Parkinson's disease (PD) is associated with cognitive and emotional disturbances, whose response to dopamine therapy is either absent or, occasionally, deleterious to the patient. By measuring cerebral blood flow (CBF), first with positron emission tomography (PET) and now with functional magnetic resonance imaging (fMRI), researchers have mapped the neurobiological substrates of cognitive and behavioral symptoms in PD. Basal ganglia processing is based on the anatomy of parallel cortico-striatal loops, each of which has a functional role. Dopaminergic overmedication may lead to cognitive impairment via an action on the relatively preserved mesocortical or mesolimbic systems. The overmedication hypothesis has been rekindled recently with the description of disorders of motivation and impulse control in PD. fMRI provides evidence that dopamine may contribute to impulse control disorders (ICDs) in PD via its role as an energizing or activating agent that assigns incentive value to stimuli and actions.