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The Relationship Between Visual Illusions and Cues to Distance

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The Relationship Between Visual Illusions and Cues to Distance Durham E-Theses The relationship between visual illusions and cues to distance Green, Frederick A. How to cite: Green, Frederick A. (1972) The relationship between visual illusions and cues to distance, Durham theses, Durham University. Available at Durham E-Theses Online: http://etheses.dur.ac.uk/9323/ Use policy The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that: • a full bibliographic reference is made to the original source • a link is made to the metadata record in Durham E-Theses • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders. Please consult the full Durham E-Theses policy for further details. Academic Support Oce, Durham University, University Oce, Old Elvet, Durham DH1 3HP e-mail: [email protected] Tel: +44 0191 334 6107 http://etheses.dur.ac.uk THE RELATIONSHIP BETWEEN. VISUAL ILLUSIONS AND CUES TO DISTANCE by Frederick A. Green A Thesis Presented for the Degree of Doctor of Philosophy ACKNOWLEDGEMENT I would like to thank all those who have helped in the preparation of this thesis. In particular, I would like to thank Dr. A. W. Still for his patient supervision and Prof. W. B. Templeton for his advice and assistance. I am indebted to the Science Research Council for prov• iding me with financial support. Lastly, I would like to thank my wife for the many ways in which she has contributed. ABSTRACT The theory of R. L. Gregory that certain visual illusions are caused by the inappropriate action of a constancy-scaling mechanism was critically examined. Several unsuccessful attempts were made to replicate his experimental findings that certain ambiguous figures, such as the M-L illusion, appear 3-dimensional in a particular way when presented in reduced cue conditions. It was noted that the depth effect reported by Gregory was not large enough to explain all the illusory distortion in his figures. It was suggested that this might be because his apparatus allowed certain cues which could be used to determine the true form of the figures and thus destroy or reduce any 3-dimensional effects. The experimental results suggested that this was not so. In later experiments it proved possible to repeat Gregory's results only by inducing Ss to adopt a specific perceptual set. If this was not done Ss tended to see the figures in different ways which often changed over time. Combined analysis of the results of all Ss on many different figures showed a slight tendency for the central part of any Gestalt or figure to appear nearer than other parts. Two possible hypotheses were advanced to explain this result but further experimentation suggested that both were inadequate. Experimental evidence is provided that the Ponzo illusion is the result, of a shrinkage of the lower line rather than an expansion of the upper line, as is generally thought. This and other evidence is interpreted as suggesting that even this illusion may not have a perspective component. Taken as a whole the results suggest that any perspective theory of the illusions will prove inadequate. It is finally suggested that further research be directed towards inhibition type theories. a TABLE OF CONTENTS Abstract i PART I - Introduction and an Overview of Gregory's Theory of Illusions A summary of Gregory's position 2 A note on Gregory's treatment of the older illusion theories h Gregory's theory examined in greater detail 9 The anthropological argument 21 Primary and secondary scaling 38 Gregory's apparatus, his experiments and related topics 3>0 An assessment of the likely effectiveness of the cues to distance present in Gregory's apparatus 65 Some notes on the size-distance hypothesis 79 PART 2 - Experimental Attempts to Replicate Gregory's Results A note on statistical procedure 86 Experiment 1 89 Experiment 2 120 PART 3 - Experiments Concerning the SDIH and the AFPP Experiment 3 lii7 Experiment h 168 Experiment £ 192 PART k - The Effects of Instruction Using Gregory's Apparatus Experiment 6 20f? An introduction to Experiment 7 222 Experiment 7 23li Experiment 8 2U7 PART 5 - Visual Acuity and Phenomenal Length Differences Experiment 9 263 PART 6 - Summary and Conclusions 286 Appendix 291 Bibliography 312 n PART 1 INTRODUCTION AND AN OVERVIEW OF .GREGORY'S THEORY OF ILLUSIONS 112 APS f 972 *-.Jk$'.. A SUMMARY OF GREGORY'S POSITION A brief summary of Gregory's position is presented at this point in the hope that it will help the reader to keep in mind the theory as a whole and that he will therefore more easily discern the relevance of the ensuing discussion to its various parts. Basic to the theory is the notion of size-constancy. This may be thought of as an internal scaling mechanism which allows size perception to remain constant even though an object's ret• inal image size changes with changes in distance. This involves the corollary that if two objects subtend the same retinal angle then the one with the greater apparent distance will be seen as larger. This is what is thought to happen in the illusion figures. Although they are flat, Gregory believes that perspective cues exist in them which trigger the constancy mechanism. Of course, in this case its functioning is inappropriate because no actual differences in depth exist, hence the distortions. The older perspective theories were very similar to this in regarding the illusion figures as flat representations of 3- dimensional objects. However, they made no attempt to explain how flat figures - and subjects do report that the figures are flat - can trigger a mechanism which is assumed to work only when differences in apparent depth are seen. Gregory extends his theory to meet this difficulty and to this end he postulates that two independent types of constancy exist. He calls them primary scaling and secondary scaling. Secondary scaling is thought to be the more normal type of constancy, well documented in the literature, which functions J. / 2 simply according to apparent distance - as set out in Emmert's Law (l88l). Primary scaling is thought to be "set directly by depth cues, even when these are countermanded by background texture so that the figure appears flat. " Were we to remove these "countermanding" background cues, the figures would then give the differences in apparent depth necessary to account for the distortions. This is testable by removing these other cues and measuring any differences in apparent depth which then occur. Background cues are easily disposed of by using luminous models in a dark room and viewing them with one eye. A large part of Gregory's writing is devoted to establishing the independence of these two scaling mechanisms. One further assumption is necessary to complete the theory. As mentioned above, illusion figures are regarded as flat repres• entations of 3-d.imensional objects, yet any illusion figure may give rise to several different depth interpretations e.g. the •long' Muller-Lyer arrow can be interpreted either as an open book or as a house roof viewed from above. These two views place the 'distorted1 shaft of the figure as alternately further and nearer than the rest of the figure. A strict perspective theory would demand that the direction of the illusion should reverse according to which interpretation was entertained. This does not happen. Gregory explains this by assuming that one particular view is more 'typical' than any other i.e. we experience it much more frequently than any other, and therefore it is this view which fixes the direction of the illusion permanently. This is known as the 'typical view' hypothesis. The majority of this thesis is concerned with examining the validity of this theory. 3 A NOTE ON GREGORY'S TREATMENT OF SOME OF THE OLDER ILLUSION THEORIES (see Gregory 196^ P 76=79) It was not until the late I9th. century that scientists began to notice the discrepancies between appearancesand actuality which we now refer to as the geometric illusion, and began to speculate as to their cause. Carter and Pollock (1968) have recently reviewed the early literature and traced the origins of the controversy which still continues today. After an initial surge of activity, which lasted several years, interest died down somewhat. However, there has recently been a sharp increase in the number of papers published on the topic (Zusne, I968) and it would seem that Gregory's theory has been instrumental in this. Paradoxically some of the new evidence has indicated that some of the older theories, which Gregory lightly dismisses, might not be so sterile after all. For instance, the eye-movement theory, first prompted by Wundt and others, has recently received a new lease of life from a paper by Festinger, White and Allyn (1968). It has been well documented that the M-L and other illusions decrease with repeated exposure (Judd and Courten,I905; Mountjoy, 1958, 1961, 1963, 1966; Mountjoy and Cordes, 1958) while Lewis noted as early as 1908 that eye-movements seemed necessary for this decrement to occur. With an exposure too fast for eye-move• ments the illusion remains undiminished. Festinger et al. replicated these results and photographed eye-movements. Saccades across the perceptually short side of the figure were found to be shorter than those across the perceptually 4 long side. Free inspection of the figure allowed the saccades to •even up1 but this did not occur if subjects fixated one point during the inspection period.
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