INPUTS TO DIRECTIONALLY SELECTIVE SIMPLE CELLS IN MACAQUE STRIATE CORTEX

It is clear that the initial analysis of visual motion takes place in the striate cortex, where directionally selective cells are found that respond to local motion in one direction but not in the opposite dire ction. Widely accepted motion models postulate as inputs to directiona l units two or more cells whose spatio-temporal receptive fields (RFs) are approximately 90 degrees out of phase (quadrature) in space and i n time. Simple cells in macaque striate cortex differ in their spatial phases, but evidence is lacking for the varying time delays required for two inputs to be in temporal quadrature. We examined the space-tim e RF structure of cells in macaque striate cortex and found two subpop ulations of (nondirectional) simple cells, some that show strongly bip hasic temporal responses, and others that are weakly biphasic if at al l. The temporal impulse responses of these two classes of cells are ve ry close to 90 degrees apart, with the strongly biphasic cells having a shorter latency than the weakly biphasic cells. A principal componen t analysis of the spatio-temporal RFs of directionally selective simpl e cells shows that their RFs could be produced by a linear combination of two components; these two components correspond closely in their r espective latencies and biphasic characters to those of strongly bipha sic and weakly biphasic nondirectional simple cells, respectively. Thi s finding suggests that the motion system might acquire the requisite temporal quadrature by combining inputs from these two classes of nond irectional cells (or from their respective lateral geniculate inputs, which appear to be from magno and parvo lateral geniculate cells, resp ectively).