goal h i e r a r chies, a n d t r a n s i t i o n analysis t o determine if these analyses a r e sensitive t o manipulations t h a t should have an e f f e c t o n subjects mental models during a t e x t e d i t o r t a s k . First, she measured t h e spatial a b i l i t y o f t h e subjects u s i n g s t a n d a r d tests t o place them in high o r low spatial a b i l i t y groups. She also placed t h e subjects in e i t h e r low o r high j o b e n r i c h ment situations d u r i n g t r a i n i n g a n d in e i t h e r high or low workload s i t u ations d u r i n g t r a n s f e r tasks. Figures 7 and 8 contain examples o f t h e goal s t r u c t u r e s f o r high and low c o g n i t i v e load conditions, respectively. These f i g u r e s were c o n s t r u c t e d u s i n g t h e techniques o u t l i n e d earlier on the CAD task. T r a n s i t i o n analysis was used t o p r o v i d e q u a n t i t a t i v e data f o r statistical analyses o f t h e effects of t h e manipulations o n t h e subjects' mental models. T h e results showed t h a t t h e number o f t r a n s i t i o n s was a sensitive dependent variable t o t h e experimental manipulations. Overall, subjects w i t h high spatial a b i l i t y h a d more transitions t h a n did subjects w i t h low spatial a b i l i t y . A l l subjects coming i n t o t h e experiment were novices f o r t h e t e x t e d i t i n g task, so t h a t a b i l i t y on t e x t e d i t i n g was constant. T h e differences in t h e spatial a b i l i t y could mean t h a t subjects w i t h low spatial a b i l i t y were f o r c e d t o have more consistent models o f t h e t a s k in o r d e r to p e r f o r m it. Spatial a b i l i t y did interact, however, w i t h j o b enrichment. H i g h spatial a b i l i t y subjects in t h e high j o b enrichment condition h a d fewer t r a n s i t i o n s t h a n those in t h e low j o b enrichment condition, a n d t h e e f f e c t was in t h e opposite d i r e c t i o n f o r low spatial ability subjects. In terms o f t h e cognitive workload condition, high workload did n o t seem t o change t h e s t r u c t u r e of t h e goal h i e r a r c h y b u t h a d more e f f e c t o n t h e q u a n t i t y of t h e lower level goals. Pause analysis a n d t r a n s i t i o n analysis seem t o b e e f f e c t i v e means f o r e x t r a c t i n g knowledge about t h e mental model used f o r t h e t a s k . If t h e goal hierarchies a r e a reflection o f t h e subjects' mental models, t h e n we would expect t h e s t r u c t u r e s t o change according t o expertise, a b i l i t y o f t h e subjects, a n d social conditions o f t h e environment. The experiments showed t h a t these changes o c c u r r e d . T h e t r a n s i t i o n analysis p r o vides a method t o measure t h e consistency o f t h e models. As would b e expected, t h e models become more consistent w i t h more expertise in t h e task. Pause analyses in t h e above tasks have implications f o r achieving high S - C - R compatibility. F o r t h e CAD task, students should b e p r e sented w i t h information so t h a t t h e y can b u i l d u p an accurate, efficient, a n d consistent mental model. T h e students should b e t a u g h t t o plan t h e i r strategies a r o u n d t h e functions and t o look a t t h e whole object Perhaps another p o s s i b i l i t y would instead of b r e a k i n g it u p i n t o p a r t s . b e t o design t h e interaction t o t a k e i n t o account t h e function-related i n t h e interaction, t h e u s e r would b e put i n t o one o f t h e strategy.

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f u n c t i o n modes a n d t h e computer would p r o m p t t h e u s e r w i t h t h e d i f f e r e n t p a r t s of t h e o b j e c t t h a t could b e w o r k e d on w h i l e in t h a t mode. T h i s would encourage t h e c o n s t r u c t i o n o f a f u n c t i o n - r e l a t e d s t r a t e g y a n d w o u l d b e compatible w i t h those users already h a v i n g t h i s s t r a t e g y . T h e purpose o f t h e Bringelson (1988) experiment was more t o examine t h e analysis techniques t h a n t o determine compatibility possibilities. No expert/novice differences were collected in t h i s case. More w o r k needs t o b e done f o r t h e t e x t e d i t i n g t a s k t o determine d i s p l a y designs t h a t would achieve high S - C - R compatibility. Summary T h e notion o f S-C-R compatibility has p r o g r e s s i v e l y changed t h r o u g h o u t t h e development o f t h i s c h a p t e r a n d t h e discussion o f t h e experimental r e s u l t s herein. I n i t i a l l y , Wickens e t al. (1983) i n t r o d u c e d t h e concept by t h e o r i z i n g t h a t each t a s k has a code t h a t lies along some s p a t i a V v e r b a l continuum a n d t h a t t h i s code does n o t change. T h e code is in some way attached t o t h e operator's mental model o f t h e t a s k . If t h e code o f t h e mental model c o u l d b e specified, t h e n t h e stimulus/central processing compatibility a n d t h e c e n t r a l processing/response compatibility could b e specified. T h i s compatibility t h e o r y p r o v i d e d a good explanation o f experimental r e s u l t s in d u a l t a s k situations. Subsequent research on t h e mental model i n d i c a t e d t h a t a s t a t i c code on a spatial/verbal continuum may n o t b e adequate t o s p e c i f y compatibility. T h e E b e r t s a n d Schneider (1985) r e s u l t s i n d i c a t e d t h a t t h e

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mental model of a task is dependent on t h e k i n d of t r a i n i n g and experience of t h e operator and t h a t t h e mental model may change over time. Additionally, good and bad models seem t o be dependent on t h e perceived consistencies i n t h e task being modeled. Designing t h e environment t o be compatible w i t h a bad mental model may not be appropriate. Designing an environment t h a t is compatible w i t h a bad mental model and, by definition, a mental model t h a t is inconsistent, may be harmful t o t h e development of a good mental model. Basic research has shown t h a t if t h e task is inconsistent, performance will not improve even w i t h vast amounts of practice. Instead, t h e environment should be designed t o be compatible w i t h t h e good mental model, which may s t i l l be i n i t s development stages. The idea of using augmenting cues t o make t h e consistencies more salient t o t h e operator was introduced f o r a second-order t r a c k i n g task i n o r d e r t o assist t h e operators i n developing a good mental model. A consistency task analysis can be used t o determine t h e form of a good mental model and t h e nature of t h e augmenting cues. Experimental evidence was p r e sented t h a t supported t h e advantages of consistent augmenting cues and t h e development of consistent mental models. Under t h e above formulation, t h e s t r u c t u r e of the mental model has assumed primary importance over t h e earlier notion of t h e code o f t h e mental model f o r a task. If t h e s t r u c t u r e can be defined, then methods Subsequent t o achieve high S-C-R compatibility could be specified. discussion was devoted t o examining methods t o e x t r a c t t h e s t r u c t u r e of the mental model f o r various kinds of tasks, subjects, and conditions. For good extraction methods, one could expect t h e following: Differences between t h e mental models would occur f o r experts and novices; the experts' mental model would be somehow more consistent than t h e novices' mental model; some individual differences would occur between t h e models extracted; and the models should be dependent on learning and social conditions. Several analysis techniques t o extract the s t r u c t u r e of t h e mental models were discussed. The f i r s t technique, situational analysis, was useful f o r extracting any r u l e information stored i n t h e mental model. The second technique, similarity judgments, was used t o determine the dimensional o r hierarchical s t r u c t u r e of an underlying mental model. The t h i r d technique, verbal protocols, was used t o extract t h e problem solvi n g techniques used i n t h e form of goal hierarchies. The final technique, pause analysis, was similar t o verbal protocols i n t r y i n g t o extract goal hierarchies b u t did not require verbal protocols and used t h e pauses between actions t o impose a s t r u c t u r e on t h e task. '

A l l the techniques had strengths and weaknesses. Situational analysis extracted rules t h a t helped t o differentiate between t h e d i f f e r e n t mental models. Whether t h e rules represent a t r u e underlying mental model o r a b y p r o d u c t of t h e processes used by the mental models is unclear. Similarity judgments were insensitive t o t h e experience of t h e subjects, which could be due t o small differences between t h e subjects. This was unlikely because verbal labeling of t h e dimensions o r nodes showed t h e differences between the two groups. Verbal protocols and pause analysis seemed t o be good techniques t o extract knowledge about the structures of t h e mental models. I n t h e experimentation reported, the extracted mental models were sensitive t o expertise differences, individual subject differences, and social environment differences. Once goal hierarchies were found, a transition analysis could be used t o determine a

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metric for t h e consistency of t h e model. The pause analysis seemed t o be more objective and less obtrusive than the verbal protocol method. Once t h e s t r u c t u r e of t h e mental model can be identified, methods Examples were p r o t o improve t h e S-C compatibility can be specified. vided. When using t h e similarity judgments t o extract knowledge about computer programming, an augmented display f o r novice computer p r o grammers was specified t h a t should allow the students t o better see t h e consistencies i n the task. Teaching techniques f o r CAD were specified a f t e r t h e expert and novice goal structures were extracted using t h e pause analysis. We are in t h e process of designing some of t h e displays f o r high S-C-R compatibility and will be collecting performance data on t h e displays. T h e previous studies w i t h second-order t r a c k i n g indicate t h a t these displays may be successful. When s t u d y i n g complex tasks, such as tracking, CAD, and comp u t e r t e x t editing, as we have studied in human factors, S-C-R compatibility must be considered i n t h e context of t r a i n i n g and the associated development of mental models. These tasks all require the development of a mental model of t h e task itself. Designing t h e environment t o be highly compatible w i t h bad mental models f o r novices is dangerous because of the inconsistencies in these models and the prospect of slow development times. Instead, t h e environment should be designed t o be compatible w i t h good mental models t h a t are consistent. These mental models can be specified t h r o u g h consistency task analyses o r t h r o u g h t h e extraction of t h e s t r u c t u r e of t h e mental models of task experts. More work needs t o be done on t h e extraction techniques and t h e relation between the extracted knowledge and t h e display specifications. Acknowledgments This review and some of t h e research which was reviewed was supported i n p a r t by t h e Information Science Division of t h e National Science Foundation under Award Number 8609418 and by t h e Design, Manufacturing, and Computer Integrated Engineering Division of t h e National Science Foundation as p a r t of t h e Presidential Young Investigator program. References Anderson, J . R . NJ: Erlbaum. Anderson, J. R. mental imagery.

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STIMULUS-RESPONSE COMPATI8lLlTY R. W. Proctor and T.G. Reeve (Editors 0 Elsevier Science Publishers 8. V. (dorth-HollandJ. 1990

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TOWARD AN ENGINEERING MODEL OF STIMULUS-RESPONSE COMPATIB I LlTY BONNIE E. JOHN Department o f Psychology Carnegie Mellon U n i v e r s i t y and ALLEN NEWELL Department o f Computer Science Carnegie Mellon U n i v e r s i t y Stimulus-response ( S - R ) compatibility effects a r e e v e r y w h e r e . A n y American who has d r i v e n a c a r in England can a t t e s t t o t h i s . D r i v i n g on t h e "wrong" side o f t h e road uncovers many d i f f e r e n t cases o f S - R incompatibility: making a right turn i n t o t h e w r o n g lane, s h i f t i n g gears w i t h t h e opposite hand, h a v i n g t h e windshield-wiper c o n t r o l s t i c k b e where t h e t u r n - s i g n a l should be. A l l these situations lead t o t h e subject i v e impression (and, we would p r e d i c t , actual performance) o f longer response time a n d more e r r o r s , t h e classic measures o f S-R compatibility effects . P u r e l y c o g n i t i v e activities a r e subject t o S - R compatibility effects. If t h e r e is a mismatch between t h e way a p e r s o n encodes information a n d t h e way new information is presented t o t h a t person, it w i l l t a k e longer t o learn t h a t information t h a n if t h e r e were a match. A n d it is n o t t h e same f o r e v e r y person. F o r example, directions t o a new r e s t a u r a n t can b e g i v e n in p r o s e o r d r a w n on a map; some people would g e t hopelessly lost w i t h t h e one but n o t t h e o t h e r . Because S - R compatibility is i n h e r e n t i n almost e v e r y a c t i v i t y , careful design o f new systems would help almost a n y ;ctivity proceed B u t w h a t does c a r e f u l design" more quickly a n d w i t h f e w e r e r r o r s . mean? Given i n d i v i d u a l differences, how c o u l d one design b e deemed " b e t t e r " t h a n another? T r a d i t i o n a l l y , t h e answers t o these questions have been: c a r e f u l design uses "common sense;" a n d one design is b e t t e r t h a n another if it is b e t t e r f o r t h e i n d i v i d u a l d e s i g n i n g t h e system. These h e u r i s t i c s d o n o t o f f e r much guidance f o r design. It would b e u s e f u l i n t h e design process t o have a t h e o r y o f S-R compatibility t h a t would make p r e d i c t i o n s about human performance. An "engineering model" o f human performance in S-R compatibility situations would have t o i n c l u d e c e r t a i n features t o b e h e l p f u l in design. The model would have t o make q u a n t i t a t i v e p r e d i c t i o n s about human performance: time t o perform, time t o learn, how many e r r o r s a n d o f what k i n d . T h i s information would allow evaluation o f t r a d e - o f f s i n design. F o r example, a p a r t i c u l a r c o n t r o l panel c o n f i g u r a t i o n m i g h t b e more expensive t h a n another configuration, but each operation u s i n g t h a t panel m i g h t b e p r e d i c t e d t o t a k e less time. Combined w i t h information about t h e number o f operations p e r year a n d t h e h o u r l y wage o f t h e panel operators, t h e capital cost c o u l d b e weighed against t h e operational cost p e r y e a r a n d t h e design t r a d e - o f f c o u l d b e evaluated q u a n t i t a t i v e l y .

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A useful model would b e approximate t o t h e level needed f o r t h e design task. One t y p e of approximation is in t h e performance chosen f o r prediction. F o r instance, an engineering model m i g h t seek t o p r e d i c t response time, r a t h e r t h a n t h e p a t t e r n o f evoked potentials, because an estimate o f task-performance time i s needed in t h e design t r a d e - o f f evaluation. A n o t h e r t y p e o f approximation is in how close t o actual p e r f o r mance t h e predictions must b e t o b e acceptable. A common engineering r u l e o f t h u m b i s t h e "80/20 rule," which states t h a t 80% o f results come f r o m 208 o f t h e e f f o r t , but it takes 80% more e f f o r t t o accomplish t h a t last 20% o f results. Therefore, a model that p r e d i c t s t o about 20% o f actual performance w i t h less t h a n 20% o f t h e e f f o r t i n v o l v e d in measuring performance i s a serviceable engineering model. Furthermore, it is necessary t o make predictions w i t h zero parameters (i.e., without running experiments on an e x i s t i n g device t o set parameters), so t h a t t h e predictions can b e made a t t h e specification stage o f design. Finally, because S-R compatibility effects a r e f o u n d in a wide range o f tasks, and because t h e design o f a system embeds S-R compatibility tasks w i t h i n o t h e r tasks, an engineering model o f S-R compatibility must b e integrated w i t h a b r o a d theory of behavior. Where m i g h t we s t a r t t o create such an engineering model? The desire f o r integration w i t h o t h e r tasks points t h e search in t h e direction o f a u n i f i e d theory o f cognition, a t h e o r y t h a t spans a wide range o f human behavior (Newell, 1987). Few such theories exist: ACT* (Anderson, 1983), Soar (Laird, Newell, E Rosenbloom, 1987), a n d t h e Model Human Processor (MHP) (Card, Moran, & Newell, 1983) are perhaps t h e b e s t known. T h e domain of S-R compatibility points t h e search in t h e d i r e c t i o n of theories o f S-R compatibility. Again, o n l y a few such theories e x i s t w i t h t h e b r e a d t h necessary f o r engineering design (see Shepard, 1961, a n d Rosenbloom & Newell, 1987, f o r examples). One t h e o r y o f S-R compatibility, f i r s t presented by Rosenbloom in 1983 (also in Rosenbloom 6 Newell in 1988), is a GOMS model, a model cast in terms o f Goals, Operators, Methads a n d Selection rules w i t h i n t h e MHP framework. Since Rosenbloom's t h e o r y o f S-R compatibility already f i t s w i t h i n a u n i f i e d t h e o r y o f cognition, his t h e o r y and t h e MHP w i l l b e t h e s t a r t i n g points f o r an engineering model o f S-R compatibility. T h e MHP Model of Immediate Behavior T h i s section describes a model o f immediate behavior, i.e., where a stimulus is mapped d i r e c t l y i n t o a response w i t h o u t problem solving o r planning. Such behavior is o f t e n e x h i b i t e d in t h e experimental w o r l d w i t h choice-reaction tasks; t h e response t o a stimulus is well known and i n i t i a t e d immediately upon presentation o f t h e stimulus, w i t h o u t deliberat i o n about w h a t response is appropriate. When sets o f stimuli a r e mapped i n t o d i f f e r e n t set's of responses, a n d t h e response time o r accuracy o f these mappings d i f f e r , S-R compatibility effects a r e observed. Thus, in situations where S-R compatibility effects a r e found, o u r model o f immediate behavior is a model o f S-R compatibility. O u r model o f immediate behavior is based on t h e MHP described by Card, Moran, a n d Newell in Chapter 2 of The Psychology of HumonComputer lnteruction (1983) a n d Rosenbloom and Newell's use o f it in t h e domain o f S-R compatibility (Rosenbloom E Newell, 1987). T h e MHP w i l l b e presented here, w i t h t h e focus on those elements t h a t we w i l l use in

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6 LTM =P Lw=K Lw= semantic

I

.

P w = 3 [2.5-4.11 chunks P wM.= 7 15-91 chunks 6m=

7 [5-2261 sec

ti WM [ichunk]= 73 L73-2261 S

Figure 1. T h e MHP processors and memories Newell, 1983, Figure 2 . 1 ) .

(from Card,

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Moran,

8

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t h e development of t h e model o f S-R compatibility. o t h e r areas can b e f o u n d in t h e o r i g i n a l w o r k .

Greater detail i n

T h e MHP i s specified in terms o f processors a n d memories, each characterized by a few q u a n t i t a t i v e parameters (see F i g u r e 1). There a r e t h r e e separate processors in t h e MHP: t h e perceptual processor, t h e c o g n i t i v e processor, a n d t h e motor processor. These processors w o r k s e r i a l l y within themselves, but c o n c u r r e n t l y with each other, subject t o serial limitations imposed by data flow requirements (e.g., t h e c o g n i t i v e processor may need information f r o m t h e perceptual processor b e f o r e it can proceed). Each processor i s d e s c r i b e d by i t s c y c l e time, I . T h e perceptual processor receives information f r o m t h e o u t s i d e w o r l d a n d deposits it i n t o a sensory memory where it is h e l d while b e i n g symbolic a l l y encoded. T h e working memory receives t h e ,symbolically encoded information a n d t h e c o g n i t i v e processor uses t h a t information, t o g e t h e r w i t h p r e v i o u s l y s t o r e d information f r o m long t e r m memory, t o make decisions about w h a t t o do. T h e c o g n i t i v e processor deposits information i n t o w o r k i n g memory t h a t initiates action by t h e motor processor. T h e motor processor acts on t h e o u t s i d e w o r l d (e.g., presses keys, responds v e r bally, e t c . ) . T h e r u d i m e n t a r y operations performed by t h e perceptual processor, t h e c o g n i t i v e processor, a n d t h e motor processor change t h e user's ment a l state o r cause p h y s i c a l changes in t h e state o f t h e w o r l d . Operators a r e defined t o b e s u f f i c i e n t l y independent o f t h e t a s k environment t h a t t h e y can b e combined t o p e r f o r m many d i f f e r e n t t a s k s . T h e operators a r e s u f f i c i e n t l y independent o f each o t h e r t h a t t h e time t o p e r f o r m each one in isolation can b e used as an estimate o f t h e time t o p e r f o r m t h a t o p e r a t o r w i t h i n a sequence o f operators. These t w o independence assumptions allow models o f complex u s e r behavior t o b e c o n s t r u c t e d f r o m t h e building blocks o f t h e MHP. T h e processors o f t h e MHP d o t h e w o r k necessary t o accomplish a n y task, i n c l u d i n g r o u t i n e t a s k s subject t o S-R compatibility effects. T h e basic p r o c e d u r e f o r an MHP analysis o f a t a s k is t o b r e a k down t h e t a s k i n t o t h e gross f u n c t i o n s t h a t each o f t h e processors must p e r f o r m a n d b r e a k these f u n c t i o n s down f u r t h e r i n t o elementary actions each p r o cessor must t a k e . Consider t h e t a s k o f g e n e r a t i n g t h e c o r r e c t abbreviation f o r a comp u t e r command (a t a s k t h a t subsequently w i l l b e shown t o e x h i b i t S - R compatibility effects). I n t h e r e a l - w o r l d situation, t h e command i t s e l f is generated by t h e u s e r when h e o r she sets a goal t o accomplish a t a s k like deleting a file. T h e n t h e abbreviation f o r "delete" must b e remembered o r f i g u r e d o u t . In an experimental situation, t h e command is g i v e n t o t h e p a r t i c i p a n t as a stimulus, specifically, as a command w r i t t e n on a C R T screen. G i v e n t h e experimental task, t h e f i r s t f u n c t i o n t h a t must b e performed is t o perceive t h e command, which is a f u n c t i o n o f t h e perceptual processor. Then, t h e abbreviation must b e remembered o r f i g u r e d out, w h i c h is a f u n c t i o n o f t h e c o g n i t i v e processor. Lastly, t h e abbreviation must b e t y p e d o u t on t h e keyboard, a f u n c t i o n of t h e motor processor. These t h r e e f u n c t i o n s must b e c a r r i e d o u t i n sequence f o r t h e c o r r e c t abbreviation t o b e t y p e d i n response t o a specific command stimulus. These f u n c t i o n s m i g h t b e c a r r i e d o u t in d i f f e r e n t ways, as demonstrated by t h e n e x t level of t a s k analysis.

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43 1

Assume, f o r o u r level o f analysis, t h a t t h e perception o f a w o r d a n d t h e execution o f muscle movements t h a t s t r i k e a k e y on a keyboard a r e elementary processes o f t h e perceptual and motor processors, respectively. However, t h e c o g n i t i v e f u n c t i o n o f generating t h e c o r r e c t abbreviation might b e accomplished in many ways. T h e person could simply r e t r i e v e t h e abbreviation d i r e c t l y f r o m l o n g t e r m memory, especially if that abbreviation was well learned a n d used f r e q u e n t l y . Alternatively, h e o r she could remember t h e abbreviation t h r o u g h some mnemonic, a process w i t h more steps than a simple r e t r i e v a l . If t h e abbreviation obeyed some rule, l i k e removing all t h e vowels f r o m t h e w o r d so delete" becomes "dlt," and t h e u s e r knew t h e rule, it could b e f i g u r e d o u t f r o m t h e spelling o f t h e command, examining each l e t t e r in t h e command a n d typing it o n l y if it was a consonant. T h i s level o f t a s k analysis details t h e steps t h e cognitive processor would have t o g o t h r o u g h t o p e r f o r m i t s f u n c t i o n in each o f these ways. As an example, t h e following steps could b e used in f i g u r i n g o u t t h e abbreviation u s i n g t h e last method mentioned above:

1. Get t h e spelling o f t h e command f r o m memory. 2. Examine t h e f i r s t l e t t e r . 3.

4. 5.

6. 7. 8.

If t h e f i r s t l e t t e r is a consonant, send a signal t o t h e motor processor t o t y p e it. If t h e f i r s t l e t t e r is a vowel, i g n o r e it. Examine t h e n e x t l e t t e r . If t h e n e x t l e t t e r i s a consonant, send a signal t o t h e motor processor t o t y p e it. If t h e n e x t l e t t e r is a vowel, i g n o r e it. Repeat steps 5, 6, and 7 until t h e r e are no more l e t t e r s in t h e command.

T h i s detailed t a s k analysis yields a t least one algorithm f o r accomIn general, o f course, t h e r e w i l l b e several alternative plishing a task. algorithms. Each specific mapping between stimuli a n d responses in t h e S-R compatibility tasks would produce d i f f e r e n t sets o f algorithms; truncation o f a command name t o t h e f i r s t t w o l e t t e r s would g i v e q u i t e d i f f e r If each of e n t algorithms than would t h e vowel deletion example above. these algorithms could b e expressed as a sequence o f steps, each o f which was assumed t o b e an elementary step o f ohe o f t h e t h r e e processors, t h e n algorithms could b e compared in terms o f t h e i r lengths, a n d length could be a p r e d i c t o r o f d i f f i c u l t y o f t h e S - R compatibility tasks. If specific values o f time could b e assigned t o t h e steps o f each o f t h e processors, t h e n t h e t o t a l amount o f time it takes t o d o a t a s k could b e p r e d i c t e d . T h e remainder o f t h i s chapter describes exactly t h i s process, b e g i n n i n g w i t h an informal language in which t o express t h e algorithms.

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Artless: An algorithmic response-time lahguagel Algorithms t h a t perform immediate behavior have several features t h a t place them within t h e GOMS framework.

1. Tasks are accomplished w i t h a sequence of operations of the MHP processors. 2 . Each operation has an identifiable duration t h a t is dependent on the processor performing t h e operation b u t independent of t h e context surrounding t h e operation.

3. Tests in conditionals must be exhaustive and explicit: no IF-THEN-ELSE construct. 4.

there is

Tests t o e x i t a loop take time only when they succeed.

5. Reasonable afgorithms are completely functional (i.e., no null operations o r empty loops).

there are

These features are embodied in an informal language called Artless. Artless has seven constructs: operators, constants, variables, assignments, blocks, branches, and loops. These constructs will be examined in t h e light of t h e computer command abbreviation encoding task. Before defining t h e seven constructs, it is useful t o introduce a data s t r u c t u r e f o r an ordered l i s t of letters. An ordered l i s t of letters has two functions defined t o act upon it: get t h e f i r s t element of the ordered l i s t (First-Letter) and get the n e x t element of the ordered l i s t (Next-Letter). If t h e l i s t is empty, First-Letter will fail. Next-Letter r e t u r n s t h e successor t o t h e member of the l i s t last returned. If the last-returned member of the l i s t is t h e last member of t h e list, then Next-Letter will fail. This data s t r u c t u r e i s used t o represent the spelling of commands and abbreviations. When a command o r abbreviation is being t y p e d out, once t h e spelling of t h e word i s gotten, the individual letters come o f f t h e ordered l i s t w i t h First-Letter and NextLetter without cost. If t h e letters are going t o be compared t o other letters o r tested t o see if t h e y are consonants o r vowels, etc., then they must be gotten explicitly w i t h a Get-First-Letter o r Get-Next-Letter. Returning t o t h e constructs, the operators do all of the real work i n the system. An operator performs a computation and optionally A typical operator i s Get-Stimulus ("Command"). The r e t u r n s a value. Get-Stimulus operator receives some characterization of what t o expect i n the stimulus display, and it returns a cdmplete description of the object found. I,? t h i s instance, t h e object was specified by the constant Command, so the operator should r e t u r n t h e description of a particular stimulus command. Artless is an informal language i n t h a t it allows algoI;ithms t o be specified t h a t ignore the details of how the concepts Command" and "complete description of the stimulus object" are represented.

'The following description of Artless i s adapted from Rosenbloom and Newell (1987).

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Operators may e i t h e r succeed o r f a i l . If an operator succeeds, it i s guaranteed t o have accomplished e v e r y t h i n g it set o u t t o do. If it fails, it may have been f o r a n y number o f reasons, such as t h e lack of needed parameters o r because some stimulus situation does n o t hold. However, t h e system o n l y knows t h a t t h e operator failed, n o t w h y it failed. Predicates a r e a subclass o f operators t h a t don't r e t u r n a value. T h e y are used t o t e s t f o r t h e t r u t h o f some condition. If t h e predicate If it fails, t h e condition may b e false o r succeeds, t h e condition is t r u e . it may b e t h a t t h e operator did n o t know how t o t e s t t h e condition in t h e c u r r e n t situation. A t y p i c a l predicate is Is-Exception? (Command), which succeeds o n l y if t h e command is an exception t o whatever r u l e i s normally used f o r c o n s t r u c t i n g i t s abbreviation. T h e assignment c o n s t r u c t i s used t o g i v e values t o variables. For example, t h e following statement assigns t h e command "delete" to t h e variable Command, g i v e n that t h e w o r d "delete" appears o n t h e C R T screen Command

+ Get-Stimulus ("delete")

In o r d e r t o do a sequence o f operators, t h e block c o n s t r u c t is used. T h e Pascal convention f o r delineating blocks is used. T h e b l o c k begins w i t h t h e k e y w o r d BEGIN, ends w i t h t h e k e y w o r d END, a n d has a l i s t of operators between. For example, t h e following block perceives a command, recalls i t s t w o - l e t t e r abbreviation, stores it in an o r d e r e d l i s t called Abbreviation, and t h e n t y p e s it o u t . BEGIN Command + Get - St imu Iu s ( "Command" ) Abbreviation t Get- Abbreviation (Command) Make-Response(Abbreviati0n [ F i r s t - L e t t e r ] ) Make-Response(Abbreviati0n [ N e x t - L e t t e r ] ) END Decisions can b e made based on t h e outcome o f a predicate (actually, of a n y operator) by employing a b r a n c h . A b r a n c h checks f o r t h e success o f a predicate and does a specific operator o r b l o c k o f o p e r For instance, an abbreviation could b e ators if t h e condition is met. r e t r i e v e d f r o m long-term memory o n l y if it is an exception t o a rule. IF-SUCCEEDED Is-Exception?(Command) THEN AbbreviationGet-Abbreviation (Command) Branches also have an optional ELSE clause t h a t allows some o t h e r However, operator o r block t o b e performed if t h e predicate failed. because t h e reason w h y a predicate fails is n o t known, if t h e ELSE clause is t o execute o n l y if t h e opposite o f t h e I F clause predicate i s t r u e , t h e ELSE clause must contain an e x p l i c i t t e s t f o r t h e opposite o f t h e I F clause. IF-SUCCEEDED Is-Exception?(Command) THEN AbbreviationGet- Abbreviation (Command) ELSE IF-SUCCEEDED Is-Not-Exception (Command) THEN.. .

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A loop executes an operator, o r block o f operators, until some p r e d i c a t e becomes t r u e . T h e r e must b e an e x p l i c i t t e s t f o r t h e predicate a t t h e e n d of t h e loop. F o r instance, if a command were t o b e t y p e d o u t in full with a " R e t u r n " a t t h e end, a n d t h e spelling o f t h a t command was s t o r e d in an o r d e r e d list, t h e n t h e following b l o c k would p e r f o r m t h a t task. BEGIN Make- Response(Command [ F i r s t - L e t t e r ] ) REPEAT Make-Response(Command[Next-Letter]) U NT IL Done? (Command 1 IF-SUCCEEDED Done?(Command)THEN Make-Response("Return") END As p r e v i o u s l y shown in t h e t a s k analysis o f g e n e r a t i n g t h e a b b r e v i ation f o r a computer command, a v a r i e t y o f algorithms a r e capable o f doing t h e t a s k . In addition, A r t l e s s introduces f l e x i b i l i t y t h a t p r o v i d e s more v a r i a n t s o f a single, general algorithm (e.g., permutations o f t h e o r d e r of decisions). Thus, many algorithms may e x i s t f o r a single t a s k . Each a l g o r i t h m represents a s t r a t e g y t h a t a person m i g h t use t o p e r f o r m t h e t a s k (see Newell, 1973, a n d Baron, 1978, f o r discussions o f t h e availBecause reaction-time t a s k s emphasize a b i l i t y of m u l t i p l e strategies). speed, a n d indeed t h e i n s t r u c t i o n s t o p a r t i c i p a n t s in such experiments o f t e n e x p l i c i t l y emphasize speed, it i s assumed t h a t people w i l l use a simp l e ( s h o r t ) algorithm if t h e y can a n d if t h e y have h a d a modest amount o f practice. T h i s assumption leads t o a simple r u l e f o r including algorithms in an analysis: If an algorithm is an o r d e r o f magnitude longer t h a n t h e o t h e r algorithms in a set t h a t p e r f o r m a single task, assume t h a t it i s not used in p r a c t i c e a n d d o n o t i n c l u d e it in t h e analysis. Even w i t h t h i s constraint, t h e r e i s o f t e n more t h a n one reasonable algorithm. In t h e absence o f data about t h e r e l a t i v e frequencies o f a l t e r n a t i v e algorithms, make t h e minimal (Laplacian) m i x i n g assumption: assume t h e algorithms a r e equally l i k e l y a n d t a k e t h e i r mean behavior. T h e r e i s a simple cost model associated w i t h algorithms a t t h i s level o f detail within t h e MHP: each o p e r a t o r takes one unit o f time associated w i t h t h e processor t h a t performs it. Thus, t h e r e a r e t h r e e basic operat o r times: a perceptual o p e r a t o r time, a c o g n i t i v e o p e r a t o r time, a n d a motor o p e r a t o r time.2 These o p e r a t o r times a r e expected t o relate t o t h e processor c y c l e times discussed by C a r d e t al. (1983). A l l c o n s t r u c t s in Artless, o t h e r t h a n operators, a r e f r e e o f charge. T h e REPEAT loop is a special case f o r c o u n t i n g operators. A l l t h e operators w i t h i n t h e loop c o u n t as t h e y a r e executed. T h e predicate in t h e U N T I L l i n e o f t h e loop does n o t count, but t h e p r e d i c a t e o u t s i d e t h e loop ( r e - t e s t i n g t h e termiT h i s c o u n t i n g p r o c e d u r e mimics t h e firnation of t h e loop) does count. ing of a p r o d u c t i o n in a p r o d u c t i o n system; t h e p r o d u c t i o n is available but doesn't c o u n t until it f i r e s . T h e a l g o r i t h m g i v e n p r e v i o u s l y f o r figuring o u t t h e abbreviation o f a command u s i n g t h e vowel deletion r u l e is shown in F i g u r e 2.

2 T h i s cost model is an improvement o v e r t h e single o p e r a t o r cost used by Rosenbloom a n d Newell (1987).

An Engineering Model of S-R Compatibility

Algorlthm

BEGIN Stimulus t Get-Stimulus("Command") Spelling c Get-Spelling(Stimu1us) Initiate-Response(Spelling[First-Letter]) Execute-Response(Spelling[First-Letter]) Next-Letter t Get-Next-Letter(Spe1ling) REPEAT BEGIN IF-SUCCEEDED Is-Consonant?(Next-Letter) THEN BEGIN Initiate-Response(Next-Letter) Execute-Response(Next-Letter) Next-Letter t Get-Next-Letter(Spel1ing) END ELSE IF-SUCCEEDED Is-Vowel?(Nexf-Letter) THEN Next-Letter t Get-Next-Letter(Spellina) .. END UNTIL Nuii?(Next-Letter) IF-SUCCEEDED Nuii?(Next-Letter) THEN BEGIN Initiate-Response("Retum") Execute-Response("Retum") END END

F i g u r e 2.

Line Number L1 L2 L3 L4 L5 L6 L7 L8 L9 L10 L11 L12 L13 L14 L15 L16 L17 L18 L19 I20 I21 L22 L23

435

Operator Type Perceptual Cognitive Cognitive Motor Cognitive Cognitive Cognitive Motor Cognitive Cognitive Cognitive Cognitive Cognitive Motor

Artless algorithm f o r f i g u r i n g o u t vowel-deletion abbreviations.

U s i n g A r t l e s s to P r e d i c t Performance As p r e v i o u s l y presented, a simple cost model is associated w i t h each operator is assigned a duration based on which Artless algorithms: processor performs t h e operation. Although these operators a r e expected t o relate t o t h e processor cycle times o f t h e MHP g i v e n by C a r d e t al. (1983), it is n o t clear, a prior;, exactly what f o r m t h i s relation w i l l take. For instance, t h e perceptual operator used in t h e Artless algorithms takes what is presented on t h e CRT screen a n d converts it t o a f o r m f r o m which t h e spelling o f t h e w o r d can b e obtained. T h i s process combines a p u r e perceptual process w i t h a c o g n i t i v e encoding process. Artless may n o t b e s u f f i c i e n t f o r specifying t h e s t r u c t u r e o f t h e encoding process f o r a complex stimulus, so these processes a r e n o t separated, but remain combined in t h e perceptual operator. Thus the duration of the perceptual operator is expected t o b e a t least t h e sum o f one perceptual processor cycle time a n d one o r more c o g n i t i v e processor cycle times. T h e durations o f t h e operators a r e t h e n t o b e empirically determined. Once t h e operator durations are estimated, t h e response times f o r d i f f e r e n t reaction-time tasks can b e predicted. Many measures could b e Because we a r e chosen t o describe t h e p r e d i c t i v e power of t h e model. interested in making predictions about human behavior t h a t can b e applied t o t h e design o f computer systems, measures o f t h e e r r o r between t h e p r e d i c t i o n and t h e observed behavior a r e more important than, say, Also, t h e absolute t h e amount of variance explained by t h e model.

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magnitude o f t h e e r r o r is n o t as important as t h e p r o p o r t i o n o f e r r o r in a prediction. Therefore, p e r c e n t e r r o r is t h e right measure t o use in evaluating predictions. Estimating Operator Durations

John, Rosenbloom, a n d Newell (1985) performed an experiment in t h e domain o f immediate behavior t o demonstrate t h a t S-R compatibility effects could b e found in a more c o g n i t i v e t a s k t h a n t h e choice reaction time tasks usually associated w i t h t h e phenomenon. T h e t a s k chosen was within t h e f i e l d of human-computer interaction t o demonstrate a relevance t o design. T h e experiment was also r u n t o demonstrate t h e explicative power of t h e MHP model o f S-R compatibility. T h e data f r o m t h i s e x p e r i ment w i l l b e used t o estimate t h e operator durations. The Task T h e t a s k was t o recall t h e abbreviation o f a computer command g i v e n t h a t you know t h e f u n c t i o n o f t h e command. For example, y o u want to move f o r w o r d one w o r d in an e d i t o r a n d have t o recall that €scope-f performs t h a t action. T h i s task, called encoding, can b e by teaching t h e commodelled in an experimental environment mand/abbreviation p a i r s a n d a s k i n g t h e p a r t i c i p a n t t o t y p e t h e abbreviation when presented w i t h t h e command name.

In vowelT w o abbreviation techniques were chosen f o r t h i s s t u d y . deletion, t h e abbreviation is t h e command name w i t h a l l t h e vowels In specialremoved (e.g., t h e abbreviation f o r "delete" is "dlt"). character- p l u s - f l r s t - l e t t e r ( hereafter ca Iled special-charac t e r ) , t h e abbreviation is t h e f i r s t l e t t e r o f t h e command name preceded by a n a r b i t r a r y special chaLacter (similar t o t h e abbreviations f o u n d in many screen editors, i.e., <Escape-key>-f" f o r " f o r w a r d one word" in Emacs). In addition t o t h e experimental conditions, t w o c o n t r o l conditions were used. No-obbreviotion, a maximally compatible task, r e q u i r e d t h e p a r t i c i p a n t to t y p e t h e f u l l command name in response t o t h e command. Nonsense, a minimally compatible task, r e q u i r e d t h e p a r t i c i p a n t t o t y p e a nonsense syllable associated w i t h each command. Qualitatively comparing vowel-deletion a n d special-character in terms of t h e complexity of t h e relation between t h e stimuli ( t h e commands) a n d t h e responses ( t h e abbreviations), t h e vowel-deletion relation appears r e l a t i v e l y simple, because t h e abbreviation can b e f i g u r e d o u t f r o m t h e command name in a s t r a i g h t f o r w a r d manner. T h e special-character relation seems more complex, because t h e a r b i t r a r y special character f o r each command cannot b e f i g u r e d o u t f r o m t h e command name i t s e l f . However, w i t h t h e GOMS model o f S-R compatibility, we can d o b e t t e r t h a n a qua!itative evaluation o f t h e complexity. A detailed t a s k analysis and algorithms can p r o v i d e a m e t r i c f o r comparison. B e g i n n i n g w i t h t h e ( i n t u i t i v e l y ) maximally compatible condition, t h e no-abbreviation condition, t h e person would have t o perceive t h e command on t h e C R T screen, access t h e spelling of that command (assuming t h a t t h e person knows how t o spell t h e command a n d does n o t have t o r e f e r back t o t h e screen f o r t h e spelling), send a signal t o t h e motor processor t o t y p e o u t each letter, a n d t y p e o u t t h e l e t t e r . An Artless algorithm f o r t h a t t a s k analysis i s shown in F i g u r e 3 .

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Algorlthm BEGIN Stimulus t Get-Stimulus ("Command") Spelling t Get-Spelling (stimulus) Initiate-Response (Spelling [First-Letter]) Execute-Response (Spelling [First-Letter]) REPEAT BEGIN Initiate-Response (Spelling [Next-Letter]) Execute-Response (Spelling [Next-Letter]) END UNTIL Done? (Spelling) IF-SUCCEEDEDDone? (Spelling) THEN BEGIN Initiate-Response ("Return") Execute-Response ("Return") END END

Line Number L1 L2 L3 L4 L5 L6 L7 L8 L9 L10 L11 L12 L13 L14 L15 L16

Operator Type Perceptual Cognitiie Cognitive Motor Cognitive Motor

Cognitive Cognitive Motor

F i g u r e 3. Word-based algorithm f o r t h e no-abbreviation condition. A n a l t e r n a t i v e algorithm came f r o m a detailed s t u d y o f t h e i n t e r k e y s t r o k e times actually produced by people p e r f o r m i n g t h i s task, In t h i s a n d involves accessing one syllable o f t h e command a t a time. case, Artless produces t h e algorithm presented in F i g u r e 4. T h e vowel-deletion condition has several d i s t i n c t algorithms, as described p r e v i o u s l y . T h e p a r t i c i p a n t could have t h e abbreviation so well learned [from previous computer experience) t h a t h e o r she could simply r e t r i e v e it from long-term memory ( u s i n g one c o g n i t i v e operator) a n d t y p e it o u t . Or, if t h e abbreviation was n o t so well learned, t h e p a r t i c i p a n t could f i g u r e it o u t by looking a t a l l t h e l e t t e r s in t h e T h i s could b e done f o r command name a n d typing o u t t h e consonants. t h e whole w o r d a t one time ( a s in t h e example algorithm presented in t h e p r e v i o u s section), o r by syllables. T h e l e t t e r s could b e t y p e d o u t as soon as t h e y a r e i d e n t i f i e d as consonants, o r all t h e l e t t e r s could b e examined and t h e n t h e abbreviation could b e t y p e d o u t in one q u i c k burst. For t h e special-character condition, it i s assumed t h a t t h e special characters a r e n o t well-learned because t h e y a r e a r b i t r a r y , have no p a t t e r n t o them, a r e n o t used in any real computer system, a n d t h e p a r t i c i pants have had o n l y a few t r i a l s o f practice. Therefore, it is unreasonable t h a t t h e characters could b e r e t r i e v e d f r o m memory w i t h one cognit i v e operator. Rather, t h e p a r t i c i p a n t p r o b a b l y uses some mnemonic o r

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Algorlthm BEGIN Stimulus t Get-Stimulus ("Command") Syllable e Get-First-Syllable (Stimulus) REPEAT BEGIN Spelling -Get-Spelling (Syllable) Initiie-Response (Spelling [First-Letter]) Execute-Response (Spelling [First-Letter]) REPEAT BEGIN Initiate-Response (Spelling [Next-Letter]) ExecuteResponse(Spelling [Next-Letter]) END UNTIL Done? (Spelling) IF-SUCCEEDED Done? (Spelling) THEN Syllable t Get-Next-Syllable (Stimulus) UNTIL Null? (Syllable) IF-SUCCEEDEDNull? (Syllable) THEN BEGIN Initiate-Response ("Return") ExecutsResponse ("Return") END END

Figure 4.

Line Number L1 L2 L3 L4 L5 L6 L7 L8 L9 L10 L l1 L12 L13 L14 L15 L16 L17 L18 L19 L20 L21

Operator Type Perceptual Cognitive Cognitive Cognitive Motor Cognitive Motor

cognitive cognitive Cognitive Cognitive Motor

Syllable-based algorithm f o r t h e no-abbreviation condition.

T h i s device, whatever it may o t h e r memory device t o a i d in t h e recall. be, i s p r o b a b l y a composite o f many cognitive operators. T h i s composite w i l l b e called a r e t r i e v a l operator. Thus, an algorithm f o r generating t h e special-character condition abbreviations consists o f p e r c e i v i n g t h e command, u s i n g a r e t r i e v a l operator t o r e t r i e v e t h e special character, thinking o f t h e spelling o f t h e command, and sending signals to t h e motor processor t o t y p e t h e special character a n d t h e f i r s t l e t t e r of t h e command. T h e n t h e motor processor t y p e s those characters. A second possible algorithm involves memorizing t h e l i s t o f commands a n d abbreviations in o r d e r a n d searching t h r o u g h them t o find t h e stimulus, t h e r e b y accessing t h e abbreviation ( u s i n g a variation o f t h e "one is a gun, t w o is a shoe" mnemonic). Again, a r e t r i e v a l operator must b e used because t h e o r d e r i n g of t h e commands is a r b i t r a r y ' a n d n o t extensively practiced. For t h e nonsense condition, t h e r e are t w o possible algorithms and t h e y parallel t h e algorithms of t h e special-character condition. In t h e f i r s t , t h e abbreviation i t s e l f (i.e., t h e nonsense syllable) i s r e t r i e v e d T h e n t h e spelling o f f r o m long-term memory u s i n g a r e t r i e v a l operator. t h e nonsense syllable is accessed and signals are sent t o t h e motor p r o In the cessor t o t y p e those l e t t e r s . T h e motor processor t y p e s them. second algorithm, t h e l i s t o f commands i s memorized and searched, w i t h t h e nonsense syllable b e i n g associated w i t h t h e command. A retrieval operator must b e used in t h i s algorithm, f o r t h e same reason as in t h e analogous algorithm f o r t h e special-character condition.

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Response time is an i n d i c a t o r o f S-R compatibility effects. It i s t h e measure used most o f t e n in t h e S-R compatibility l i t e r a t u r e , a n d t h e In fact, t w o response times a r e o f t h e o r y i s in terms o f response time. Inltlal-response time i s t h e time t o i n i t i a t e t h e abbreviation interest. g i v e n t h e command name (measured by t h e time t o hit t h e f i r s t k e y in a It reflects t h e time t o p e r f o r m t h e mapping between t y p e d response). t h e stimulus a n d i t s response, a n d thus, reflects S-R compatibility. Execution time is t h e time between t h e initial-response a n d t h e typing of t h e last l e t t e r o f t h e abbreviation. If a l l t h e characters in an abbreviat i o n a r e n o t planned o u t b e f o r e t h e i n i t i a l c h a r a c t e r i s typed, t h e n execution time includes t h e time t o p e r f o r m t h e mapping between t h e stimulus a n d t h e subsequent characters o f t h e response. Thus, execut i o n time may r e f l e c t b o t h t h e d i f f i c u l t y o f t h e response i t s e l f a n d t h e compatibility o f t h e S-R mapping. H a v i n g t w o times o f possible i n t e r e s t complicates a q u a l i t a t i v e A person m i g h t remember t h e vowelassessment o f t h e S-R relations. deletion abbreviation faster, but complete t h e abbreviation more slowly because t h e r e a r e more l e t t e r s t o t y p e . However, t h e MHP model o f S-R compatibility has no d i f f i c u l t y analyzing t h i s situation a n d making separ a t e q u a n t i t a t i v e p r e d i c t i o n s f o r each o f these response times. The initial-response time i s simply t h e time between t h e presentation o f t h e stimulus (which i s when t h e perception of t h e command is assumed to start, i.e., t h e f i r s t step in t h e algorithm) a n d t h e e n d o f t h e motor o p e r a t o r t h a t t y p e s o u t t h e f i r s t l e t t e r o f t h e abbreviation. The execution time is measured by t h e number o f operators needed t o complete t h e typing o f t h e abbreviation. Thus, b o t h times can b e accounted f o r . T h i s analysis w i l l t r e a t b o t h initial-response a n d execution times. T h e numbers o f d i f f e r e n t t y p e s o f operators (perceptual, cognitive, motor, a n d r e t r i e v a l ) f o r b o t h o f t h e response times, f o r each o f t h e algorithms, were counted a n d averaged t o g e t h e r f o r each o f t h e conditions. If an a l g o r i t h m h a d more t h a n one v e r s i o n because o f permutations o f decisions, these versions were f i r s t averaged t o g e t h e r t o determine an operator c o u n t f o r t h a t t y p e o f algorithm. Then, t h e d i f f e r e n t t y p e s o f algorithms were averaged t o g e t h e r . T h e r e s u l t s o f t h i s analysis a r e in Table 1.

Results

A one-way ANOVA compared t h e performance o n n o n - e r r o r t r i a l s between conditions: no-abbreviation, vowel-deletion, special-character, a n d n ~ n s e n s e . ~It was assumed t h a t t h e cell variances were n o t equal, a n d b o t h t h e Welch a n d B r o w n - F o r s y t h e tests4 show s i g n i f i c a n t differences between t h e conditions f o r t h e initial-response time ( F = 38.29, p < 0.01 f o r Welch; F = 33.54, p < 0.01 f o r B r o w n - F o r s y t h ) a n d f o r t h e execution time ( F = 38.29, p < 0.01; f o r Welch; f = 13.66, p < 0.01 f o r B r o w n - F o r s y t h ) . T h e means a r e g i v e n in Table 2.

3T-tests show no s i g n i f i c a n t d i f f e r e n c e between t h e t w o e x p e r i mental g r o u p s f o r t h e t w o c o n t r o l t a s k s . Therefore, t h e data was pooled across all subjects in t h e c o n t r o l conditions. 4Run w i t h t h e BMDP7D statistical p r o g r a m .

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Table 1 Average Number of Operators f o r Each Abbreviation Technique

Abbreviation Technique

No. o f Algorithms

Average No. of Operators to F i r s t Letter (Initial-Response Time)

Average No. Of Operators between F i r s t & Last Letter (Execution Time)

P

C

R

M

P

C

R

M

No Abbreviation

2

1

2.50

0

1

0

6.71

0

5.21

Vowel Deletion

7

1

12.63

0

1

0

9.75

0

2.75

Special Character

6

1

3.83

1

1

0

1.50

0

1

Nonsense

2

1

4.25

1

1

0

2

0

2

Note

P =. Perceptual operator C = Cognitive operator

R = Retrieval operator M = Motor operator

Table 2 Descriptive Statistics f o r Experiment 1

Condition

No-Abbreviation

Vowel-Deletion

Special-Character

Nonsense

Initial-Response Time: Mean (ms) S.D. (ms) n

844 227 24

1096 283 12

1839 713 12

2225 622 24

1321 462 24

1389 709 12

353 124 12

813 502 24

Execution Time: Mean (ms) S.D. (ms) n

Thus, w i t h respect t o t h e initial-response time, the f o u r conditions can be ordered as follows. No-abbreviation i s the fastest (844 ms), with vowel-deletion 30% slower (1096 ms), special-character 118% slower (1839 For execution ms) and nonsense t h e slowest (167% slower a t 2225 m s ) . time, t h e order i s special-character the fastest (353 ms), with nonsense 1308 slower (813 ms), no-abbreviation 274% slower (1321 m s ) , and voweldeletion t h e slowest (293% slower at 1389 m s ) . Table 1 gives the number of different types of operators f o r each algorithm f o r the initial-response and the execution times. An estimate

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44 1

30

30

theoretical (ms) F i g u r e 5.

Theoretical v s . o b s e r v e d response times.

f o r t h e d u r a t i o n of each t y p e o f o p e r a t o r was obtained by minimizing t h e least-squares d i f f e r e n c e between t h e o b s e r v e d performance a n d t h e r e s u l t i n g p r e d i c t i o n s o f initial-response time a n d execution time. Both t h e initial-response time data a n d t h e execution-time data w e r e used, t o p r o v i d e t h e maximum number o f independent data points. The resulting estimates were t h a t a perceptual o p e r a t o r takes 340 ms, a c o g n i t i v e o p e r a t o r takes 53 ms, a r e t r i e v a l operator takes 1249 ms, a n d a motor U s i n g these results, a p l o t of theoretical v s . operator takes 230 ms. observed initial-response times a n d execution times ( F i g u r e 5) has an average absolute p e r c e n t e r r o r o f 15.7%. Discussion T h i s experiment set o u t t o fulfill t h r e e goals: demonstrate S-R comp a t i b i l i t y effects in a c o g n i t i v e t a s k w i t h i n human-computer interaction; demonstrate t h e e x p l i c a t i v e power o f t h e MHP model o f S-R compatibility; a n d p r o d u c e parameter estimates f o r f u t u r e p r e d i c t i o n s . It c l e a r l y accomplished those goals. T h e S - R compatibility effects were demons t r a t e d by t h e l a r g e a n d s i g n i f i c a n t differences between t h e i n i t i a l response times o f t h e d i f f e r e n t abbreviation techniques. Initial-response times a r e most important i n demonstrating S-R compatibility effects, because initial-response time i s t r a d i t i o n a l l y t h e d e f i n i n g measure f o r S-R compatibility. However, t h e MHP model allowed us t o g o b e y o n d a t r a d i tional i n t u i t i v e statement o f S-R compatibility t o say q u a n t i t a t i v e l y how complex each o f t h e d i f f e r e n t abbreviation techniques is a n d t o i d e n t i f y T h i s is differences in execution time as well as initial-response time. important in design, because comparing these t w o response times highl i g h t s design t r a d e - o f f s n o t apparent f r o m i n t u i t i v e analysis. For example, i n t u i t i v e l y , vowel-deletion seems more compatible t h a n specialcharacter, a n d it is, when o n l y i n i t i a l response time i s considered. However, when t h e execution time is also examined, vowel-deletion abbreviations a r e shown t o t a k e much longer t o complete t h a n t h e specialcharacter abbreviations. T h i s execution time d i f f e r e n c e is due, in p a r t , t o t h e additional l e t t e r s r e q u i r e d i n t h e vowel-deletion abbreviation. T h i s

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i s n o t t h e whole s t o r y , however, because t h e no-abbreviation condition r e q u i r e s still more l e t t e r s a n d it takes less time t o execute t h a n t h e vowel-deletion condition. Therefore, although vowel-deletion may b e an easy r u l e f o r people t o recognize a n d learn, it is n o t a good choice f r o m t h e p o i n t o f view of e x p e r t performance time. Thus, t r a d e - o f f s i n v o l v i n g novice t r a i n i n g time a n d peak e x p e r t performance time must b e considered in t h e selection o f an abbreviation technique. T h e theoretical calculations leading t o predictions w i t h i n 20% o f t h e o b s e r v e d data v e r i f y t h e e x p l i c a t i v e power o f t h e MHP model o f S-R compatibility. Estimates o f t h e o p e r a t o r parameters were generated f r o m t h e regression analysis, but t h e y should b e examined t o determine t h e numb e r of s i g n i f i c a n t d i g i t s it i s u s e f u l t o c a r r y o v e r t o o t h e r S-R compatibility experiments and, eventually, t o o t h e r t a s k domains. A traditional e n g i n e e r i n g d e f i n i t i o n o f s i g n i f i c a n t d i g i t s o r f i g u r e s is: As y o u proceed f r o m l e f t t o right in a number . . ., y o u w i l l e v e n t u a l l y encounter a digit w h i c h is d o u b t f u l d u e t o t h e w a y it was measured o r determined . . . A l l d i g i t s in a number, f r o m l e f t t o right, up t o a n d including t h e d o u b t f u l one, a r e said t o b e s i g n i f i c a n t f i g u r e s . (Beakley, Evans, & Keats, 1986, pp. 251-252) Since t h e A r t l e s s language is a v a r i a n t o f t h e MHP model, t h e operator values estimated f r o m t h i s experiment can b e seen as estimates o f t h e MHP processor cycle times. T h e C a r d e t al. (1983) estimates o f these c y c l e times shed light o n t h e u n c e r t a i n t y i n h e r e n t in t h e parameter estimates. T h e c o g n i t i v e operator, estimated in t h i s analysis t o b e 53 ms, can b e considered t o b e an elementary o p e r a t o r o f t h e c o g n i t i v e processor. Estimates o f t h e MHP c o g n i t i v e cycle time r a n g e f r o m 25-170 ms; 53 ms is well w i t h i n t h a t r a n g e a n d close t o t h e 70 ms C a r d e t at. chose f o r t h e i r t y p i c a l value. However, t h e r a n g e o f values c i t e d by C a r d e t al. i n c l u d e r e s u l t s f r o m experiments t h a t were n o t designed t o i d e n t i f y an elementary c o g n i t i v e o p e r a t o r a n d should b e viewed w i t h suspicion, especially a t t h e high e n d o f t h e range, where composite operators m i g h t account f o r t h e long time estimates. I n addition, t h e C a r d e t al. numbers a r e ranges, n o t estimates o f v a r i a t i o n . Thus, t h e estimates o f v a r i a t i o n a r e inside t h e ranges g i v e n . Therefore, t h e c o g n i t i v e o p e r a t o r is u n c e r t a i n in t h e ten's digit a n d w i l l b e t a k e n t o be 50 ms f o r t h e remainder o f t h e analyses in t h i s c h a p t e r . T h e perceptual o p e r a t o r is estimated t o b e 340 ms by regression. T h i s o p e r a t o r encodes t h e words on t h e C R T screen as well as perceives them. Thus, it i s a combination o f a perceptual processor cycle time (50-200 ms, 100 ms t y p i c a l value) a n d a t least one c o g n i t i v e processor cycle time (50 ms). Again, t h e numbers g i v e n by C a r d e t al. a r e t h e range, a n d t h e v a r i a t i o n i s inside that, making t h e perceptual processor cycle time u n c e r t a i n in t h e tens digit. A sum o f values s i g n i f i c a n t t o t h e t e n s digit i s also s i g n i f i c a n t t o t h e tens digit. Therefore, t h e perceptual o p e r a t o r w i l l b e t a k e n t o b e 340 ms (where t h e u n i t s digit is n o t s i g n i f i cant).

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443

T h e motor o p e r a t o r is estimated t o b e 230 ms, high f o r t h e r a n g e of motor processor cycle times (30-100 ms, t y p i c a l v a l u e 70 ms). However, t h e p a r t i c i p a n t s in t h i s experiment were r e l a t i v e l y inexperienced t y p i s t s (average 31 gwpm as measured by an independent typing test).5 This could mean some o f t h e a c t i v i t i e s c o n t r i b u t i n g t o t h e high average motor o p e r a t o r estimate m i g h t b e hunting f o r t h e c o r r e c t k e y a n d p e c k i n g a t it, as well as t h e more s t r a i g h t - f o r w a r d activities used t o estimate t h e motor processor cycle time. I n t h e absence o f data illuminating t h e motor a c t i v i t i e s actually involved, t h e o p e r a t o r d u r a t i o n estimate w i l l remain a black-box estimate o f t h e time t o hit a k e y on a k e y b o a r d f o r a population w i t h a 31 gwpm typing speed. Since t h e motor, cognitive, a n d p e r ceptual processor cycle times a r e s i g n i f i c a n t in t h e tens digit, a sum o f several o f those operators would also b e s i g n i f i c a n t in t h e t e n s digit. Therefore, t h e parameter estimate c a r r i e d along w i l l b e 230 ms (where t h e u n i t s digit is n o t s i g n i f i c a n t ) .

A r e t r i e v a l operator, estimated a t 1249 ms, performs t h e recall o f an a r b i t r a r y connection between t w o items. T h i s is essentially an a r b i t r a r y paired-associate task, t h o u g h done i n t e r n a l l y (hence w i t h o u t perception a n d motor components). A f e w studies of simple p a i r e d associates tasks r e p o r t latencies w i t h means r a n g i n g between 1615 a n d 2020 ms ( B r o w n & Huda, 1961; Millward, 1964; Williams, 1962). These times, o f course, a r e f o r a complete task, a n d t h e r e t r i e v a l o p e r a t o r is j u s t one component. T h e simplest algorithm f o r a paired-associate recall t a s k would have one perception operator, one r e t r i e v a l operator, one c o g n i t i v e o p e r a t o r t o i n i t i a t e t h e response, a n d one motor o p e r a t o r . Thus, s u b t r a c t i n g 620 ms ( t h e d u r a t i o n o f one perceptual o p e r a t o r p l u s one c o g n i t i v e o p e r a t o r p l u s one motor operator) f r o m t h e latency data gives an estimate o f t h e r e t r i e v a l o p e r a t o r in t h e r a n g e 995 to 1400 ms. T h i s b r a c k e t s t h e r e t r i e v a l o p e r a t o r values o f 1249 ms a n d casts d o u b t on t h e h u n d r e d s digit. In accord w i t h t h i s analysis, t h e r e t r i e v a l o p e r a t o r reflects many individualized methods f o r accomplishing t h e r e t r i e v a l . These d i f f e r e n t methods make t h e r e t r i e v a l o p e r a t o r more t h a n j u s t t h e sum o f a constant number of c o g n i t i v e operators, it increases t h e v a r i ance o f t h e r e s u l t i n g estimate. Thus, o n l y t h e f i r s t t w o s i g n i f i c a n t d i g i t s should b e c a r r i e d along w i t h t h e r e t r i e v a l o p e r a t o r duration, making t h a t estimate 1200 ms. T h i s analysis in terms o f paired-associate recall makes t h e value of t h e r e t r i e v a l o p e r a t o r reasonable, but it does n o t h i n g t o explain i t s magnitude. Much w o r k on mediation i n paired-associates l e a r n i n g indicates t h a t t h i s o p e r a t o r is composite (Jensen & Rohwer, 1963; Montague, Adams, E Kiess, 1966; R u n q u i s t E Farley, 1964). F o r a detailed v i e w o f how n a t u r a l language mediation m i g h t work, see P r y t u l a k (1971). Supp o r t f o r t h e i n t e r p r e t a t i o n o f a r e t r i e v a l o p e r a t o r as a composite o f seve r a l c o g n i t i v e operators also comes f r o m protocol data collected in t h e n e x t experiment. A subject remembered t h a t t h e abbreviation f o r t h e command "apply" was "al" because "apply" reminded him o f a p p l y i n g f o r a j o b a n d t h a t reminded him o f his f r i e n d , Al, who h a d j u s t done so. Decomposing t h e r e t r i e v a l o p e r a t o r is an important task, but t h e f u n c t i o n o f t h e composite o p e r a t o r is s u f f i c i e n t l y clear t h a t it can b e employed successfully in c o n s t r u c t i n g t a s k algorithms.

5Gentner, 1983, d e f i n e d e x p e r t t y p i s t s as those typing a t 60 wpm o r b e t t e r , a n d t h a t b e g i n n i n g t y p i s t s k e y e d about 12 wpm.

444

B.E. John and A. Newell

The new operator duration estimates, applied t o the average operator counts f o r t h e conditions in t h i s experiment, yield predictions w i t h an average absolute e r r o r of 15.8%. This result is essentially t h e same as t h a t obtained w i t h t h e original, unrounded estimates, which is t o be expected, because t h e e x t r a digits simply add random e r r o r t o t h e predictions. The rounded operator duration estimates, shown in Table 3, will be carried t o t h e n e x t section in t h e second step toward an engineering model w i t h predictive power, t h a t of making a priori predictions f o r a new task. Table 3 Perceptual, Cognitive and Motor Parameter Definitions and Estimated Durations

Parameter

Definition

Duration

Perceptual Operator

Reading a word of about 6 letters and encoding it i n t o an ordered l i s t of letters

Cognitive Operator

A cognitive processor cycle time

Retrieval Operator

Recalling an a r b i t r a r y association t h a t is not well-learned

1200 ms

Motor Operator

T y p i n g a character on an alphanumeric keyboard at a rate of about 30 gwpm

230 ms

340 ms

50 ms

Making Predictions o f Response-Time Performance A f t e r estimating operator-duration parameters, those parameters can be used t o predict response-time performance on other routine immediate behavior tasks. Four such tasks will b e explored: a second commandabbreviation recall task w i t h different abbreviation techniques, two spatial S - R compatibility tasks, and a symbolic S - R compatibility task. The command-abbreviation task analysis predicts performance reported i n John and Newell (1987). The other three tasks predict performance reported i n the S-R compatibility literature and are the same tasks examined by Rosenblmm and Newell (1987).

A Second Command-Abbreviation Experiment A second experiment (John & Newell, 1987) studied two additional abbreviation techniques. Truncation t o the minimum number of letters t o distinguish between commands (hereafter, MTD f o r minimum to distinguish) maps t h e commands compile, create, define, and delete into co, c r , def, and dei, respectively. Truncation t o two letters, with exceptions t o distinguish between ambiguous commands (hereafter, 2 letter) maps t h e same commands i n t o co, c r , (as does MTD) and de and d l , respectively. T h e r u l e f o r t h e exception, when two commands cannot

An Engineering Model of S-R Compatibility

Choice 1 RecallAbbrev.

Choice 2

Choice 3

Choice 4

One-Method Combined

By-exception

By-category

Marked-by-word

'Marked-by-word Marked-by-letter

One-Method Combined

Choice

5 Type Wait

Marked-by-letter

RecallFeature

445

By-exception

Marked-by-word

Type Wait Type Wait Type Wait Type Wait Type Wait Type Wait Wait

By-category Wait Wait FigureOut

One-Method Wait

Combined

By-exception

Marked-by-word Marked-by-letter

By-category

Marked-by-word Marked-by-letter

Figure 6.

Choices for generating abbreviation methods

w ..-.. Type Wait Type Wait Type Wait

446

B.E. John and A. Neweii

be distinguished by t h e i r f i r s t two letters ( i n this case, define and delete), is t o use t h e f i r s t distinguishing letter as t h e second letter i n t h e abbreviation, f o r t h e command second i n alphabetical order. Predictions o f response time f o r a new S-R compatibility task can be made by doing a detailed analysis of t h e task, w r i t i n g algorithms, counting t h e operators in t h e algorithm, and multiplying those counts by t h e previously determined parameters. As previously discussed, many algorithms are possible for a task, and d i f f e r e n t people can be expected t o use d i f f e r e n t algorithms. In general, it i s not possible t o predict a prior1 which algorithms will be used by any given individual. The simple solution, used successfully w i t h t h e John e t al. (1985) experiment and adopted here, is t o generate a population of d i f f e r e n t algorithms f o r the task and then average them together. If t h e major algorithms are generated, and t h e population is relatively large, then t h e absence of In t h e absence of better r a r e algorithms will make l i t t l e difference. information, all distinctly d i f f e r e n t algorithms are weighted equally. Algorithms f o r t h e MTD and 2-letter encoding task were generated according t o a series of f i v e design choices, as shown i n Figure 6. T h e f i r s t choice i s t o recall the abbreviation f o r a command directly, as if it were a paired associate (the recall-abbreviation option); t o recall a feat u r e of t h e abbreviation, f o r example, how many letters it contains (recall-feuture); o r t o f i g u r e o u t the abbreviation knowing t h e rules given above (figure-out) . A second choice is between treating all commands t h e same way (single-method) o r using d i f f e r e n t methods for different cases (combined). A third choice arises in how t o do t h e combining. If only a few commands do not fit t h e primary r u l e (the abbreviation being the f i r s t two letters), then those few commands can be marked as exceptions, and the abbreviations can be either recalled o r figured-out whenever an exception i s encountered (by-exception) . Alternatively, if approximately an equal number of commands have different types of abbreviations (e.g., first-two-letters, first-three-letters and f i r s t - f o u r - l e t t e r s i n the MTD condition), then categories can be set u p t o reflect these types, and each command can be marked with i t s category; once the category is retrieved, t h e abbreviation can be either recalled o r f i g u r e d o u t ( b y category). A f o u r t h choice arises because exceptions and categories can be associated w i t h either of two features of the commands: the whole word (mark-by-word) or t h e f i r s t letter (mark-by-letter). For example, i n the 2-letter condition, if a by-exception method i s used with the whole word, delete will be flagged as an exception, b u t not define. However, if t h e f i r s t letter is used t o mark the exception, both delete and define will be flagged. A f i f t h choice arises when the response begins. The participant can t y p e as soon as a letter of the abbreviation is generated ( t y p e ) , o r t h e participant can wait u n t i l t h e entire abbreviation is generated and then t y p e (wait).

An Engineering Model of S-R Compatibility

447

Each one o f these d i f f e r e n t methods f o r g e n e r a t i n g t h e abbreviations can b e embodied in an a l g o r i t h m w r i t t e n in A r t l e s s . However, when t h e details a r e w o r k e d out, n o t all o f these possible methods a r e u n i q u e algorithms. F o r instance, 2 - l e t t e r by-category, m a r k e d - b y - l e t t e r i s n o t u n i q u e because t h e t w o commands s t a r t i n g with t h e same l e t t e r a r e in d i f f e r e n t abbreviation categories (e.g., d e f i n e w o u l d b e in a f i r s t - t w o l e t t e r s category a n d delete would b e in a f i r s t - a n d - t h i r d - l e t t e r category), w h i c h i s equivalent t o m a r k - b y - w o r d . D i f f e r e n t versions o f these algor i t h m s a r i s e because e v e r y time a t e s t i s made in an algorithm, a v e r s i o n o f t h a t algorithm could b e c o n s t r u c t e d t h a t permutes t h e o r d e r o f t h e t e s t s . In all, t h e r e a r e 71 algorithms f o r t h e M T D condition a n d 53 algor i t h m s f o r t h e 2 - l e t t e r condition (John, 1988). Predictions a r e made by g e t t i n g an average number o f operators f o r t h e d i f f e r e n t abbreviation t a s k s . 6 These averages a r e combined w i t h t h e parameter estimates determined in t h e J o h n e t al. experiment w i t h t h e following equation:

T h e p r e d i c t i o n s w i l l b e r o u n d e d t o t h e nearest 10 ms. The predictions f o r t h e t w o new abbreviation techniqyes a n d f o r a noabbreviation c o n t r o l condition a r e f o u n d in Table 4. Tabl e 4 Predictions of Response Times

Condition

T y p e of R T

Number of Operators Perception

Cogni t i ve

Retrieval

Prediction Motor

(ms)

No Abbr evia tio n

initial response

1

2.50

0

1

No Abbr evia tio n

execution

0

6.81

0

5.31

1560

Minimum-to Distingush

ini t i al response

1

4.31

0.30

1

1150

Minimum-to D i s t i ngus h

execution

0

3.88

0.06

2

730

2- L e t t e r

i ni t i al response

1

4.27

0.28

1

1120

2- L e tte r

execution

0

3.27

0.10

1

510

7Gil

6As in t h e John e t al. (1985) experiment, versions o f t h e algorithms created by p e r m u t i n g t h e o r d e r o f decisions were averaged Then t o g e t h e r f i r s t t o p r o v i d e an estimate f o r t h a t t y p e of algorithm. t h e d i f f e r e n t t y p e s o f algorithms were g i v e n equal weight. 7As in t h e John e t al. (1985) experiment, t h e p a r t i c i p a n t s were r e l a t i v e l y inexperienced t y p i s t s (average speed o f 26 gwpm). T h i s simil a r i t y in s k i l l j u s t i f i e s u s i n g t h e p r e v i o u s l y estimated d u r a t i o n f o r t h e motor operator.

B.E. John and A. Newell

448

Table 5 Observations and Predictions of Response Times f o r t h e John & Newell Experiment

Condition

No Abbreviation No Abbreviation MTD MTD 2-Letter 2- Letter

T y p e of RT

initial execution initial execution initial execution

Observed (ms)

955 1554 1064 525 1032 299

SO

Prediction

(ms)

(ms)

397 700 799 1560 375 1150 389 730 442 1120 216 510 Average absolute % e r r o r

% Error

-

-

-

36.7 0.4 8.1 39.0 8.5 70.6 27.2

2000-

predicted (ms) F i g u r e 7. Predicted v s . o b s e r v e d response times f o r t h e John & Newell experiment. Table 5 presents t h e observed response times a n d t h e theoretical p r e d i c t i o n s f o r t h e initial-response times a n d execution times. Figure 7 p l o t s t h e p r e d i c t e d times v s . observed times, w i t h an average absolute p e r c e n t e r r o r o f 27.28. T h e Fitts a n d Seeger T a s k F i t t s a n d Seeger (1953) s t u d i e d an immediate behavior t a s k w h i c h r e q u i r e d a p a r t i c i p a n t t o move a s t y l u s in response t o t h e lighting o f a lamp. F i t t s a n d Seeger crossed t h r e e stimulus conditions w i t h t h r e e response conditions. O n l y t w o o f those stimulus a n d response conditions w i l l b e examined here, because t h e third stimulus a n d response conditions

An Engineering Model of S-R Compatibility

449

i n v o l v e d motor-movement complexities t h a t have n o t y e t been i n c o r p o r a t e d i n t o t h e model. T h e details o f t h e t a s k a r e d e f i n e d by t h e apparatus used: Stimulus Set A a n d Response Set A each consisted o f e i g h t permutations o f d i r e c t i o n f r o m a c e n t r a l reference p o i n t . The stimulus panel contained e i g h t l i g h t s f o r m i n g t h e o u t l i n e o f a c i r c l e . T h e response panel contained e i g h t pathways r a d i a t i n g f r o m a c e n t r a l p o i n t l i k e t h e spokes o f a wheel. Each light a n d each pathway were separated f r o m t h e i r neighbors by an angle o f 45O. Stimulus panel 6 consisted o f f o u r l i g h t s separated by 90°. It p r o v i d e d f o u r single-light-positions, plus the four - two-light Response panel 6 combinations formed by adjacent p a i r s o f l i g h t s . consisted o f f o u r pathways o r i g i n a t i n g a t a c e n t r a l point a n d Each path, as seen in Fig. 1 [here, separated by 90° i n t e r v a l s . Figure 81, b r a n c h e d in a T a n d p e r m i t t e d S5 t o move f r o m t h e c e n t e r p o i n t t o one o f e i g h t terminal positions. The four corner p o i n t s o f t h e response panel c o u l d b e reached by t w o a l t e r n a t i v e pathways; f o r example, t h e u p p e r - r i g h t c o r n e r c o u l d b e reached e i t h e r by a right-up sequence o r an up-right sequence. T h e S5 were t o l d t h a t these were equivalent responses . . . A t t h e center o f each response panel was a 1/8-in. diameter metal disc, s u r r o u n d e d by a thin nonconductive ring. Reaction time was measured as t h e time t a k e n by S t o move t h e s t y l u s o f f t h i s metal b u t t o n ( F i t t s & Seeger, 1953, pp. 201-202).

Sa

Ra F i g u r e 8. T h e t w o stimulus panels a n d t h e t w o response panels used in t h i s analysis, f r o m F i t t s & Seeger, 1953.

B.E. John and A. Newell

450

Algorithms can b e w r i t t e n in terms of perceptual, cognitive, and motor operators t h a t accomplish these tasks. However, t h e perceptual and motor operators are d i f f e r e n t from those i n t h e command abbreviation experiments because t h e perceptual and motor tasks are d i f f e r e n t . To make a prior/ predictions of t h e behavior in these tasks with no parameters, some independent estimate of appropriate perceptual and motor-operator durations must be made. I n t h e absence of data about t h e perceptual and motor operators f o r t h i s task, t h e typical value of t h e MHP perceptual and motor-processor cycle times will be used (100 ms and 70 ms, respectively). The perceptual process is complex, involving both p u r e perception and encoding i n t o meaningful symbols. Assuming t h a t t h e estimate of t h e perceptual processor cycle time is an estimate o f the p u r e perceptual portion of t h i s process, cognitive operators must be included in t h e algorithms t o account f o r t h e encoding process. With simple stimuli, like t h e lighting of one o u t of eight lights, t h e encoding process could b e a It simple discrimination process, subject t o information-theory analyses. will be assumed t h a t f o r such a stimulus, and f o r t h e stimuli in t h e next two analyses as well, encoding is accomplished w i t h one cognitive operator f o r each bit of information necessary t o discriminate between stimuli. Thus, w i t h eight possible stimuli, three b i t s of information are needed, and three cognitive operators are included in t h e algorithms t o perform t h e encoding. The algorithms f o r t h e d i f f e r e n t S-R conditions i n t h i s experiment The predictions arising from these can be found in Appendix I. algorithms, combined w i t h t h e operator-duration estimates (perceptual = 100 ms, cognitive = 50 ms, motor = 70 ms, all significant i n t h e tens digit), along w i t h t h e actual observed response times, are presented i n Table 6 and Figure 9. These predictions have an absolute average percent e r r o r of 16.1%. The Duncan Task Duncan (1977) studied a routine immediate behavior task i n which t h e participant had t o press a button i n response t o the l i g h t i n g of a line on an oscilloscope. His apparatus and task were as follows: The stimuli were f o u r vertical lines, arranged i n a horizontal row on an oscilloscope . . . (Fig. l ) [ h e r e , Figure 101 . . . the stimuli have been numbered 1 t o 4 from l e f t t o r i g h t . Stimuli 1 and 4 will be termed "Outer" and Stimuli 2 and 3 " I n n e r . " Table 6 Predicted a n d O b s e r v e d Response Times f o r t h e F i t t s fi Seeger Experiment

Condition

(ms)

Sa-Ra Sa-Rb Sb-Ra Sb-Rb

Number of Operators

Observed

390 430 450 410

Prediction

Perception

Cognitive

Motor

1 .oo 1 .oo

5.00 7.00 7.50

1 .oo 1 .OO

1.50 1 .OO

5.M)

1 .oo

% Error

(ms)

420

-7.7

520 6M)

-20.9 -33.3 -2.4

1 .oo 420 A v e r a g e absolute % e r r o r

16.1

An Engineering Model of S-R Compatibility

45 1

h

E

600-

W

=u

a 400> L a v)

9

0

2007

O

I 0

/

200

400 600 8 10 predicted (ms)

F i g u r e 9. Predicted v s . observed response time f o r t h e F i t t s E Seeger experiment.

CONDITION P - 0

~

1 2 3 4

Responses

A

B C

Responses

D

A

B C D

CONDITIONM Stimuli

0 0 0 I]

Stimuli

x 1 2 3 4

Responses

1 2 3 4 Responses

A B C D

F i g u r e 10. S-R mappings Duncan, 1977, p. 51.

[I 0 I] I]

in t h e

four

1x1 A B C D

experimental conditions

from

Responses were made w i t h f o u r keys a r r a n g e d t o l i e u n d e r t h e fore- a n d middle f i n g e r s o f each hand . . . t h e responses have been labeled f r o m A to D f r o m l e f t t o right. F i g u r e 1 [here Figure 101 illustrates t h e f o u r S-R mapping rules employed. A r r o w s connect stimuli t o t h e i r appropriate responses.. .

. . . t h e following terminology w i l l b e employe?. S-R p a i r s w i l l b e termed e i t h e r "Corresponding',', or Opposite. A n exampk, of Corresponding p a i r is "1-A, a n d o f a n Opposite p a i r 1-D. Mappings which contain o n l y one t y p e o f p a i r w i l l b e termed "Pure," while those which contain b o t h w i l l b e termed "Mixed."

B.E. John and A. Newell

452

.

.

.

. . Each subject served in . one of t h e f o u r experimental conditions. A t t h e s t a r t the S-R mapping was described by numbering t h e stimuli from 1 t o 4, and showing which key corresponded t o each number. (Duncan, 1977, pp. 50-51) As in t h e algorithms f o r t h e Fitts and Seeger tasks, t h e perceptual process is broken down i n t o one p u r e perceptual operator of 100 ms and one cognitive operator f o r each bit of information necessary t o accomplish In t h i s case, t h e participant must discriminate one o f f o u r t h e task. Thus, there possible vertical lines, r e q u i r i n g two b i t s of information. are two cognitive operators t o perform t h e discrimination in each of t h e ,algorithms. T h e motor operator will again be assumed t o take 70 ms, even though t h e response of pressing a key i s d i f f e r e n t from t h e stylus This typical value of t h e movement in t h e Fitts and Seeger experiment. motor-processor cycle time will be used as a best estimate whenever there are no data t o support a d i f f e r e n t estimate. T h e algorithms f o r t h i s experiment can be found i n Appendix I I . The predictions and actual observed response times are presented i n Table 7 and Figure 11. These predictions have an absolute average p e r cent e r r o r of 8.3%.

Table 7 Predicted and Observed Response Times f o r t h e Duncan Experiment

Condition

Number of Operators

O bserved (ms)

Prediction

Perception

Cogni t i ve

Motor

(ms)

% Error

Pur eC o r r e sp o n d ing

411

1.oo

4.00

1 .oo

370

10.0

Pure-Opposite

461

1.00

5. 00

1

.oo

420

8.9

MixedC or r espondin g

485

1 .@I

5.50

1 .oo

450

7.2

Mixed-Opposite

539

1 .oo

6.50

1 .oo

500

7.2

Average absolute % e r r o r

8.3

An Engineering Model of S-R Compatibility

0 F i g u r e 11. experiment.

Predicted

vs.

453

200 400 6 I0 predicted (ms) observed

response time

for

the

Duncan

T h e M o r i n a n d F o r r i n Task T h e t a s k used by M o r i n a n d F o r r i n (1962) was f o r t h e p a r t i c i p a n t t o respond v e r b a l l y w i t h a numeral when p r e s e n t e d v i s u a l l y w i t h a numeral o r a symbol (cross, square, circle, o r t r i a n g l e ) . F i g u r e 12 shows t h e f i v e experimental conditions. T h e f i r s t element in each p a i r s e r v e d as t h e stimulus a n d t h e second element as t h e response. Response times were analyzed in t r i a l s o f t h e " c r i t i c a l p a i r s " only, t o p r o d u c e t h e R T performance recorded in each cell o f t h e f i g u r e . In t h i s experiment, t h e discrimination o f t h e visual stimuli i s d i f f e r In conditions I a n d I I , ent, depending on t h e experimental condition. t h e r e a r e o n l y t w o possible stimuli, a n d t h u s o n l y one c o g n i t i v e o p e r a t o r is needed, c o r r e s p o n d i n g t o one bit o f information. I n conditions IV a n d V, f o u r d i f f e r e n t stimuli a r e possible, r e q u i r i n g t w o c o g n i t i v e operators

I

EXPERIMENTA 'OND

I 111

I"

CR'T'CAL CONTEXT PAIRS

2-2 8-8 2-2 8-8

2-2 8-8

I

EXPERIMENT B COND

II

+-4

111

1-7

4-4 7-7

v

c~~~~

1

CONTEXT

+ -4 I-7 +-4 1-7 +-4

1-7

2-2 8-8

0-2 A-8

F i g u r e 12. Schematic diagram o f t h e experimental conditions f r o m M o r i n & F o r r i n , 1962, p. 138.

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B.E. John and A. Newell

t o perform t h e encoding. I n conditions I I l a and Illb, two cognitive operators are also needed: one t o discriminate between numeral and symbol, and one t o discriminate between either t h e two numerals o r t h e two symbols. Again, t h e typical value of t h e motor-processor cycle time, 70 ms, will be used as the estimate f o r t h e motor-operator duration. The algorithms f o r t h i s experiment can be found i n Appendix I l l . The predictions and actual observed response times are presented in Table 8 and Figure 13. These predictions have an absolute average p e r cent e r r o r of 23.4%.

Table 8 Pr edicted and Observed Response Times f o r t h e Morin & F o r r i n Experiment

C o n d ition

I

II llla lllb

Number of Operators

O nserved

Prediction

(ms)

Perception

Cognitive

Motor

(ms)

520 590

1 .oo 1 .oo

3.00 5.50

1 .M) 1.00 1 .OO 1 .OO 1.00 1 .OO

320 450

600

IV

710 490

V

720

1 .M) 1 .oo

1 .oo

1 .oo

800

6.50

9.00 4.00 7.50

500

620 370 550 Average absolute % e r r o r

% Error

38.5 23.7 16.7 12.7 24.5 24.3 23.4

i

600

400 200

0

predicted (ms) Figure 13. Predicted vs. Forrin experiment.

observed response time f o r the Morin and

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Results of t h e Prediction Exercise Zero-parameter predictions of response time f o r f o u r d i f f e r e n t types of tasks were made from t h e GOMS model of immediate behavior. The stimuli in these tasks included lights in an a r r a y (Duncan, 1977; Fitts E Seeger, 1953), numerals and geometric symbols (Morin & Forin, 1962). The responses included and computer commands (John & Newell, 1987). moving a stylus (Fitts & Seeger, 19531, s t r i k i n g keys (Duncan, 1977), naming a number (Morin & Forin, 1962). and typing (John & Newell, 1987). The average absolute percent e r r o r s between predictions and observed response times ranged from 8.3% t o 27.28, w i t h an overall average of 18.8% (see Figure 14). This e r r o r percentage i s i n keeping w i t h t h e goal of an engineering model, t o be able t o make zerozparameter predictions averaging within about 208 of actual performance. In aggregate, t h e results are good, and t h e theory is adequate f o r engineering purposes. However, examination of t h e predictions of t h e individual experiments provides a clue f o r improvement of t h e theory. Each of t h e t h r e e experiments taken from t h e S-R compatibility literature (Duncan, 1977; Fitts & Seeger, 1953; Morin & Forrin, 1962) shows a bias i n the predictions. Performance on t h e Fitts and Seeger task is uniformly overpredicted, and performances on t h e Morin and F o r r i n task and on the Duncan t a s k are uniformly underpredicted by the GOMS model. However, no such uniform bias exists i n t h e prediction of t h e John and Newell (1987) command-abbreviation task. Predictions f o r t h e John and Newell task were made using estimates of the perceptual and motor operators based on similar operators i n t h e John, Rosenbloom, and Newell (1985) t a s k . Predictions f o r the other three tasks, however, used the typical perceptual and motor-processor cycle times estimated by Card

1500n

o John & Newell (1987) Fitts & Seeaer (1953) Duncan (1677)’ x Morin & Forrin‘(1962)

2

U

u 1000-

$ B 8

500-

OY 0

’

I

I

500

1000

I

1500

I

2000

predicted (ms) Figure 14. experiments.

Predicted

vs.

observed

response

time

for

all

four

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e t al. (1983). These estimates a r e v e r y crude. One way t o r e f i n e t h e model would b e t o use regression analysis t o estimate d i f f e r e n t operator durations f o r a wide range o f perceptual and motor tasks. These estimates, combined i n t o nomographs o r tables o f parameters, could t h e n b e used by designers to make task-specific predictions. Engineering Use of t h e GOMS Model of Immediate Behavior a n d S-R Compatibility How well does t h e MHP model meet t h e requirements f o r a useful engineering model? T h e requirements stated in t h e i n t r o d u c t i o n t o t h i s chapter were (a) b e i n g able t o make zero-parameter, q u a n t i t a t i v e p r e d i c tions o f several measures o f human performance t o w i t h i n about 20% o f actual performance, (b) b e i n g able t o make predictions f o r a wide range o f tasks, a n d (c) b e i n g a t a level o f approximation appropriate f o r design. I n response t o requirement 1, it has been demonstrated t h a t t h e model can make zero-parameter predictions o f response time t o w i t h i n about 20% o f actual performance. T h e model does not, however, make a n y predictions about t h e number o r t y p e s o f e r r o r s t h a t m i g h t b e made when p e r f o r m i n g an immediate response task, n o r does it c u r r e n t l y make predictions about learning time, the effects of practice, or speed/accuracy t r a d e - o f f . A l l o f these measures a r e important when making design decisions, a n d t h e model must b e extended t o cover these areas.

In response t o requirement 2, predictions have been made f o r a v a r i e t y o f tasks: manually responding t o p a t t e r n s o f lights, v e r b a l l y responding t o symbolic stimuli, remembering computer command abbreviations, a n d typing them on a keyboard. T h e model has also been extended t o include predictions o f response time f o r dozens of t r a n s c r i p t i o n typing tasks (John, 1988). Framing t h e model o f S - R compatibility w i t h i n a u n i f i e d t h e o r y o f cognition, l i k e t h e MHP, was a successful s t r a t e g y when considering b r e a d t h o f prediction. T h e remaining requirement, t h a t o f being a t a level o f approximat i o n f o r design, can o n l y b e answered by making design decisions u s i n g t h e model a n d seeing how t h e y w o r k o u t i n practice. However, we can go t h r o u g h a hypothetical case t o see how such a n evaluation m i g h t look. Consider t h e design t a s k o f choosing a computer command-abbreviation technique f r o m among t h e six techniques ( f o u r real abbreviation techniques p l u s n o abbreviation a n d nonsense syllables) examined in t h e experiments r e p o r t e d here. Predictions have already been made f o r t h e i n i t i a l response time a n d t h e execution time in t h e experimental situation investigated. In a design situation, t h e real task is f o r t h e computer u s e r t o remember t h e abbreviation when she o r he has already decided what command i s t o b e implemented. T h e real task does n o t have t h e perceptual component t h a t is included in t h e experimental situation, in which t h e command was presented t o t h e subject on t h e computer screen. Therefore, t h e perceptual operators are removed f r o m t h e predictions of t h e response times (i.e., o n l y t h e i n i t i a l response times a r e affected because t h e predictions f o r t h e execution time included no perceptual operators). Total time can b e p r e d i c t e d by u s i n g t h e e n t i r e algorithm f o r

An Engineering Model of S-R Compatibility

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determining t h e abbreviation a n d typing it out--essentially t h e sum o f These i n i t i a l and execution times, subject t o significant digit r o u n d i n g . predictions appear in Table 9. T h e p a t t e r n o f results in Table 9 i s complex, w i t h n o obvious A t f i r s t glance, t h e M T D choice for an optimal abbreviation technique. and 2 - L e t t e r conditions seem t o b e most a t t r a c t i v e because o f t h e i r relat i v e l y s h o r t total times. However, special-character has t h e shortest execution time a n d no-abbreviation has t h e shortest initial-response time. O n l y t h e nonsense a n d vowel-deletion conditions seem t o b e real losers, p r e d i c t e d t o b e longer t h a n t h e others in all time measures. Table 9 Predictions o f Initial, Execution, a n d Total Response Times Once t h e Command is Known.

T y p e o f Response Time

Condition

2-Letter MTD Special Character No Abbreviation Vowel Deletion Nonsense

Initial (msec)

Execution (msec)

800 800 1600 360 860 1600

500 700 300 1560 1120 560

Total (msec)

1300 1500 1900 1920 1980 2200

T o decide between t h e remaining contenders, no-abbreviation, special-character, MTD, a n d 2-Letter, t h e designer would have t o r e t u r n t o an analysis o f t h e t a s k situation a n d t o t h e algorithms t h a t make t h e predictions. Tables 1 a n d 4 indicate t h a t r e t r i e v a l operators a r e a l a r g e r percentage o f t h e p r e d i c t e d times in which t h e y p l a y a n y role a t all (60% of t h e Special-character initial-response time, 45% o f t h e M T D i n i t i a l response time, 10% o f t h e M T D execution time, 42% of t h e 2 - L e t t e r i n i t i a l Because a response time and 24% o f t h e 2-Letter execution time). r e t r i e v a l operator i s defined as a n operator t h a t performs an a r b i t r a r y association t h a t is n o t well learned, t h e y would o n l y a p p l y in situations where t h e abbreviation is new o r r a r e l y used. T h i s information is important t o t h e computer-system designer when combined w i t h knowledge about t h e user's t a s k .

If t h e real-world t a s k were t o b e done by casual users o f a system (people who use t h e system o n l y occasionally), then the retrieval operators would remain in t h e algorithms and t h e predictions would s t a y as t h e y a r e presented. I n t h a t case, considering response time alone, t h e designer would p i c k e i t h e r t h e M T D o r 2 - L e t t e r abbreviation technique, because t h e y a r e lowest in t o t a l response time and a r e n o t distinguishable w i t h i n 20%. O t h e r considerations m i g h t t h e n d r i v e t h e decision (e.g., perhaps t h e r e i s a v e r y r e s t r i c t i v e device f o r feedback t o

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t h e user, l i k e a two-character d i s p l a y on a n inexpensive telephone, which would f a v o r t h e 2-Letter technique).

If, on t h e o t h e r hand, t h e t a s k was t o b e performed f r e q u e n t l y a n d f o r long periods of time ( l i k e t e x t - e d i t i n g in a word-processing e n v i r o n ment), t h e abbreviations would eventually become well-learned a n d t h e r e t r i e v a l operators would d r o p o u t o f t h e algorithms. Then the initialresponse times become essentially equal and t h e execution times depend In t h a t case, t h e M T D o n l y on t h e number o f characters t y p e d . technique would b e eliminated because it requires more characters t o b e typed. T h e decision between special-character a n d 2 - L e t t e r would t h e n b e made o n t h e basis o f o t h e r considerations (e.g., perhaps t h e keyboard has no special characters). T h i s example of a p p l y i n g t h e MHP model o f immediate behavior t o a real-world t a s k shows a t least t h a t t h e model i s a t a level o f approximat i o n useful f o r design-decision making. T h e q u a n t i t a t i v e predictions separate some o f t h e techniques, even t h o u g h t h e predictions a r e uncertain t o about 20%. T h e o t h e r techniques can t h e n b e separated by examining t h e algorithms, t h e i r operators, a n d t h e i r implications in d i f f e r e n t realThis level of analysis adds important information f o r t h e w o r l d tasks. designer a n d i s t h u s a t an appropriate level f o r design, meeting r e q u i r e ment 2 f o r engineering models. T h e command-abbreviation example o f applying t h e MHP model t o design i s a simplistic one, p r i m a r i l y because t h e t h e o r y does n o t y e t make predictions about errors, a v e r y important aspect in human factors design. However, t h e example demonstrates t h e techniques t h a t can b e used w i t h t h e model, as is, t o generate some information f o r design. These analysis techniques could b e used in t h e f u t u r e w i t h a more complete model t o generate more complete information. Summary Engineering models t h a t can make zero-parameter, quantitative, a p r i o r i predictions o f human performance are necessary f o r design. In response t o t h i s need, a GOMS model o f immediate behavior a n d S - R comp a t i b i l i t y was presented. Parameters o f t h e model were estimated w i t h an experiment examining t h e recall o f computer command abbreviations, These parameters, together w i t h o t h e r estimates taken f r o m C a r d e t at. (19831, were used t o p r e d i c t response times in f o u r d i f f e r e n t t y p e s o f experimental tasks r e p o r t e d in t h e l i t e r a t u r e : moving a s t y l u s in response t o a light ( F i t t s & Seeger, 1953), s t r i k i n g a k e y in response t o a light (Duncan, 19771, naming a number in response t o a numeral o r geometric symbol (Morin & Forrin, 19621, and typing a n abbreviation in response t o a computer command (John & Newell, 1987). T h e average absolute p e r c e n t e r r o r s between predictions a n d observed response times ranged f r o m 8.3% t o 27.2%, w i t h an overall average of 18.8%. T h e potential use o f t h i s model was demonstrated by a p p l y i n g it t o hypothetical design situations.

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Acknowledgements T h e bulk o f t h i s c h a p t e r is f r o m 6 . E . John's doctoral dissertation, 1988. T h i s research was, in p a r t , s u p p o r t e d by t h e O f f i c e o f Naval Research, Perceptual Science Programs, C o n t r a c t Number N00014-87K0432. T h e views a n d conclusions contained in t h i s document a r e those of t h e a u t h o r s a n d should n o t b e i n t e r p r e t e d as r e p r e s e n t i n g t h e o f f i c i a l policies, e i t h e r expressed o r implied, o f t h e O f f i c e o f Naval Research o r t h e U.S. Government. References Anderson, J. R. (1983). T h e architecture of cognition. MA: H a r v a r d U n i v e r s i t y Press.

Cambridge.

Baron, J. (1978). Intelligence a n d general strategies. In G. Underwood (Ed. ) , Strategies of information processing (pp. 403-450). London: Academic Press. Beakley, G. C., Evans, D. L., & Keats, J. B . (1986). Engineering: An introduction to a creative profession. New Y o r k : Macmillan. Brown, J., & Huda, M. (1961). Response latencies p r o d u c e d by massed a n d spaced l e a r n i n g o f a paired-associate l i s t . Journal of Experimental Psychology, 61, 360-364. (1983). T h e psychology of Card, S. K., Moran, T. P., & Newell, A . human computer interaction. Hillsdale, NJ: Lawrence Erlbaum. (1977). Response selection r u l e s in spatial choice reaction Duncan, J. tasks. In S . D o r n i c (Ed.), Attention and performance V I (pp. 4961). Hillsdale, NJ: Lawrence Erlbaum. (1953). S-R compatibility: Spatial c h a r Fitts, P. M., & Seeger, C . M. acteristics o f stimulus a n d response codes. Journal of Experimental Psychology, 46, 199-210. Jensen, A. R., & Rohwer, W. D . (1963). V e r b a l mediation in p a i r e d associate a n d serial l e a r n i n g . Journal of Verbal Learning and Verbal Behavior, 1 , 346-352. (1988). Contributions to engineering models of humanJohn, 6. E. computer interaction. Doctoral dissertation, Carnegie-Mellon University. John, B . E . , & Newell, A. (1987). P r e d i c t i n g t h e time t o recall comp u t e r command abbreviations. In J. M. C a r r o l l & P . P . Tanner, (Eds.), CHI * GI 1987 (Toronto, A p r i l 5-9). New Y o r k : ACM, 33-40. John, 6. E . , Rosenbloom, P. S., & Newell, A. (1985). A t h e o r y of stimulus-response compatibility applied t o human-computer interaction. In L. Borman a n d 6. C u r t i s (Eds.), CH1'85 Human Factors I n Computing Systems (San Francisco, April 14-18). New Y o r k , ACM, 213219.

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Laird, J. E., Newell, A., & Rosenbloom, P. S. (1987). Soar: A n intelligence. Artificial Intelligence, 33(1), 1a r c h i t e c t u r e for general -

64.

Millward, R. (1964). Latency in a modified paired-associate learning experiment. Journal of Verbal Learning and Verbal Behavior, 3, 309316. (1966). Forgetting Montague, W. E., Adams, J. A., & Kiess, H. 0. a n d n a w r a l language mediation. Journal of Experimental Psychology, 72, 829-833. Morin, R. E., & Forrin, B. (1962). M i x i n g t w o t y p e s o f S-R association in a choice reaction time t a s k . Journal of Experimental Psychology, 64, 137-141. Newell, A. (1973). Production systems: Models of c o n t r o l s t r u c t u r e s . In W. G. Chase (Ed.), Visual information processing ( p p . 463-526). New Y o r k : Academic. (1987) Unified theories of cognition. T h e William James Newell, A . Lectures. H a r v a r d U n i v e r s i t y , Spring, 1987. (Available in videocassette, Psychology Department, H a r v a r d ) . P r y t u l a k , L. S. (1971). Psychology, 2 , 1-56.

Natural

language mediation.

Cognitive

Rosenbloom, P. S., & Newell, A . (1987). A n integrated computational In G. H. model of stimulus-response compatibility a n d practice. Bower (Ed.), The psychology of learning and motivation (Vol. 21) (pp. 1-52). New Y o r k : Academic. Runquist, W. N., & Farley, F. H . (1964). The use o f mediators in the learning o f v e r b a l p a i r e d associates. Journal of Verbal Learning and Verbal Behavior, 3, 280-285. Shepard, R. N. compatibility.

(1961). Role o f generalization in stimulus-response Perceptual and Motor Skills, 13, 59-62.

J. P. (1962). A t e s t of t h e all-or-none hypothesis for v e r b a l learning. Journal of Experimental Psychology, 64, 158-165.

Williams,

An Engineering Model of S-R Compatibility

46 1

Appendices T h e following appendices p r e s e n t t h e Artless algorithims used t o analyze t h e t h r e e tasks from t h e classic S-R compatibility l i t e r a t u r e (Duncan, 1977; F i t t s & Seeger, 1953; Morin & Forrin, 1962). These comp l e t e l y w o r k e d - o u t examples demonstrate t h e use o f t h e GOMS analysis tech ni q u e . T h e l e t t e r s in t h e l e f t margin indicate t h e t y p e o f operator: P represents a perceptual operator t a k i n g 100 ms; C, a c o g n i t i v e operator t a k i n g 50 ms; M, a motor operator t a k i n g 70 ms.

Appendix I: Artless Aigorlthms for the Fitts and Seeger Task Stimulus A - Response A P C C C C C M

BEGIN Stimulus t Perceive-Stimulus("0n-Light") Stimulus c Encode-Position-Bit-1 Stimulus t Encode-Position-Bit-2 Stimulus c Encode-Position-Bit-3 Angle t Get-Value(Stimu1us. "Angle") Initiate-Response("Push-Lever",Angle) Execute-Response("Push-Lever",Angle) END # Perceptual Operators: 1 #Cognitive Operators: 5 #Motor Operators: 1

Predicted response time 5 420 ms

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Stlmulus A - Response B: Algorithm a P C C C C C C C C C C M C C M

BEGIN Stimulus t Perceive-StimuIus("On-Light") Stimulus t EncodsPosition-Bit-1 Stimulus t Encode-Position-Bit-2 Stimulus t Encode-Position-Bit-3 Stimulus-Angle t Get-Value(Stlmulus,"Angle") IF-SUCCEEDED Is-Divisible?(Stimulus-Angle,90) THEN BEGIN Initiate-Response("Psh-Lever",Stimulus-Angle) Execute-Response("Push-Lever",Stimulus-Angle) END ELSE IF-SUCCEEDED Is-Not-Divisible?(Stimulus-Angle,90) THEN BEGIN Response-Angle t Add(Stimulus-Angle,45) Initiate-Respo~e("Push-Lever", Response-Angle) ExewtsResponse("Push-Lever",Response-Angle) ResponseAngle t Add(Stimu1us-Angle, -45) Initiate-Response("Push-Lever",ResponseAngle) Execute-Response("Push-Lever",Response-Angle) END END # Perceptual Operators: # Cognitive Operators: # Motor Operators:

Divisible-by-90 Not-Divisible-by-90 1 1 6 8 1 1

AVERAGE FOR THIS ALGORITHM: #Perceptual Operators: 1 # Cognitive Operators: 7 # Motor Operators: 1 Predicted response time = 520 ms

An Engineering Model of S-R Compatibility Stimulus A - Response B: Algorithm b P C C C C C C C M C C M

C C M

BEGIN Stimulus t Perceive-Stimulus("0n-Light") Stimulus t Encode-Posltion-Bit-1 Stimulus t Encode-Position-Bit-2 Stimulus t Encode-Position-Bi-3 Stimulus-Angle t Get-Value(Stirnulus, "Angle") IF-SUCCEEDED Is-Not-Divisible?(Stimulus-Angle, 90) THEN BEGIN Response-Angle t Add(Stirnu1us-Angle, 45) Initiate-Response("Push-Lever",ResponseAngle) Execute-Response("Push-Lever",Response-Angle) Response-Angle t Add(Stimu1us-Angle, -45) Initiate-Response("Push-Lever",Response-Angle) Execute-Response("Push-Lever",ResponseAngle) END ELSE IF-SUCCEEDED Is-Divisible?(Stimulus-Angle, 90) THEN BEGIN Initiate-Response("Push-Lever",Stimulus-Angle) Execute-Response("Push-Lever",Stimulus-Angle) END END # Perceptual Operators: # Cognitive Operators: # Motor Operators:

Divisible-by-90 1 7 1

Not-Divisible-by-90 1 7 1

AVERAGE FOR THIS ALGORITHM: 1 # Cognitive Operators: 7 # Motor Operators: 1 # Perceptual Operators:

Predicted response time = 520 rns

Average for the Stimulus A - Response B condition = 520 ms

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-

Stimulus B Response A: Algorithm a

P C C C C C C M C C C C

c

C C C M

BEGIN Stimulus t Perceive-Stimulus("0n-Light") Stimulus c Encode-One-or-Many IF-SUCCEEDED One-Light-On?() THEN BEGIN Stimulus c Encode-Position-Bit-1 Stimulus c Encode-Position-Bib2 Angle t Get-Value(Stimulus, "Angle") Initiate-Response("Push-Lever",Angle) Execute-Response("Push-Lever",Angle) END ELSE IF-SUCCEEDED Many-Lights-On()? THEN BEGIN Stimulus t Encode-Position-Bit Angle t Get-Value(Stimulus, "Angle") Stimulus2 c Get-Second-Stlmulus("0n-Light",Angle) Stimulus2 t Encode-Position-Bit Angle2 c Get-Value(Stimulus2, "Angle") Angle t Average(Angle, Angle2) Initiate-Response("Push-Lever",Angle) Exearte-Response("Push-Lever",Angle) END END

# Perceptual Operators: # Cognitive Operators: # Motor Operators:

One-Light 1

AVERAGE FOR THIS ALGORITHM: 1.5 #Cognitive Operators: 7.5 # Motor Operators: 1

# Perceptual Operators:

Predicted response time = 600 ms

6 1

Many-Lights 2 9 1

An Engineering Model of S-R Compatibility Stimulus B - Response A: Algorithm b

C C C C C M

C C C C C M

BEGIN Stimulus t Perceive-Stimulus("0n-Light") Stimulus t Encode-One-Or-Many IF-SUCCEEDED Many-Lights-On()? THEN BEGIN Stimulus t Encode-Position-Bit Angle t Get-Value(Stimuius, "Angle") Stimulus2 t Qet-Second-Stlmulus("0n-LigM",Angle) Stimulus2 t Encode-Position-Bit Angle2 t Get-Value(Stimuius2, "Angle") .Angle t Average(Angle, Angle2) Initiate-Response("Push-Lever",Angle) Execute-Response("Push-Lever",Angle) END ELSE IF-SUCCEEDEDOne-Light-On?() THEN BEGIN Stimulus t Encode-Position-Bit-1 Stimulus t Encode-Position-Bit-2 Angle t Get-Value(Stlmulus, "Angle") Initiate-Response("Push-Lever",Angle) Execute-Response("Push-Lever". Anale) - . END END # Perceptual Operators: # cognitive Operators: # Motor Operators:

One-Light 1 7 1

Many-Lights 2 8

1

AVERAGE FOR THIS ALGORITHM: # Perceptual Operators: 1.5 # Cognitive operators: 7.5 # Motor Operators: 1 Predicted response time = 600 ms

Average for the Stimulus B - Response A condition = 600 ms

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Stimulus B - Response B: Algorithm la P C C C C M C C C C C C C M

BEGIN Stimulus t Perceive-Stimulus("0n-Light") Stimulus t Encode-Position-Bit-1 Stimulus t Encode-Position-Bit-2 Angle t Get-Value(Stimulus, "Angle") Initiate-Response("Push-Lever",Angle) Execute-Response("Push-Lever",Angle) Stimulus t Encode-One-@-Many IF-SUCCEEDED One-Light-On?() THEN BEGIN END ELSE IF-SUCCEEDED Many-Lights-On()? THEN BEGIN Stimulus t Get-Second-Stimulus("0n-Light",Angle) Stimulus t Encode-Position-Bit Angle t Get-Value(Stlmulus."Angle") Initiate-Response("Push-Lever", Angle) Execute-Response("Push-Lever",Angle) END END One-Light Many-Lights # Perceptual Operators: # Cognitive Operators:

1 4

4

# Motor Operators:

1

1

AVERAGE FOR THIS ALGORITHM: # Perceptual Operators: 1 # Cognitive Operators: 4 # Motor Operators: 1 Predicted response time = 370 ms

1

An Engineering Model of S-R Compatibility Stimulus B - Response B: Algorithm Ib P C C C C M C C C C C M C

BEGIN Stimulus t Perceive-Stimulus("0n-Light") Stimulus t Encode-Position-Bit-1 Stimulus t Encode-Posltion-Bit-2 Angle t Get-Value(Stimulus, "Angle") Initiate-Response("Push-Lever",Angle) Execute-Response("Push-Lever", Angle) IF-SUCCEEDED Many-Lights-On()? THEN BEQIN Stimulus t Get-Second-Stimulus("0n-Light", Angle) Stimulus t Encode-Position-Bit Angle c Get-Value(Stimulus,"Angle") Initiate-Response("Push-Lever",Angle) Execute-Response("Push-Lever",Angle) END ELSE IF-SUCCEEDED One-tight-On?() THEN BEGIN END END

# Perceptual Operators:

# Cognitive Operators: # Motor Operators:

One-Light 1 4 1

AVERAGE FOR THIS ALGORITHM: # Perceptual Operators: 1 # Cognitive Operators: 4 # Motor Operators: 1 Predicted response time = 370 ms

Many-Lights 1 4 1

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Stimulus B - Response 6: Algorithm Ila P C C C C C C M C C

C C

M C C C C

M

BEGIN Stimulus t Perceive-Stimulus("0n-Light") Stimulus t EncodsOne-Or-Many IF-SUCCEEDED one-LigM-On?(j THEN BEGIN Stimulus t Encode-Position-Bit-1 Stimulus t Encode-Position-Bit-2 Angle t Get-Value(Stimulus, "Angle") Initiate-Response("Push-Lever",Angle) Execute-Response("Push-Lever",Angle) END ELSE IF-SUCCEEDED Many-Lights-On()? THEN BEGIN Stimulus t Encode-Position-Bit Angle t Get-Value(Stimulus, "Angle") Initiate-Response("Push-Lever",Angle) Execute-Response("Push-Lever",Angle) Stimulus c Get-Second-Stimulus("0n-Light", Angle) Stimulus t Encode-Position-Bit Angle t Get-Value(Stimulus,"Angle") Initiate-Response("Push-Lever", Angle) Execute-Response("Push-Lever",Angle) END END One-Light Many-Lights 1 1 6 6 1 1

# Perceptual Operators: # Cognitive Operators: # Motor Operators:

AVERAGE FOR THIS ALGORITHM: # Perceptual Operators: 1 #Cognitive Operators: # Motor Operators:

6 1

Predicted response time = 470 ms

An Engineering Model of S-R Compatibility Stimulus B - Response 8: Algorithm Ilb P C C C C C M C C C C M

BEGIN Stimulus t Perceive-Stimulus("0n-Light") Stimulus t Encode-One-Or-Many IF-SUCCEEDED Many-Lights-On()? THEN BEQIN Stimulus t Encode-Position-Bit Angle t Get-Value(Stimulus, "Angle") Initiate-Response("Push-Lever",Angle) Execute-Response("Push-Lever",Angle) Stimulus t Get-SecondStimulus("On-UgM", Angle) Stimulus t Encode-Position-Bit Angle t Get-Value(Stimulus,"Angie") Initiate-Response("Push-Lever", Angle) Execute-Response("Push-Lever",Angle) END ELSE IF-SUCCEEDED One-Light-On?() THEN BEGiN Stimulus t Encode-Position-Bit-1 Stimulus t Encode-Position-Bit-2 Angle t Get-Value(Stimulus, "Angle") initiate-Response("Push-Lever",Angle) Execute-Response("Push-Lever",Angle) END END

# Perceptual Operators: # Cognitive Operators: # Motor Operators:

One-Light Many-Lights 1 1 5 7 1 1

AVERAGE FOR THIS ALGORITHM: # Perceptual Operators: 1 # Cognitive Operators: 6 # Motor Operators: 1 Predicted response time = 470 ms Average for the Stimulus B - Response B condition = 420 ms

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Appendix II: Artless Algorithms for the Duncan Task Corresponding P C C C C M

BEQIN Stimulus t Perceive-Stimulus("Line") Stimulus t Encode-Postion-Bit-1 Stimulus t EncodsPostion-Bit-2 Horizontal-Location tQet-Value(Stimulus, "Horlzontal-Location") Initiate-Response("Press-Button",HorizontaCLocatlon) ExecutsResponse("Press-Button",Horizontal-Location) END # Perceptual Operators: 1 # Cognitlve Operators: 4 #Motor Operators: 1

Predicted response time = 370 ms

An Engineering Model of S-R Compatibility Opposite P C C C C C M

BEGiN Stimulus t Perceive-Stlmuius("Line") Stimulus t Encode-Postion-Bit-1 Stimulus t Encode-Postion-Bit-2 Horizontal-Location t Get-Value(Stimulus, "Horizontal-Location") Horizontal-Location t Negate(Horironta1-Location) initiate-Response("Press-Button",Horizontal-Location) ExecutsResponse("Press-8utton",Horizontal-Location) END # Perceptual Operators: 1 # Cognitive Operators: 5 # Motor Operators: 1

Predicted response time = 420

47 1

472

B.E. John and A. Newell Mixed-1 P C C C C C

C M C

C M

BEGIN Stimulus t Perceive-Stimulus("Line") Stimulus t Encode-Postion-Bit-1 Stimulus t Encode-Postion-Bit-2 Horizontal-Location c Get-Value(Stimulus, "Horizontal-Location") IF-SUCCEEDED In-Middle?(Horizont-Location) THEN BEGIN Horizontal-Location t Negate(Horizontal-Location) Initiate-Response("Press-Button", Horizontal-Location) Execute-Response("Press-Button", Horizontal-Locatlon) END ELSE IF-SUCCEEDED Outside-OtMiddle?(Ho~zontal-LOCaUon) THEN BEGIN Initiate-Response("Press-Butlon",Horizontal-Location) Execute-Response("Press-B~on",Horizontal-Location) END END Corresponding # Perceptual Operators: # Cognitive Operators: # Motor Operators:

Opposite

1 6

1 6

1

1

An Engineering Model of S-R Compatibility

473

Mixed-2 P C

BEGIN Stimulus c Perceive-Stimulus("Line") Stimulus t Encode-Postion-BR-1 Stimulus c Encode-Postion-Bit9 Horizontal-Location tGet-Value(Stimuius, "Horizontal-Location") IF-SUCCEEDED Outside-Of-Middle?(Horizontal-Location) THEN BEGIN Initiate-Response("Press-Button", Horizontal-Location) Execute-Response("Press-Button",Horizontal-Location) END ELSE IF-SUCCEEDED in-Middle?(Horizontai-Location) THEN BEGIN Horizontal-Location c Negate(Horizonta1-Location) Initiate-Response("Press-Button",Horizontal-Location) Execute-Response("Press-Button",Horizontal-Location) END END

# Perceptual Operators: # Cognitive Operators:

#Motor Operators:

Corresponding 1 5 1

AVERAGE FOR THE MIXED ALGORITHMS: Corresponding # Perceptual Operators: 1 # Cognitive Operators: 5.5 # Motor Operators: 1 Predicted response times:

450 ms

Opposite 1 7 1

Opposite 1 6.5 1 500 ms

B.E. John and A. Newell

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Appendix 111: Artless Algorithms for the Morin and Forrln Task Condition I P C C C M

BEQIN Stimulus t Perceivestimulus Stimulus t EncodeNumber Name t Qet-Value(Stimlus, "Name") Initiate-Response("Say-Name",Name) Execute-Response(SaName", Name) END # Perceptual Operators: 1 # Cognitive Operators: 3 # Motor Operators: 1

Predicted response time = 320 ms

An Engineering Model of S-R Compatibility Condition II BEGIN Stimulus t Perceive-Stimulus Stimulus t Encode-Symbol Name t Get-Value(Stimlus, "Name") IF-SUCCEEDED Is-Plus?(Name) THEN Name t Qet-Plus-Name ELSE IF-SUCCEEDED Is-Square?(Name) THEN Name t Get-SquarsName Initiate-ResDonse("Sav-Name".Name) ExecutsResponse("Say-Name",Name) END # Perceptual Operators: 1 # Cognitive Operators: 5.5 # Motor Operators: 1

Predicted response time = 450 ms

47 5

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B.E. John and A. Newell Condition 111 test-number-first P C C C C C C C C C C C C

C M

BEGIN Stimulus c Perceive-Stlmulus SHmulus t EncodeNumber-Or-Symbol Class t Qet-ValuefStimulus. "Class") IF-SUCCEEDED Is-humtmr?(class) ' THEN BEQIN Stimulus c Encode-Number Name t Get-Value(Stimulus,"Name") END ELSE IF-SUCCEEDED Is-Symbol?(Class) THEN BEGIN Stimulus t Encode-Symbol Name t Get-Value(Stimulus,"Name") IF-SUCCEEDED Is-Plus?(Nam) THEN Name t Get-Plus-Name ELSE IF-SUCCEEDED Is-Square?(Name) THEN Name c Get-Square-Name END Initiate-Response("Say-Name",Name) ExecutsResponse("Say-Name",Name) END

# Perceptual Operators: # Cognitive Operators:

# Motor Operators:

Number 1 6 1

Symbol 1 9.5 1

An Engineering Model of S-R Compatibility Condition 111 test-symbol-first P C C C C C C C C C C C

c

C M

BEGIN Stimulus t Perceive-Stimulus Stimulus t Encode-Number-Or-Symbol Class t Get-Value(Stlmulus, "Cia&) IF-SUCCEEDED Is-Symbol?(Class) THEN BEGIN Stimulus t Encode-Symbol Name t Get-Value(Stimulus, "Name") IF-SUCCEEDED Is-Plus?(Name) THEN Name t Get-Plus-Name ELSE IF-SUCCEEDED Is-Square?(Name) THEN Name t Get-Square-Name END ELSE IF-SUCCEEDED Is-Number?(Class) THEN BEGIN Stimulus t Encode-Number Name t Get-Value(Stimulus, "Name") END Initiate-Response("Say-Name",Name) Execute-Response("Say-Name",Name) END Number

Symbol

# Perceptual Operators:

1

1

# Cognitive Operators: # Motor Operators:

7 1

8.5 1

AVERAGE FOR THE CONDITION 111 ALGORITHMS: Number Symbol # Perceptual Operators: 1 1 # Cognitive Operators: 6.5 9 # Motor Operators: 1 1 Predicted response times:

500 ms

620 ms

477

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B.E. John and A. Newell Condition IV P C C C C M

BEGIN Stimulus c Perceive-Stimulus Stimulus c Encode-Number-Bit-1 Stimulus t Encode-Number-Bit-2 Name c Get-Value(Slmlus, "Name") initiate-Response("Say-Name",Name) Execute-Response("Say-Name", Name) END # Perceptual Operators: 1 # Cognitive Operators: 4 #Motor Operators: 1

Predicted response time = 370 ms

An Engineering Model of S-R Compatibility Condition V P C C C C C

BEGIN Stimulus t Perceive-Stimulus Stimulus t Encode-Symbol-BI-1 Stimulus t Encode-Symbol-Bib2 Name t Get-Value(Stimlus, "Name") IF-SUCCEEDED Is-Plus?(Name) THEN Name t Get-Plus-Name ELSE IF-SUCCEEDED Is-Square?(Name) THEN Name t Get-Square-Name ELSE IF-SUCCEEDED Is-Circle?(Name) THEN Name t Get-Circle-Name ELSE IF-SUCCEEDED Is-Triangle?(Name) THEN Name t Get-Triangle-Name Initiate-Response("Say-Name",Name) Execute-Response("Say-Name",Name) END #Perceptual Operators: 1 # Cognitive Operators: 7.5 # Motor Operators: 1 Predicted response time = 550 ms

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PART VI AN INTEGRATED PERSPECTIVE T h e f i n a l c h a p t e r summarizes t h e p r i m a r y research f i n d i n g s a n d explanations described in t h e p r e c e d i n g chapters. General p o i n t s o f consensus a r e identified, t h e most important o f w h i c h i s t h a t stimulusresponse compatibility effects r e f l e c t basic c o g n i t i v e processes. T h e o r e t i cal models a r e evaluated, w i t h t h e conclusion t h a t t h e mental representations o f t h e stimulus a n d response sets p l a y a c r u c i a l role in compatibility effects. T h e p e r s p e c t i v e developed i n t h i s c h a p t e r emphasizes t h a t S-R compatibility effects a r e o f fundamental importance t o u n d e r s t a n d i n g human cognition.

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STIMULUS-RESPONSE COMPATIBILITY R. W. Proctor and T.G. Reeve (Mitors @ Elsevier Science Publishers B. V. (tdorth-HolIandJ. 1990

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RESEARCH ON STIMULUS-RESPONSE C O M P A T I B I L I T Y : TOWARD A COMPREHENSIVE ACCOUNT R o b e r t W. Proctor Department o f Psychologica I Sciences Purdue University T. Gilmour Reeve Motor Behavior Center Auburn University O v e r v i e w of C h a p t e r s Research on stimulus-response (S-R) compatibility has a rich history, as documented in t h e Foreword t o t h i s volume by A . M. Small a n d in t h e i n t r o d u c t o r y c h a p t e r s by A l l u i s i a n d Warm ( C h a p t e r 1) a n d Simon (Chapter 2 ) . A s described by Small, contemporary i n t e r e s t in compatibility phenomena arose f r o m t h e applied problems faced by humanA l t h o u g h Small's f a c t o r s engineers in d e s i g n i n g m i l i t a r y - r e l a t e d systems. i n t e r e s t was p r i m a r i l y in stimulus-stimulus compatibility (e.g., relations between stimulus p r o p e r t i e s i n m u l t i - s e n s o r y displays), t h e research o f Paul F i t t s a n d his colleagues changed t h e emphasis t o one o f S-R compatibility. As indicated by A l l u i s i a n d Warm, considerable research on compatibility effects w i t h l i g h t - p a t t e r n e d stimulus sets a n d motor response sets was conducted in t h e 1950s, n o t o n l y by F i t t s a n d his colleagues but also by D a v i d Grant, William Garvey, a n d t h e i r respective co-workers. These e a r l y studies demonstrated c l e a r l y t h e f a c t that, f o r spatial-location stimuli a n d responses, t h e r e l a t i v e compatibility is p r i m a r i l y a f u n c t i o n o f t h e degree o f d i r e c t physical correspondence. Another factor illustrated in t h e e a r l y studies was t h a t compatibility is a f u n c t i o n o f t h e e x t e n t t o which S-R p a i r i n g s a r e consistent w i t h population stereotypes (i.e., tendencies t o make p a r t i c u l a r responses to stimuli i n f r e e response situations). From these beginnings, research on S - R compatibility b r a n c h e d o u t t o consider a v a r i e t y o f situations. Even when apparent physical c o r r e spondence was lacking, c e r t a i n sets o f responses c o u l d b e executed f a s t e r a n d more accurately t o a set o f stimuli t h a n c o u l d o t h e r sets o f responses. F o r example, vocal number-naming responses could b e made f a s t e r t o numerical stimuli t h a n c o u l d k e y - p r e s s responses ( A l l u i s i & Muller, 1958). In addition, when t h e spatial locations o f stimuli were i r r e l e v a n t t o t h e task, these locations s t i l l were shown t o p r o d u c e comp a t i b i l i t y effects. T h i s l a t t e r phenomenon was an accidental d i s c o v e r y made by Simon (Chapter 2 ) , w h i c h h e subsequently investigated thoroughly. T h e phenomenon has come t o b e r e g a r d e d as a d i s t i n c t t y p e o f compatibility effect, now r e f e r r e d t o as t h e "Simon e f f e c t . " It reflects a basic response-selection tendency t o react t o w a r d t h e source of stimulation. T h e implication of t h e Simon e f f e c t f o r system designers is t h a t i r r e l e v a n t location cues can i n t e r f e r e w i t h human information processing.

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Despite t h e substantial research conducted on S-R compatibility effects, until r e c e n t l y r e l a t i v e l y l i t t l e p r o g r e s s has been made t o w a r d t h e development o f detailed, theoretical accounts. However, in recent years, several systematic research programs have focused on examining t h e n a t u r e o f compatibility effects, w i t h these programs collectively p r o v i d i n g new i n s i g h t i n t o t h e mechanisms t h a t u n d e r l i e S-R compatibility. A l t h o u g h these research programs a r e diverse, t h e y can b e categorized b r o a d l y as examinations o f mental representation, psychophysiological indices and neurophysiological mechanisms, the relation of S-R compatibility t o motor performance, a n d applications t o human f a c t o r s .

Mental Representation Studies o f t h e role o f mental representation in S-R compatibility have focused p r i m a r i l y on "coding" explanations. Such explanations have been developed because, as A l l u i s i a n d Warm (Chapter 1) emphasize, compatibility e f f e c t s a r e a f u n c t i o n o f t h e e x t e n t t o w h i c h " p a i r i n g s of stimulus a n d response alphabets c o r r e s p o n d t o one another in a d i r e c t conceptual sense." T h a t is, compatibility is n o t merely a f u n c t i o n o f physical correspondence, but o f a correspondence between a b s t r a c t mental codes t h a t a r e formed t o r e p r e s e n t t h e stimulus a n d response sets. Because many studies o f S - R compatibility have used spatial-location stimuli a n d responses, emphasis has been placed on spatial coding, b e g i n n i n g w i t h F i t t s a n d Seeger's (1953) i n f l u e n t i a l w o r k . Among t h e earliest demonstrations i n s u p p o r t o f a spatial-coding account were those He dissociated spatial locations f r o m p r o v i d e d by Wallace (1971, 1972). response effectors by h a v i n g subjects p e r f o r m a two-choice t a s k w i t h t h e arms crossed. T h e c r u c i a l outcome was t h a t t h e fastest responses s t i l l o c c u r r e d when t h e right light signaled t h e right response location a n d t h e l e f t light signaled t h e l e f t response location. Subsequent t o Wallace's (1971, 1972) finding, t h e n a t u r e o f spatial c o d i n g a n d t h e w a y in w h i c h such coding influences responding have been examined in detail. Umilta a n d Nicoletti (Chapter 3) note t h a t when t h e stimulus a n d response sets a r e composed o f l e f t a n d right elements, t h e designation "left" a n d "right" r e f e r s t o b o t h egocentric a n d r e l a t i v e locations. U m i l t l a n d Nicoletti unconfounded these t w o location dimensions f r o m t h e stimulus set a n d f o u n d t h a t each independently p r o d u c e d S - R compatibility effects. For t h e response set, t h e egocentric dimension has y e t t o b e evaluated, b u t compatibility effects have been demonstrated t h a t a r e a t t r i b u t a b l e t o t h e relative position o f t h e response. Moreover, d i s t i n g u i s h i n g between t h e r e l a t i v e position o f t h e e f f e c t o r a n d t h e r e l a t i v e position of t h e response goal showed t h a t S-R compatibility effects a r e a f u n c t i o n o f t h e l a t t e r . O t h e r research by Umiltti a n d Nicoletti demonstrates t h a t compatibility effects can b e obtained f o r t h e above-below dimension, as well as f o r t h e l e f t - r i g h t dimension, but t h a t t h e l a t t e r dimension dominates when t h e stimulus a n d response sets o v e r l a p in b o t h dimensions. U m i l t l and Nicoletti conclude t h a t spatial compatibility effects p r o d u c e d by a relevant location dimension r e f l e c t t h e time f o r t r a n s l a t i o n between stimuli a n d responses, whereas spatial compatibility effects p r o d u c e d by an i r r e l e v a n t location dimension (i.e., t h e Simon effect) most l i k e l y r e f l e c t competition in t h e selection o f t h e a p p r o p r i a t e response.

Toward a Comprehensive Account

485

Heister, Schroeder-Heister, a n d Ehrenstein ( C h a p t e r 4) show t h a t similar evidence f o r spatial c o d i n g is obtained when subjects r e s p o n d w i t h t w o f i n g e r s f r o m a single hand, e i t h e r p r o n e o r supine. They disting u i s h between spatial f i n g e r distance a n d anatomical f i n g e r distance, finding t h a t spatial distance is crucial, as would b e expected by a spatial-coding accaunt. In addition, Heister e t al. r e p o r t evidence f o r "spatio-anatomical mapping," w h i c h involves i n t e r n a l coding o f anatomical l e f t - r i g h t classifications. Heister e t at. propose a model, in w h i c h c o d i n g of k e y position, coding o f e f f e c t o r position, a n d spatio-anatomical mapping a r e a r r a n g e d in an i n t e r a c t i v e h i e r a r c h y . LBdavas ( C h a p t e r 5) also s u p p o r t s a hierarchical model similar t o t h a t proposed by Heister e t al. She emphasizes t h a t whereas spatial coding dominates in most situations, t h e anatomical status o f t h e r e s p o n d i n g h a n d becomes i m p o r t a n t when t h e r e is no dimensional o v e r l a p between stimulus a n d response locations. Moreover, LBdavas shows t h e presence o f spatial compatibility effects i n y o u n g c h i l d r e n who have n o t y e t acquired t h e capacity t o label l e f t a n d r i g h t . Thus, spatial S-R comp a t i b i l i t y effects a p p a r e n t l y a r e n o t a consequence o f v e r b a l l y labeling t h e positions o f stimuli a n d responses. Reeve a n d Proctor ( C h a p t e r 6) r e p o r t f u r t h e r evidence f o r spatial coding in more complex, four-choice s p a t i a l - p r e c u i n g tasks. I n such tasks, d i f f e r e n t i a l p r e c u i n g benefits f o r p a r t i c u l a r p a i r s o f responses a r e a f u n c t i o n o f t h e spatial locations t h a t a r e c u e d a n d n o t o f w h e t h e r t h e f i n g e r s a r e f r o m t h e same o r d i f f e r e n t hands. Similar evidence f o r spatial coding is apparent when two-dimensional, symbolic stimuli a r e assigned t o t h e f o u r response locations. Additionally, w i t h b o t h spatiallocation a n d symbolic stimuli, evidence f o r h a n d coding i s obtained when t h e d i s t i n c t i o n between t h e hands i s made salient. Reeve a n d Proctor propose t h a t t h e i r f i n d i n g s a r e c a p t u r e d by a salient-features coding principle, according t o w h i c h t h e r e l a t i v e reaction times ( R T s ) a r e a f u n c t i o n o f t h e e x t e n t t o which t h e salient features o f t h e stimulus set a n d t h e salient features o f t h e response set correspond. Psychophysiological Indices a n d Neurophysiological Mechanisms O t h e r researchers have related t h e compatibility effects t h a t a r e observed f o r R T s t o psychophysiological indices a n d have examined t h e neurophysiological bases f o r t h e effects. Most o f t h e psychophysiological research has examined e v e n t - r e l a t e d potentials. Bashore ( C h a p t e r 7), Ragot (Chapter 8), a n d B r e b n e r ( C h a p t e r 9) summarize evidence t h a t these indices of c e r e b r a l a c t i v i t y p r o v i d e information t h a t i s n o t r e d u n d a n t w i t h t h e R T measure a n d t h a t can b e used t o d i s t i n g u i s h between a l t e r n a t i v e accounts f o r compatibility effects. T h e psychophysiological research has focused p r i m a r i l y o n t h e P300 wave. T h e P300 latency has been shown t o b e influenced s t r o n g l y by stimulus processing. T h e evidence r e g a r d i n g w h e t h e r t h e P300 is affected by response-selection processes is f a r less clear. Some studies have f o u n d l i t t l e o r no e f f e c t o f such processes o n t h e P300 latency, wheres o t h e r studies have shown s t r o n g e r effects. F o r example, Coles, Gratton, Bashore, Eriksen, a n d Donchin (1985) f o u n d t h a t t h e latency o f t h e P300 was increased when f l a n k i n g noise l e t t e r s indicated a response t h a t was i n c o n g r u e n t w i t h t h e response indicated by a t a r g e t l e t t e r . Bashore ( C h a p t e r 7) describes a s t u d y i n w h i c h S-R compatibility was

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manipulated in t h e f l a n k e r t a s k by u s i n g d i f f e r e n t assignments o f l e f t a n d r i g h t - p o i n t i n g a r r o w s t o responses. P300 latency was influenced by b o t h t a r g e t - n o i s e incongruity a n d S-R compatibility, a n d t h e latency of another component, t h e N200, was affected o n l y by S-R compatibility. Bashore notes t h a t t h e P300 latency was n o t influenced by S-R compatibility when t h e f l a n k i n g noise stimuli were n e u t r a l asterisks. Thus, his r e s u l t s imply t h a t S-R compatibility affects P300 latency o n l y f o r situat i o n s in w h i c h stimuli a c t i v a t e competing responses. Ragot ( C h a p t e r 8) describes research f r o m his l a b o r a t o r y t h a t leads t o a similar conclusion. He summarizes several studies i n w h i c h t h e P300 was f o u n d t o b e delayed by spatial incompatibility when stimulus location was i r r e l e v a n t (i.e., t h e situation in w h i c h t h e Simon e f f e c t is obtained). Ragot concludes t h a t t h e d i s t i n g u i s h i n g f a c t o r between studies t h a t obtained an e f f e c t o f S-R compatibility a n d those t h a t did n o t is whether an i n c o n g r u e n t response is indicated by an i r r e l e v a n t stimulus dimension. Thus, t h e ERP studies suggest t h a t t h e P300 i s sensitive t o response competition effects a n d t h a t t h e basis o f t h e Simon e f f e c t is similar t o t h a t o f t h e f l a n k e r interference. B r e b n e r (Chapter 9) examines i n d i v i d u a l differences in t h e P300 latency, n o t i n g t h a t i n t r o v e r t s a n d e x t r a v e r t s d i f f e r on t h i s measure. I n t r o v e r t s have much s h o r t e r P300 latencies t h a n d o e x t r a v e r t s , y e t t h e i r RTs a r e longer. T h i s p a t t e r n o f r e s u l t s i s consistent w i t h a model of introversion/extraversion developed by B r e b n e r a n d Cooper (1974). A c c o r d i n g t o t h e model, i n t r o v e r t s d e r i v e excitation f r o m stimulus analysis a n d i n h i b i t i o n f r o m response organization, with extraverts showing t h e opposite relation. B r e b n e r ' s f i n d i n g s suggest t h a t i n d i v i d u a l differences p l a y an i m p o r t a n t r o l e in compatibility effects a n d t h a t p s y chophysiological measures may b e useful i n r e s o l v i n g t h e n a t u r e o f t h e differences. Speculation about t h e neurophysiological basis o f S-R compatibility effects has t a k e n t w o forms. F i r s t , compatibility effects have been used as estimates o f interhemispheric transmission time (Bashore, C h a p t e r 7 ) . T h e logic, a r t i c u l a t e d i n i t i a l l y by Poffenberger (19121, i s t h a t R T s should b e s h o r t e r if b o t h t h e reception o f t h e stimulus a n d t h e execution o f t h e response o c c u r w i t h i n t h e same hemisphere, because cross-commissural communication i s n o t r e q u i r e d . However, Bashore summarizes evidence i n d i c a t i n g t h a t anatomical c o n n e c t i v i t y is n o t t h e p r i m a r y determinant o f compatibility effects in choice-RT t a s k s . T h a t is, in choice tasks, spatial-locations a r e crucial, r a t h e r t h a n t h e p a r t i c u l a r e f f e c t o r . For simple RT, a small e f f e c t o f anatomical c o n n e c t i v i t y does seem t o b e obtained reliably, w i t h t h e r e s u l t i n g estimate o f interhemispheric t r a n s mission time b e i n g 1-3 ms. Verfaellie, Bowers, a n d Heilman ( C h a p t e r 10) p r o v i d e an a l t e r n a t i v e neurophysiological account f o r t h e compatibility effects t h a t a r e obtained in choice-RT tasks, because t h e interhemispheric account is n o t able t o explain t h e data f r o m these tasks. Verfaellie e t al.'s hypothesis is t h a t each hemisphere controls d i f f e r e n t aspects o f attention in contralateral hemispace. When stimulus processing a n d response selection a r e mediated by t h e same hemisphere, R T s w i l l b e f a s t e r t h a n when these processes a r e mediated by d i f f e r e n t hemispheres. Verfaellie e t al. report performance data b o t h f o r patients w i t h hemispatial neglect a n d f o r normal subjects. These data suggest t h a t in a f r e e - f i e l d situation, each hemisphere d i r e c t s attention a n d i n t e n t i o n t o w a r d t h e contralateral side o f

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space. When stimuli a r e p r e s e n t e d t o one side o f space, t h e r e l a t i v e location is crucial, w i t h t h e more lateral stimulus f a v o r e d by t h e contralateral hemisphere. Verfaellie e t al. r e p o r t an experiment examining t h e Simon effect, in w h i c h e i t h e r t h e location o f t h e stimulus (attentional set) o r location o f t h e response (intentional set) o r b o t h w e r e cued. Compatibility effects were obtained f o r v a l i d - c u i n g t r i a l s o n l y when intentional but n o attentional information was p r o v i d e d . Verfaellie e t al. i n t e r p r e t these a n d o t h e r r e s u l t s as s u g g e s t i n g t h a t intentional set primes t h e hemisphere t h a t controls t h e c u e processing, t h u s inducing compatibility effects as a f u n c t i o n o f w h e t h e r t h e stimulus location i s in t h e hemispace attended by t h e activated hemisphere. These a u t h o r s conclude t h a t t h e process by w h i c h motor responses a r e p r e p a r e d i s an important f a c t o r i n S-R compatibility. S-R Compatibility a n d Motor Performance Studies t h a t e x p l i c i t l y i n v e s t i g a t e d t h e r o l e o f S-R compatibility in motor performance have been concerned p r i m a r i l y w i t h c o n t r o l l i n g comp a t i b i l i t y effects, so t h a t motor-programming effects c o u l d b e studied. Zelaznik a n d F r a n z (Chapter 11) emphasize t h a t S-R compatibility effects a r e p e r v a s i v e in such studies. T h e effects influence t h e p a t t e r n s o f R T s obtained f o r p r e c u i n g various movement parameters when spatial incomp a t i b i l i t y is present, as well as when symbolic stimuli a r e mapped a r b i t r a r i l y t o responses. Similarly, when comparing across S-R sets o f diff e r e n t sizes, compatibility is a c o n f o u n d i n g f a c t o r . When t h e S-R assignment is incompatible, decreases in R T can o c c u r simply as a f u n c t i o n o f a r e d u c t i o n in set size. Zelaznik a n d Franz propose t h a t an o p e r ational t e s t o f w h e t h e r S-R t r a n s l a t i o n processes o r motor-programming processes a r e determining t h e p a t t e r n s o f R T s i s t o manipulate S-R comp a t i b i l i t y , along w i t h w h e t h e r t h e p r e c u e d movement parameters v a r y f r o m trial-to-trial o r are fixed. If t h e o r d e r i n g o f RTs is d i f f e r e n t f o r t h e v a r i e d a n d f i x e d methods, t h e n t h e p r e c u e d variables a r e i n f l u e n c i n g t r a n s l a t i o n processes, as well as possibly motor processes. O n l y if t h e o r d e r i n g i s t h e same across methods can t h e r e s u l t s b e i n t e r p r e t e d relat i v e l y unambiguously in terms o f motor processes. Spijkers ( C h a p t e r 12) demonstrates t h e d i s t i n c t i o n between response selection a n d motor programming by showing t h e independence o f compatibility effects f r o m movement-precuing effects. In h i s experiments, n e i t h e r spatial compatibility n o r semantic compatibility affected t h e influence o f average movement v e l o c i t y on R T . S p i j k e r s t h u s concludes t h a t t h e response codes selected in t h e t r a n s l a t i o n stage a r e abstract, nonmot o r i c representations. Motor programming t h e n involves elaboration o f t h e specific parameters o f t h e movement. Heuer ( C h a p t e r 13) also shows t h e independence o f S-R compatibility effects a n d motor-programming effects, demonstrating that, u n l i k e S-R compatibility effects, response-response (R-R) compatibility effects (differences in r e s p o n d i n g as a f u n c t i o n o f t h e a l t e r n a t i v e members o f t h e response set) a r i s e in motor programming. When d i f f e r e n t movements, r a t h e r t h a n t h e same movements, a r e assigned t o t h e t w o hands, a t r i a d o f R-R compatibility effects is observed: Mean RTs a r e longer, mean R T v a r i a b i l i t y is larger, a n d f r e q u e n c y of choice e r r o r s is less. However, these effects d o n o t o c c u r when t h e movements a r e t h e same, but t h e f i n g e r s w i t h which t h e y a r e executed a r e d i f f e r e n t . These R-R compatibility effects a r e a t t r i b u t e d by Heuer t o interactions t h a t o c c u r during t h e processes i n v o l v e d in simultaneously programming t w o responses.

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Whereas most studies o f S-R compatibility emphasize central, cognitive functioning, Gordon (Chapter 14) provides a special case o f compatibility in speech perception a n d p r o d u c t i o n t h a t arises f r o m lowlevel l i n k s between t h e perceptual a n d motor systems. He r e p o r t s t w o experiments in which perceptual-motor interactions a r e shown f o r b o t h t h e nasal/stop consonant d i s t i n c t i o n a n d t h e nasal/fricative distinction. Gordon concludes t h a t such interactions occur o n l y f o r features t h a t a r e perceptually salient a n d presents a n interactive-activation model t o account f o r t h e effects. Application to Human Factors

S-R compatibility has a pervasive influence in "'real-wortd" situations, as shown by t h e numerous examples p r o v i d e d by Kantowitz, Triggs, a n d Barnes (Chapter 15). These authors summarize a v a r i e t y o f They distinguish t y p e s o f S-R incompatibilities f o u n d in e v e r y d a y life. between t y p e s o f incompatibility t h a t arise f r o m frames (general knowledge used t o i n t e r p r e t experience), rules ( r e l a t i v e l y specific knowledge about simple S-R relations), a n d response tendencies (more specific tendencies f o r stimuli t o elicit implicit responses). Kantowitz e t al. stress t h e importance o f developing models t h a t relate t h e basic, laboratory f i n d i n g s t o applied settings. Eberts a n d Posey (Chapter 16) p r e s e n t such a model. T h e y elabor a t e t h e concept o f stimulus-central processing-response (S-C-R) compatibility proposed by Wickens and h i s associates (e.g., Wickens, Sandry, & Vidulich, 1983). Whereas Wickens stressed t h e n a t u r e o f t h e mental code (spatiaVverbal) r e q u i r e d t o p e r f o r m a task, Eberts and Posey emphasize t h e s t r u c t u r e o f t h e mental model. T h e y note t h a t t h e mental model is dependent o n t h e t r a i n i n g of t h e operator and may change o v e r time. Several analysis techniques t o e x t r a c t t h e n a t u r e of t h e mental model are discussed. Eberts a n d Posey p r o v i d e examples o f how t o improve S-R compatibility once t h e s t r u c t u r e o f t h e mental model has been identified. T h e authors emphasize t h e importance o f designing t h e environment t o b e compatible w i t h good mental models. John and Newell (Chapter 17) present another model intended t o relate basic f i n d i n g s t o applied settings. T h e i r model i s based on a t h e o r y of S-R compatibility by Rosenbloom a n d Newell (1987) t h a t is cast i n terms o f Goals, Operators, Methods, and Selection (GOMS) rules w i t h i n t h e Model Human Processor (MHP) framework (Card, Moran, E Newell, 1983). T h e i r analysis relies on t h e view t h a t d i f f e r e n t mappings o f stimuli t o responses r e s u l t in d i f f e r e n t sets o f algorithms f o r accomplishing t h e t a s k . John and Newell d e r i v e d parameter estimates from an experiment examining t h e recall of computer command abbreviations. These parameters, and parameter estimates f r o m C a r d e t al. (1983). t h e n were used t o p r e d i c t performance in f o u r d i f f e r e n t t y p e s of S - R compatibility tasks. T h e average absolute percent e r r o r s between predictions a n d observations was 18.8%. T h e authors i l l u s t r a t e t h e potential use o f t h e model by a p p l y i n g it t o hypothetical design situations.

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Summary Several general p o i n t s o f consensus including t h e following:

emerge f r o m t h e chapters,

1. S-R compatibility effects a r e u b i q u i t o u s . T h e y o c c u r in a wide v a r i e t y o f situations, f r o m basic perceptual-motor t a s k s t o highly complex programming t a s k s .

2 . Conceptual correspondence, r a t h e r t h a n p h y s i c a l correspondence, is t h e source o f S-R compatibility e f f e c t s . Compatibility effects have been shown f o r stimulus sets a n d response sets t h a t have n o spatiallocation dimension, f o r symbolic stimuli assigned t o k e y p r e s s responses, a n d f o r d i f f e r e n t p a i r s o f c u e d responses w i t h i n s p a t i a l l y compatible stimulus a n d response sets.

3. F o r spatial-location stimuli assigned t o responses a t d i f f e r e n t locations, compatibility e f f e c t s o c c u r regardless o f w h e t h e r t h e stimulus location is r e l e v a n t o r i r r e l e v a n t f o r d e t e r m i n i n g t h e c o r r e c t response. Whether these t w o t y p e s o f compatibility e f f e c t s have similar bases is less clear. 4. S - R compatibility e f f e c t s a r i s e p r i m a r i l y f r o m a stage o f informat i o n processing t h a t is r e f e r r e d t o as t h e t r a n s l a t i o n stage or t h e response-selection stage. These effects a r e independent f r o m effects o f R - R compatibility t h a t a r i s e in a response-programming stage. 5. T h e codings o f stimulus a n d response sets, a n d how t h e codings relate, p l a y an important r o l e in most S-R compatibility e f f e c t s . These codings determine how q u i c k l y a n d a c c u r a t e l y a stimulus code can b e t r a n s l a t e d i n t o a response code.

6. T h e c o d i n g system is hierarchical, but f l e x i b l e . In most situations f o r which spatial-location stimuli a r e assigned t o spatial-location responses, r e l a t i v e location c o d i n g dominates. However, spatio-anatomical mapping (e.g., t h e d i s t i n c t i o n between t h e l e f t a n d right hands) dominates when it relates systemically t o t h e salient stimulus f e a t u r e but t h e response locations d o n o t . Models of S-R Compatibility

T h e models o f S - R compatibility summarized in t h e respective chapters o f t h i s book have been developed f r o m d i s t i n c t research programs. As a consequence, t h e terminology varies, a n d d i f f e r e n t issues a r e stressed. T h e p r e s e n t section compares a n d c o n t r a s t s t h e v a r i o u s models, w i t h t h e i n t e n t o f emphasizing t h e p o i n t s o f agreement a n d disagreement. T r a d i t i o n a l l y , most models o f S-R compatibility a r e characterized as p r o v i d i n g "attentional" o r "coding" accounts. A third c a t e g o r y o f models can b e d e s c r i b e d as p r o v i d i n g "general information processing" accounts. A t t e n t i o n a l Models T h e attentional models o r i g i n a t e d w i t h Simon's ( C h a p t e r 2) research o n t h e influence o f i r r e l e v a n t spatial locations. These models emphasize t h e d i r e c t i o n o f attention t o locations. Simon's i n i t i a l account proposed an i n n a t e t e n d e n c y t o react in t h e d i r e c t i o n o f a stimulus. The buffer

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model t h a t h e p r e s e n t s i n C h a p t e r 2 is a more elaborate account t h a t describes t h e response-selection process as i n v o l v i n g a scanning o f response b u f f e r s . A c c o r d i n g to t h e model, a response b u f f e r is established f o r each assigned response in a choice-RT t a s k . Thus, f o r two-choice t a s k s t h a t use l e f t - h a n d a n d r i g h t - h a n d responses, t w o b u f f e r s a r e established t h a t each contain a representation o f t h e assigned stimulus. T h e b u f f e r s a r e assumed t o b e searched in a serial o r d e r when a stimulus is presented, w i t h a response b e i n g i n i t i a t e d when a match is found. T h e i r r e l e v a n t location o f t h e stimulus affects responding by biasing t h e subject t o search f i r s t t h e b u f f e r o f t h e corresponding response location. A c c o r d i n g t o Simon, t h i s b u f f e r model d i f f e r s f r o m t h e coding accounts in minimizing t h e c o g n i t i v e component o f S - R compatibility a n d s t r e s s i n g a more p r i m i t i v e response tendency, w i t h t h i s tendency b e i n g a f u n c t i o n o f r e l a t i v e location, r a t h e r t h a n absolute location. Verfaellie e t al. ( C h a p t e r 10) relate t h e t e n d e n c y t o respond in t h e d i r e c t i o n o f t h e stimulus t o activation o f t h e c o r r e s p o n d i n g hemisphere o f t h e b r a i n . A c c o r d i n g t o them, t h e i n t e n t i o n t o execute a response in one hemispace produces activation in t h e contralateral hemisphere. T h i s activ a t i o n t h e n facilitates t h e processing of a stimulus t h a t occurs in t h e same hemispace. One f i n d i n g complicates explanations o f t h e Simon e f f e c t in terms o f an automatic t e n d e n c y t o respond in t h e d i r e c t i o n o f t h e stimulus. This finding i s t h a t t h e e f f e c t occurs when t h e t w o stimulus a n d response locations a r e t o t h e same side o f b o d y midline (Umiltd a n d Nicoletti. 1985; see also C h a p t e r 3 ) . Simon concludes t h a t t h i s finding does n o t r e a l l y create a problem f o r t h e attentional account, but h e does n o t p r o v i d e an e x p l i c i t explanation of t h e finding. Verfaellie e t al. (Chapter 10) do cons i d e r more e x p l i c i t l y how an attentional account can e x p l a i n t h e f a c t t h a t t h e Simon e f f e c t occurs when t h e a l t e r n a t i v e locations a r e t o t h e same side o f t h e b o d y midline. T h e y p r e s e n t evidence that, in such situations, t h e more lateral stimulus is p r e f e r r e d by t h e contralateral hemisphere a n d t h e less lateral one by t h e ipsilateral hemisphere. Thus, Verfaellie e t al. conclude t h a t t h e c o n t r o l o f attention by t h e hemispheres is dynamic, r a t h e r t h a n b e i n g a f i x e d f u n c t i o n o f a s t r i c t spatial dichotomy.

Coding Models Most a u t h o r s who f a v o r c o d i n g models view them as b e i n g complete explanations o f compatibility effects t h a t a r e n o t mediated by attentional factors. Umiltd a n d Nicoletti (Chapter 3) have obtained evidence e x p l i c i t l y i n t e n d e d t o r u l e o u t attentional accounts. I n place o f such accounts, t h e y propose t h a t spatial compatibility effects, b o t h when stimulus locations a r e relevant a n d i r r e l e v a n t , a r e d u e t o t h e mental codes used t o r e p r e s e n t t h e r e l a t i v e positions o f t h e response keys and/or t h e positions o f t h e effectors. Heister e t al. ( C h a p t e r 4), Ladavas ( C h a p t e r 51, a n d Reeve a n d Proctor ( C h a p t e r 6) a r e i n agreement w i t h Umiltd a n d Nicoletti t h a t relat i v e spatial c o d i n g i s c r u c i a l t o spatial-compatibility effects. In addition, t h e y conclude t h a t spatio-anatomical mapping is e v i d e n t in c e r t a i n circumstances. Thus, coding is viewed as hierarchical, w i t h spatial coding b e i n g dominant a n d anatomical coding b e i n g used e i t h e r when spatial coding cannot b e o r when t h e anatomical f e a t u r e is made more salient.

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Even w i t h i n spatial coding, t h e r e seems t o b e a h i e r a r c h y o f p r e f e r r e d codes. Umilt6 a n d Nicoletti indicate t h a t above/below c o d i n g i s used when it i s t h e o n l y dimension, whereas l e f t / r i g h t c o d i n g dominates when stimuli c o u l d b e coded on e i t h e r dimension. Similarly, Reeve a n d P r o c t o r note t h a t w i t h i n p r e c u e d stimulus displays, a h i e r a r c h y e x i s t s f o r t h e r e l a t i v e p r e c u i n g b e n e f i t s t h a t o c c u r f o r t h e a l t e r n a t i v e p a i r s of p r e c u e d locations. Whereas t h e pvimary evidence f o r t h e c o d i n g model comes f r o m spatial-location stimulus a n d response sets, evidence also has been obtained when t h e stimuli a r e symbolic. Differences in R T o c c u r as a consequence o f t h e assignment o f t h e features o f two-dimensional, symbolic stimuli t o response locations (Reeve & Proctor, C h a p t e r 6 . ) Responses a r e f a s t e r a n d p r e c u i n g b e n e f i t s g r e a t e r when t h e salient features o f t h e stimulus a n d response sets correspond. Similarly, f o r a u d i t o r y speech stimuli a n d vocal responses, t h e responses a r e f a s t e r when t h e y s h a r e salient phonetic features w i t h t h e stimuli t h a n when t h e y d o n o t (Gordon, C h a p t e r 14). General Information-Processing Models T h e third c a t e g o r y involves models t h a t have been developed by human-factors engineers t o enable consideration o f S-R compatibility in t h e design process. These models have as a goal t h e a b i l i t y t o p r e d i c t performance i n a v a r i e t y o f r e a l - w o r l d s e t t i n g s . As a consequence, t h e y e x p l a i n compatibility effects w i t h i n t h e c o n t e x t of more general models o f human -information processing .

A s discussed p r e v i o u s l y , Kantowitz e t al. ( C h a p t e r 15) d i s t i n g u i s h between compatibility e f f e c t s t h a t a r i s e f r o m frames, rules, a n d response tendencies. T h e y emphasize t h a t t h e effects t h a t a r i s e as a f u n c t i o n o f frames a n d r u l e s a r e o f most concern in human factors, but t he m a j o r i t y o f l a b o r a t o r y research has focused o n effects a t t h e level o f response tendencies. A n i n t e g r a t e d account o f S-R c o m p a t i b i l i t y must consider all levels. E b e r t s a n d Posey ( C h a p t e r 16) note t h a t Wickens e t al. (1983) t o o k an i n i t i a l step t o w a r d a d d r e s s i n g t h e h i g h e r level determinants o f compatibility in p r o p o s i n g t h e i r S-C-R compatibility model. Wickens e t al. recognized t h e important f a c t t h a t stimuli h a d t o b e i n c o r p o r a t e d i n t o a n d retained by t h e mental models o f t h e operators o f complex machines. T h e S-C-R compatibility model t h u s emphasizes t h e importance o f t h e code o f c e n t r a l representation in compatibility effects. A c c o r d i n g t o t h e model, t h e code can l i e on a v i s u a l - v e r b a l continuum. By k n o w i n g t h e n a t u r e o f t h e c e n t r a l code, i n p u t displays a n d response devices can b e designed t o b e consistent w i t h t h e code. E b e r t s a n d Posey b u i l d o n t h e S-C-R compatibility model by c h a n g i n g t h e emphasis f r o m t h e n a t u r e o f t h e mental code t o one o f t h e s t r u c t u r e o f t h e mental model o f t h e o p e r a t o r . T h a t is, t h e mental model is t h e conceptual representation o f a system t h a t t h e o p e r a t o r has a c q u i r e d f r o m t h e system documentation a n d f r o m i n t e r a c t i o n w i t h t h e system. In E b e r t s a n d Posey's account, several s t r u c t u r e s can b e used f o r t h e mental models: (a) image-based spatial mental models; (b) framebased mental models; (c) p r o d u c t i o n systems; a n d (d) goal hierarchies. Good a n d b a d mental models, w h i c h lead t o good o r b a d performance, a r e dependent on t h e p e r c e i v e d consistencies in t h e t a s k . T h e mental model

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approach enables t h e development o f techniques t o determine t h e s t r u c t u r e o f t h e mental model f o r a t a s k and, thus, specification o f t h e methods t o improve S-R compatibility. John a n d Newell's ( C h a p t e r 17) e n g i n e e r i n g model is based o n t h e GOMS t h e o r y o f Rosenbloom a n d Newell (1987) t h a t was developed w i t h i n a general model o f cognition, t h e MHP ( C a r d e t al., 1983). T h e MHP model specifies t h r e e separate processors : a perceptual processor, a c o g n i t i v e processor, a n d a motor processor. T h e basic p r o c e d u r e of an MHP analysis is t o determine t h e gross f u n c t i o n s a n d elementary operations t h a t each processor must p e r f o r m in o r d e r t o accomplish a task. Each elementary operation is assigned a d u r a t i o n (through an estimation procedure), t h u s enabling p r e d i c t i o n . o f response times f o r different tasks. J o h n a n d Newell estimated t h e o p e r a t o r d u r a t i o n s f r o m a t a s k i n v o l v i n g human-computer interaction. Compatibility was v a r i e d by t h e directness w i t h w h i c h t h e commands related t o t h e actions t h a t were t o b e performed. T h e p r e d i c t i o n s o f t h e GOMS model fit t h e r e s u l t s r e l a t i v e l y well. Moreover, u s i n g these a n d o t h e r estimated o p e r a t o r durations, t h e model p r e d i c t e d r e l a t i v e l y accurately t h e response times in (a) a second command abbreviation experiment, (b) t w o spatial-compatibility studies, a n d (c) a s t u d y r e q u i r i n g a v i s u a l number-naming response t o e i t h e r numeric stimuli o r t o nonalphanumeric forms. Thus, t h e GOMS model is able t o make r e l a t i v e l y accurate p r e d i c t i o n s across a wide r a n g e o f tasks. Summary Most o f t h e models o f S-R compatibility advocated in t h e p r e s e n t volume r e l y o n mental coding. These models explain S-R compatibility effects in terms o f t h e mental representations used t o p e r f o r m t h e tasks. T h e representations determine t h e d u r a t i o n o f processi.ng t h a t must b e performed and, hence, R T . Considerable evidence f o r coding accounts has been g a t h e r e d f r o m t a s k s t h a t r a n g e f r o m phoneme identification, t o choices between spatial-location stimuli a n d responses, t o computer p r o gramming. Moreover, considerable p r o g r e s s has been made in determining t h e n a t u r e o f t h e codes a n d t h e situations in w h i c h p a r t i c u l a r t y p e s o f codings w i l l b e used. Most i n t e r e s t i n g l y , t h e r e s u l t s f r o m simple p e r ceptual-motor t a s k s seem t o b e explainable in terms similar t o t h e r e s u l t s f r o m much more c o g n i t i v e l y complex, human-machine interactions. In short, t h e coding operations revealed by studies o f S-R compatibility seem t o r e f l e c t fundamental c o g n i t i v e processes o f b r o a d g e n e r a l i t y . A t present, attentional mechanisms a r e n o t as w i d e l y accepted as a r e c o d i n g models. T h e p r i m a r y evidence f o r attentional mechanisms comes f r o m t h e Simon e f f e c t . Yet, even f o r t h a t effect, t h e evidence implicates f l e x i b l e control, r a t h e r t h a n a f i x e d t e n d e n c y t o respond t o w a r d a source. If a t e n d e n c y t o a t t e n d t o p a r t i c u l a r locations is needed t o augment t h e c o d i n g accounts, as Simon ( C h a p t e r 2) a n d Verfaellie e t at. ( C h a p t e r 10) argue, t h i s attentional c o n t r o l also must b e based o n c o g n i t i v e representations o f t h e t a s k . A n I n t e g r a t e d Perspective T h a t S-R compatibility phenomena a r e d i v e r s e is apparent in t h e p r e s e n t volume. Yet, despite t h i s d i v e r s i t y , research i s c o n v e r g i n g on specific u n d e r l y i n g mechanisms a n d common e x p l a n a t o r y p r i n c i p l e s . The

Toward a Comprehensive Account

493

most consistent p o i n t t h a t emerges f r o m t h e c h a p t e r s i s t h a t compatibility effects r e f l e c t basic c o g n i t i v e processes t h a t influence human performance in a v a r i e t y o f situations, r a n g i n g f r o m simple perceptual-motor t a s k s to complex c o g n i t i v e tasks. Performance is a f u n c t i o n o f t h e manner in which t h e a l t e r n a t i v e stimuli a n d responses a r e mentally represented. These representations a r e based on salient features o f t h e stimulus a n d response sets. T h e r a t e o f t r a n s l a t i o n between t h e stimulus a n d response representations determines r e l a t i v e compatibility, w i t h t h e most compatible situations b e i n g those f o r w h i c h t h e correspondence between codes is most d i r e c t . T h e more t h a n 30 y e a r s o f research on S-R compatibility has p r o duced a substantial b o d y o f data. A l t h o u g h a theoretical explanation o f compatibility phenomena has been slow in coming, t h e u n d e r s t a n d i n g o f t h e phenomena has increased s u b s t a n t i a l l y . F o r example, t h e knowledge now has attained a level a t w h i c h it is possible t o p r e d i c t w i t h reasonable accuracy t h e r e l a t i v e compatibilities i n seemingly u n r e l a t e d situations. Perhaps more importantly, t h e u n d e r s t a n d i n g has emphasized t h e c e n t r a l role o f compatibility in human-information processing. T h a t is, because S-R compatibility phenomena a r i s e p r i m a r i l y f r o m t r a n s l a t i o n processes t h a t mediate between perceptual representations a n d motor representations, t h e phenomena p r o v i d e i m p o r t a n t evidence regarding the representations a n d operations t h a t u n d e r l i e human cognition. References Alluisi, E. A., & Muller, P. F . , J r . (1958). V e r b a l a n d motor responses t o seven symbolic v i s u a l codes: A s t u d y in S-R compatibility. Journal of Experimental Psychology, 5 5 , 247-254. Brebner, J., & Cooper, C . (1974). T h e effects o f a low r a t e o f r e g u l a r signals upon t h e reaction times of i n t r o v e r t s a n d e x t r a v e r t s . Journal of Research i n Personality. 8 , 263-276.

(1983). The psychology of Card, S . K., Moran, T. P., G Newell, A . human-computer interaction. H i IIsdale , NJ : E rlbaum . Coles, M. G . H., Gratton, G . , Bashore, T . R . , Eriksen, C. W., & Donchin, E. (1985). A psychophysiological investigation o f t h e cont i n u o u s f l o w model o f human information processing. Journal of Experimental Psychology: Human Perception and Performance, 1 1 , 529-

553. Fitts, P. M., & Seeger, C . M. (1953). S-R compatibility: Spatial c h a r acteristics o f stimulus a n d response codes. Journal of Experimental P s y ~ h o l o g y ,46, 199-210. (1912). Reaction time t o Poffenberger, A . T. Archlves of Psychology, 2 3 , 1-73.

retinal

stimulation.

(1987). A n i n t e g r a t e d computational Rosenbloom, P. S . , & Newell, A. model o f stimulus-response compatibility a n d practice. In G. H. Bower (Ed.), T h e psychology of learning and motivation (Vol. 21, pp. 1-52). New Y o r k : Academic Press.

R. W. Proctor and T.G. Reeve

494

Umilte, C . , & Nicoletti, R. (1985). Attention and coding effects in S-R compatibility due t o irrelevant spatial codes. I n M. I. Posner & 0. S. M. Marin (Eds.), Mechanisms of attention: Attention and performance X I (pp. 457-471). Hillsdale, NJ: Erlbaum. Wallace, code,

R. J.

(1971). S - R compatibility and the idea of the response Journal of Experimental Psychology, 88, 354-360.

Wallace, R. J. (1972). Spatial S-R compatibility effects involving kinesthetic cues. Journal of Experimental Psychology, 93, 163-168. Compatibility and Wickens, C . D., Sandry, D., & Vidulich, M. (1983). resource competition between modalities of input, central processing, and output: Testing a model of complex task performance. Human Factors, 25, 227-248.

495

AUTHOR INDEX Acosta, E., 36, 48, 53-56, 58, 59, 69, 92, 107, 120, 155 Adams, J. A., 279, 443 Adams, 0. S . , 10 Agens, R. M., 74 Albert, M. L., 264 14, 17 Alcorn, M. B., Alexandrov, I. O., 253 Allison, T., 192, 193, 211 Alluisi, E. A,, 3, 5-7, 10-16, 18-20, 483, 484 254 Amabile, G., Anderson, J. R., 394, 395, 428 Anderson, N. H., 5 Andreassi, J. L., 194 Annet, M., 150, 151 Anson, J. G., 280, 282 Anzola, G . P., 75, 91, 94, 99, 100, 118, 138, 145, 146, 148, 158, 163, 185, 187, 261, 262 11 Attneave, F., Baba, D. M., 329 Babkoff, H., 74 Baddeley, A. D., 8, 17, 358 Barber, P . , 10, 75 Barnes, H. J., 317, 324 Baron, J., 434 Barr, A., 382 74, 99, 100, Bashore, T. R., 183, 184, 188, 190, 193, 201, 204, 207, 209, 230, 233, 485, 486 Bauer, D. W . , 75, 120 Beakley, G . C., 442 Belmont, L., 150, 153 Benton, A. L., 150 Berlucchi, G . , 99, 101, 138, 146, 185, 262 Berlyne, D. E., 11, 311 Berman, A. J., 327 Bernstein, I. H., 75 Bertera, J. H., 6, 17, 19, 75 Bertoloni, G., 75, 94, 118, 145, 185, 261 Biederman, I . , 50, 55, 171 Birch, H. G . , 150, 153 Black, J. B., 413 Blackman, A. R . , 91

Blumstein, S., 348 10 Boff, K. R., Boone, D. R., 150 242 Boring, E. G., Bourguet, M., 211 Bowers, D., 108, 261, 264-267, 486 Boyle, J . , 370 Bradley, T. V., 244 Bradshaw, J. L., 74, 99, 100, 138, 186, 187, 270 Braida, L. D. 345 Brainard, R. W. 14 Braren, M. 226 Braune, R . 201 Brebner, J. 20, 75, 94, 100, 102, 117, 118, 120, 133, 145, 146, 158, 187, 241, 244-251, 254, 255, 261-263, 485, 486 Briggs, G. E. 18 Bringelson, L. 418, 419 Broadbent, D. E. 11, 298 Brookhuuis, K. A . 198 Brooks, L. R. 241, 242 Brown, J. 439, 443 Bruner, J . 150, 155 Brunia, C. H. M. 315 Buchtel, H. A. 75, 94, 118, 145, 185, 261 Bunnel, T . 346 Bunz, H. 332 Bury, K. 370 Cairney, P. 20, 75, 94, 117, 187, 241, 246, 261 Caldwell, C. 407, 411 C a l l a n , J. 6, 17, 75, 118, 120, 133 Callaway, E. 198, 201 Camarda, R. 271 Campbell, J. 368 Caramazza, A. 396 Card, S . K., 413, 428, 429, 434, 435, 442, 455, 458, 488, 492 Carello, C., 280 Carlson, R. A., 406 Carnahan, H., 290 Carpenter, M. B., 210

496

Author Index

Carron, R., 186 Carter, M., 286, 290, 315 Casper, P. A., 365 Chapanis, A., 9, 369 Cheron, G., 211, 212 Chesney, G. L., 253 Chistovich, L. A., 346 Christensen, C., 233 Christina, R. W., 280 Cirillo, L., 155 Clancey, W. J., 406 Clapman, R. M., 190 Cloninger, L., 186 Cohen, D., 211 Cohen, G., 74 Cohen, L., 332 Cohen, S. L., 298 Cole, R. A., 349, 350 Coles, M. G. H., 188, 193,

198, 201, 202, 205, 206, 208, 210, 215, 233, 485 Conover, D. W., 10, 21 Cooper, C., 254, 255, 257, 486 Cooper, F. S., 343, Cooper, W. E., 345, 349, 350 Corcos, D. A., 329 Coslett, H. B., 266 Cotton, B., 19, 75, 120, 188 Courjon, J., 211 C r a f t , J. L., 19, 20, 31, 36, 39, 42, 45-52, 54, 57, 58, 69, 73-76, 108, 117-119, 262, 263 Crea, F., 99, 138, 185, 262 Critchley, M., 264 Crossman, E. R . F. W., 14 Cuffin, B. N., 211 Dascola, I . , 271 Dawson, G. D., 225 Deboeck, M., 126 Declerk, J., 343 Deecke, L., 206 Deininger, R. L., 4, 5, 13, 19,

89, 90, 183, 298 P. C., 348 S., 349, 350, 354 W. N., 11 A., 328 J. E., 211-215

Delattre, Dell, G. Dember, Derwort, Desmedt,

Dewey, G. I., 406 D i Stefano, M., 99, 138, 185,

262 Dixon, N. F., 185 Donchin, E., 188, 190, 193,

197-202, 208, 226, 230, 233235, 253, 254, 485 Donders, F. C., 6 Dooling, R . J.; 349 Dubowski, K. M., 75 Dulany, D. E., 406 Dumais, S. T., 94, 397 Duncan, J., 299, 386, 450-453, 455. 458 Duncan-Johnson, C. C., 197, 198, 230, 232, 235, 254, 255 Durlach, N. I . , 345 Dyer, F. N., 92 389, 392-394, 396, 403, 405, 413, 414, 419 Edelstein, B. A., 75 Egeth, H. E., 350 Ehrenstein, W. ti., 20, 117, 120, 122, 126, 132, 485 El-Zahhar, N., 14 Elias, J. W., 74 Elias, M. F., 74 Elliott, D., 290 Eberts, R. E.,

Elkerton, J., 10 Elman, J. L., 354, 355, 358 Elmes, D. G., 368 Ericsson, K. A., 396 Eriksen, B. A., 201, 203 Eriksen, C. W., 201-203, 205,

206, 208, 210, 215, 485 Erwin, C. I . , 297, 299 Evans, D. L., 442 Evarts, E. V., 271 Evey, R . J., 370 Faber, H. E. L., 75, 119 Falkenberg, L. E., 297, 299 Farley, F. H., 443 Farwell, L. F., 190 Fearnley, S., 331 Feigenbaum. E . , 382 Ferrell, W. R., 59 Fiori, N., 233, 316

Author Index Fischman, M. G., 280 Fitts, P. M., 4, 5, 8-14, 18,

19, 21, 31, 50, 56, 58, 89, 90, 117, 120, 164, 171, 183, 234, 261, 298, 346, 366, 368370, 375, 382, 386, 406, 448, 449, 451, 452, 455, 458, 483, 484 Fletcher, R . , 73 Fober, G. W., 201 Ford, J. M., 201, 205, 233 Forrin, B., 453-455, 458 Fraenkel, G. S., 73 Frank, J. S., 329 Franz, E., 279, 487 Friedin, B. D., 298 Frowein, H. W., 226

497

Grice, H. P., 345 Guiard, Y., 75, 101, 234 Gunia, H., 120 Gunn, C. G., 73, 75 Gunn, D. L., 73, 75 Haagh, S . A . V. M., 299, 315 Hahn, R., 283, 286, 299, 307 Haken, H., 332 Halgren, E., 226 Halle, M., 343, 358 Halvachs, P., 186 Hammond, N., 75 H a r d y c k , C., 19, 75, 120, 188 Harner, P. F., 191 Harvey, L. O., 186, 187 Hasbroucq, T., 75, 90, 113,

234 Gabel, M., 413 Gallaher, A. J., 75 Gardner, E. B., 74, 210 Garner, W. R., 1 1 , 18, 48 Garvey, W. D., 4, 5, 31, 164,

483 Gawryszewski, L. G.,

75, 102,

118, 146 Genesee, F., 74 Genkina, 0. A . , 253 Genser, S., 74 Gentilucci, M., 266 Gentner, D. R., 281, 292 Giami, S., 233 Gielen, S. C. A. M., 329 Glaser, E., 190 Gloerich, A . B. M., 198 Goff, W. R., 192, 193 Goldberg, M. E., 264 Goodman, D., 164, 280, 284,

285, 290, 291, 298, 313, 329 P. C., 343-345, 347350, 352-357 Goto, H., 253 Gottsdanker, R . , 245 Grant, D. A., 5, 366-368, 483 Gratton, G., 193, 201, 202, 206, 208-210, 485 Green, B., 396 Greene, 8 . C., 345 Greenwald, A. G., 346, 347 Gregg, L. W., 14 Greim, D. M., 100, 119, 159, 262, 297, 322 Gordon,

Hausner, H . H., 242 Hawkins, B., 297, 329 Hawkins, F., 375, 380 Hawkins, H. L., 298 Heath, R. A., 320 Hedge, A . , 17, 19, 20, 31,

6468, 75, 90, 92, 120, 145, 229, 249-252, 261 Heffley, E. F., 190 Hegge, F . W., 74 Hefiman, K. M., 108, 261, 263267, 486 Heister, G., 20, 75, 117, 120122, 124, 126, 128, 129, 132, 134-136, 262, 485 H e n d r i k x , A . J. P., 306, 386 H e n r y , F. M., 282 297 Heron, W., 99, 185, 262 Heuer, H . , 281, 292, 307, 311317, 319-326, 328-332, 334-336 Hick, W. E., 14, 17, 285 H i l l y a r d , S. A., 271 H i r i d o r f f , V., 311 H i n r i c h s , J. V., 20, 45-47, 58, 75, 108, 118, 229, 316 Hitch, G., 8 Hoff, A . L., 185 Hollan, J. D., 394, 395 Holley, S. L., 298 Holt, K . G., 280 Hopkins, W. F., 201 Horst, R. L., 190, 253 Hosobuchi, Y., 226 Hu, J . M., 56, 92, 155

Author Index

498 Huda, M., 443 Hueting, J., 126 Huy, N. T., 211 Hylkema, H., 297 Hyman, R., 99, 185, 262, 285 Hyvarinen, J., 271

trby, T. S., Izdebski, K . ,

5, 14 74

Jakimik, J., 349, 350 Jasper, H. H., 190 Jeeves, M. A., 185, 262 Jensen, A. R., 73, 92, 443 John, B. E., 427, 436, 444,

446, 447, 448, 455, 456, 458, 459 John, E. R., Johnson, R.,

226 197, 198, 200, 201, 226, 230, 232, 234, 235, 253-255 Jones, R. E., 370 Kahneman, D., 75 Kaplan, R., 50, 55, 69 Kantowitz, 8. H., 253, 365,

366, 368, 383, 385 Karis, D., 193, 253 Katz, A. N., 19, 75, 76, 120,

121 Kaufman, F . , 253 Kaufman, L., 10 Kaufman, R., 253 Kay, B. A., 332 Keats, J. B., 442 Keele, S. N., 10, 279,306 Kelso, J. A . S., 164 280, 281,

284, 285, 290, 291, 298, 306, 313, 329, 332 Kephart, N. C., 150 K e r r , B., 280, 282-284, 297, 325 Keuss, P. J. G., 75, 119 Kiess, H . O., 443 Kinkade, R . G., 10, 21 Kinsbourne, M., 74, 75, 185, 186, 266 Klapp, S. T., 100, 101 119, 126, 127, 131, 132, 137, 159, 262, 281, 282, 289, 290, 297299, 306, 322, 329

Klatzky, R. L., 74 Kleinman, M., 186 Klisz, D., 6, 75, 118 Knoll, R . L., 298, 306 Knowles, W. B., 4, 5, 17, 164 Koenig, R . S., 119, 159, 262 Kok, A., 253 Kopell, B. S., 201, 230, 232,

233, 235, 255 S., 90, 109, 1 1 1 , 113, 282, 313, 315, 316, 331 Kornhuber, H. H., 206 Koubek, R. J., 411 Kozhevnikov, V. A., 346 Krebs, M. J., 20, 106, 183, 285 Kruskal, J. B., 407 Kornblum,

Kugler, P. N., 280 Kulp, R . A., 18 Kutas. M.. 200. 208 Ladavas,

E., 75, 90, 100, 103, 117, 126, 132, .134-136, 145149, 156, 159, 160, 178, 187, 188, 485 Ladefoged, P., 343, 344 Laird, J. E., 428 Landis, T . , 128 Lang, G . T . , 413 Larish, D. D., 164, 279, 283, 285-287, 298, 302, 307, 313 Ledlow, A., 74, 185, 186, 196, 197, 201, 215 Lee, R . D., 290 Leikind, B. J., 329 LeMay, R . P., 19, 31 Leonard, J. A., 14, 17, 285 Lesevre, N., 75, 206, 210, 215, 225, 230, 231, 235, 237 Levy, J., 138 Lib.erman, A . M., 343-345, 348, 357 Lindau, M., 343 Lindblom, B., 358 Lines, C . R., 185, 194-196, 203, 207 Lingo, W. M., 281 Liotti, M., 94, 95, 97, 98, 105, 106, 108-110, 120 Logan, G . D., 290 Longstreth, L. E., 14, 17 Lorenz, K., 73 Loveless, N . E., 245, 374

Author Index Luce, R . D., 11 L u p k e r , S . J., 19, 75. 76 Luppino, G . , 75, 91, 118, 148,

163. 261 Lutz,.M.

C.,

9

MacKay, D. G . , 332, 354 MacKay, S. L., 298 MacKenzie, C . L., 327, 329,

331 MacNeilage, P. F., 344 Magliero, A., 188, 201, 233 Maksimova, N . E., 253 Marsh, N. W. A., 17, 19, 20

31, 64-68, 75, 90, 92, 120, 145, 229, 249-252, 261 Marshburn, E. A., 297, 322 Marslen-Wilson, W., 346 Marteniuk, R . G., 327, 329, 331 Martin, H . B., 13, 19 Martin, J., 346 Martinerie, J. M., 190 Marzi, C . A., 103, 118, 185 Matelli, M., 266 Mattingly, I . G., 343, 344, 357 Mauguiere, F., 211 Mayer, R. E., 394 McCallum, W. C., 190, 193 McCarthy, G., 188, 190, 193, 199-201, 230, 233-235 McClelland, J. L., 318, 354, 355, 358 McCloskey, M. 396 McConkie, A. 18, 75, 386 McFarquhar, R. 297 McGuinness, D. 264 McKeever, W. F. 185 Megaw, E. D. 281, 282, 284 Mendicino, C. M. 100, 119, 159, 262 Merckx, H . 126 Merisalo, A . 315, 325 Merton, P . A. 281 Meulenbroek, R . G . J. 297 Mewaldt, S. P., 53, 54, 56, 58, 59, 62, 69.92, 107, 120, 155 Meyer, D. E., 344, 345, 347350, 352-357 Michaels, C. F., 20, 280 Miller, G . , 349, 350, 352, 353, 356

499

18, 75, 120, 164, 166, 167, 171, 173, 175, 288. 333' Miller, J. D., 349 Miller, J.,

Millward, R., 443 Milner, A . D., 185, 194 Minsky, M. L., 382, 395 Mitnick, L. L., 5, 31 Mohs, R. C., 201 Molzberger, P., 393 Monsell, S., 298, 306 Montague, W. E., 343 Moran, T. P., 394, 413, 428,

429, 488 Morelli, M., 185 Morgan, €5. B., 18 Morin, R . E., 5, 366-368, 453-

455, 458 20, 75, 103, 117, 118, 134-136, 147, 159, 160, 178, 186-188 Mowbray, G . H., 14 Mudd, S. A., 56, 57 Mulder, L. J. M., 198, 201 Muller, P. F., 5-7, 11, 12, 19, 483 Munro, E . M., 311 Munte, T. F., 271 Myers, R . E., 146 Moscovitch, M.,

Naatanen, R., 315, 325 Nathan, G., 270 Neal, A , , 370 Neisser, U., 311 Nettleton, N. C., 270 Neville, H. J., 271 Newell, A., 395, 413, 427, 428,

429, 434, 436, 444, 448, 455, 458, 488, 492 Newell, K . M., 297, 299 Newlin, E . P., 5 Newman, J. E., 349, 350 Nicely, P., 349, 350, 352, 353, 356 Nickerson, R. S., 75 Nicoletti 19, 20, 75, 76, 89-91, 94, 97, 100, 101, 103-111, 118, 120, 132, 133, 146, 148, 155, 163, 187, 188, 216, 261263, 267, 484, 490, 491 Niemi, P . , 315 Nissen, M. J., 271

Author Index

500 Norman, D. h., 354, 394 Nunnally, J., 368, 369 Nusbaum, H. C., '345 Nystrom, C. O., 5

Quastler, H., Quinn, J. T.,

11 297, 329

Rabbitt, P. M. A.,

120, 331,

332 O'Gorman, J. G., 254 Ogden, G. D., 264 Ogden, J. A., 18 Okamura, H., 194 O m a n , A., 90, 113, 188, 201,

202, 204, 209, 230 Pachella, R. G., 55, 287, 321 Paillard, J., 254 Paivio, A., 394 Papcun,. G., . 343 Parsons, 0. A., 6, 75, 118,

349 Penry, J. K., 194 Perriment, A . D., 186, 187 Petropoulos, H., 245, 254 Pew, R. W., 280, 290 Pfefferbaum, A., 201, 205, 233,

235 Picheny, M. A., 345 Picton, T. W., 190, 193 Pierson, J. M., 270 Pieters, J. M., 75 Pishkin, V., 6, 75 Pisoni, D. B., 345 Poffenberger, A. T., 486 Polich, J., 233, 235, 253 Posner, M. I., 9, 11, 13, 264,

266, 279, 281 Poulton, E. C., 398 Pouraghabagher, A. R.,

20,

36, 69 Preilowski, T. E., 327 Prescott, T. E., 150 Pribram, K., 264 Prinz, W., 311 Proctor, R. W., 18, 75, 159,

163, 166-171, 173-177, 188, 189, 201, 288, 291, 307, 315, 333, 485 Provine, R. R., 150, 155 P r y t u l a k , L. S., 443 Puleo, J. S., 75 Putnam, C. A., 329 Pylyshyn, Z. W., 394

75, 201, 205, 206, 210, 215, 225-231, 233, 235, 254, 255, 485, 486 Rapcsak, S. Z., 266 Rapp, P. E., 190 Rastatter, M. P., 75 Razran, G . , 73 Reeve, T. G . , 18, 75, 159, 163, 166-171, 173-177, 188, 189, 288, 291, 307, 315, 333, 485 Regard, M., 123 Reich, P. A., 354 Remond, A., 75, 206, 225, 226, 229, 230, 235 Renault, B., 75, 206, 225, 226, 230, 233, 235 Repp, B. H . . 344 Requin, J., .316 Rhoades, M. V., 14 Riggio, L., 75, 102, 1 1 1 , 118, 119, 131, 133, 146, 158, 271 R i t t e r , W., 190, 193, 198, 200 Rizzolatti, G., 75, 91, 94, 99, 118, 145, 148, 163, 185, 261, 262, 266, 271 Robertson, D., 211, 214 Robertson, S. P., 413 Robinson, C. P., 392, 393 Rodriguez, G., 306 Roediger, H. L., 368 Rogers, D. E., 282, 297 Rohwer, W. D., 73, 92, 443 Roscoe, S. N., 370 Rosenbaum, D. A., 164, 280, 282-287, 289-291, 297, 298, 307, 311, 313, 315-317, 324, 328, 331, 332 Rosenbloom., P. S., 428, 436, 444, 455, 488, 492 Roth, K . , 329 Roth, W. T., 201 Rubin, P., 280 Ruchkin, D. S., 190, 198, 253 Rudell, A. P., 19, 20, 31, 75, 76, 118, 119 Ragot, R.,

Author Index

501

Rugg, M. D., 194-197, 201, 215 Rumelhart, D. E., 354, 355 Runquist, W. N., '443

Silver, K . , 255 Simon, C . W., 18 Simon, H. A., 386, 395, 396 Simon, J. R., 17, 19, 20, 22,

Salamy, A., 194 Salapatek, P., 253 Saltzman, E . L., 332 Salvendy, G., 8, 10 Sanders, A . F., 91, 120, 226,

31, 33-37, 39, 42, 45-59, 62, 65-69, 71, 73-78, 90-92, 94, 97, 107, 108, 117-120, 126, 133, 145-147, 149, 155, 163, 170, 187-189, 216, 229,.234, 235, 248, 249, 251, 254, 261263, 267, 270, 483 Simson, R . , 193, 200 Slivinske, A . J., 5 Slotta, J. D., 317, 324 Sly, P. E., 17, 66, 90, 145, 187, 229, 263 Small, A . M., Jr., 19, 35, 39, 42, 47, 48, 50, 52, 54, 56, 57, 64, 65, 74, 78, 91, 119, 229, 230, 234, 235, 262 Small, A. M., Sr., 4, 21, 117, 483 Smith, E. E., 14, 16 Smith, F. O., 185 Smith, L. C., 186 Smith, P., 75, 246, 250, 251 Smith, R. P., 14 Smyth, M. M., 297 Soderberg, G. A., 332 Soetens, E., 126 Sorkin, R . D., 365, 366 Southard, D . L., 329 Speidel, C. R., 59, 107, 120 S p e r r y , R. W., 146 Spijkers, W. A. C., 297, 299, 300, 302, 303, 306 Squires, N . K., 190, 193 Stanovich, K . E., 287 Stern, M., 194 Sternberg, S., 18, 55, 69, 78, 91, 120, 226, 282, 286, 298, 299, 306 Stevens, A., 395 Stevens, K . W., 343, 348 Stevens, S. S., 10 Steyvers, F., 297, 299, 306 Stoffels, E. J., 75, 119 Strain, G. S., 14, 15 S t r a y e r , D. L., 201 Studdert-Kennedy, M. G., 343 Stuss, D. T., 190, 193 Sudalaimuthu, P., 31, 145 Sutton, S., 198, 226

298, 299, 303, 305-307, 311 Sandow, B., 244, 254 Sandry, D. L., 389, 390, 488 Sannit, T., 191 Santee, J., 350 Scandolara, C., 266 Schaefer, C. F., 75 Scheirs, J. G. M., 315 Schlosberg, H., 8, 74 Schmidt, R. A., 279, 280, 283,

290, 297, 306, 329 329 94, 96 394, 396, 397, 403, 419 Schoner, G . , 332 Schroeder-Heister, P., 20, 75, 117, 120, 122, 126, 128, 134136, 485 Schultz, D. W., 201 Seeger, C. M., 4, 5, 13, 19, 21, 31, 50, 56, 58, 89, 117, 120, 164, 183, 234, 261, 298, 346, 366, 448, 449, 451, 452, 455, 458, 484 Sekiyama, K . , 148 Semjen, A , , 316 Senders, J. W., 245 Senders, V. L., 1 1 Shankweiler, D. P., 343 Shapiro, D. C., 286, 290, 315 Sheldon, P. E., 75 Shepard, R. N., 350, 352, 353, 356, 428 Shephard, M., 20, 75, 94, 117, 145, 187, 241, 246, 261, Sheridan, T. B., 59 Sherwood, D. E., 329 S h i f f r i n , R. M., 94, 96, 290, 397 Shipp, T., 74 Shostakovich, G. S., 253 Shulman, H. G., 18, 75, 386 Sidorsky, R. C., 5, 18 Schmidtke, H., Schneider, R., Schneider, W.,

Author Index

502 Swanson, J. M.,

74, 75, 185,

186 Switzer,

G.,

8, 10, 14

Tassinari, G., 99,138, 185, 262 Taub, E., 327 Taylor, D . A., 55 Teichner, W. H., 20, 106, 183,

285 Theios, J., 299 Thomas, J. P., 10 Thurmond, J. B., 14, 15, 18 T i n b e r g e n , N., 73 Tressoldi, E. P., 103, 118 Tueting, P., 198 T u r v e y , M. T., 281, 292 Tzeng, 0.J. L., 19, 75, 120,

188 20, 74-76, 89-91, 94, 95, 97-100, 102-110, 118, 120, 132, 138, 146, 148, 155, 160, 163, 185, 187, 188, 216, 261-263, 267, 271, 484, 490, 491

Umilta, C.,

Valenstein, E., 263-266 V a n Boxtel, A., 299 V a n Cott, H. P., 10, 21 v a n Dellen, H. J., 198 V a n Den Abell, T., 266 Van den Boogaart, B., 299 v a n d e r Molen, M., 75, 119 V a n Galen, G . P., 297 Vaughan, H . G., 192, 193, 200 Vercruvssen, M. J.. 280 Verfaeliie, M., 108; 261, 264,

267, 269, 486, 487 V i c k e r s , D., 319, 320 Vidulich, M., 389, 390, 488 Vilapakkam, S., 17, 66, 90,

Walter, A., 297, 303, 306 Wapner, S., 155 Ward, A. W., 74, 210 Ward, P . , 254 Warm, J. S., 3, 11, 14, 483,

484 Warrick, M. J., 242, 243, 245 Watson, R. T., 263-266 Webster, J. R., 48, 49, 51, 57,

73, 76 Welford, A. T.,

14, 17, 106,

132 Wenegrat, B . G., 201 Westerman, J. A , , 150, 155 Whelan, H., 266 Whitaker, L. A . , 19, 20, 75,

188 8-10, 21, 201, 389-391, 393, 397, 403, 404, 419, 488, 491 Wicker, J. E., 392 Wier, C. C., 349 Williams, J. P., '443 Williams, M., 395 Williges, B. H., 10 Williges, R . C., 10 Wilson, L. E., 270 Wish, M., 407 Wolf, J. D., 31, 117, 126 Wolpaw, J. R., 194 Wood, C. C., 193, 211 Woods, D. L., 193 Woodson, W. E., 10, 2, 242-244 Woodworth, R . S., 8, 74, 186, 346 Woody, C . D., 225, 230, 232. 233, 236 Wright, C. E., 298, 306 Wyatt, E . P., 281

Wickens, C . D.,

145, 187, 229, 263 Viviani, P., 328 v o n Holst, E . , ' 346 Vyas, S. M., 331, 332

Yarita, M., 211 Y i n g l i n g , C. D., 226 Young, A . W., 69-71, 74

Wakelin, D . R., 331 Wakita, H., 343 Wallace, R. J., 6, 17, 19, 75,

Zelaznik, H. N.,

76, 91, 97, 100, 101, 106, 107, 110, 117, 118, 120, 126, 146, 148, 149, 158, 163, 248, 262, 263, 484

279, 281-283, 285-287, 290, 291, 297, 299, 307, 313, 315, 328, 329, 487 Ziglar, R. A., 39, 50, 52, 119 Z u b i n , J., 226

503 SUBJECT INDEX

432, 435, 436, 438, 441, 446, 447, 456, 458, 488 acceleration, 398-401 accumulator models, 320 accuracy, 4, 89, 153, 183, 190, 201, 202, 208, 209 241, 268, 269, 313, 314, 316, 317, 320, 321, 325, 328, 329, 331, 335, 352, 358, 374, 396, 428, 456, 493 acoustic cues, 344, 348, 349 action, 10, 31, 211, 264, 270, 280, 281, 289-291, 311, 372, 383, 384, 395, 403-406, 429, 430, 436 abbreviations,

advance-specification assumption,

315-318, 320-322, 324, 327, 331 afferent, 193, 210-212, 214, 215, 265 age, 69, 70, 150, 153, 155, 193 aging, 20, 69, 70, 78 aimed movements, 284, 329-331 aircraft, 370-372, 375, 376, 378, 390, 391 algorithms, 431, 432, 434-439, 443, 446, 447, 450, 452, 454, 457, 458, 488 alphabets, 3-11, 17-22, 484 anatomical distance, 122, 123, 125, 133 anatomical factors, 99, 121, 138 apparent position, 40, 119 arousal, 225, 264 articulatory, 343, 344 Artless, 432, 434-437, 442, 447 attention, 18, 76, 105, 107, 108, 122, 123, 130, 196, 214, 215, 229, 242, 263-268, 270, 271, 280, 283, 290, 383, 390, 393, 486, 489, 490 auditory, 5, 6, 8, 17, 31, 34, 36, 45-50, 55, 56, 58, 60, 71, 73, 74, 76, 77, 91, 105, 118120, 192, 193, 210, 333, 343349, 352, 353, 355-358, 391393, 491 automatic processing, 96, 97, 105, 106, 397

biases, 63, 75, 76, 90, 185, 255 bimanual reactions, 120, 128,

1 33 190, 199, 225, 235, 255, 265, 270, 490 b u f f e r , 62-64, 75, 76, 78, 299, 489, 490 brain,

categorical perception, 344, 348 central processing, 8, 62, 389-

393, 397, 419, 488 central nervous system,

225,

327, 336 cerebral lateralization,

128, 129,

138 20, 145, 149, 150, 153, 155, 156, 159, 379, 485 choice reaction, 60, 89, 106, 127, 145, 170, 186, 195, 196, 201, 208, 261, 346, 436 coarticulatory information, 346, 350 cockpits, 380, 392 coding, 3, 6, 8, 20, 75, 76, 102, 106, 107, 110-113, 117, 120-122, 126-128, 130-136, 138, 146, 148, 149, 158, 159, 163, 164, 167-171, 173, 174, 177, 178, 189, 262-264, 271, 303, 390, 393, 484, 485, 489492 cognition, 10, 395, 428, 456, 492, 493 cognitive psychology, 3, 22, 31, 311, 386, 394 computer programming, 393, 411, 422, 492 conjunctive preparation, 31 6 consistency, 390, 397-399, 402, 404, 418, 421, 422 consistency task analysis, 398, 402, 404, 421 continuous flow models, 201, 202, 205, 206, 210, 318 contralateral, 46, 53, 99, 101, 150, 155, 194-197, 206, 208, 210-212, 214, 215, 227, 228, 236, 254, 262-267, 270, 327, 486, 487, 490 children,

Subject Index

504 control panel,. 47, 48, 56, 427 control led processing, 94-96,

105, 390, 397 22, 31, 73, 147, 148, 242, 243, 245, 298, 346, 347, 372, 374-378, 380, 381, 383, 384, 486, 487 correspondence, 4-6, 8, 9, 17, 19-22, 31, 36, 45-47, 50, 63, 64, 66-69, 73, 77, 78, 91, 106, 145-149, 153, 155, 170, 175, 176, 178, 183, 187, 188, 241, 245, 246, 263, 376, 483, 484, 489, 493 criterion, 6, 202, 319, 336 controls,

cross-commissural communication,

184, 486 crossed arms,

46, 126, 132,

133, 135-137 crossed fingers, 133 crossed-hand effect, 101, 111,

146, 153, 155 crossed sticks, 119, 133 cuing, 181, 264, 268, 269, 487 decision making, 40, 458 design practices, 9 designers, 73, 78, 242, 456,

483 detection,

185, 186, 190, 197,

319, 349 90, 91, 94, 96, 102, 109, 159, 485 directional cues, 10, 73, 74, 78 discriminability, 7, 18, 55, 69, 149, 201, 215 dimensional overlap,

display-control arrangement correspondence, 66-69, 78 displays, 22, 31, 36, 50, 58,

73, 74, 94, 147, 148, 165, 242-245, 345, 370, 372-376, 378-380, 382, 384, 393, 402, 404, 422, 483, 491 dual tasks, 392 duration, 60, 184, 185, 194, 230, 284-286, 289, 298-300, 303-307, 328-331, 346, 432, 435, 441, 443, 444, 450, 454, 492 electoencephalogram (EEG),

225, 255

190,

100-102, 106, 111, 112, 118, 119, 126, 131-133, 146, 158, 159, 247, 248, 263, 484 -486 encoding, 4, 54, 55, 69-71, 78, 120, 136, 149, 242, 299, 345, 347, 432, 435, 436, 444, 445, 450, 454 endogenous components, 193, 197, 215 engineering model, 427, 428, 444, 455, 456, 492 ensembles, 3-6, 11, 13-17, 20, 22, 90-92, 94, 96-98, 102, 103, 105-108, 111, 112, 183, 298 environment, 150, 153, 253, 264, 271, 387, 390-392, 395, 404, 418, 421, 422, 430, 436, 458, 488 evoked potentials (EPs), 193, 197, 225, 230, 235, 236, 428 effector,

event-related potentials, (ERPs)

189, 190, 192-195, 200, 203, 204, 211-215, 225, 236, 237, 485 errors, 4, 51-53, 7, 123, 153, 154, 156, 157, 242, 252, 270, 303, 313, 317, 318, 322, 332, 351-353, 370, 374, 380-382, 389, 391, 413, 415, 418, 427, 455, 456, 458, 487, 488 excitation, 254, 255, 356, 357, 486 execution time, 439-441, 456, 457 exogenous components, 193, 197 experts, 365, 386, 393, 398, 402, 404, 407, 411-415, 418, 421, 422 extraverts, 254, 255, 486 eye movements, 184, 255 34, 36, 37, 48, 107, 110, 137, 297, 298, 347-350 fast movements, 298, 299 feedback, 279, 347, 358, 412, 457 figure-ground, 370, 371 finger compatibility, 120, 123, 126 finger distance, 122, 125, 137, 138, 485 facilitation,

Subject Index f i r s t - o r d e r system, 399 Fitts law, 284 foreperiod, 300, 303-306 frames, 135, 147, 365, 382-387,

395, 396, 488, 491 frequency-space stereotype,

56-

59, 78 102, 131, 366, 368, 369, 413, 418, 428, goals, operators, selection rules 428, 432, 436, 488. 492 goals,

133, 158, 345, 394, 395, 411441, 488 methods, and (GOMS), 413, 455, 456, 458,

184-188, 194, 196, 197, 215 hand compatibility, 120, 133, 134, 136 hand placement, 165-176 handedness, 31, 32, 149, 150 hardware, 3, 129 head tilt, 135, 136 hemispace, 187-189, 215, 263270, 486, 487, 490 hemispatial neglect, 265, 266, 486 hemispheres, 19, 262, 266, 486, 490 Hick's law, 17 hierarchical model, 117, 119, 131-133, 136, 138, 287, 288, 485 hierarchy, 178, 365, 386, 394, 411-413, 418, 485, 491 high workload, 403, 418 homologous coupling, 331, 332, 334 human -computer interaction , 413, 428, 436, 441, 492 human factors, 3, 4, 8,31 365368, 385, 386, 393, 422, 458, 484, 488, 491 human-machine systems, 3, 10, 20, 21, 389 human performance, 8, 22, 390, 427, 456, 458, 493 half-field,

505

images, 148, 395 incompatibility, 118,204,

205, 229-231, 233, 234, 237, 241, 248, 251, 255, 311, 369, 373, 375, 378, 381, 389, 427, 486488 individual differences, 251, 255, 257, 384, 421, 427, 486 information processing, 10, 11, 18, 31, 34, 36, 39, 47, 49-52, 54, 58-60, 70-74, 76-78, 137, 163, 183, 201, 210, 226, 235, 253, 264, 266, 283, 291, 298, 299, 318, 366, 382, 383, 385, 389, 393, 403, 483, 489, 491, 493 information t r a n s f e r , 31, 56, 120, 183 information transmission, 7, 11, 12, 59, 226 inhibition, 110, 137, 202, 254, 255, 297, 327, 354, 357, 486 initial-response time, 439-441, 457 input, 183-185, 193, 210, 215, 264, 265, 311, 354-356, 358, 389, 391, 392, 396-402, 404406, 408, 491 intensity, 43, 44, 77, 150, 155, 193-197, 225, 303, 306 intention, 264-267, 325, 486, 490 interactive-activation model, 354, 355, 359, 488 interference, 20, 34, 36-39, 48, 54, 73, 76, 94, 96, 111, 137, 234, 326, 327, 336, 393, 486 interhemispheric transmission, 184, 186, 215, 262, 486 intermanual interactions, 311, 312, 327-329, 331-337 introverts, 255, 486 ipsilateral, 33, 34, 46, 53, 76, 99, 150, 155, 185, 186, 194, 196, 197, 208, 215, 249, 254, 261, 262, 265-267, 270, 336, 490 judgment,

102

74, 350, 391, 407,

Subject Index

506 118, 119, 126, 130, 131, 485 kinesthetic, 6, 17 key position,

knowledge extraction techniques,

404 left-right dominance, 103-105 linguistic units, 343 logical recoding, 64, 66-69 long-term memory, 9, 356, 433,

437, 438 mental chronometry,

189, 190,

199, 236 389, 390, 393399, 402-407, 411, 412, 415, 418, 419, 421, 422, 488, 491, 492 mental representation, 148, 149,

oculomotor, 6, 17, 19 operators, 245, 345, 369, 370,

374, 376-378, 380, 381, 386, 389, 395, 397, 398, 402, 404, 421, 427, 428, 430, 432-435, 438-440, 442, 443, 446, 447, 450, 452-458, 488, 491 orienting reflex, 73, 119 orthogonal S-R relations,. 126, 128 orthographic , 233 output, 35, 184, 185, 187, 193, 230, 264, 355-358, 369, 391, 392, 396-399, 404-406, 411

mental model,

484 model human processor (MHP),

428-430, 432, 434-436, 439, 441, 442, 450, 456, 458, 488, 492 monaural, 35, 36, 39, 40, 4648, 50, 52, 61, 63, 68, 69, 71, 74, 194 motor control, 10, 279, 280, 291, 297, 327, 336, 344 motor performance, 279, 484, 487 motor preparation, 227, 228, 236, 264, 306, 333, 336 motor programming, 225, 230, 254, 279-283, 285, 286, 288292, 297-299, 303, 305-307, 312, 348, 353, 487 motor theory of speech perception, 343, 358 movement features, 280 multiple preparation, 315, 324 multiple sub-tasks, 375 nasal/f ricative distinction,

352,

353, 488 127, 138, 146, 183-185, 196, 262 neurons, 190, 267, 271 novice, 407, 408, 410, 411, 414416, 418, 419, 422, 442 nuclear power plant, 372, 377, 378, 380, 382 numerals, 6, 8, 11-14, 454, 455 neuroanatomical,

P300, 193, 194, 197-202, 204206, 210, 214, 215, 226, 228235. 237. 485. 486 perception ’ 50, .343-349. 353-359, 431, 439, 443, 450, 488 perceptual-motor priming procedure, 347, 352-354, 356 perceptual salience, 349, 350,

352, 353, 356 127, 130, 133, 135, 138, 147, 149, 156, 167 personality, 254 phonetic perception, 344, 345, 358 phonological, 233 physical correspondence, 5, 6, 8, 483, 484, 489 pilots, 370, 375, 392 place features, 348, 349, 357 plans, 358, 394 population stereotype, 9, 10, 89, 119, 148, 245, 367, 377, 146 practice, 4, 9, 13, 14, 16, 51, 52, 119, 123, 129, 151, 175177, 201, 203, 205, 248, 251, 289, 290, 301, 302, 333, 365, 366, 368, 381, 385, 397, 398, 403, 421, 434, 437, 456 precuing, 164-171, 173, 175177, 279, 282-291, 298, 299, 307, 312, 313, 315, 317, 324, 327, 332-334, 485, 487, 491 preparatory processes, 271, 315 problem solving, 395, 404, 411, 412, 421, 428 perpendicular,

Subject Index 346, 347, 395, 491 nteractions .Droarammina-i assumptick, 315, 318, 320, 322, 324, 327, 331 prone, 121, 127, 132, 133, 137, ' 138; 485 . psychophysiological measures, 183, 202, 210, 484-486 production systems,

random-walk models, 320 readiness potential, 190, 206,

215, 226, 236 10, 20, 64, 66-69, 183, 249-251, 257, 298, 302, 303 response activation, 201, 202, 206, 210 response alternatives, 282, 284, 286, 287, 311, 315 response competition, 202, 205, 206, 486 response position, 112, 119, 128, 135, 147, 197 response selection, 54, 62-64, 70, 71, 78, 106, 108, 110, 111, 113, 163, 186, 216, 226, 253. 261, 270, 279-284, 286, 289, 297-299, 306, 347, 486, 487, 490 response set, 89-92, 94, 96, 101-104, 106, 109, 110, 112, 148, 158, 169, 170, 174, 175, 177, 178, 183, 189, 200, 201, 264, 283, 449, 484, 485, 487 response tendencies, 51, 53, 58, 59, 242, 245, 255, 366, 378, 382-387, 488, 491 retrieval, 290, 291, 431, 438, 439, 441, 443, 444, 457, 458 rules, 102, 241, 245, 250, 251, 343, 365, 378, 380, 382-387, 395, 396, 404-406, 421, 428, 446, 451, 488, 491 recoding,

S - R relations, 50, 64, 65, 126, 128, 147, 229, 234, 237, 241, 298, 302, 314, 375, 376, 378, 381, 383, 439, 488 saccadic eye movements, 184 salient features, 164, 170, 178, 356, 485, 491, 493 schema, 249, 254

507

398, 399, 403, 405 selective adaptation, 328, 345, 348 selective attention, 18, 214, 215, 264 sensory, 37, 39, 47, 53, 77, 119, 184, 185, 193, 210-212, 225, 226, 228, 242, 265-, 354, 430, 483 sentences, 241, 343 short-term memory, 8, 9, 96, 242, 358 similarity, 105, 241, 243, 249, 313, 319-322, 404, 407, 421,422 Simon effect, 19, 20, 22, 31, 36, 37, 52-55, 58, 62, 65-69, 71, 73, 75-78, 90, 145, 146, 149, 188, 189, 216, 234, 235, 249, 251, 254, 261, 262, 267, 270, 483, 484, 486, 487, 490, 492 simple reaction, 184-187, 194, 196 s imu Ita neou s movements , 312, 327, 329, 330, 337 single preparation, 316, 317 skilled behavior, 9, 150, 390, 397 slow movements, 297, 299, 304, 306, 332 software, 3, 10 spatial distance, 122, 123, 133, 485 spatial representation, 263, 271, 391 spatio-anatomical mapping, 117, 126, 128, 130-136, 138, 485, 489, 490 speech, 8, 31, 74, 343-348, 350, 353-359, 391-393, 397, 488, 491 speed-accuracy tradeoff, 313, 317, 352 stages of processing, 54, 59, 71 stereotype, 4, 9, 10, 21, 31, 34, 42, 48, 51, 55-60, 62, 74, 77, 78, 89, 119, 245, 262, 367, 377, 380 stop/nasal distinction, 350, 352 strategies, 291, 345, 412-414, 418, 434 second-order system,

508

Su@ect Index

4, 21, 94, 170, 290, 381, 488 supine, 121, 122; 127, 132-134, 137, 138. 485 symbolic stimuli, 159, 163, 164, 170, 171, 175, 177, 178, 189, 456, 485, 487, 489, 491

t y p e IV ensembles,

190, 312, 313, 317, 325, 326, 328, 329, 331 task analysis, 398, 402-405, 421, 430, 431, 434, 436, 444 technology, 3, 297, 365, 369 temporal overlap, 325-327 timing, 98, 106, 193, 201, 205, 206, 210, 215, 216, 226, 229, 234, 235, 254, 281, 290; 328; 329, 331, 348. 349 tracking, .284, '393, 398, 399, 402-406, 421, 422, 304, 378, 380, 381, 395, 397, 398, 402, 403, 405, 418, 421, 422, 442, 488 trajectory, 279, 328 translation, 20, 31, 59, 75, 106, 107, 110, 112, 113, 163, 164, 170, 175, 177, 183, 216, 284-286, 288, 289, 291, 297, 299, 300, 303, 391, 484, 487, 489, 493 type I ensembles, 90 type II ensembles, 91, 94, 96, 98, 102, 103, 105-108, 111, 112 type I l l ensembles, 91, 96-98, 108

velocity,

stress,

tapping,

92, 96, 98, 103, 105-108, 111, 112

underadditivity, 287, 288, 290 unilateral neglect, 264 unimanual reactions, 120

297-306, 398-403, 405,

406, 487 90, 145, 149, 150, 156, 159 verbal protocols, 396, 404, 411413, 421 visual field, 36-38, 60, 74, 91, 93, 97, 99, 100, 109, 117, 119, 120, 122, 125, 127-129, 145, 146, 148, 186, 188, 215, 265, 266 visual stimuli, 5, 18, 20, 55, 62, 68, 71, 94, 99, 105, 147, 155, 188, 193, 347, 453 vocal responses, 8, 11-13, 14, 19, 128, 347, 351-354, 356, 357, 491 voicing features, 347, 349, 353, 357 verbal labels,

Warrick's Principle,

242, 243,

245 working memory, 8, 429, 430 workload, 4, 21, 390, 392, 403,

418-420 zero-order system,

399