McGraw-Hill Paperbacks Third edition: revised and updated World University R.L Gregory Library Eye and the psychology of seeing PRINT GREGORY, Richard Langton 152.1 Eye and brain G Copy 1 PRINT 152.1 G GREGORY, Richard Langton Eye and brain C1978 Copy 1 World University Library The World University Library is an international series of books, each of which has been specially commissioned. The authors are leading scientists and scholars from all over the world who, in an age of increasing specialization, see the need for a broad, up-to-date presentation of their subject. The aim is to provide authoritative introductory books for university students which will be of interest also to the general reader. Publication of the series takes place in Britain, France, Germany, Holland, Italy, Spain, Sweden and the United States. Frontispiece The visual region of the brain — the area striata. Here we see a small part of the nnechanisnn of the brain highly magnified. These cell bodies with their connections handle information from the eyes, to give us knowledge of the world Eye and Brain The psychology of seeing R.LGregory World University Library McGraw-Hill Book Company New York Toronto First published 1966 Reprinted 1966 Reprinted 1967 Reprinted 1969 Reprinted 1970 Second edition 1973 Reprinted 1974 Reprinted 1976 Third edition 1978 Library of Congress Cataloging in Publication Data Gregory, Richard Langton Eye and brain (World university library) Bibliography: p. 1. Vision. I. Title BF241.G7 1973 152.1'4 72-3792 ISBN 0-07-024665-3 paperbound © 1966, 1972 and 1977 R. L. Gregory Photoset by BAS Printers Limited, Wallop, Hampshire Manufactured by LIBREX, Milan, Italy Contents Foreword to the third edition 7 1 Seeing 9 2 Light 15 3 In the beginning . 27 4 The brain 37 5 The eye 49 6 Seeing brightness 77 7 Seeing movement 93 8 Seeing colour 119 9 Illusions 133 10 Art and reality 163 11 Do we have to learn how to see? 189 12 Seeing and believing 219 13 Eye and brain machines 226 Bibliography 241 Acknowledgments 251 Index 253 Digitized by the Internet Archive in 2011 with funding from National Federation of the Blind (NFB) http://www.arGhive.org/details/eyebrainpsycholoOOrlgr Foreword to the Third Edition May I take this opportunity to thank the pubhshers for producing in the first place, and now revising, Eye and Brain. Appearing in 1966, it was the first volume of the World University Library. This was the imaginative conception of Lord Weidenfeld. Production could be made quite lavish, with many colour pictures, by printing in several countries and as many languages simultaneously. The more financially successful books of the series supported the more specialised, which could not be expected to command economic sales with the production standards of this conception. The series was originally edited by Colin Haycraft, who has remained a close and valued friend. Research on eyes and brains - on how our predecessors and ourselves on the incredible ladder of Hfe see and understand - has made exciting advances during the ten years since this book was first written. I can only hope that some of this has been captured and encapsulated in this edition. I have also taken this opportunity to rephrase passages which seemed less than happy. May I thank particularly John Curtis. He was responsible for editing - and made immense contributions to - The Intelligent Eye, which appeared in 1970. With his usual care and imaginative judgment, he has now produced this new edition of Eye and Brain. R.L.G. 1 Seeing We are so familiar with seeing, that it takes a leap of imagination to realise that there are problems to be solved. But consider it. We are given tiny distorted upside-down images in the eyes, and we see separate solid objects in surrounding space. From the patterns of stimulation on the retinas we perceive the world of objects, and this is nothing short of a miracle. The eye is often described as like a camera, but it is the quite uncamera-like features of perception which are most interesting. How is information from the eyes coded into neural terms, into the language of the brain, and reconstituted into experience of surrounding objects? The task of eye and brain is quite different from either a photographic or a television camera converting objects merely into images. There is a temptation, which must be avoided, to say that the eyes produce pictures in the brain. A picture in the brain suggests the need of some kind of internal eye to see it - but this would need a further eye to see its picture . and so on in an endless regress of eyes and pictures. This is absurd. What the eyes do is to feed the brain with information coded into neural activity - chains of electrical impulses - which by their code and the patterns of brain activity, represent objects. We may take an analogy from written language : the letters and words on this page have certain meanings, to those who know the language. They affect the reader's brain appropriately, but they are not pictures. When we look at something, the pattern of neural activity represents the object and to the brain is the object. No internal picture is involved. Gestalt writers did tend to say that there are pictures inside the brain. They thought of perception in terms of modifications of electrical fields of the brain, these fields copying the form of perceived objects. This doctrine, known as isomorphism, has had unfortunate effects on thinking about perception. Ever since, there has been a tendency to postulate properties to these hypothetical brain fields such that visual distortions, and other phenomena, are 'explained'. ; But it is all too easy to postulate things having just the right properties. There is no independent evidence for such brain fields, and no independent way of discovering their properties. If there is no evidence for them, and no way of discovering their properties, then they are highly suspect. Useful explanations relate observables. The Gestalt psychologists did however point to several important phenomena. They also saw very clearly that there is a problem in how the mosaic of retinal stimulation gives rise to perception of objects. They particularly stressed the tendency for the perceptual system to group things into simple units. This is seen in an array of dots (figure 1.1). Here the dots are in fact equally spaced, but there is a tendency to see, to 'organise', the columns and rows as though they are separate objects. This is worth pondering, for in this example we have the essential problem of perception. We can see in ourselves the groping towards organising the sensory data into objects. If the brain were not continually on the look-out for objects, the cartoonist would have a hard time. But, in fact, all he has to do is present a few lines to the eye and we see a face, complete with an expression. The few lines are all that is required for the eye - the brain does the rest: seeking objects and finding them whenever possible. Sometimes we see objects which are not there : faces-in-the-fire, or the Man in the Moon. Figure 1 .2 is a joke figure which brings out the point clearly. Just a set of meaningless fines? No - it is a washer-woman with her bucket! Now look again: the lines are subtly different, almost solid - they represent objects. The seeing of objects involves many sources of information beyond those meeting the eye when we look at an object. It generally involves knowledge of the object derived from previous experience, and this experience is not limited to vision but may include the other senses touch, taste, smell, hearing and perhaps also temperature or pain. Objects are far more than patterns of stimulation: objects have pasts and futures ; when we know its past or can guess its future, an object transcends experience and becomes an embodiment of knowledge and expectation without which life beyond the simplest is not possible. Although we are concerned with how we see the world of objects, it is important to consider the sensory processes giving perception - 10 1 .1 This array of equally spaced dots is seen as continually changing patterns of rows and squares. We see something of the active organising power of the visual system while looking at this figure. 1 .2 A joke figure - what is it? When you see it as an object, not merely meaningless lines, it will suddenly appear almost solid - an object, not a pattern. what they are, how they work and when they fail to work properly. It is by coming to understand these underlying processes that we can understand how we perceive objects. There are many familiar so-called 'ambiguous figures', which illustrate very clearly how the same pattern of stimulation at the eye can give rise to different perceptions, and how the perception of objects goes beyond sensation. The most common ambiguous figures are of two kinds : figures which alternate as 'object' or 'ground', and those which spontaneously change their position in depth. Figure 1.3 shows a figure which alternates in figure and ground - sometimes the black part appears as a face, the white being neutral background, and at others the black is insignificant and the white surround dominates and seems to represent an object. The well-known Necker cube (figure 1.4) shows a figure alternating in depth. Sometimes the face 12 : 1 .3 This figure alternates spontaneously, so that sometimes it is seen as a pair of faces, sometimes as a white urn bounded by meaningless black areas - the faces.