This study will examine and compare what happens in the brains of patients with Parkinson's disease with that of healthy normal subjects while they train to react as fast as possible to the appearance of a visual signal. Particularly, we will measure the amount of the chemical dopamine released in the brain as well as the electrical activity during training. Indeed, patients with Parkinson's disease frequently complain of slowness and early fatigue during movements. These symptoms are believed to be related to a decrease of dopamine in the brain which may be associated with abnormalities in cerebral electrical activity.