Perturbation of Macaque Supplementary Motor Area Produces Context-Independent Changes in the Probability of Movement Initiation.

The contribution of the supplementary motor area (SMA) to movement initiation remains unclear. SMA exhibits premovement activity across a variety of contexts, including externally cued and self-initiated movements. Yet SMA lesions impair initiation primarily for self-initiated movements. Does SMA influence initiation across contexts or does it play a more specialized role, perhaps contributing only when initiation is less dependent on external cues? To address this question, we perturbed SMA activity via microstimulation at variable times before movement onset. Experiments used two adult male rhesus monkeys trained on a reaching task. We used three contexts that differed regarding how tightly movement initiation was linked to external cues. Movement kinematics were not altered by microstimulation. Instead, microstimulation induced a variety of changes in the timing of movement initiation, with different effects dominating for different contexts. Despite their diversity, these changes could be explained by a simple model where microstimulation has a stereotyped impact on the probability of initiation. Surprisingly, a unified model accounted for effects across all three contexts, regardless of whether initiation was determined more by external cues versus internal considerations. All effects were present for stimulation both contralateral and ipsilateral to the moving arm. Thus, the probability of initiating a pending movement is altered by perturbation of SMA activity. However, changes in initiation probability are independent of the balance of internal and external factors that establish the baseline initiation probability. The role of the supplementary motor area (SMA) in initiating movement remains unclear. Lesion experiments suggest that SMA makes a critical contribution only for self-initiated movements. Yet SMA is active before movements made under a range of contexts, suggesting a less-specialized role in movement initiation. Here, we use microstimulation to probe the role of SMA across a range of behavioral contexts that vary in the degree to which movement onset is influenced by external cues. We demonstrate that microstimulation produces a temporally stereotyped change in the probability of initiation that is independent of context. These results argue that SMA participates in the computations that lead to movement initiation and does so across a variety of contexts.