ILLUSORY PERCEPTION AS A CONSTANCY PHENOMENON.

Abstract

GREGORY1 offers a general account of visual illusions, relating the apparent distortions in them to the perceptual process of size constancy. He observes that Tausch is the only previous writer to have considered constancy in relation to illusions. However, Gibson2 has incorporated illusory perceptions within the context of size constancy, and suggests that perception of size is a by-product of a constant scale, which Gregory calls ‘constancy scaling’, at different distances. Arguing that illusory figures are “flat projections of typical views of objects lying in three-dimensional space”, Gregory notes that “the parts of the figure corresponding to distant objects are expanded and the parts corresponding to nearer objects reduced”. He states the same principle more simply when he writes: “Those parts of the figures which would normally be further away in 3-D space appear too large in the illusion figures”3. These principles are applied to a series of illusions, including those of Müller-Lyer, Ponzo and Hering. Gregory therefore postulates a common process modifying retinal images in constancy scaling and in the perception of illusory figures.