Supplementary motor area of the monkey's cerebral cortex: short- and long-term deficits after unilateral ablation and the effects of subsequent callosal section.

The short-term and long-term behavioral effects of unilateral lesions of the supplementary motor area (SMA) were studied in five monkeys (Macaca fascicularis ssp). A monkey with a unilateral lesion of the premotor area (PM) served as a control. In all animals, general behavior was unaffected by the lesions. For a few weeks postoperatively, all monkeys showed a clumsiness of forelimb movements, bilaterally, which involved both the distal and proximal muscles. Two SMA-lesioned monkeys (but not the PM-lesioned one), studied for up to 1 year postoperatively, showed a characteristic deficit of bimanual coordination where the two hands tended to behave in a similar manner instead of sharing the task between them. This deficit was more pronounced after a lesion contralateral to the nonpreferred hand. The deficit was interpreted as indicating that the intact SMA now influenced the motor outflow of both the ipsilateral hemisphere and the contralateral one through the corpus callosum. Callosal section immediately abolished the bimanual deficit, although the clumsiness returned transiently. The results imply that SMA may give rise normally to discharges informing the contralateral hemisphere of intended and/or ongoing movements via the corpus callosum. This may prevent duplication of responses during unimanual tasks and help to coordinate the behavior of both hands during a bimanual task.