TOPOGRAPHY, ARCHITECTURE, AND CONNECTIONS OF SOMATOSENSORY CORTEX IN OPOSSUMS - EVIDENCE FOR 5 SOMATOSENSORY AREAS

Microelectrode maps of somatosensory inputs were related to cortical a rchitecture and patterns of cortical connections to provide evidence f or five subdivisions of the somatosensory or sensorimotor cortex in No rth American opossums (Didelphis marsupialis). Microelectrode recordin gs revealed three systematic representations of the body surface. A la rge mediolaterally oriented representation was identified as the prima ry somatosensory area (S1) by its relative position, somatotopy, archi tecture, and connections. S1 represented the hindlimb, trunk, forelimb , and face in a mediolateral sequence. Two additional representations of cutaneous receptors were found caudolateral to S1, each with face r epresentations adjacent to the border of lateral S1 and other body-par t representations progressing more caudally toward the auditory cortex . We identified the more dorsal field as the second somatosensory area (S2) and the more ventral field as the parietal ventral area (PV). Tr acers injected into S1 labeled neurons and terminals in architectonica lly distinct fields rostral and caudal to S1, the somatosensory caudal area (SC) and the somatosensory rostral area (SR). Movements could be evoked by microstimulation from sites scattered over S1, SR, and the frontal cortex, but thresholds were high and uncharacteristic of motor cortex. S2 and PV merged caudally with the cortex responsive to audit ory stimuli, possibly Al, and neurons in some caudal recording sites i n PV were activated by both auditory and cutaneous stimuli. Primary (V 1) and secondary (V2) visual areas were also identified by microelectr ode mapping, architecture, and connections. In addition, at least part of the cortex between V2 and the somatosensory cortex had visual conn ections. Thus, most of the dorsolateral cortex of opossums appears to be somatosensory, auditory, or visual. (C) 1996 Wiley-Liss, Inc.