Seeing Black and White Alan Gilchrist OXFORD UNIVERSITY PRESS Seeing Black and White This page intentionally left blank SEEING BLACK AND WHITE Alan Gilchrist 1 2006 1 Oxford University Press, Inc., publishes works that further Oxford University’s objective of excellence in research, scholarship, and education. Oxford New York Auckland Cape Town Dar es Salaam Hong Kong Karachi Kuala Lumpur Madrid Melbourne Mexico City Nairobi New Delhi Shanghai Taipei Toronto With offices in Argentina Austria Brazil Chile Czech Republic France Greece Guatemala Hungary Italy Japan Poland Portugal Singapore South Korea Switzerland Thailand Turkey Ukraine Vietnam Copyright ᭧ 2006 by Oxford University Press, Inc. Published by Oxford University Press, Inc. 198 Madison Avenue, New York, New York 10016 www.oup.com Oxford is a registered trademark of Oxford University Press 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 permission of Oxford University Press. Library of Congress Cataloging-in-Publication Data Gilchrist, Alan. Seeing black and white / by Alan Gilchrist. p. cm. (Oxford psychology series; no. 40) Includes bibliographical references and index. ISBN 0-19-518716-4 ISBN-13 978-0-19-518716-8 1. Imagery (Psychology). I. Title. II. Series. BF241.L54 2006 153—dc22 2006006283 987654321 Printed in the United States of America on acid-free paper This book is dedicated to the memory of Irvin Rock, my wonderful mentor and warm friend. This page intentionally left blank Foreword And God said let Gilchrist be and all was light. —V. S. Ramachandran I feel honored to be asked to write this foreword for Professor Alan Gilchrist’s book, as I have followed his work on perception with keen interest over the years. The study of perception has a long and venerable history dating back to the great 19th-century German physicist, physiologist, and ophthalmologist Hermann Von Helmholtz, who more than anyone else pointed out that the visual image is inherently ambiguous. A circular image in the retina can be produced by an infinity of oval objects of different widths, each tilted by a certain amount. Black print in sunlight reflects more light than a white page seen in artificial light at night, yet it looks black. The only way the brain could solve such a problem, Helmholtz realized, was by using certain built-in knowledge or “assumptions” about the statistics of the natural world (“built in” can either mean built in by learning or by genes; Helmholtz empha- sized the former, and Hering, the latter). These assumptions are used by the visual system to eliminate an infinity of improbable solutions and home in effortlessly on the correct solution. Since these mecha- nisms are unconscious—on autopitot, as it were—Helmholtz called them “unconscious inferences,” a phrase that has never been im- proved on. The study of perception, then, is the study of these as- sumptions and of their implementation in the neural hardware of the brain. One approach to perception is to study visual illusions, which have the same effect on psychologists as the smell of burning rubber on an engineer: an irresistible urge to find the cause (as Peter Medawar once said of philosophers). Illusions are not mere curiosities; they are clev- erly contrived stimuli that reveal the hidden assumptions that drive viii Foreword perception and allow us to discover underlying rules of operation. This book is full of lovely new examples of illusions of brightness and lightness, many of which were discovered by the author. In the last century the experimental study of perception went through three distinct stages. First, in the early 20th century the Ge- stalt psychologists (including Wertheimer, Koffka, Ko¨hler, and Anstis) used astonishingly simple displays to uncover what they called the “laws” of perception, reminding us that great science is driven mainly by ingenuity and experimental cunning rather than fancy equipment. Unfortunately, some of them—although not the best among them— had the habit of calling every observation a “law,” with the air of having actually explained something. (Such explanations, as Peter Medawar might have said, are “mere analgesics that dull the ache of incomprehension without removing the cause.”) It is to Gilchrist’s credit that he doesn’t fall into this trap. But at least the Gestaltists discovered and studied perceptually compelling phenomena; what they did was interesting. The same can- not be said of the next big movement, called (ironically) “classical psychophysics,” which was championed by Stevens at Harvard. This movement, which completely eclipsed Gestalt psychology, was partly the result of the pernicious effects of behaviorism and a general sus- picion of “introspective” psychology. It became unfashionable, for sev- eral decades, to ask the subject what he or she was actually seeing in a visual display. Which is ironic, given that that’s why most of us study perception in the first place! The emphasis was placed, instead, on obtaining detailed quantitative measurements of small second- order effects, in order to plot what are pompously referred to as “psy- chometric functions” (i.e., graphs). A time will come when this whole movement will be seen as a curious anomaly in the history of psy- chology, a manifestation of “physics envy” and the accompanying be- lief that the mere act of measuring something makes it scientific (what I call “researchmanship” rather than research). When I was a graduate student in Cambridge, in the late seventies, the use of any perceptual stimuli, other than blurred stripes, was con- sidered taboo. (To be sure, sine wave gratings were useful stimuli for providing more complete and accurate descriptions of visual perfor- mance than two-point discrimination, resulting in the use of modulation-transfer functions. But it’s fair to say people got a bit car- ried away.) Giles Brindley told us there are two types of experiments: Class A (obtaining “psychometric functions”) and Class B (simply observing carefully what you see—for example, the work of Bela Ju- lesz or Edwin Land). He warned us not to trust Class B “because they are no better than dreams.” (It’s a good thing he wasn’t around when Newton passed white light through a prism, Galileo saw the moons of Jupiter, or Faraday moved a magnet to and fro within a coil. He might have tapped them on the shoulder and said, “This isn’t science Foreword ix until you have obtained a graph,” thereby aborting the birth of phys- ics.) And as for biology, David Hubel once reminded me that the most important book ever published—Darwin’s Origin of Species—doesn’t have a single graph in it! This desire to “ape” physics is especially ironic given that physics itself had to initially go through a qualitative “Faraday” stage before it reached a mature quantitative “Maxwell” stage. Psychology has to— and will—pass through the same stages, and there is simply no point in trying to jump ahead. Fortunately, this obsession with blurred stripes turned out to be a temporary aberration. In the last three decades there has been a tre- mendous resurgence of interest in those phenomena that drew us all to the study of perception in the first place. This was spearheaded by Irvin Rock, Richard Gregory, J. J. Gibson, George Sperling, Bela Julesz, Julie Hochberg, Gaetano Kanizsa, and Donald MacKay in the gener- ation previous to mine. Although their work was initially ignored by the “classical psychophysics” types, (remember Kenneth Ogle’s dis- missal of Julesz and Leo Hurvich’s contempt for Edwin Land?), it has now become part of “mainstream” research in perception. The study of perception became interesting once again. When I tell younger col- leagues about the Ogle/Julesz debate, the usual reaction I get is, “Who’s Ogle?” Exactly my point. Following this, there was a sort of “neo-gestalt” revolution—a movement that owes its existence largely to a handful of contempo- rary researchers: Ken Nakayama, Pat Cavanagh, Randy Blake, Chris Tyler, Ted Adelson, Lothar Spillman, and, now, Alan Gilchrist, Dale Purves, and Pawan Sinha. Thanks to all these researchers (and many whose names I’ve left out) there has been a renaissance of interest in such phenomena as illusory contours, occlusion, shape-from-shading, binocular rivalry, apparent motion, and perceptual “constancies”—in- trinsically fascinating phenomena that were eclipsed by “Class A” psychophysics for nearly three decades. (But to be fair to Brindley, Class A research has had its victories, especially in the study of color vision. The discovery of the detailed laws of Trichromacy is one of the great triumphs of visual science, although—as David Hubel noted— its practitioners study it with a passion that seems grossly out of pro- portion to its evolutionary importance.) But it’s the revival of interest in what used to be called “illusions” that has fired the imagination of physiologists (no mean feat!) and AI researchers. If you look at the big picture, these last two decades have been heady times for perceptual psychologists. The neo-gestalt revolution is here! One of the foremost among these revolutionaries is the author of this book. What I especially like about his work is that, unlike many of us who jump around from topic to topic, he has devoted his whole life to the single-minded pursuit of the laws that govern our percep- x Foreword tion of lightness/brightness. Anyone acquainted with Gilchrist knows that he doesn’t merely study lightness; he lives it. (I remember driving around Manhattan with him. Any advertisement or newly painted or oddly illuminated wall becomes an “experiment” for him as he jumps out of the car excitedly, oblivious to the traffic; high on science, high on life, and just high!) The result of his lifelong obsession is this book, the most comprehensive and thorough monograph on this topic that has ever been published, a tome that Helmholtz and Hering would have taken great delight reading in their bathrooms.