Visual cortical mechanisms responsible for direction selectivity.

We measured the response of some 171 directionally selective (DS) neurons in visual area 17 of the cat. Sequences of long duration (400 msec) stimulus onsets and offsets were employed to analyze the underlying mechanisms. We corroborate the existence of sub-regions of localized, anisotropic inhibition as the basic mechanism of directional tuning. At extremely small and at rather large asynchronies, DS neurons inhibit virtually non-selectively. Optimal directional selectivity occurs when the second response peak of S1 coincides with the first response peak of S2 (usually at an asynchrony of approximately 75 msec). The mechanism appears to involve the linear temporal convolution of impulse response functions.