Cortical connections of the somatosensory fields of the lateral sulcus of macaques: evidence for a corticolimbic pathway for touch.

The ipsilateral corticocortical connections of the somatosensory fields of the lateral sulcus of macaques were examined with both anterograde and retrograde axonal transport methods. In most cases, the field of interest was identified prior to the injection of the tracer substance by recording neuronal responses to somatic stimulation. The results show that the second somatosensory area (S2) is reciprocally connected with the retroinsular area (Ri), area 7b, and the granular (Ig) and dysgranular (Id) insular fields. Ri is also reciprocally connected with Ig. Previously reported connections were confirmed between S2 and areas 3a, 3b, 1, and 2 and between area 5 and both area 7 and Ri. Moreover, the portions of Ig and Id that receive somatic inputs were shown to project to the amygdaloid complex. Id projects, in addition, to the perirhinal cortex, which supplies input to the hippocampal formation. The corticocortical projections were found to have two distinct laminar patterns of termination. One is characterized by heavy terminations in layers IV and IIIb and the other by heavy terminations in layer I, but no terminations in layers IV and IIIb. These two patterns were typically found to be reciprocally related. The results suggest that somatosensory information is processed by a series of cortical fields, including areas 3a, 3b, 1, 2, 5, 7b, S2, Ig, and Id. These fields have access to the amygdaloid complex and the hippocampal formation. Thus, a ventrally directed tactile processing pathway can be followed from S1 to the temporal lobe limbic structures via relays in S2 and the insula; this corticolimbic pathway may subserve tactile learning and memory.