Mirroring avatars : dissociation of action and intention in human motor resonance

Observation of others' actions induces a subliminal activation of motor pathways (motor resonance) that is mediated by the mirror neuron system and reflects the motor program encoding the observed action. Whether motor resonance represents the movements composing an action or also its motor intention remains of debate, as natural actions implicitly contain their motor intentions. Here, action and intention are dissociated using a natural and an impossible action with the same grasping intention: subjects observe an avatar grasping a ball using either a natural hand action ('palmar' finger flexion) or an impossible hand action ('dorsal' finger flexion). Motor-evoked potentials (MEPs), elicited by single transcranial magnetic stimulation of the hand area in the primary motor cortex, were used to measure the excitability modulation of motor pathways during observation of the two different hand actions. MEPs were recorded from the opponens pollicis (OP), abductor digiti minimi (ADM) and extensor carpi radialis (ECR) muscles. A significant MEP facilitation was found in the OP, during observation of the grasping phase of the natural action; MEPs in the ADM were facilitated during observation of the hand opening phase of the natural action and of both opening and grasping phases of the impossible action. MEPs in the ECR were not affected. As different resonant responses are elicited by the observation of the two different actions, despite their identical intention, we conclude that the mirror neuron system cannot utilize the observer's subliminal motor program in the primary motor cortex to encode action intentions.