Subcortical afferents to the lateral mediodorsal thalamus in cynomolgus monkeys.

The mediodorsal (MD) nucleus of the thalamus has long been known to provide the principal source of subcortical input to the primate prefrontal cortex, as well as to other areas of the frontal lobe that are thought to contribute to higher-order cognitive functions. In this study, we used injections of retrograde tracers in the lateral portion of the monkey MD to assess the locations of labeled cells in subcortical structures. Three main patterns were identified in the distribution of subcortical connections. We found that the claustrum, superior colliculus and ventral midbrain regions were heavily labeled in the cases with injections in caudoventral MD. In these cases, labeled cells were also found in either the periaqueductal gray or zona incerta, depending on the specific case. In one case with an injection in anterodorsal MD, labeled cells were most numerous in the structures of the ventral midbrain, especially the ventral tegmental area. Finally, the claustrum and superior colliculus contained the largest percentage of labeled subcortical cells in cases with injections in ventrolateral MD. These three patterns of subcortical label corresponded to three equally distinctive trends in the distribution of MD connections with the cortex in these same cases [J Comp Neurol 473 (2004) 107]. Very few labeled cells were found in other areas such as the amygdala, globus pallidus and deep cerebellar nuclei, suggesting that pathways leading from these structures to dorsolateral and dorsomedial frontal cortices are not likely to include the lateral divisions of MD. In concert, these findings show that particular locales within lateral MD receive distinct profiles of subcortical afferents, and project into specific neocortical domains, suggesting that these different sites within lateral MD may participate in functionally distinct circuits of information processing.