Motor areas in the frontal lobe of the primate.

There has been a substantial change in our concepts about the cortical motor areas. It is now clear that the frontal lobe of primates contains at least six premotor areas that project directly to the primary motor cortex (M1). Two premotor areas, the ventral premotor area (PMv) and the dorsal premotor area (PMd), are located on the lateral surface of the hemisphere. Four premotor areas are located on the medial wall of the hemisphere and include the supplementary motor area (SMA) and three cingulate motor areas. Each of these premotor areas has substantial direct projections to the spinal cord. Corticospinal axons from the premotor areas terminate in the intermediate zone of the spinal cord, and some also terminate in the ventral horn around motoneurons. In this respect, the premotor areas are like M1 and appear to have direct connections with spinal motoneurons, particularly those innervating hand muscles. Furthermore, it is possible to evoke movements of the distal and proximal forelimb using intracortical stimulation at relatively low currents in the premotor areas. Thus, the premotor areas appear to have the potential to influence the control of movement not only at the level of M1, but also more directly at the level of the spinal cord. For these reasons, we have suggested that the premotor areas may operate at a hierarchical level comparable to M1. We propose that each premotor area is a functionally distinct efferent system that differentially generates and/or controls specific aspects of motor behavior.