Continuous perspective transformations and the perception of rigid motion.

An exploratory survey of the varieties of optical motion which could serve as stimuli for the perception of motions in the world (6, 7) suggested the hypothesis that one kind of geometrical motion in a plane yields an impression of a rigid motion in space but that any other kind of geometrical motion in a plane does not. The stimulus pattern was always a texture, that is, a grouping of dark shapes on a light background. If, on a motion picture screen, it underwent a continuous sequence of perspective transformations in any of six ways, it gave a perception of a rigid surface moving in one of six ways—the three transpositions and three rotations which, in combination, exhaust the possibilities of mechanical movement. If it underwent continuous transformations not of this geometrical kind (but only a few examples were presented) it aroused perceptions of nonrigid or elastic surface motions, of the kind exemplified in the movements of organisms. Of the six rigid phenomenal motions, three (rotation around the line of sight, transposition up or down, and transposition right or left) are 1 This experiment was reported by the first