Changes in corticospinal excitability associated with motor learning by observing.

While many of our motor skills are acquired through physical practice, we can also learn how to make movements by observing others. For example, individuals can learn how to reach in novel dynamical environments ('force fields', FF) by observing the movements of a tutor. Previous neurophysiological and neuroimaging studies in humans suggest a role for the motor system in motor learning by observing. Here, we tested the role of primary motor cortex (M1) in motor learning by observing. We used single-pulse transcranial magnetic stimulation to elicit motor-evoked potentials (MEPs) in hand muscles at rest. MEPs were elicited before and after participants observed either a video showing a tutor adapting her reaches to an FF or a control video showing a tutor performing reaches in an unlearnable FF. During MEP acquisition, participants fixated a crosshair while their hand muscles were relaxed. We predicted that observing motor learning would result in greater increases in offline M1 excitability compared to observing movements that did not involve learning. We found that observing FF learning resulted in subsequent increases in MEP amplitudes recorded from right first dorsal interosseous and right abductor pollicis brevis muscles at rest. There were no changes in MEP amplitudes after control participants observed a tutor performing similar movements but not learning. The observed MEP changes can thus be specifically linked to observing motor learning. These results are consistent with the idea that observing motor learning produces functional changes in M1, corticospinal networks or both.