Anisotropy in an ambiguous kinetic depth effect

A set of animated stimuli (Lissajous figures), each element of which is physically consistent with two different three-dimensional shapes undergoing rigid rotations about orthogonal axes, is described. Human observers typically show a preference for one shape or the other; this preference may be biased by manipulating various parameters of the stimulus. Fairly good predictions of which shape will be seen are made by using an adaptation of Hildreth's smoothest-velocity-field computation. When a given stimulus is rotated 90 degrees in the picture plane, the resolution of the ambiguity is often different, demonstrating anisotropy in the processing of the figures. The nature of this bias is such that for certain figures subjects see a three-dimensional object rotating about a vertical axis regardless of which two-dimensional orientation is used to present the stimulus. This bias is not predicted by the Hildreth model. One interpretation of the results is that the ambiguity in two-dimensional visual motion (i.e., the aperture problem) is not resolved before the interpretation of the three-dimensional structure.