Projections of the precentral motor cortex and other cortical areas of the frontal lobe to the subthalamic nucleus in the monkey.

Corticofugal projections were examined by means of the autoradiographic tracer method in 21 macaca fascicularis. The labeled material was injected into the main body representation areas of the precentral motor cortex and into various regions of Brodmann's areas 6, 8 and 9 of the frontal lobe. The ipsilateral subthalamic nucleus receives a moderately strong and somato-topically organized projection from Woolsey's precentral motor cortex. This projection is mainly restricted to the lateral moiety. The remaining nucleus is occupied by less intensive projections from premotor and prefrontal areas. It is concluded that the subthalamic nucleus is a convergence site of pallidal and corticomotor and frontal projections. Cortical afferents may exert an influence on the pallido-subthalamic-pallidal inhibitory feedback loop.