Believable change: Bistable reversals are governed by physical plausibility

Planar motion flows can induce the illusory appearance of a volume rotating in depth (depth from motion; G. Sperling, & B. A. Dosher 1994). This appearance changes spontaneously from time to time, reversing simultaneously its depth and its direction of rotation. We investigated asymmetric illusory volumes, which reverse more frequently at some angles of view than at others. In three experiments, we studied spontaneous joint reversals of depth and motion, as well as induced reversals of either motion or depth alone. We find that depth reversals occur exclusively when the illusory volume is depth symmetric (so that the shape of the volume remains unchanged). In contrast, motion reversals occur at all view angles, but their frequency varies with the motion speed. The probability of joint reversals is well approximated by the product of the individual reversal probabilities, suggestive of two independent random processes. We hypothesize that reversals of illusory volumes are conditioned by prior experience of physical transformations in the visual world.