Corticospinal Excitability Modulation to Hand Muscles During the Observation of Different Actions

Transcranial magnetic stimulation (TMS) studies have shown that the observation of an action causes subliminal activation within the motor system. However, 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. Does our motor system merely resonate for what we see or for what we would have actually done if acting under similar circumstances? We test this by using TMS, by asking participants to observe video-clips representing reach-to-grasp actions in which the adopted type of grasp might be either appropriate or inappropriate for interacting with the target object (small or a large). Remarkably, such an effect was independent from the appropriateness of the observed action. We contend that, in action observation tasks, the human cortico-spinal system mediating action observation effects, code merely the visual aspects of the observed action.