Anxiety & Depression Association of America

for all participants: - All genders, races, and ethnic origins, aged between 18 and 45 - Capable of providing written informed consent, and fluent in English - Right-handed - Absence of any psychotropic medications for at least 2 weeks - Has a smartphone (iPhone or Android) (needed for Ecological Momentary Assessment) Inclusion Criteria for "Remitted MDD" group: - Meets inclusion criteria for all subjects, plus: - History of MDD as defined by DSM-5 - Absence of anxiety disorder for the past two months Inclusion Criteria for "Current MDD" group: - Meets inclusion criteria for all subjects, plus: - Presence of MDD as defined by DSM-5 - Absence of anxiety disorder for the past two months

for all participants: - Subjects with suicidal ideation where outpatient treatment is determined unsafe by the study clinician. These patients will be immediately referred to appropriate clinical treatment - Pregnant women or women of childbearing potential who are not using a medically accepted means of contraception (defined as oral contraceptive pill or implant, condom, diaphragm, spermicide, IUD, s/p tubal ligation, or partner with vasectomy) - Serious or unstable medical illness, including cardiovascular, hepatic, renal, respiratory, endocrine, neurologic or hematologic disease - History of seizure disorder - History of psychiatric illnesses, other than depression or anxiety disorders among the Current MDD and Remitted MDD groups - History of substance use disorder or alcohol use disorder (as these terms are defined by DSM-5); except depressed subjects may have a history of 'Mild' substance/alcohol use disorder only if it ended as least 12 months ago - History of cocaine or stimulant use or dopaminergic drugs - History or current diagnosis of dementia, or a score of < 26 on the Mini Mental State Examination at the screening visit; - Patients with mood congruent or mood incongruent psychotic features - Current use of other psychotropic drugs - Clinical or laboratory evidence of hypothyroidism - Patients with a lifetime history of electroconvulsive therapy (ECT) - Failure to meet standard MRI safety requirements - Abnormal ECG and lab results - History of seizure disorder - Contraindications for arterial line (e.g., abnormal result on Allen test, Raynaud's syndrome, history of anemia or bleeding disorder, history of fainting from blood draws).