The geometry of the occluding contour and its effect on motion interpretation.

Form information related to occlusion is needed to correctly interpret image motion. This work describes one of a series of investigations into the form constraints on motion perception. In the present study, we focus specifically on the geometry of the occluding contour, and in particular on whether its influence on motion can be accounted for merely by its effect on perceived occlusion. We used an occluded square moving in a circle, holding the T-junctions at points of occlusion constant while manipulating the occluding contour. We found evidence for two main influences of occluding contour geometry on motion interpretation and occlusion: the convexity of the occluding contour and additional static T-junctions that are formed elsewhere on the occluding contour. Our results suggest that convex occluding contours are more occlusive than concave ones, and that T-junctions along the contour increase or decrease the strength of occlusion depending on their orientation. Motion interpretation is influenced by both factors, but their effect on motion appears to be dominated by interactions occurring at an intermediate "semilocal" scale, which is larger than the scale at which junctions are defined, but smaller than the scale of the whole moving figure. We propose that these computations are related to occlusion but are not identical to the computations that mediate static occlusion judgments.