Subcortical projections to the frontal pole in the marmoset monkey.

The subcortical projections to the marmoset frontal pole were mapped with the use of fluorescent tracer injections. The main thalamic projections, which originated in both the magnocellular and parvocellular subdivisions of the mediodorsal nucleus, were topographically organized. Our results suggest the existence of a third, caudal subdivision of this nucleus, which is likely to be homologous to the macaque's pars densocellularis. A substantial, but not topographically organized, projection to Brodmann's area 10 originated in the medial part of the ventral anterior nucleus. Minor thalamic projections originated in the medial pulvinar nucleus and in the midline/intralaminar nuclei. Finally, the posterior thalamic group (including the limitans and suprageniculate nuclei) sent a small projection to rostral area 10 that has not previously been documented in primates. The main extrathalamic projections stemmed from the claustrum, which contained as many as 50% of all subcortical labelled neurons. Minor connections originated in the hypothalamus (mainly in the lateral anterior and lateral tuberal regions), dorsal periaqueductal grey matter, basal forebrain (nucleus basalis of Meynert and horizontal limb of the diagonal band of Broca), and amygdala (basal, accessory basal and lateral nuclei). The present results, combined with recent data on the cortical projections to area 10, reveal the frontal pole as a region that integrates information from multiple neural processing systems, including high-level sensory, limbic and working memory-related structures. Although the pattern of subcortical projections is similar to that previously described in the macaque, suggesting a homologous organization, the present data also suggest functional distinctions between medial and lateral sectors of area 10.