Supplementary Motor Area - Functions

Functions

Penfield and Welch in 1951 first described SMA in the monkey brain and the human brain as a representation of the body on the medial wall of the hemisphere. Woolsey and colleagues in 1952 confirmed SMA in the monkey brain, describing it as a rough somatotopic map with the legs in a posterior location and the face in an anterior location. The representations of different body parts were found to overlap extensively. Stimulation of many sites evoked bilateral movements and sometimes movements of all four limbs. This overlapping somatotopic map in SMA was confirmed by many others.

Four main hypotheses have been proposed for the function of SMA: the control of postural stability during stance or walking, coordinating temporal sequences of actions, bimanual coordination, and the initiation of internally generated as opposed to stimulus driven movement. The data, however, tend not to support an exclusive role of SMA in any one of these functions. Indeed, SMA is demonstrably active during non-sequential, unimanual, and stimulus-cued movements.

SMA in the monkey brain may emphasize locomotion, especially complex locomotion such as climbing or leaping. This suggestion was based on studies in which stimulation on a behaviorally relevant time scale evoked complex, full body movements that resembled climbing or leaping. This hypothesis is consistent with previous hypotheses, including the involvement of SMA in postural stabilization, in internally generated movements, in bimanual coordination, and in the planning of movement sequences, because all of these functions are heavily recruited in complex locomotion. The locomotion hypothesis is an example of interpreting the motor cortex in terms of the underlying behavioral repertoire from which abstract control functions emerge, an approach emphasized by Graziano and colleagues.