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.