Following the baseline PET scans, patient participants will begin medication treatment with either lithium or lamotrigine, based on the clinical judgement of the treating psychiatrist. Psychological measures will be obtained every 2 weeks. After 6 weeks of medication treatment at a therapeutic dose, patients will be assessed for remission (defined as a 50% decrease in the HDRS score from baseline). If this criteria is met, patient participants will then have follow-up PET scans (one CUMI and one DASB). If this criteria is not met, the patient will be switched to the other medication under study and will be reevaluated after an additional 4 weeks of medication treatment. Patients who still do not demonstrate a 50% decrease in their HDRS will be considered non-responders and will have repeat CUMI and DASB scans.