Neural activity of supplementary and primary motor areas in monkeys and its relation to bimanual and unimanual movement sequences

A chronic single-unit study of motor cortical activity was undertaken in tw o monkeys trained to perform a bimanually coordinated task. The hypothesis was tested that the supplementary motor area plays a specific role in coord inating the two hands for common goal-oriented actions. With this objective , a special search was made for neurons that might exhibit properties exclu sively related to bimanual task performance. Monkeys learned to reach for a nd to pull open a spring-loaded drawer with one hand, while the other hand reached out to grasp food from the drawer recess. The two hands were precis ely coordinated for achievement of this goal. Monkeys also performed, in se parate blocks of trials, only the pulling or grasping movements, using the same hands as in the bimanual task. Task-related activity of 348 neurons fr om the supplementary motor area and 341 neurons from the primary motor area , each examined in the bimanual and in both unimanual tasks, was recorded i n the two hemispheres. Most neurons from the supplementary motor area were recorded within its caudal microexcitable portion. Contrary to expectation, the proportion of neurons with activity patterns related exclusively to th e bimanual task was small, but somewhat higher in the supplementary motor a rea (5%) than in the primary motor cortex (2%). Another group of neurons th at were equally modulated during the bimanual as well as to both unimanual task components might also contribute in controlling bimanual actions. Such "task-dependent" rather than "effector-dependent" activity patterns were m ore common in neurons of the supplementary motor area (19%) than of the pri mary motor cortex (5%). Bilateral receptive fields were also more numerous among the supplementary motor area neurons. However, a large majority of ne urons from primary and supplementary motor areas had activity profiles clea rly related only to contralateral hand movements (65% in the primary motor and 51% in the supplementary motor area). A similar group of neurons showed an additional slight modulation with ipsilateral movements; they were equa lly common in the two areas (14% and 16%, respectively) and their significa nce for bimanual coordination is questionable. Summed activity profiles of all neurons recorded in the primary and supplementary motor areas of the sa me hemisphere were compared. The modulations of the three histograms, corre sponding to the two unimanual and the bimanual tasks, were similar for the two motor areas, i.e. prominent with bimanual and contralateral movements a nd weak with ipsilateral movements. It is concluded that the supplementary motor area is likely to contribute t o bimanual coordination, perhaps more than the primary motor cortex, but th at it is not a defining function for the former cortical area. Instead, it is suggested that the supplementary motor area is part of a callosally inte rconnected and distributed network of frontal and parietal cortical areas t hat together orchestrate bimanual coordination. (C) 1999 IBRO. Published by Elsevier Science Ltd.