The role of the supplementary motor area in the control of voluntary movement

The contribution of the supplementary motor area (SMA) to the preparation of voluntary movement has been revealed by various experimental methods. These include studies of movement-related cortical potentials recorded from surface and subdural electrodes, extracellular recordings from SMA neurones in monkeys, studies of regional cerebral blood flow, clinical studies of movement deficits associated with SMA lesions and disruption of basal ganglia output to the SMA in Parkinson's disease. The SMA is found to be especially involved in self-paced, or well-learnt and predictable movements which can be internally-determined. In Parkinsonian subjects, however, the SMA is only involved in non-cued movements which must be internally-determined; this may reflect both the reliance on external cues, and the deficit in using internal predictive models to guide movement, which are associated with Parkinson's disease. SMA involvement is also more reliant upon timing than on spatial cues, indicating its role in the temporal organisation of sequential movements, rather than the programming of spatial movement parameters. These observations suggest an internal-cuing mechanism, involving interaction between the SMA and the basal ganglia to mediate the temporal organisation of voluntary and internally-determine sequential movement.