In a series of anatomical experiments, we have identified two routes by which somatosensory information reaches limbic structures important for somatic 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. A second route courses ventrally from the postcentral strip through SII to the insular cortex and then to the amygdala and indirectly to the hippocampus through rhinal cortex. We now have evidence that this second pathway is analogous to the occipitotemporal pathway for object vision. 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. By recording neuronal activity from SII in hemispheres that received lesions of selected portions of the postcentral body representation, we demonstrated a loss of the corresponding representation from SII, indicating a serial flow of information from the postcentral strip to SII. An unexpected finding in this study was that, following a postcentral lesion, the deactivated SII region undergoes major functional reorganization. In another experiment we recorded neuronal activity from the postcentral strip in 4 monkeys 10-12 years after the dorsal roots representing the upper limb were severed. The results indicated that the entire representation of the upper limb had been replaced by an expanded face representation, an expansion an order of magnitude greater than previously thought possible. Both of these experiments, involving perturbations of the central nervous system, resulted in a previously unrecognized degree of cortical plasticity in adult mammals and suggest that the potential for reorganization is greater after central than peripheral nervous system manipulations.
