Objective: In the central nervous system, short-term plasticity can be defined as a change in connection strength that is induced by receiving presynaptic inputs. Dopamine (DA), a neuromodulatory neurotransmitter, is believed to play a role in short-term plasticity. [18F]dopa positron emission tomography (PET) studies have shown progressive impairment of presynaptic dopaminergic (DAergic) function in both healthy elderly subjects (HES) and patients with Parkinson's disease (PD). Both PD and HES are characterized by a progressive decline in performance on some motor tasks and changes in activity in brain areas involved in motor control (including motor control by cognitive functions) as shown by functional imaging studies. However, little is known about the exact neurobiological mechanisms linking DAergic function, brain activity and motor (and cognitive) performances in PD and HES. In the present study, we hypothesize that impaired performance on some motor tasks in those subject groups is related to a dysfunction of task-specific functional connections between areas participating in motor control. In addition, the brain activation with fMRI across sessions and across subjects in the healthy group will be examined to investigate the reliability of the fMRI techniques.