Two Distinct Regions of Secondary Somatosensory Cortex in the Rat: Topographical Organization and Multisensory Responses

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Abstract
In rodents, as in other species, regions of secondary somatosensory cortex (SII) may be distinguished from primary cortex (SI) both anatomically and electrophysiologically. However, the number of rodent SII subregions, their somatotopic organization, and their function are poorly understood. The presence of multisensory responsive neurons in some areas of SII suggests that one of its roles may be in the integration of somatosensory information with information from other sensory modalities. In this study, we used auditory, somatosensory, or combined auditory/somatosensory stimuli, and high-resolution epipial-evoked potential maps of rat SII to identify the number of spatially discrete subregions, estimate their somatotopic organization, and delineate regions with multisensory response properties. Maps revealed two distinct subregions within SII, one rostral and the other caudal, which were situated lateral to the posteromedial barrel subfield. Distinct somatotopies were evident at both SII loci, and analysis of evoked responses within both areas indicated multisensory interactions. These data are consistent with the presence of classically defined rostral SII regions and provide functional evidence for a lesser known, but distinct, caudal SII area. Furthermore, evidence for multisensory interactions within SII suggests that both secondary areas may process features specifically associated with multisensory integration in parallel with unimodal processing in primary areas.