Subjects. In this prospective study, MSA patients will be included if they met Gilman criteria for probable MSA. All those subjects will be distinguished between parkinsonian form (MSA-P) and cerebellar form (MSA-C). All will underwent Unified Parkinson's Disease Rating Scale UPDRS and International Cooperative Ataxia Rating Scale ICARSS. All patients will have a poor response to levodopa (<30% of the UPDRS score). Patients with PD will be included if they suffered from idiopathic PD according to the criteria of UKPDSBB and had a positive response to levodopa (≥ 30% improvement on UPDRS part III). All healthy subjects will have normal neurological examination and none will have a history of neurological, cardiovascular or psychiatric disturbance. For all subjects, handedness will be determined by the Edinburg test. For all patients (MSA and PD) a MRI brain scan will be realized PET activation study PET investigations will be performed during two pharmacological conditions: OFF (e.g after 12 hours of usual dopaminergic treatment discontinuation) and ON (after an acute oral levodopa challenge) in all subjects. During each PET there will be two motor conditions: rest (no movement, hand and wrist lying on the joystick) and a right-hand movement, consisting of moving joystick in 4 four different directions avoiding sequence repetition. Movement will be paced by an auditory stimulus at a frequency of 0.33 Hz. Each patient will be trained to perform the joystick movement before the PET. During PET investigation, angular wrist speed and angular wrist acceleration will be recorded using a computer recording connected to a joystick. The movement will start 30 seconds before image acquisition. Rest and Movement scan conditions will be replicated, making a total number of 6 six scans per patient in OFF condition and 6 six scans per patient in ON condition. The order of OFF and ON sessions and motor conditions will be fully counterbalanced across subjects to eliminate time and order effects.