The aim of the present study was to define the origin of the thalamocortical projections to each of the mesial and superior area 6 areas. To this purpose, restricted injections of neuronal tracers were made into areas F3, F6, F2, and F7 after physiological identification of the injection sites. The results showed that each of these areas receives afferents from a set of thalamic nuclei and that this set differs, qualitatively and quantitatively, according to the injected area. The main inputs to F3 [supplementary motor area properly defined (SMA-proper)] originate in the nuclei ventral lateral, pars oralis (VLo), ventral posterior lateral, pars oralis (VPLo), and ventral lateral, pars caudalis (VLc) as well as in caudal parts of the VPLo and VLc (VPLo/VLc complex). F6 (pre-SMA) is mainly the target of nucleus ventral anterior, pars parvocellularis (VApc), and area X of Olszewski. The input to F2 originates mainly in the VPLo/VLc complex, in VLc, and in VLo. The dorsal part of F7 (supplementary eye field) mainly receives from area X, VApc, and nucleus ventral anterior, pars magnocellularis (VAmc), whereas the ventral F7 is connected with VApc, area X, VLc, and the VPLo/VLc complex. All of the injected areas receive a strong projection from the medial dorsal nucleus (MD). It is concluded that each cortical area is a target of both cerebellar and basal ganglia circuits. F3 and F2 are targets of the so-called "motor" basal ganglia circuit and a cerebellar circuit originating in dorsorostral sectors of dentate and interpositus nuclei. In contrast, F6 and ventral F7 receive a basal ganglia input mainly from the so-called "complex" circuit and a cerebellar input originating in the ventrocaudal sectors of dentate and interpositus nuclei. Finally, with respect to the rest of F7, dorsal F7 also receives a basal ganglia input from the "oculomotor circuit."
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