Lateralization of motor cortex excitability in stroke patients during action observation: a TMS study.

Action observation activates the same motor areas as those involved in the performance of the observed actions and promotes functional recovery following stroke. Movement observation is now considered a promising tool for motor rehabilitation, by allowing patients to train their motor functions when voluntary movement is partially impaired. We asked chronic-stroke patients, affected by either left (LHD) or right hemisphere (RHD) lesions, to observe either a left or right hand, while grasping a small target (eliciting a precision grip) or a large target (eliciting a whole hand grasp directed towards a target object). To better understand the effects of action observation on damaged motor circuits, we used transcranial magnetic stimulation (TMS) to induce motor evoked potentials (MEP) from two muscles of the unaffected hand in 10 completely hemiplegic participants. Results revealed that LHD patients showed MEP facilitation on the right (contralesional) M1 during action observation of hand-object interactions. In contrast, results showed no facilitation of the left (contralesional) M1 in RHD patients. Our results confirm that action observation might have a positive influence on the recovery of motor functions after stroke. Activating the motor system by means of action observation might provide a mechanism for improving function, at least in LHD patients.