Thalamic projections to the macaque caudal ventrolateral prefrontal areas 45A and 45B

We studied the sources of thalamic projections to the caudal ventrolateral prefrontal areas 45A and 45B, which display markedly distinct cortical connections [M. Gerbella et al. (2010) Cereb. Cortex, 20, 141-168], and compared them with those to area 8/FEF (frontal eye field). Both areas 45A and 45B were the targets of highly predominant projections from the mediodorsal nucleus (MD) and of additional projections, mostly from the magnocellular ventral anterior and the medial pulvinar nucleus. The projection profiles from different MD subdivisions clearly distinguished these two areas from one another and from area 8/FEF. Area 45A was the target of predominant projections from parvicellular MD and of minor, albeit robust, projections from magnocellular MD. The opposite was true for area 45B: magnocellular MD was the major source of projections and parvicellular MD contributed minor, albeit robust, projections. Furthermore, area 45B, but not area 45A, was targeted by robust projections from multiform MD, the principal thalamic nucleus for area 8/FEF. These results provide further evidence for the distinctiveness of areas 45A and 45B, and support the idea that area 45B is affiliated with the frontal oculomotor system, challenging the proposed homology of this area with part of the human language-related area 45 (rostral part of Broca's region). Furthermore, the present data provide evidence for potentially robust trans-thalamic (via magnocellular MD) afferent, as well as direct and reciprocal, amygdaloid connections of areas 45A and 45B, suggesting the contribution of emotional information to the differential role of these two areas in non-spatial information processing.