PET and MRI imaging will be used to investigate the aims described above in patients who have bipolar disorder or unipolar depression and are currently experiencing a depressive episode. Both healthy controls and depressed participants with bipolar disorder or unipolar depression will be recruited. Patients who are on medication before enrolling in the study will have a three week washout. At baseline, healthy controls and patients will have an MRI consisting of both structural and functional sequences. Psychological measures will also be obtained at baseline. Within one week of the MRI, both patients and healthy controls will have one CUMI and one DASB PET scan.