Suppression on neuronal responses by a metacontrast masking stimulus in monkey V4.

We studied the temporal characteristics of suppression in area V4 of the monkey using a visual stimulus for metacontrast masking. Visual responses of V4 neurons to a brief test stimulus presented within the receptive field were recorded, and the effect of a mask stimulus that did not spatially overlap the test stimulus was examined. Responses to the test stimulus were suppressed by the mask stimulus, which either preceded or followed the test stimulus. To study the temporal characteristics of suppression, the interval between the onset of the test stimulus and that of the mask stimulus (stimulus onset asynchrony, SOA) was varied. Maximum suppression occurred with a simultaneous presentation of the two stimuli, and the suppression gradually weakened as the SOA increased. The suppressive effect of the mask stimulus lasted on average about 77 ms in the negative SOA (forward masking) and 65 ms in the positive SOA (backward masking). These results indicate that surround suppression in V4 neurons has considerable temporal width, which is longer than that previously reported in areas V1 and V2. There were marked differences between the time course of suppression in V4 neurons in the present study and those reported in human metacontrast masking.