Occipital and inferotemporal responses to visual signals in the monkey

This study analyzes cellular and field-potential responses in striate and inferotemporal cortex to visual stimuli in monkeys performing a memory task (delayed matching-to-sample). Each trial was initiated by a brief alerting diffuse flash preceding presentation of the memorandum (sample); the latter was a lighted circle (red or green, 1.5 s) to be retained by the animal during a subsequent delay for correct behavioral response (color match). The alerting flash evoked distinct excitatory cell responses and field potentials in the occipital cortex; those two orders of phenomena were broadly related to each other in temporal terms. By contrast, most cells in the inferotemporal region were inhibited by the flash, although the local evoked field potential had a configuration similar to that of the occipital potential. In each region, the sample stimuli elicited excitatory unit responses which summed to a unimodal distribution with an initial component roughly corresponding in time course to the local field potential. Although the shortest response latencies were found in occipital cortex, considerable temporal overlap of the sample-related activities in the two cortices was observed. The finding that most inferotemporal cells, unlike occipital cells, treated only the sample with excitatory response indicates that the inferotemporal cortex is selectively attuned to visual detail. However, the largely simultaneous activation of both cortical regions following the onset of the sample suggests that discriminative visual information is processed by hierarchic interactions of the two cortices through their reciprocal connections.