Structural constraints: further evidence from apparent motion in depth.

The three-dimensional (3-D) apparent motion of lines, outline triangles, and light points was studied in four experiments. The stimulus sequences were beginning and end patterns of 3-D motions of a line and a triangle. Light-point patterns corresponded to the ends of the lines and the vertices of the triangles. Perceived motion of lines and light-point pairs resembled the distal motions that were used to construct the proximal patterns. The correspondence was striking for configurations that appeared to move in depth. Outline triangles and light-point triplets produced a strong correspondence between distal and perceived motions when the three sides appeared to be translating in depth. The correspondence was reasonably good for the other motion patterns when scoring included an appropriate second category. The results support the conception of structural or internalized constraints: light points were processed as if they were connected (unity constraint) and proximal change in linear size (or distance between light points) was perceived as rigid 3-D motion (rigidity constraint).