To identify a route by which tactile information could reach limbic structures in the temporal lobe, we used axonal transport techniques to trace the connections between somatosensory cortical fields. On the basis of the laminar patterns of these corticocortical connections, we identified them as 'forward' or 'backward' by analogy to similar designations in the visual system, where they have been shown to have functional validity. The analysis indicated that a forward-projecting route could be traced from the subdivisions of the primary somatosensory cortex to the second somatosensory area, SII; from SII to the granular and dysgranular fields of the insula; and from the insula directly to the amygdala and indirectly to the hippocampus via rhinal cortex. This multisynaptic cortico-limbic pathway in the somatosensory system is thus organized in a manner analogous to the multisynaptic cortico-limbic pathway in the visual system. To assess the functional importance of the pathway, we studied the somatosensory receptive fields of neurons in SII cortex following selective ablations within the primary somatosensory cortex and found that elimination of any given representation of the body surface in the postcentral strip eliminated it also in SII. For example, removing the hand representation from the postcentral strip resulted in its disappearance from SII cortex; conversely, removing all other body representations from the post central strip (i.e. except that of the hand) resulted in the preservation in SII of the hand representation only. The electrophysiological data thus provide strong support for the conclusion, based originally on the anatomical data, that tactile information is processed sequentially along a corticortical pathway. The electrophysiological experiments also revealed a surprising degree of functional reorganization in SII cortex following the postcentral cortical ablations. After each partial removal, the vacated representation in SII was filled in by the expansion of the intact, neighboring representations. These last findings point to a previously unrecognized degree of cortical plasticity in adult primates following brain injury.
