The symbolic side of motor resonance

Background: A large body of research indicates that observing actions made by others is associated with corresponding motor facilitation of the observer's corticospinal system. However, it is still controversial whether this matching mechanism strictly reflects the kinematics of the observed action or its meaning. Methods: To test this issue, motor evoked potentials (MEPs) induced by single-pulse transcranial magnetic stimulation (spTMS) were recorded from hand and leg muscles while participants observed a symbolic action carried out with the index finger, but classically performed with the leg (i.e. a penalty kick). A control condition in which participants observed a similar (but not symbolic) hand movement was also included. Results: Results showed that motor facilitation occurs both in the observer's hand (first dorsal interosseus) and leg (quadriceps femoris) muscles. The present study provides evidence that both the kinematics and the symbolic value of an observed action are able to modulate motor cortex excitability. The human motor system is thus not only involved in mirroring observed actions, but is also finely tuned to their symbolic value