Inclusion Criteria 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
Exclusion Criteria 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).