Current research and theories have highlighted that the parameters linked to movement planning, e.g. the decision to select a movement goal, and adaptation, e.g. the ability to update control dependent on a novel dynamical context, are update quickly and within an ongoing movement in the general population. In parallel, it has been suggested that the evaluation of movement costs is a function of the basal ganglia, and it is impaired in Parkinson disease (PD). Here the investigators want to test whether these mechanisms also alter the ability of patients to update control during an ongoing action. A positive result would confirm that movement-related costs and dynamical representations depend on the integrity of the basal ganglia, in contrast a negative result would indicate that the deficit in PD is more specifically related to movement initiation, which would invite one to reconsider the cause of bradykinesia in this population.