There are two kinds of motion perception. Previously it was assumed that both kinds feed into a common stage. However, the results of a series of psychophysics experiments, using specially developed stimuli and tasks, have shown otherwise. For example, observers found it easy to recognize shape from optic flow (structure from motion) with first order (linear, or short-range) motion. But they consistently failed to do so when the optic flow patterns were constructed from second order motion.

Analogies between the two kinds of motion and the two kinds of stereopsis (exchanging motion displacement with binocular disparity) are strikingly consistent across a number of tasks and variations in stimulus parameters.

This higher order correspondence, both for stereopsis and for motion perception, can be considered as a pairing of attended visual directions, or visual indices. These change constantly as an observer moves about, leaving open several questions regarding interrelations between the two kinds.