A pathway by which somatosensory information could reach the limbic structues in the temporal lobe known to be critical for tactile memory has now been delineated. To trace this pathway, the anatomical connections of electrophysiologically identified somatosensory fields lying in or near the lateral sulcus of the macaque were investigated with both anterograde and retrograde axonal transport techniques. The data show that a series of parallel tactile processing pathways converge on the insular cortex; this region, in turn, projects directly to the amygdala and indirectly to the hippocampus via the rhinal cortex, thus linking the somatosensory cortices with the limbic structures of the temporal lobe. To further describe the functional properties and relationships of the various somatic fields, electrophysiological studies have been undertaken. Neurons of the posterior insula have response properties typical of higher order processing fields in other modalities. Further, the somatic fields in the lateral sulcus appear to be dependent on SI for their somatic input because neurons in these fields no longer respond to tactile stimulation following ablation of SI. To examine this problem from a comparative perspective, the cortical connections of the somatic fields have been studied in the cat. The cat and monkey have similar connectional patterns, but, in the cat, there are more widespread connections among these fields. The contribution of thalamic inputs to the functional properties of the somatic fields have been studied with axonal transport techniques. The findings shed new light on the organization of thalamocortical connections by showing that different thalamic nuclei project to the first (SI) and second (SII) somatic fields and that somatic fields outside of SI receive their inputs from an array of thalamic nuclei rather than just one, as had been thought previously.
