DETECTION OF SPATIAL DISCONTINUITIES IN FIRST-ORDER OPTICAL-FLOW FIELDS

We investigated the extent to which the human visual system can detect discontinuities in first-order optical flow fields. We constructed tw o types of spatial discontinuities: a circular split field with a stra ight edge and a disk with annular surround. We used two different firs t-order optical flow components: an expansion and a rotation. We found an intriguing difference in the detection thresholds for straight and circular discontinuities. Whereas straight discontinuities yielded th resholds of 10%-50% difference in expansion or rotation, circular disc ontinuities could, at first, only be detected at extreme differences ( much greater than 100%). After a learning period, thresholds for such stimuli decreased, but they remained significantly higher than thresho lds for the straight edge. Thresholds rose for stimuli that formed a g radual transition between a circular and a straight edge, and they dec reased with increasing eccentricity of the circular discontinuity. Res ults suggest that symmetry in the stimulus, defined by the coincidence of the center of expansion or rotation and the center of the circular discontinuity, was responsible for the difference in thresholds for c ircular and straight discontinuities.