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Topographically divergent and convergent connectivity between premotor and primary motor cortex.

This study was undertaken to determine the topographic organization of connections between the forelimb representations of the ventral premotor cortex (PMv) and the primary motor cortex (M1). Intracortical microstimulation techniques were used in three experimentally naive squirrel monkeys to delineate the M1 and PMv forelimb representations in the hemisphere contralateral to the dominant hand. Small amounts of biotinylated dextran amine (BDA) were then injected in the PMv distal forelimb representation. Following tangential sectioning, the location of the injection core in PMv and BDA-labeled cell bodies and synaptic boutons in M1 were documented in relation to functional topography. Whereas the injection core was mainly located within the distal forelimb representation in PMv, BDA-labeled cell bodies and terminals were distributed over comparable proportions of proximal and distal forelimb representations in M1. These results suggest that neuronal populations within PMv send topographically divergent outputs to M1 and receive topographically convergent inputs from M1. Finally, we found that PMv projections to M1 were not evenly distributed but rather were directed consistently to three domains within the rostro-lateral portion of M1. To our knowledge, this is the first description of such a consistent clustering of PMv terminals within M1.