Self and other in the human motor system

Observation of another's action can selectively facilitate the brain's motor circuits for making the same action [1-3]. A "mirror-matching mechanism" might map observed actions onto the observer's own motor representations [4, 5]. Crucially, this view suggests that the brain represents others' actions like one's own. However, this hypothesis has been difficult to test because the experience of one's own body differs from that of others' bodies with respect to viewpoint, morphological features, familiarity, and the hallmark feature of kinaesthetic experience. We used an established method for manipulating the sense of body ownership ("rubber-hand illusion") to compare effects of observing actions that either were or were not illusorily attributed to the subject's own body. We show that observing another's actions facilitated the motor system, whereas observing identical actions, which Were illusorily attributed to the subject's own body, showed the opposite pattern. Thus, motor facilitation strongly depends on the agent to whom the observed action is attributed. this result contradicts previous concepts of equivalence between one's own actions and actions of others and suggests that social differentiation, not equivalence, is characteristic of the human action system.