With training, an observer's ability to discriminate similar directions of motion gradually improves. A series of studies reveals that this improvement, 1. (1) is restricted to the trained direction and other, similar directions, 2. (2) persists for at least several months, 3. (4) shows appreciable, but not complete, transfer between the two eyes, and 4. (5) is largely restricted to the stimulated region of the field. Moreover, the improvement in direction discrimination does not produce a concomitant change in detection thresholds. In all likelihood, most of the improvement in direction discrimination represents a change in visual function, rather than changes in nonsensory processes. © 1987.