Motor cortex excitability is tightly coupled to observed movements

Although facilitation of the corticospinal (CS) system during action observation is a widely accepted phenomenon, it is still controversial if facilitation reflects the replica of observed movement kinematics or the tension to achieve a particular goal. In this study, transcranial magnetic stimulation (TMS)-induced motor evoked potentials (MEPs) were recorded from the abductor digiti minimi (ADM) and the first dorsal interosseus (FDI) muscles while participant-volunteers observed a model grasping a small target eliciting a precision grip or a large target eliciting a whole hand grasp directed toward an isolated object or flanked by different sized objects (i.e., distractor). A detailed movement analysis revealed that the model's kinematics were influenced by the distractor's size. Video clips filming the scene were edited in such a way that the distractor was removed from the scene. Participant-volunteers were asked to observe actions characterized by the same goal but performed using different kinematical patterns. Although the differences in movement kinematics were not noticed by the participant-volunteers, they nonetheless elicited distinct configurations of corticospinal activation. Detailed motor matching seems to recruit the same muscles in the onlooker as in the person actually carrying out the action during observation of grasping actions. These effects appear to be elicited by very subtle, imperceptible aspects of observed actions pointing to a finely tuned mechanism that specifically encodes body parts.