TRANSCALLOSAL CONNECTIONS OF THE DISTAL FORELIMB REPRESENTATIONS OF THE PRIMARY AND SUPPLEMENTARY MOTOR CORTICAL AREAS IN MACAQUE MONKEYS

The goal of the present neuroanatomical study in macaque monkeys was t wofold: (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 transcallosa l 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 retrogra de tracer substances in order to label the neurons of origin in the co ntralateral motor cortical areas (four monkeys) or, with an anterograd e tracer, to establish the regional distribution and density of termin al fields in the opposite motor cortical areas (two monkeys). The main results were: (I) The hand representation of M1 exhibited a modest ho motopic 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 opposit e 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 cal losal projections from the contralateral rostral and caudal cingulate motor areas, moderate projections from the lateral premotor cortex, an d sparse projections from M1. (3) After injection of an anterograde tr acer (biotinylated dextran amine) in the hand representation of MI, on ly 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, ho wever, resulted in dense and widely distributed axonal terminal fields in the opposite SMA, premotor cortex, and cingulate motor areas, whil e labeled terminals were clearly less dense in M1. It is concluded tha t the hand representations of the SMA and M1 strongly differ with resp ect 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 bilater ally organized system, possibly contributing to bimanual coordination.