A THEORY OF SHAPE CONSTANCY BASED ON PERSPECTIVE INVARIANTS

Shape constancy refers to the phenomenon in which the percept of the s hape of a given object remains constant despite changes in the shape o f the object's retinal image. The phenomenon of shape constancy is con sidered from historical, theoretical and empirical perspectives in thi s paper. First, four prior theories are discussed; specifically, (1) H elmholtzian theory, which assumes that shape constancy is achieved by taking an object's orientation into account, (2) Gestalt theory, which assumes that shape constancy involves a relationship between the perc eived shape and perceived orientation of an object, (3) Gibsonian theo ry, which assumes that shape constancy is based on projective invarian ts and (4) multiple view theory, which assumes that shape constancy is achieved by memorizing a large set of different views of the object. It is shown, by an analysis of the prior literature, that none of thes e theories can actually explain the phenomenon of shape constancy. A n ew theory, which is based on new perspective invariants of a flat shap e, is then proposed. The new Perspective Invariants Theory can account for all prior shape constancy experiments. New experiments, testing p redictions of the Perspective Invariants Theory are then described. Th ese experiments showed that: (1) a novel shape can be matched with its single perspective image in the absence of depth cues, (2) perceptual processing of shape is impaired when the range of possible values of tilt is wide, (3) perceptual processing of shape is not affected by th e width of the range of possible values of slant. These results suppor t predictions of Perspective Invariants Theory.