Microstimulation Activates a Handful of Muscle Synergies

SummaryMuscle synergies have been proposed as a mechanism to simplify movement control. Whether these coactivation patterns have any physiological reality within the nervous system remains unknown. Here we applied electrical microstimulation to motor cortical areas of rhesus macaques to evoke hand movements. Movements tended to converge toward particular postures, driven by synchronous bursts of muscle activity. Across stimulation sites, the muscle activations were reducible to linear sums of a few basic patterns—each corresponding to a muscle synergy evident in voluntary reach, grasp, and transport movements made by the animal. These synergies were represented nonuniformly over the cortical surface. We argue that the brain exploits these properties of synergies—postural equivalence, low dimensionality, and topographical representation—to simplify motor planning, even for complex hand movements.

► Microstimulation-evoked hand movements converged on postures specific to each site
► Microstimulation-evoked muscle activity was also invariant and unique to each site
► The muscle patterns could be decomposed into synergies found in natural behavior
► These shared synergies were represented nonuniformly over the cortical surface