Contents 1. Introduction
It is customary in the recognition of images to treat the two spatial dimensions x and y symmetrically. This is justified by the statistics of natural images, which are to a first approximation isotropic—all orientations are equally likely—and shift-invariant [41, 26]. But what about video signals ? Motion is the spatiotemporal counterpart of orientation [2], but all spatiotemporal orientations are not equally likely. Slow motions are more likely than fast motions (indeed most of the world we see is at rest at a given moment) and this has been exploited in Bayesian accounts of how humans perceive motion stimuli [58]. For example, if we see a moving edge in isolation, we perceive it as moving perpendicular to itself, even though in principle it could also have an arbitrary component of movement tangential to itself (the aperture problem in optical flow). This percept is rational if the prior favors slow movements.
