Visual Sensitivity to the Shape and Size of a Moving Object: Implications for Models of Object Perception

Abstract

Subjects adapted to rectangular targets whose opposite edges moved in opposite directions at any given instant (as when an object moves directly towards or away from the head). The distance that adaptation spread from the vertical edges was up to three times less when the horizontal edges were also moving than when the horizontal edges were stationary. Furthermore, for a square target (1 deg × 1 deg), adaptation spread least when the horizontal edges moved at the same speed as the vertical edges, whereas for a target whose height was half its width (0·5 deg × 1 deg) adaptation spread least when the horizontal edges moved at half the speed of the vertical edges. We propose that the human visual system acts as though it contains detectors sensitive to the size and shape of an object and that these detectors enhance this sensitivity to shape and size by comparing the velocities of the horizontal and vertical edges. When a nonrotating solid object moves in three dimensions its shape severely restricts the possible relationships between the velocities of the vertical and horizontal edges of its retinal image. Our hypothetical detectors utilise these geometrically determined velocity relationships as a basis for their selective sensitivity to shape. More speculatively, object perception and the perception of shape and size in everyday vision might involve visual sensitivity to the relationship between the velocities of an object's edges as well as sensitivity to the locations of these edges: the visual system may recognise which of the many edges in the visual field belong to a single object by comparing the velocities of orthogonal pairs of edges.