The organization and connections of somatosensory cortex in the brush-tailed possum (Trichosurus vulpecula): evidence for multiple, topographically organized and interconnected representations in an Australian marsupial

Microelectrode mapping techniques were used to determine the organization o f somatosensory cortex in the Australian brush-tailed possum (Trichosurus v ulpecula). The results of electrophysiological mapping were combined with d ata on the cyto- and myeloarchitecture, and patterns of corticocortical con nections, using sections cut tangential to the pial surface. We found evide nce for three topographically organized representations of the body surface that were coextensive with architectonic subdivisions. A large, discontinu ous cutaneous representation in anterior parietal cortex was termed the pri mary somatosensory area (SI). Lateral to SI we found evidence for two furth er areas, the second somatosensory area (SII) and the parietal ventral area (PV). While neurones in all of these areas were responsive to cutaneous st imulation, those of SI were non-habituating, whereas those in SII and PV of ten habituated to the stimuli. Moreover, neuronal receptive fields in SII a nd PV were, in general, larger than those in SI. Neurones in cortex adjacen t to the rostral and caudal boundaries of SI, including cortex that interdi gitated between the discontinuous SI head and body representations, require d stimulation of deep receptors in the periphery to elicit responses. Withi n the region of cortex containing neurones responsive to stimulation of dee p receptors, body parts were represented in a mediolateral progression. Inj ections of anatomical tracers placed in electrophysiologically identified l ocations in SI revealed ipsilateral connections with other parts of SI, as well as cortex rostral to, caudal to, and interdigitating between, SI. Inje ctions in SI also resulted in labelling in PV, SII, motor cortex, posterior parietal cortex and perirhinal cortex. The patterns of contralateral proje ctions reflected those of ipsilateral projections, although they were relat ively less dense. The present findings support recent observations in other marsupials in which multiple representations of the body surface were desc ribed, and suggest that multiple interconnected sensory representations may be a common feature of cortical organization and function in marsupials.