The analyses of visual data by stereo and motion modules have typically been treated as separate parallel processes which both feed a common viewer-centered 2.5-D sketch of the scene. When acting separately, stereo and motion analyses are subject to certain inherent difficulties; stereo must resolve a combinatorial correspondence problem and is further complicated by the presence of occluding boundaries, motion analysis involves the solution of nonlinear equations and yields a 3-D interpretation specified up to an undetermined scale factor. A new module is described here which unifies stereo and motion analysis in a manner in which each helps to overcome the other's short-comings. One important result is a correlation between relative image flow (i.e., binocular difference flow) and stereo disparity; it points to the importance of the ratio ¿ ¿, rate of change of disparity ¿ to disparity ¿, and its possible role in establishing stereo correspondence. The importance of such ratios was first pointed out by Richards [19]. Our formulation may reflect the human perception channel probed by Regan and Beverley [18].