The organization and connections of somatosensory cortex in primates

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The use of microelectrode mapping methods, in conjunction with studies of cortical architecture, patterns of connections, and the response properties of single neurons, has led to a greatly increased understanding of the functional organization of somatosensory cortex in mammals. All mammals appear to have the traditionally defined first and second somatosensory representations, SI and SII, but at least some mammals have additional body representations as well. Furthermore, the area traditionally regarded as “SI” in monkeys actually includes four functionally distinct representations. Within “SI,” four striplike architectonic fields, areas 3a, 3b, 1, and 2 have long been recognized. It is now clear that areas 3b and 1 form separate parallel and largely mirror‐image representations of the body surface, each with major inputs from a single representation of the body surface in the ventroposterior nucleus (VP) of the thalamus. VP appears to relay information from both slowly and rapidly adapting peripheral neurons to area 3b, while information from rapidly adapting neurons is relayed to area 1. Neurons with properties that could reflect Pacinian receptor influences are found in area 1, but not area 3b. The anatomical pathways mediating Pacinian‐like responses are unknown. Area 3a contains a representation of largely deep receptors, probably muscle spindle afferents, relayed from part of the thalamus dorsal to VP. Area 2 forms a representation of both deep and cutaneous body receptors. The somatosensory cortex receives input from other regions as well, including the anterior pulvinar and ventroposterior inferior nucleus of the thalamus. Other portions of the somatosensory cortex are less well understood.