Recordings were made of the natural dischages of neurones in the supplementary motor area (SMA) of conscious monkeys trained to perform a stereotyped motor task with either hand. 2. Eighty % of the total population of cells showed modulation of their activity during particular movements of either limb. Two thirds of this group had a similar pattern of modulation regardless of whether the contralateral or ipsilateral hand was used. 3. The number of cells whose activity was related to movements of distal joints was approximately equal to that whose discharges occurred with proximal movements. 4. Only 5% of cells tested sent their axons into the pyramidal tract, and only 15% of units investigated showed responses to passive manipulation of the limbs. The effective afferent input usually was of a rather complex kind. 5. The findings suggest that the discharges of a large number of neurones in SMA are changing during particular movements of either arm, and that only a small number of cells receive afferent sensory input. These results contrast with those obtained in the primary motor area and suggest a different role for SMA the control of movement.
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