A dopaminergic system may play a role in some forms of motor learning. At a subcortical level, studies in animals and humans suggest that the striatum and its dopaminergic innervation participate in both the acquisition and retrieval of programs for sequential movements. Moreover, as the activity of mesencephalic dopaminergic neurons is modulated by reward predictability, the nigrostriatal dopaminergic system has been proposed to participate in reinforcement of learning. At a cortical level, it has been suggested that, in humans, the dopaminergic system might modulate cortico-cortical connectivity during volitional movements. The first purpose of work is to investigate the dynamic involvement of striatal dopamine (DA) release during different phases of implicit motor sequence learning in patients with Parkinson's disease (PD) compared with healthy subjects (HS) matched for age, gender, handedness, and level of education. Indeed, PD is mainly known as a dopamine deficiency disorder-nigral projections to the motor striatum being most affected-and patients suffering from PD usually have difficulties in learning and execution of sequential motor tasks. The second purpose of this study is to assess cortico-cortical connectivity during implicit motor sequence learning in both HS and PD patients. Implicit motor sequence learning will be studied using a probabilistic serial reaction time (SRT) task in which the sequence presentation of the stimuli is based on a finite-state grammar. Implicit motor sequence learning will be studied using a probabilistic serial reaction time (SRT) task in which the sequence presentation of the stimuli is based on a finite-state grammar. Learning will be assessed by improvement in reaction time. Striatal DA release will be assessed in vivo with positron emission tomography (PET) using a DA D2 receptor radioligand: [(11)C]raclopride. Striatal [(11)C]raclpride binding potential (BP) will be measured in both HS and PD under 3 main conditions: at rest and during early and advanced learning phases of the SRT task. At the same time, we will use EEG recording to test the effect of learning the SRT task on cortico-cortical coherence in both HS and PD patients. This study should provide new information on the (path)physiological role of dopamine in the formation of motor memory.