During each training session, subjects will have encephalographic (EEG) recordings to measure the electrical activity of the brain. In addition, they will have one or two positron emission tomography (PET) scans during the first training session, and some will also have one or two PET scans during the last session. For the PET scan, the subject will be injected with a substance called raclopride, which is taken up by the brain. The raclopride is tagged with a radioactive substance so that it can be detected by the PET camera. The amount of raclopride detected in the brain will provide an indirect measure of the amount of dopamine released during training.