A SUSTAINED INPUT TO THE DIRECTION-SELECTIVE MECHANISM IN CAT STRIATECORTEX NEURONS

Direction-selective neurons in cat striate cortex were tested with bar -shaped stimuli, sequentially flashed at spatially displaced positions chosen to elicit maximal direction selectivity. Temporally overlappin g flash exposures of prolonged duration (400-1000 ms) were employed at a series of onset asynchronies to explore the nature of temporal tuni ng of the direction-selective mechanism. In most neurons studied, dire ction selectivity was found to be supported by a surprisingly broad ra nge of stimulus onset asynchronies, which was greater for longer expos ure durations. These findings imply the existence of a sustained input to the direction-selective mechanism, in spite of the relatively tran sient nature of most cortical neurons' step responses. A model is desc ribed to illustrate how different front-end temporal filters can affec t the dependence of two-flash direction selectivity on stimulus onset asynchrony. The versions of the model which successfully predict the f orm of the observed responses are those which combine inputs from sust ained and transient filters.