Science
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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. ⋯ 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 explanation, which I call the general constancy theory, satisfactorily encómpasses all known illusions, but merely that it is more comprehensive than alternative explantions I conclude that any stimulus which serves to maintain perceptual constancy of a property of an object as the visual representation of that property varies will, when independently manipulated with the retinal image not varied, produce an illusion. This general principle predictS the conditions under which illusory effects will occur and has wide explanatory application.