Abstract
The effects of pursuit eye-movements on perceived depth from motion can generally be explained by reductions in translational velocity with invariance of motion shear. We report some interesting new effects of pursuit eye-movements on perceived 3-D shape. A starry night pattern that provides no cues for shape-from-texture was folded into one cycle of vertical or horizontal sinusoidal corrugation, rotated between +/− 30 degrees for 3 sec around its orthogonal axis through the zero-crossings, and viewed monocularly in perspective. On each trial, a fixation dot was either set in the center, or moved across the central extent of the stimulus left to right, right to left, top to bottom, or bottom to top. Two observers reported the shape (concave or convex) of each half cycle for 10 trials in each condition. For the vertical corrugation with central fixation, both observers perceived two convex half-cycles rotating in opposite directions, violating a rigidity assumption, but consistent with the motion-perspective assumption that higher local velocities indicate nearer segments. However, while pursuing fixation dots moving in any of the four directions, observers perceived rigid corrugations with the top rotating downwards, suggesting a prior preference for an object falling towards rather than away from the observer. For the horizontal corrugation, with central fixation both observers perceived a rigidly rotating corrugation, but the corrugation appeared invariably convex on top to one observer and concave on top to the other, irrespective of the simulated phase and rotation direction. Horizontal pursuit eye-movements did not alter the percepts. However, for upwards pursuit, the upper half-cycle was perceived as convex like a ledge, and for downwards pursuit, the lower half-cycle was perceived as convex like a base by both observers. Different directions of voluntary eye-movements thus seem to activate different ecologically relevant prior expectations for object shape and motion.