The detection and analysis of mo tion is one of the fundamental tasks of vision, because practically every thing of interest in the visual world moves. Although motion analysis of a high order is evident in such sim ple visual systems as the fly's, it is only in primates that a well-defined anatomical division of the central vi sual pathways can be seen to be spe cialized for the analysis of motion.1 The best defined area in this path way is an extrastriate area known as MT(orV5). Unlike other areas of the monkey's extrastriate visual cortex, almost all neurons in MT are direc tion selective, meaning that they typically respond best to motion within a given range of directions, and respond not at all (or with inhi bition) to motion in the opposite di rection. The activity of MT neurons has been linked to a variety of mo tion-related tasks, including the analysis of the motion of complex patterns, the detection of target mo tion relative to the background, and the generation of signals for smooth pursuit eye movement.2