Patients will be re-assessed every month following the initiation of treatment for the 3 months following baseline. Subjects will be re-assessed with the information processing assessment, neurocognitive battery , behavioral and side effect ratings, and information about drug dosage and compliance with treatment at each visit. Moreover, subjects will be asked to participate in a PET study of dopamine D2 occupancy at the third month visit . Trained raters blind to patient's drug condition will conduct behavioral and side effect ratings.