The goal of the present neuroanatomical study in macaque monkeys was twofold: (1) to clarify whether the hand representation of the primary motor cortex (M1) has a transcallosal projection to M1 of the opposite hemisphere; (2) to compare the topography and density of transcallosal connections for the hand representations of M1 and the supplementary motor area (SMA). The hand areas of M1 and the SMA were identified by intracortical microstimulation and then injected either with retrograde tracer substances in order to label the neurons of origin in the contralateral motor cortical areas (four monkeys) or, with an anterograde tracer, to establish the regional distribution and density of terminal fields in the opposite motor cortical areas (two monkeys). The main results were: (1) The hand representation of M1 exhibited a modest homotopic callosal projection, as judged by the small number of labeled neurons within the region corresponding to the contralateral injection. A modest heterotopic callosal projection originated from the opposite supplementary, premotor, and cingulate motor areas. (2) In contrast, the SMA hand representation showed a dense callosal projection to the opposite SMA. The SMA was found to receive also dense heterotopic callosal projections from the contralateral rostral and caudal cingulate motor areas, moderate projections from the lateral premotor cortex, and sparse projections from M1. (3) After injection of an anterograde tracer (biotinylated dextran amine) in the hand representation of M1, only a few small patches of axonal label were found in the corresponding region of M1, as well as in the lateral premotor cortex; virtually no label was found in the SMA or in cingulate motor areas. Injections of the same anterograde tracer in the hand representation of the SMA, however, resulted in dense and widely distributed axonal terminal fields in the opposite SMA, premotor cortex, and cingulate motor areas, while labeled terminals were clearly less dense in M1. It is concluded that the hand representations of the SMA and M1 strongly differ with respect to the strength and distribution of callosal connectivity with the former having more powerful and widespread callosal connections with a number of motor fields of the opposite cortex than the latter. These anatomical results support the proposition of the SMA being a bilaterally organized system, possibly contributing to bimanual coordination.

The goal of the present study was to clarify whether the primary motor cortex (M1) and the supplementary motor cortex (SMA) both receive, via the motor thalamus, input from cerebellar and basal ganglia output nuclei. This is the first investigation that explores the problem by direct comparison, in the same animal, of thalamic zones that 1) project to M1 and SMA and 2) receive cerebellar-nuclear (CN) and pallidal (GP) afferents. These four zones were mapped in two monkeys by means of two retrograde tracers for M1 and SMA injections and of two anterograde tracers for CN and GP injections. All injections were performed under electrophysiological control (microstimulation and multiunit recordings). Injections in cortical areas were restricted to the hand/arm representation; in the SMA, the tracer deposit was within the "SMA-proper" (or "area F3") and did not include its rostral extension ("pre-SMA" or "area F6"). It was found that zones of all four types formed a number of highly complex patches of labeling that were usually not confined to one cytoarchitectonically defined thalamic nucleus. The overlap of clusters of labeled terminals and perikarya was evaluated morphometrically (area measurements) on a number of coronal sections along the anteroposterior extent of the motor thalamus. In line with previous studies, the thalamic territories innervated by CN and GP afferents rarely overlapped. However, zones projecting to M1 and/or to SMA included thalamic regions receiving CN as well as GP projections, providing the first evidence of such overlap from individual animals. The present observations support the previous conclusion from this laboratory (based on transsynaptic labeling) that the SMA receives, apart from its strong pallidal transthalamic input, a CN transthalamic input. These present findings that both M1 and SMA are recipients of transthalamic inputs from GP and CN thus support the concept that a mixed subcortical input consisting of weighted contributions from cerebellum, basal ganglia, substantia nigra, and spinothalamic tract is directed to each functional component of the sensorimotor cortex.

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