Projections of the claustrum to the primary motor, premotor, and prefrontal cortices in the macaque monkey.

The claustrum is interconnected with the frontal lobe, including the motor cortex, prefrontal cortex, and cingulate cortex. The goal of the present study was to assess whether the claustral projections to distinct areas within the frontal cortex arise from separate regions within the claustrum. Multiple injections of tracers were performed in 15 macaque monkeys, aimed toward primary motor area (M1), pre-supplementary motor area (pre-SMA), SMA-proper, rostral (PMd-r) and caudal (PMd-c) parts of the dorsal premotor cortex (PM), rostral (PMv-r) and caudal (PMv-c) parts of the ventral PM, and superior and inferior parts of area 46. The distribution of retrogradely labeled neurons showed no clear segregation along the rostrocaudal axis of the claustrum; they were usually located along the entire anteroposterior extent of the claustrum. For all motor cortical areas, there was a general trend of the labeled neurons to occupy the dorsal and intermediate parts of the claustrum along the dorsoventral axis. The same territories were labeled after injection in area 46, but in addition numerous labeled neurons were found in the most ventral part of the claustrum. At higher resolution, however, there was clear evidence that the territories projecting to pre-SMA and SMA-proper formed separate, interdigitating, clusters along the dorsoventral axis. A comparable local segregation was observed for the two subdivisions of area 46, whereas there was more local overlap among the subareas of PM. The projections from the claustrum to the multiple subareas of the motor cortex and to area 46 arise from largely overlapping territories, with, however, some degree of local segregation.