Space-time separability of direction selectivity in cat striate cortex neurons.

The relationship between spatial and temporal factors underlying direction selective velocity tuning in cat striate cortex neurons was studied quantitatively using flashed and smoothly moving light bar stimuli. Responses to double-flash stimuli were obtained for all combinations of a series of spatial and temporal intervals between the two flashes. Nonlinear interactions underlying the unit's direction selectivity were estimated by subtracting the responses to the component single flashes. The extent of systematic interaction between spatial and temporal variables was assessed qualitatively, with contour plots, and quantitatively, by plotting the optimum displacement as a function of temporal separation. Both methods indicated interactions (space-time nonseparability) which were generally very small in magnitude, and uncorrelated either in magnitude or sign with the direction selective velocity tuning measured in the same neurons. These results demonstrate characteristic optimum spatial and temporal intervals for responses to the two-flash stimuli. The optimum spatial offset was independent of the temporal offset chosen, indicating space-time separability.