Two motion perception mechanisms revealed through distance-driven reversal of apparent motion.

We demonstrate two kinds of visual stimuli that exhibit motion in one direction when viewed from near and in the opposite direction from afar. These striking reversals occur because each kind of stimulus is constructed to simultaneously activate two different mechanisms: a short-range mechanism that computes motion from space-time correspondences in stimulus luminance and a long-range mechanism in which motion computations are performed, instead, on stimulus contrast that has been full-wave rectified (e.g., on the absolute value of contrast).