Aim 2. To examine the relationship between dopamine release and the Reward Positivity (RewP) potential. Subjects will be presented with the option to participate in an event-related potential (ERP) study. The investigators will correlate the amount of DA release with the amplitude of the RewP potential, an ERP that is enhanced in reward versus non-reward circumstances. Based on previous studies, the investigators hypothesize that RewP amplitude will positively correlate with dopamine release in the nucleus accumbens.