FUNCTIONAL ANATOMY OF THE SOMATOSENSORY CORTEX OF THE MONKEY

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By use ot anatomical markers-to trace the connections between 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 occipitbtemporal 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 rhe 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, bedomes 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 insular cortex 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 comparisions suggest that sensory- limbic interaction through a hierarchically organized cortical pathway is a mechanism common to memory formation in all sensory modalities.