Visual Spatial Illusions: A General Explanation

Abstract

Representation at the visual receptors of such properties of the object as its size, shape, orientation, and movement undergo considerable variation as the distance, bearing, posture, and motion of the observer, relative to the object, changes. However, despite these gross and frequent deformations of the image, perceived properties remain extraordinarily stable. Such constancy has obvious biological utility; the observer perceives his world according to its fixed physicalfeatures rather than in terms of its variable sensory representation. Constancy of apparent size, shape, orientation, and movement is dependent on information for the distance, bearing, lateral tilt, and movement of the observer, respectively. Thus, as the retinal image shrinks with distance, constancy of apparent size is maintained by five classes of distance, stimuli that operate singly or in various combinations. Likewise, visual orientation and visual stimuli for the orientation of the observer, and movement constancy on visual and nonvisual stimuli for the movement of the observer. Illusion occur when stimuli that normally preserve constancy are operative but with the image of the object not varied. Thus if retinal disparity, convergence-accommodation, projected stimuli, or other distance stimuli are varied with the image not varied, illusions of size occur. Those rsluting from variation of projected stimuli are the. well-known geometrical size illusions and include the Mü llerLyer group. In essentially the same manner, independent manipulation of stimuli for the orientation and the motion of the observer, with the orientation and the motion of the image at the retina not varied, gives rise to illusory orientations and movements of the object. Limited attempts to explain size illusions in terms of the projected stimuli that preserve perceptual constancy are by no means new; Thiéry (51) proposed such a view in the later part of the last century, and in recent times there has been a spate of such proposals including the " misapplied constancy hypothesis" advanced by Gregory (2). However, Gregory's theory is confined largely to geometrical size illusions and invoks only distance scaling given by a limited number of projected stimuli. Furthermore, the Müller-Lyer illusion is seen by him to be a consequence of distance scaling resulting from the converging arrows. There is no recognition of the range of such effects with various attached elements, as shown in Fig3, and no attention is accorded the recently established difference between the illusions with inward and outward-directed elements. The theory of spatial illusions outlined here distinguishes between classes of illusory effect and, in linking each to its particular class of spatial constancy, offers a general and testable explanation. Failure to recognize classes of illusion(and perceptual constancy), such as those of size, orientation, and movement, can be regarded as among the major deficiencies of recent attempts (2, 44) to explain illusory effects. I do not claim that this explanati...

The Ponzo illusion and the perception of orientation

“…This claim is controversial. On the one hand, Day (1972) suggested that the tilt induction effect, the Zöllner illusion, and the tilted room illusion of Asch and Witkin (1948) were all caused by the same mechanism. On the other hand, Howard proposed that although the Zöllner illusion and the tilt induction effect were caused by one mechanism, context illusions that involved large stimuli (the rod-and-frame effect and the tilted room illusion) were caused by a different mechanism.…”

“…The Zöllner illusion has long been considered an example of the tilt induction effect (Day, 1972;Howard, 1982, pp. 156-157).…”

A comparison of the Ponzo illusion with a textural analogue

“…It was found that length estimates were significantly different between the horizontals of the Ponzo figure and control stimuli, but not between the texture figure and a context-free control. The results suggest that inappropriate constancy scaling plays a minor role at best in the perception of geometric illusions.One category of theory that attempts to explain the operation of geometric illusions assumes that the failure to equate physicaUy identical elements in two-dimensional displays may be attributed to inappropriate constancy scaling (Day, 1972; Gergory, 1963;Tausch, 1954). Such a theory explains the Ponzo illusion by proposing that the angular portion of the figure suggests depth in the manner of linear perspective, and so the more "distant" of the two lines is perceived as larger when both lines subtend equal visual angles.…”

The Müller-Lyer illusion explained by the statistics of image–source relationships

“…Although the effects produced by these several variants have not been quantitatively compared, there is a general agreement that the shaft or the corresponding interval in the ''outward'' figure always appears longer than its counterpart in the ''inward'' figure. As a result, there has been much controversy about the genesis of the Müller-Lyer effect (6,(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20), which still has no generally accepted explanation (21,22).…”