Conscious and unconscious representations of observed actions in the human motor system

Action observation activates the observer's motor system. These motor resonance responses are automatic and triggered even when the action is only implied in static snapshots. However, it is largely unknown whether an action needs to be consciously perceived to trigger motor resonance. In this study, we used single-pulse TMS to study the facilitation of corticospinal excitability (a measure of motor resonance) during supraliminal and subliminal presentations of implied action images. We used a forward and backward dynamic masking procedure that successfully prevented the conscious perception of prime stimuli depicting a still hand or an implied abduction movement of the index or little finger. The prime was followed by the supraliminal presentation of a still or implied action probe hand. Our results revealed a muscle-specific increase of motor facilitation following observation of the probe hand actions that were consciously perceived as compared with observation of a still hand. Crucially, unconscious perception of prime hand actions presented before probe still hands did not increase motor facilitation as compared with observation of a still hand, suggesting that motor resonance requires perceptual awareness. However, the presentation of a masked prime depicting an action that was incongruent with the probe hand action suppressed motor resonance to the probe action such that comparable motor facilitation was recorded during observation of implied action and still hand probes. This suppression of motor resonance may reflect the processing of action conflicts in areas upstream of the motor cortex and may subserve a basic mechanism for dealing with the multiple and possibly incongruent actions of other individuals.