Global shape coding for motion-defined radial-frequency contours.

Abstract

The visual system is highly skilled at recovering the shape of complex objects defined exclusively by motion cues. But while low-level and high-level mechanisms involved in shape-from-motion have been studied extensively, intermediate computational stages remain poorly understood. In the present study, we used motion-defined radial-frequency contours--or motion RFs--to probe intermediate stages involved in the computation of motion-defined shape. Motion RFs consisted of a virtual circle of Gabor elements whose carriers drifted at speeds determined by a sinusoidal function of polar angle. Motion RFs elicited vivid percepts of shape, and observers could detect and discriminate radial frequencies up to approximately five cycles. Randomizing Gabor speeds over a small contour segment impaired detection and discrimination performance significantly more than predicted by probability summation. Threshold comparisons between spatial-RF and motion-RF contours ruled out that motion-induced shifts in perceived position (i.e., the DeValois effect) determine shape perception in motion RFs. Together, results indicate that the shape of motion RFs is processed by synergistic mechanisms that perform a global analysis of motion cues over space. These results are integrated with data on perceptual interactions between motion RFs and spatial-RFs and are discussed in terms of cue-specific and cue-invariant representations of object shape in human vision.