Somatic and motor components of action simulation

Seminal studies in monkeys report that the viewing of actions performed by other individuals activates frontal and parietal cortical areas typically involved in action planning and execution [1-3]. That mirroring actions might rely on both motor and somatosensory components is suggested by reports that action observation and execution increase neural activity in motor [4-13] and in somatosensory areas [8-10, 1417]. This occurs not only during observation of naturalistic movements (4-17] but also during the viewing of biomechanically impossible movements that tap the afferent component of action, possibly by eliciting strong somatic feelings in the onlooker [18, 19]. Although somatosensory feedback is inherently linked to action execution [20], information on the possible causative role of frontal and parietal cortices in simulating motor and sensory action components is lacking. By combining low-frequency repetitive and single-pulse transcranial magnetic stimulation, we found that virtual lesions of ventral premotor cortex (vPMc) and primary somatosensory cortex (S1) suppressed mirror motor facilitation contingent upon observation of possible and impossible movements, respectively. In contrast, virtual lesions of primary motor cortex did not influence mirror motor facilitation. The reported double dissociation suggests that vPMc and S1 play an active, differential role in simulating efferent and afferent components of observed actions.