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The Psychology of Visual Illusion

Dover Publications, Inc.

INTRODUCTION

The study of visual illusions dates further back than the recognition of psychology as a separate discipline and a large proportion of all that is in this book was described by Helmholtz (1856) in his Handbuch der Physiologischen Optik. Boring (1942) and Titchener (1901) have both reviewed the early literature. The volume of writing on the subject, particularly the geometrical optical illusions, in the late nineteenth century can give the impression that interest faded after the turn of the century. This impression is expressed by Tolanski (1964), but Zusne (1968) did a count of all such publications over the years. This clearly shows that the number of papers published on the geometrical optical illusions has increased steadily, quite closely in step with the total number of papers published in psychology.

The steadily increasing number of papers has not, however, found an echo in an increase in the publication of books. I have found in the English language only two, Tolanski (1964) and Luckiesh (1922), published since the First World War. Although Tolanski's book is the more recent it is the lighter of the two and is neither extensive nor detailed in its treatment. There seems to be a need for a work drawing together the very large literature on illusions and attempting to assess the various theoretical positions which have more recently been described.

The act of perception may be characterised as a decision which is based largely (though not solely) on information, picked from a display, about the 'real' nature of the display (Gregory, 1970, talks of the perceptual system 'making a bet'). The information consists of cues about, for example, whether the display contains objects, how many objects, whether they are large or small, near or distant. There is also movement information, cues about whether parts of the objects are moving with reference to other parts, whether the objects as wholes are moving, whether the observer's eyes, head, or whole body is moving.

The perceptual decision is biased by states of the organism such as hunger, emotion, expectancy and so on. There is a large literature on such bias, but it is beyond the scope of this book. More important for my argument is ambiguity in the visual display. More than one perceptual decision is possible for most displays. Displays can vary from complete ambiguity to partial ambiguity. The latter are ones in which a particular interpretation is by far the most likely one, but other interpretations are probable and still others, although bizarre, are possible. Paucity of cues, brought about, for example, by the brevity of the period of sampling or by reduced illumination, can increase ambiguity.

One potent source of illusions is misinterpretation of cues in a situation in which there is at least a small amount of ambiguity. It is easy to invent displays in which the ambiguity is permanent and unresolvable (several are mentioned in Chapter 7) and which make the perceptual system continue indefinitely and mysteriously to vacillate between the possible perceptual conclusions about the 'real' nature of the display. But most of the illusions of this sort which one meets in everyday life involve situations which are ambiguous mainly because of brevity of sampling or paucity of cues. In these circumstances the perceptual decision is more difficult and cues arriving after the first perceptual decision often cause a revocation of it. Most of us must have accosted a stranger believing him to be a friend because in the first flash of seeing him we have 'recognised' a friend. Here the first sample of cues was brief. We resolved any ambiguity it contained and made a decision, but subsequent cues showed the decision to be wrong.

Cues in the illusion displays described in this book are generally rather sparse. Line drawings have nothing of the richness we are used to in visual scenes, nor have small lights and luminous lines in the dark. Ambiguity is therefore both more likely to be present and more likely to be resolved in a way which, by other evidence, is erroneous.

Another way in which illusions are thought to occur is by the inappropriate operation of some mechanism which generally works in favour of veridicality (truthfulness). This probably accounts for most of the illusions involving simple line-figures, though the precise nature of the process is unknown. Gregory, for example, supposes that constancy, a mechanism which generally helps us to judge size and distance, is accidentally set off by the line illusions and leads to distortions (see Chapter 6). Other writers maintain that a process which helps us to see edges, lateral inhibition in the visual system, also misleads us when lines run close together in the visual field.

A third way in which illusions occur is by the failure of the visual system to cope with input. Dixon's spinning chequered disc (Chapter 10) comes first to mind. When this disc is spun at more than about 20 rev/min the checks are no longer seen and a slowly rotating grey cross takes their place. The most common everyday example of this is probably ordinary visual blurring of fast-moving objects, but I would hesitate, perhaps unreasonably, to call this an illusion.

That psychologists have taken such a steady and enduring interest in visual illusions might be taken as an indication of long-standing intellectual perversity, but there are arguments against such a view. A very important strategy in finding out how correct perception operates is to observe situations in which misperceptions occur; to test, that is, the limits of the satisfactory function of perception whilst carefully altering the conditions under which it is working. It is fairly easy to formulate a theory which is consistent with the facts of correct perception, but it is a much more demanding task to produce a theory which is capable of predicting the failures as well as the successes of the perceptual system. People have long been aware of this and probably the chief cause of the continued interest in illusions is that they have been used as test instruments for theory, particularly by the Gestalt school. They have also in themselves furnished important ideas. Thus Mach, having observed the bright and dark bands which now bear his name (Chapter 8), advanced the idea of lateral inhibition in the visual system which has since been demonstrated physiologically and has become accepted as an important factor in seeing.

There are many practical perceptual situations where illusions, stemming from misleading cues or misleading interactions of stimuli, are likely to lead to serious error. This happens particularly when there is a shortage of visual information. A man flying an aircraft or driving at night or in fog is relatively error prone because cues about the correct situation are relatively scarce. However, the practical importance of illusions should not be overestimated; most perceptual environments are too rich in information to give rise to illusions.

Illusions might also have potential as a means of preventing error. One example of this, reported by Denton (1971), is a method of road marking which causes drivers to overestimate their speed. This is proposed as a way of counteracting the underestimation of speed to which drivers leaving a motorway are prone. Such 'correction' of perception by the use of the principles of illusion is, of course, not new. In the ancient world stone columns were shaped in such a way as to affect perception of their height.

This book begins with an extensive treatment of the so-called geometrical optical illusions. In Chapters 2 and 3 I review studies of these illusions and try to pick out important themes. In Chapter 2 the themes are the Müller-Lyer effect, a framing effect and a contrast effect. These three among them seem capable of accounting for the illusions described in that chapter. No suggestion is made of how the effects are wrought. This is left to the theorists in Chapter 5. In Chapter 3, which deals with illusions involving angles and direction, again I pick out themes that seem to me to be important. These are the effect of the apex of an angle on the lines comprising the angle, and the effect of the orientation of those lines. In this chapter too I make clear the sort of explanation of this type of illusion which I favour; it is an explanation in terms of specifically tuned orientation analysers which extract from the display information about the orientation of lines contained in it and are misled by certain configurations. This is only a general type of explanation and could not at this stage be advanced as a theory of illusions.

Some subject variables are treated in Chapter 4, but there was room for only a selection of those that might have been included. The treatment of figural after-effects in Chapter 5 is a prelude to Chapter 6 in which some of the theories of geometrical optical illusions described regard figural after-effects and illusions as the results of the same processes. It is often (though not invariably) the case that theories have a characteristic career; an author discovers that one particular illusion figure or small set of figures can be accounted for by a simple principle or a simple mathematical expression. He finds it difficult to apply this successfully to other figures and so introduces another, generally more vague, principle, auxilliary to the first. The nature of this second principle makes experimental test of the theory very difficult and contributions to the literature cease.

The remaining chapters deal with illusions arising in displays other than line displays. Illusions stem from misperceptions of depth, contrast and movement as well as from fast-moving or intermittently lit displays. These have tempted authors into theory a little less often and in some cases, for example in illusions involving cues of depth, it is fairly clear how the perceptual system is being misled.

THE GEOMETRICAL OPTICAL ILLUSIONS (I)

The term 'geometrical optical illusion', a translation of the German 'geometrisch-optische Tauschung', seems to have been coined by Oppel (1855) in a paper about the overestimation of an interrupted as compared with an uninterrupted spatial extent, later called the Oppel—Kundt illusion because Kundt also studied it (Kundt, 1863). The term has been used for any illusion seen in line drawings. Interest in such illusions was intense during the second half of the nineteenth century, particularly in Germany, and the literature on the subject has increased steadily ever since.

From the beginning, and particularly in recent years, a great deal of interest has been directed towards theories of illusions. It has clearly been the hope of some to find a general theory which would account for all the geometrical optical illusions. There is no better indicator of the forlornness of this hope than a thorough review of the illusions themselves. Such a review follows. It avoids theory as far as it is reasonable to do so, and gives some idea of the breadth of the field and the diversity of both phenomena and research results. (The reader hungry for a systematic treatment of theory may wish to turn forthwith to Chapter 6. Some reference back would then be necessary, but for a quick reading this course is probably a good one.)

I shall present a large collection of this type of illusion. It does not pretend to be comprehensive because that would probably be impossible. It would not be too bold to claim that stimuli in the visual field almost always interact, especially if they are close together or concurrent. Thus, judgments of the degree of separation and the orientation of lines or areas is influenced by the degree of separation and the orientation of other lines or areas in the visual field, especially if they are close by. This makes it easy to invent variations of illusion figures once one has appreciated the essential configuration that gives rise to the illusion. To do this there is no need to understand in any deeper way why the illusion occurs. This collection does however contain most of the published line illusions, with the exception of those which seem to be trivial variations and have not had special attention in the literature.

THE CLASSIFICATION OF ILLUSIONS

Classification is a taxonomic exercise and does not itself provide explanations. It may help in the process of finding them by ordering the material in a way that makes thinking easier, but it could also obscure important similarities or differences.

Boring (1942) summarises early work, and those interested in the history of the subject will find his contribution worth reading. It is sufficient here to accept his claim that all classifications up to that time boil down to two groups: illusions of extent (where size or length is misjudged) and illusions of direction (where orientation of a line or figure is misjudged), 'to which may be added some special complicated cases' (p. 243). The argument here is broadly in agreement with this classification though it is difficult fully and clearly to justify it.

Neither Luckiesh (1922) nor Tolanski (1964) justifies the classifications he uses. Luckiesh classifies his rather small collection into (1) the effect of the location in the visual field, which includes the vertical-horizontal illusion; (2) illusions of interrupted extent, which includes both Ebbinghaus's figure (Fig. 2.70) and also Fig. 2.12; (3) illusions of contour, which includes several of Müller-Lyer's figures (Figs. 3.87 and 3.92); (4) illusions of contrast which includes both Fig. 2.14 and the Ponzo illusion (Fig. 2.31); and (5) illusions of perspective, which includes the Sander parallelogram (Fig. 2.23) and two figures (Figs. 3.9 and 3.10) which are clear relatives of the Zöllner figure (Fig. 3.1).

Some of these classifications seem illogical. The vertical—horizontal illusion is a result of orientation of lines, not of their position in the visual field. In Ebbinghaus's figure the filled space is overestimated, whereas in Fig. 2.12 it is underestimated. This latter figure is much better classified as a variant of the Müller-Lyer illusion. To classify Figs. 3.9 and 3.10 under illusions of perspective is to espouse a particular theory of illusions (which will be dealt with in the next chapter) and therefore to pre-empt a decision about their explanation.

Tolanski deals with fairly straightforward examples of a small number of well-known illusions and these dictate his classification. His chapter on 'the effect of weak wings' contains almost solely the vertical—horizontal illusion. His chapter on 'convergence—divergence' concerns what I shall call the Müller-Lyer effect. He includes Fig. 2.14 but is misled by this into including Wundt's area illusion (Fig. 2.77) which clearly operates on a different principle. In this illusion it is always the bottom figure which apparently has the greater area, whereas in Fig. 2.14 the bottom line of the smaller figure looks shorter than the top line of the larger figure, regardless of their relative positions. Tolanski also includes chapters on the Poggendorff illusion, on illusions related to the Zöllner illusion, on 'irradiation illusions' (Fig. 3.72) and on the moon illusion.

With the exception mentioned above, Tolanski's classes seem homogeneous, but the collection is not large. It would not have been possible to fit into his classes all the illusion figures appearing in this book, indeed, even with his collection he had to add a chapter entitled 'Some Further Illusions' which is itself heterogeneous.

Oyama (1960), again writing on a fairly limited range of illusions, uses a classification in part like that mentioned by Boring, but he subdivides the classes. Oyama's main classes are (1) illusions of length and distance, (2) illusions of angle, direction, straightness and curvature, and (3) illusions of size and area. These broad classes are very heterogeneous, but his subclasses are much less so.

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Excerpted from The Psychology of Visual Illusion by J. O. Robinson. Copyright © 1998 J. O. Robinson. Excerpted by permission of Dover Publications, Inc..
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