Thalamic connectivity of the second somatosensory area and neighboring somatosensory fields of the lateral sulcus of the macaque.

The thalamocortical relations of the somatic fields in and around the lateral sulcus of the macaque were studied following cortical injections of tritated amino acids and horseradish peroxidase (HRP). Special attention was paid to the second somatosensory area (S2), the connections of which were also studied by means of thalamic isotope injections and retrograde degeneration. S2 was shown to receive its major thalamic input from the ventroposterior inferior thalamic nucleus (VPI) and not, as previously reported, from the caudal division of the ventroposterior lateral nucleus (VPLc). Following small injections of isotope or HRP into the hand representation of S2, only VPI was labeled. Larger injections, which included the representations of more body parts, led to heavy label in VPI, with scattered label in VPLc, the central lateral nucleus (CL), and the posterior nucleus (Po). In addition, small isotope injections into VPLc did not result in label in S2 unless VPI was also involved in the injection site, and ablations of S2 led to cell loss in VPI. Comparison of injections involving different body parts in S2 suggested a somatotopic arrangement within VPI such that the trunk and lower limb representations are located posterolaterally and the hand and arm representations anteromedially. The location of the thalamic representations of the head, face, and intraoral structures that project to S2 may be in the ventroposterior medial nucleus (VPM). The granular (Ig) and dysgranular (Id) fields of the insula and the retroinsular field (Ri) each receive inputs from a variety of nuclei located at the posteroventral border of the thalamus. Ig receives its heaviest input from the suprageniculate-limitans complex (SG-Li), with additional inputs from Po, the magnocellular division of the medial geniculate n. (MGmc), VPI, and the medial pulvinar (Pulm). Id receives its heaviest input from the basal ventromedial n. (VMb), with additional inputs from VPI, Po, SG-Li, MGmc, and Pulm. Ri receives its heaviest input from Po, with additional input from SG-Li, MGmc, Pulm, and perhaps VPI. Area 7b receives its input from Pulm, the oral division of the pulvinar, the lateral posterior n., the medial dorsal n., and the caudal division of the ventrolateral n. These results indicate that the somatic cortical fields, except for those comprising the first somatosensory area, each receive inputs from an array of thalamic nuclei, rather than just one, and that individual thalamic somatosensory relay nuclei each project to more than one cortical field.