Velocity selectivity in the cat visual system. II. Independence from interactions between different loci.

To investigate the dependence of velocity characteristics on spatiotemporal interactions the velocity selectivity of 15 geniculate and 72 cortical cells (areas 17 and 18) was examined with light and dark bars before and after masking all but the most sensitive part of the receptive field. The use of a 0.3 degree window proved effective in eliminating enough spatiotemporal interactions to abolish cortical direction selectivity. The same window improved the responsiveness at high velocities in only 26% of the cortical cells preferring low velocities and having a receptive field with nonoverlapping ON and OFF subregions. The remaining 74% showed various degrees of velocity-independent decrease in response amplitude. The only two geniculate cells that had a velocity upper cutoff lost this cutoff when tested with the mask. Cortical units preferring high velocities lost their responsiveness at high velocities in the mask condition, provided that their receptive fields contained nonoverlapping ON and OFF subregions. Cortical units, which responded best at intermediate velocities and which had receptive fields with nonoverlapping subregions, lost their sharp velocity tuning when tested with a mask. We conclude that inhibitory spatiotemporal interactions can account for the preference for low velocities in at most a quarter of the cells with nonoverlapping subregions. In contrast, facilitatory interactions seem to be important for cortical cells preferring high or intermediate velocities and having receptive fields with nonoverlapping subregions.