Mirror and (absence of) counter-mirror responses to action sounds measured with TMS

Sonification of combined action observation and motor imagery: Effects on corticospinal excitability

Action observation and motor imagery are valuable strategies for motor learning. Their simultaneous use (AOMI) increases neural activity, with related benefits for motor learning, compared to the two strategies alone. In this study, we explored how sonification influences AOMI. Twenty-five participants completed a practice block based on AOMI, motor imagery and physical execution of the same action. Participants were divided into two groups: An experimental group that practiced with sonification during AOMI (sAOMI), and a control group, which did not receive any extrinsic feedback. Corticospinal excitability at rest and during action observation and AOMI was assessed before and after practice, with and without sonification sound, to test the development of an audiomotor association. The practice block increased corticospinal excitability in all testing conditions, but sonification did not affect this. In addition, we found no differences in action observation and AOMI, irrespective of sonification. These results suggest that, at least for simple tasks, sonification of AOMI does not influence corticospinal excitability; In these conditions, sonification may have acted as a distractor. Future studies should further explore the relationship between task complexity, value of auditory information and action, to establish whether sAOMI is a valuable for motor learning.

How Task Interactivity Shapes Action Observation

Action observation triggers imitation, a powerful mechanism permitting interpersonal coordination. Coordination, however, also occurs when the partners’ actions are nonimitative and physically incongruent. One influential theory postulates that this is achieved via top-down modulation of imitation exerted by prefrontal regions. Here, we rather argue that coordination depends on sharing a goal with the interacting partner: this shapes action observation, overriding involuntary imitation, through the predictive activity of the left ventral premotor cortex (lvPMc). During functional magnetic resonance imaging (fMRI), participants played music in turn with a virtual partner in interactive and noninteractive conditions requiring 50% of imitative/nonimitative responses. In a full-factorial design, both perceptual features and low-level motor requirements were kept constant throughout the experiment. Behaviorally, the interactive context minimized visuomotor interference due to the involuntary imitation of physically incongruent movements. This was paralleled by modulation of neural activity in the lvPMc, which was specifically recruited during the interactive task independently of the imitative/nonimitative nature of the social exchange. This lvPMc activity reflected the predictive decoding of the partner’s actions, as revealed by multivariate pattern analysis. This demonstrates that, during interactions, we process our partners’ behavior to prospectively infer their contribution to the shared goal achievement, generating motor predictions for cooperation beyond low-level imitation.

Evidence for a dyadic motor plan in joint action

What mechanisms distinguish interactive from non-interactive actions? To answer this question we tested participants while they took turns playing music with a virtual partner: in the interactive joint action condition, the participants played a melody together with their partner by grasping (C note) or pressing (G note) a cube-shaped instrument, alternating in playing one note each. In the non-interactive control condition, players’ behavior was not guided by a shared melody, so that the partner’s actions and notes were irrelevant to the participant. In both conditions, the participant’s and partner’s actions were physically congruent (e.g., grasp-grasp) or incongruent (e.g., grasp-point), and the partner’s association between actions and notes was coherent with the participant’s or reversed. Performance in the non-interactive condition was only affected by physical incongruence, whereas joint action was only affected when the partner’s action-note associations were reversed. This shows that task interactivity shapes the sensorimotor coding of others’ behaviors, and that joint action is based on active prediction of the partner’s action effects rather than on passive action imitation. We suggest that such predictions are based on Dyadic Motor Plans that represent both the agent’s and the partner’s contributions to the interaction goal, like playing a melody together.

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 aimed at reconciling these results. We measured 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 them by performing 400 free-choice button presses. We then 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 correspond to the button-tone contingencies, whilst longer-term practice results in representations that rely on lower-level intrinsic parameters associated with the kinematics of specific movements.