Single-Trial Dynamics of Competing Reach Plans in the Human Motor Periphery

Contemporary motor control theories propose competition between multiple motor plans before the winning command is ex-ecuted. While most competitions are completed before movement onset, movements are often initiated before the competi-tion has been resolved. An example of this is saccadic averaging, wherein the eyes land at an intermediate location between two visual targets. Behavioral and neurophysiological signatures of competing motor commands have also been reported for reaching movements, but debate remains about whether such signatures attest to an unresolved competition, arise from aver-aging across many trials, or reflect a strategy to optimize behavior given task constraints. Here, we recorded EMG activity from an upper limb muscle (m. pectoralis) while 12 (8 female) participants performed an immediate response reach task, freely choosing between one of two identical and suddenly presented visual targets. On each trial, muscle recruitment showed two distinct phases of directionally tuned activity. In the first wave, time-locked ;100 ms of target presentation, muscle activ-ity was clearly influenced by the nonchosen target, reflecting a competition between reach commands that was biased in favor of the ultimately chosen target. This resulted in an initial movement intermediate between the two targets. In contrast, the second wave, time-locked to voluntary reach onset, was not biased toward the nonchosen target, showing that the competition between tar-gets was resolved. Instead, this wave of activity compensated for the averaging induced by the first wave. Thus, single-trial analysis reveals an evolution in how the nonchosen target differentially influences the first and second wave of muscle activity.

Automated summary

Contemporary motor control theories propose competition between multiple motor plans before the winning command is executed. This study shows that muscle activity during immediate response reach task is influenced by the nonchosen target and reveals different phases of directionally tuned activity, indicating an evolution in how the nonchosen target influences muscle activity.