Motor activation during action perception depends on action interpretation

Since the discovery of motor mirroring, the involvement of the motor system in action interpretation has been widely discussed. While some theories proposed that motor mirroring underlies human action understanding, others suggested that it is a corollary of action interpretation. We put these two accounts to the test by employing superficially similar actions that invite radically different interpretations of the underlying intentions. Using an action-observation task, we assessed motor activation (as indexed by the suppression of the EEG mu rhythm) in response to actions typically interpreted as instrumental (e.g., grasping) or referential (e.g., pointing) towards an object. Only the observation of instrumental actions resulted in enhanced mu suppression. In addition, the exposure to grasping actions failed to elicit mu suppression when they were preceded by speech, suggesting that the presence of communicative signals modulated the interpretation of the observed actions. These results suggest that the involvement of sensorimotor cortices during action processing is conditional on a particular (instrumental) action interpretation, and that action interpretation relies on inferential processes and top-down mechanisms that are implemented outside of the motor system.