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 Â¿ Â¿, rate of change of disparity Â¿ to disparity Â¿, and its possible role in establishing stereo correspondence. The importance of such ratios was first pointed out by Richards . Our formulation may reflect the human perception channel probed by Regan and Beverley .