Subjects will be recruited who are drug-naive or who have not received psychotropic drugs for at least 3 weeks (8 weeks for fluoxetine) prior to screening. Subjects also will be excluded if they have: a) serious suicidal ideation or behavior, or current delusions or hallucinations, b) inability to provide informed consent, c) serious, unstable illnesses including hepatic, renal, gastroenterologic, respiratory, cardiovascular (including ischemic heart disease, endocrinologic, neurologic, immunologic, or hematologic disease, d) a history of drug or alcohol abuse within 6 months or alcohol or drug dependence in the last five years (DSM IV criteria), e) not using a medically accepted means of contraception and are a woman of childbearing potential, f) current pregnancy (documented by pregnancy testing prior to each brain scan to avoid exposing a fetus to radiation or to a research MRI scan that is not medically necessary), g) current breast feeding, h) history of ulcerative colitis or toxic megacolon, i) vision and/or hearing problems severe enough to interfere with testing, j) electrocardiographic evidence of ischemia, arrhythmia, conduction defect, or myocardial infarction, k) current blood pressure of greater than 160 mm Hg or less than 90 mm Hg systolic, or greater than 90 mm Hg diastolic, l) clinically significant cerebrovascular or cardiovascular disease, hypertension, congestive heart disease, angina pectoris, clinic evidence of cerebrovascular disease, gross neurological impairment, hyperthyroidism, known hypersensitivity or idiosyncracy to anticholinergic agents (e.g. skin rashes), glaucoma, renal or hepatic impairment, m) current nicotine use or nicotine dependence within last six months (due to the effects of nicotine on the cholinergic system) n) narrow angle glaucoma (due to the possibility of exacerbation of this condition by scopolamine) o) age greater than 55 years (to reduce the biological heterogeneity encompassed by the MDD and BD criteria, since subjects with a late age-at onset for depression have a far greater likelihood of having MRI correlates of cerebrovascular disease than age-matched, healthy controls or age-matched, early-onset depressives), p) exposure within two weeks to medications likely to affect mood or cognition or likely to interact with scopolamine (e.g. narcotics or anti-cholinergic agents)- as verified by history and urine drug screen, q) HIV positive status, r) history of gastric or intestinal obstructions, s) history of urinary retention or bladder obstruction. During the course of this study, participants will be unable to take some medications, including antidepressant or antianxiety agents, sleep aids, diphenhydramine (e.g. Benedryl) or cough/cold preparations that contain diphenhydramine or antihistamines. A detailed list of allowed and not allowed medications is provided in Appendix B in the protocol.