Neuronal firing in the inferotemporal cortex of the monkey in a visual memory task

The objective of this study was to elucidate the functional role of neurons in the inferotemporal cortex of the primate. Single unit activity was recorded with microelectrodes in monkeys performing a visual delayed matching-to-sample task. On each trial, the animal was exposed briefly to a color - the sample - and, after a period of delay, had to select the same color among two or four colors simultaneously presented. Thus, correct performance of the task required perception, retention, and recognition of the sample color for every trial. A large number of inferotemporal units were seen to react to the stimuli with changes of firing frequency. Many units showed color-dependent reactions, suggesting their involvement in perception and discrimination of colors. A substantial contingent of cells showed increased, sustained, and in some cases, color-dependent discharge during the delay which was not necessarily preceded or succeeded by firing changes in sample or match periods. It is proposed that those cells were engaged in temporary retention of the sample stimulus. Since most of the inferotemporal units examined showed firing changes in more than one period of a trial, they appeared to be involved in more than one of the operations required by the task. Thus, the data do not support a clear-cut topographic separation of visual functions within the inferotmeporal cortex. However, neurons that appear to participate in visual memory, either exclusively or in addition to other functions, are concentrated in the cortex of the lower banks of the superior temporal sulcus.