THE ROLE OF THE SUPPLEMENTARY MOTOR AREA IN THE CONTROL OF VOLUNTARY MOVEMENT

The contribution of the supplementary motor area (SMA) to the preparat ion of voluntary movement has been revealed by various experimental me thods. These include studies of movement-related cortical potentials r ecorded 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 an d 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-le arnt and predictable movements which can be internally-determined. In Parkinsonian subjects, however, the SMA is only involved in non-cued m ovements which must be internally-determined; this may reflect both th e reliance on external cues, and the deficit in using internal predict ive models to guide movement, which are associated with Parkinson's di sease. SMA involvement is also more reliant upon timing than on spatia l 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 in teraction between the SMA and the basal ganglia to mediate the tempora l organisation of voluntary and internally-determine sequential moveme nt.