Processing speed in the cerebral cortex and the neurophysiology of visual masking

In experiments to investigate the duration of the time for which cortical neurons respond when the identification of a visual stimulus is just possible, we presented a test face stimulus for 16 ms, and followed it at different intervals by a masking stimulus (either an N-O pattern or a face) while recording from single neurons in the temporal visual cortex of macaques. When there was no mask the cells responded to the 16 ms of the test stimulus for 200-300 ms, far longer than the presentation time. We suggest that this reflects the operation of a short-term memory system implemented in cortical circuitry. If the mask was a stimulus which did not stimulate the cells (either a non-face pattern or a face which was a non-effective stimulus for that cell), then, as the interval between the onset of the test stimulus and the onset of the mask stimulus (the stimulus onset asynchrony) was reduced, the length of time for which the cells fired in response to the test stimulus was reduced. It is suggested that this is due to the mask stimulating adjacent cells in the cortex which by lateral inhibition reduce the responses of the cells activated by the test stimulus. When the stimulus onset asynchrony was 20 ms, face-selective neurons in the inferior temporal cortex of macaques responded for a period of 20-30 ms before their firing was interrupted by the mask. With the same test-mask stimulus onset asynchrony of 20 ms, humans could just identify which of six faces was shown. These results provide evidence that a cortical area can perform its computation necessary for the recognition of a visual stimulus in 20-30 ms, and provide a fundamental constraint which must be accounted for in any theory of cortical computation.