Corticospinal excitability modulation to hand muscles during the observation of appropriate versus inappropriate actions

Transcranial magnetic stimulation (TMS) studies have shown that the observation of an action causes subliminal activation within the motor system. However, the issue of whether such an effect is modulated by the match between the observed action and that the observer would have exhibited if acting under similar circumstances remains unclear. We address this issue by recording motor potentials evoked by single-pulse TMS from the first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles during the observation of video-clips representing prehensile actions towards small or large objects. In a separate behavioral study, participants were asked to evaluate which type of grasp would be the most appropriate for the tested objects. The TMS data revealed a selective motor facilitation during the observation of movements recruiting the targeted digits. We contend that, in action observation tasks, the human corticospinal system mediating action observation effects codes merely for the visual aspects of the observed action.