From goals to muscles: motor familiarity shapes the representation of action-related sounds in the human motor system.

Numerous studies corroborated the idea that the sound of familiar motor acts triggers a muscle-specific replica of the perceived actions in the listener’s brain. We recently contradicted this conclusion by demonstrating that the representation of newly-learned action-related sounds is not somatotopically organised but rather it corresponds to the goal a particular action aims to achieve. In the present study, we reconciled these contrasting results by showing that a higher degree of motor familiarity obtained via long-term practice with auditory- motor associations shapes a somatotopically organised representation of action-related sounds. We measured motor-evoked potentials (MEPs) to TMS as an index of the functional correspondence between the sensory stimulation and the activity in the listener’s motor cortex. Participants heard two tones of different pitch, void of previous motor meaning, before and after an acquisition phase in which they generated these tones by performing 400 free-choice button presses. After that, we disentangled the representation of the action goal (button–tone association) from the somatotopic (muscle–tone) association by reversing the muscle–button contingencies. Our result supports the hypothesis that the neuronal representations of action-related sounds depends on motor familiarity: perceptuomotor representations of newly-learned actions are muscle– independent and corresponded to the button–tone contingencies; whilst longer-term practice results in representations that relied on lower-level intrinsic parameters associated with the kinematics of specific movements.