PROCESSING SPEED IN THE CEREBRAL-CORTEX AND THE NEUROPHYSIOLOGY OF VISUAL MASKING

In experiments to investigate the duration of the time for which corti cal neurons respond when the identification of a visual stimulus is ju st possible, we presented a test face stimulus for 16 ms, and followed it at different intervals by a masking stimulus (either an N-O patter n or a face) while recording from single neurons in the temporal visua l cortex of macaques. When there was no mask the cells responded to th e 16 ms of the test stimulus for 200-300 ms, far longer than the prese ntation time. We suggest that this reflects the operation of a short-t erm 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 noneffective stimulus for that cell), then, as the interval between the onset of the test stimulus and the onset of t he mask stimulus (the stimulus onset asynchrony) was reduced, the leng th 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 re sponses of the cells activated by the test stimulus. When the stimulus onset asynchrony was 20 ms, face-selective neurons in the inferior te mporal cortex of macaques responded for a period of 20-30 ms before th eir firing was interrupted by the mask. With the same test-mask stimul us onset asynchrony of 20 ms, humans could just identify which of six faces was shown. These results provide evidence that a cortical area c an perform its computation necessary for the recognition of a visual s timulus in 20-30 ms, and provide a fundamental constraint which must b e accounted for in any theory of cortical computation.