Corticothalamic connections of paralimbic regions in the rhesus monkey

This study addressed the issue of whether paralimbic regions of the cerebral cortex share common thalamic projections. The corticothalamic connections of the paralimbic regions of the orbital frontal, medial prefrontal, cingulate, parahippocampal, and temporal polar cortices were studied with the autoradiographic method in the rhesus monkey. The results revealed that the orbital frontal, medial prefrontal, and temporal polar proisocortices have substantial projections to both the dorsomedial and medial pulvinar nuclei, whereas the anterior cingulate proisocortex (area 24) projects exclusively to the dorsomedial nucleus. These proisocortical areas also have thalamic connections with the intralaminar and midline nuclei. The cortical areas between the proisocortical regions on the one hand and the isocortical areas on the other, that is, the posterior cingulate region (area 23) and the posterior parahippocampal gyrus (areas TF and TH), project predominantly to the dorsal portion of the medial pulvinar nucleus, the anterior nuclear group (AV, AM), and the lateral dorsal (LD) nucleus. Additionally, the posterior cingulate and medial parahippocampal gyri (area TH) have projections to the lateral posterior (LP) nucleus. Thus, it appears that the proisocortical areas, which are characterized by a predominance of infragranular layers and an absence of layer IV, have common thalamic relationships. Likewise, the intermediate paralimbic areas between the proisocortex and isocortical regions, which also have a predominance of infragranular layers but in addition have evidence of a fourth layer, project to the medial pulvinar and to the so-called limbic nuclei, AV, AM, LD, as well as a modality-specific nucleus, LP.