The somatotopic organization of the supplementary motor area: intracortical microstimulation mapping

The somatotopic organization of the supplementary motor area (SMA) is commonly held to consist of a rostrocaudal sequence of orofacial, forelimb, and hindlimb representations. Recently, however, this somatotopy has been questioned. Studies of regional cerebral blood flow in humans and the movements evoked by intracortical electrical stimulation in cynomolgus monkeys have been unable to reveal evidence of distinct orofacial, forelimb, and hindlimb representations rostrocaudally situated along the medial cortex of the hemisphere. Partly on the basis of those results, it has been suggested that the SMA functions as a nontopographically organized "higher-order" motor center. The present study reexamines SMA organization by observing stimulation-evoked movements. The medial frontal cortex of 2 rhesus monkeys was mapped using a modified intracortical microstimulation technique. We observed a forelimb representation mainly on the medial surface of the hemisphere in both animals. Rostral or rostrolateral to the forelimb representation, depending on the individual, we evoked orofacial movements (including eye movements). Hindlimb movements were evoked from tissue overlapping, but largely caudal to, the forelimb representation. Thus, we conclude that there is a clear rostrocaudal progression of orofacial, forelimb, and hindlimb movement representations in the SMA.