Convergence of pallidal and cerebellar outputs on the frontal motor areas

The general pattern of the organization of the thalamo-cortical projections to the agranular frontal cortex of the monkey is still matter of debate. An important issue is whether each motor area is the target of a single thalamic nucleus or it receives afferences from multiple thalamic sources. In this light it is of interest to determine whether the basal ganglia and cerebellar outputs, which are segregated in the thalamus, remain segregated also at the cortical level or, on the contrary, both converge on the same cortical areas. In the present article we present data concerning the thalamic input to mesial area 6 obtained with cortical injections of retrograde neural tracers. This cortical sector, classically considered as coextensive with the so called supplementary motor area (SMA), was recently found to be formed by two independent anatomo-functional areas: F3 (SMA-proper) and F6 (pre-SMA). On the basis of the neurophysiological properties of the two areas we have proposed that F6 plays a hierarchically higher role in motor control than F3. The present results allow us the following main conclusions: a) Each motor area is the target of a distinct set of thalamic nuclei. b) Each area is the target of both basal ganglia and cerebellar outflows. c) As far as the basal ganglia input is concerned, F3 is a part of the so called "basal ganglia motor loop", whereas, F6 belongs to the "basal ganglia complex loop". This differential basal ganglia input provides further evidence in favor of a higher hierarchical role of F6 in comparison to F3.