A selective representation of the meaning of actions in the auditory mirror system

Mirror neurons in the monkey's premotor cortex respond during both execution and observation of actions and are thought to be critical for understanding others' actions. Human studies have shown premotor cortex activation while viewing actions, hearing their sounds, listening to or reading action-related sentences, and have compared execution and observation of similar actions. However, we still lack direct evidence in humans of the most striking and theoretically relevant feature of mirror neurons, i.e., that they map seen/heard actions onto motor representations of the same actions at an abstract level. Here we combine fast event-related functional magnetic resonance imaging with an unconscious semantic priming paradigm and show that the human auditory mirror system also holds an abstract representation of the meaning of heard actions. We analyzed the effect on brain activity of trial-by-trial semantic congruency between a target sound denoting a hand or mouth action (or an environmental event) and a briefly flashed written word acting as an unconscious cross-modal prime. Left inferior frontal and posterior temporal regions selectively responded to action sounds in a non-somatotopic fashion and were modulated by semantic congruency only in action sound trials. We also observed regions selective for either hand or mouth actions, which however did not show a corresponding effector-specific effect of semantic congruency. These results provide evidence that the human mirror system represents the meaning of actions (but not of other events) (a) at an abstract, semantic level, (b) independently of the effector, and (c) independently of conscious awareness.