FUNCTIONAL ANATOMY OF THE SOMATOSENSORY CORTEX OF MACAQUES

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By use of anatomical markers to trace the connections etween somatosensory cortical fields, we have identified two possible routes by which somatosensory information could reach limbic structures important for somatosensory memory. One route courses dorsally from the postcentral strip through posterior parietal cortex and has access to the limbic system via cingulate cortex. This pathway may be functionally analogous to the occipitoparietal pathway for spatial vision. The second route courses ventrally from the postcentral strip through SII and insular cortex to the amygdala and indirectly to the hippocampus through rhinal cortex. We now have strong evidence that this second pathway is analogous to the occipitotemporal pathway for object vision. First, by recording single- and multi-unit activity from the SII region in hemispheres that had received lesions of selected portions of the postcentral body representation, we have demonstrated loss of the corresponding representation from SII, indicating that at least this part of the ventrally directed somatosensory pathway is arranged as a cortical hierarchy. An unexpected finding in this study was that, following a postcentral lesion, the SII region undergoes major functional reorganization, in that the SII tissue vacated by the original representation does not remain silent but, instead, becomes occupied by representations of different body parts, providing evidence for a previously unrecognized degree of cortical plasticity in adult primates. Our neurobehavioral evidence indicates that bilateral insular lesions cause a severe tactile recognition deficit, consistent with the suggestion that insularcortex is a critical link in a parieto-insulo-limbic pathway for tactile recognition, and so occupies a position analogous to that of area TE in the occipito-temporo-limbic pathway for visual recognition. These comparisons suggest that sensory limbic interactions through a hierarchically organized cortical pathway is a mechanism common to memory formation in all sensory modalities.