Corticospinal excitability during action observation in task-specific dystonia: a transcranial magnetic stimulation study.

Observation of actions performed by other individuals activates the onlooker's motor system in a way similar to real movement execution. The functioning of this mechanism in the pathological domain is not clear yet. The aim of this study was to explore whether action observation activates the motor system of patients affected by a task-specific form of dystonia, such as writer's cramp. Transcranial magnetic stimulation was applied over the primary motor cortex and motor evoked potentials were recorded from hand (FDI and ADM) and forearm (FCR) muscles at baseline and during observation of actions (grasping and writing) or images. Writing actions could be performed with healthy or dystonic movement patterns. Results showed a highly specific and reversed pattern of activation in the FDI muscle of the two groups. Differences between the two writing conditions were significantly opposite in the two groups: control subjects had higher activation during observation of the dystonic compared to the healthy action, whereas in patients observation of the healthy writing led to higher activation than the dystonic writing. This opposite corticospinal modulation might be explained by a different self-attribution of the observed actions in the two groups.

Expertise with pathological actions modulates a viewer's motor system.

Brain mechanisms for action understanding rely on matching the observed actions into the viewer's motor system. Health professionals, who treat patients affected by movement disorders as dystonia, frequently see hyperkinetic action patterns characterized by an overflow of muscle co-contractions. To avert an overload of the motor system during observation of those actions, they might need to look at dystonic motor symptoms in a cool, detached way. To investigate whether visual expertise about atypical movement kinematics influences the viewer's motor system, we applied transcranial magnetic stimulation to clinicians and to naive subjects, while they observed handwriting actions performed with two different kinematics: fluent and non-fluent. Crucially, the latter movement pattern was easily recognized by the clinicians as a typical expression of writer's cramp, whereas it was unknown to the naive subjects. Results showed that clinicians had similar corticospinal activation during observation of dystonic and healthy writings, whereas naive subjects were hyper-activated during observation of dystonic movements. Hyper-activation was selective for the muscles directly involved in the dystonic co-contractions and inversely correlated with subjective movement fluency scores, hinting at a fine-tuned association between the breakdown of observed movement fluency and corticospinal activation. These findings suggest that observation of unusual pathological actions differently modulates the viewer's motor system, depending on knowledge, visual expertise, and ability in recognizing suboptimal movement kinematics.