A reaction time task, known as the "Simon task" in the literature, allows one to study the influence of irrelevant visual information on decision making. In the most common version of this task (used in the prosed study), the subjects have to choose between a left- and a right-hand keypress according to the color of a visual signal presented either to the left or to the right of a fixation. The to be established association is said "congruent" when the response is ipsilateral to the stimulus and "incongruent" when the response is contralateral to the stimulus. In healthy volunteers, EMG investigations have revealed that in a significant numbers of trials, the contraction of the response agonist is preceded by a infra-liminal contraction of the agonist involved in the non-required response. Such "partial errors" demonstrate that the nervous system is able to detect, abort and correct a part of its errors, thereby revealing the existence of an on-line executive control in simple decision tasks. Behavioral studies performed in Parkinson disease patients, suggest that these patients may experience a deficit in such a control.