Comparison of the spatial limits on direction selectivity in visual areas MT and V1.

We recorded responses to apparent motion from directionally selective neurons in primary visual cortex (V1) of anesthetized monkeys and middle temporal area (MT) of awake monkeys. Apparent motion consisted of multiple stationary stimulus flashes presented in sequence, characterized by their temporal separation (delta t) and spatial separation (delta x). Stimuli were 8 degrees square patterns of 100% correlated random dots that moved at apparent speeds of 16 or 32 degrees/s. For both V1 and MT, the difference between the response to the preferred and null directions declined with increasing flash separation. For each neuron, we estimated the maximum flash separation for which directionally selective responses were observed. For the range of speeds we used, delta x provided a better description of the limitation on directional responses than did delta t. When comparing MT and V1 neurons of similar preferred speed, there was no difference in the maximum delta x between our samples from the two areas. In both V1 and MT, the great majority of neurons had maximal values of delta x in the 0.25-1 degrees range. Mean values were almost identical between the two areas. For most neurons, larger flash separations led to both weaker responses to the preferred direction and increased responses to the opposite direction. The former mechanism was slightly more dominant in MT and the latter slightly more dominant in V1. We conclude that V1 and MT neurons lose direction selectivity for similar values of delta x, supporting the hypothesis that basic direction selectivity in MT is inherited from V1, at least over the range of stimulus speeds represented by both areas.