The motor system resonates to the distal goal of observed actions: testing the inverse pliers paradigm in an ecological setting

Does motor mirroring in humans reflect the observed movements or the goal of the observed motor acts? Tools that dissociate the agent/object dynamics from the movements of the body parts used to operate them provide a model for testing resonance to both movements and goals. Here, we describe the temporal relationship of the observer's motor excitability, assessed with transcranial magnetic stimulation (TMS), with the observed goal-directed tool actions, in an ecological setting. Motor-evoked potentials (MEPs) to TMS were recorded from the opponens pollicis (OP, thumb flexor) and the extensor indicis proprius (EIP, index extensor) muscles of participants while they observed a person moving several small objects with a pair of normal pliers (closed by finger flexion) or reverse pliers (opened by finger flexion). The MEPs were a significant predictor of the pliers' kinematics that occurred in a variable time interval between -400 and +300 ms from TMS. Whatever pliers' type was being observed, OP MEPs correlated positively and EIP MEPs correlated negatively with the velocity of pliers' tips closure. This datum was confirmed both at individual and at a group level. Motor simulation can be demonstrated in single observers in a "real-life" ecological setting. The relation of motor resonance to the tool type shows that the observer's motor system codes the distal goal of the observed acts (i.e., grasping and releasing objects) in terms of its own motor vocabulary, irrespective of the actual finger movements that were performed by the observed actor.