Adaptive tDCS for Treatment-Resistant MDD

Inclusion Criteria: 1. Adults age 18 to 65 2. History of major depressive disorder with current depressive symptoms, as defined by DSM-IV criteria 3. Baseline score of Hamilton Depression Rating Scale 17-item (HDRS-17) score of 17 or higher 4. Failure of at least 3 prior lifetime antidepressant trials 5. Good command of the English language Exclusion Criteria: 1. Lifetime history of the following, defined by DSM-IV criteria per study physician: 1. Moderate or severe substance use disorder in the past 6 months with the exception of nicotine use disorder 2. Dementia or major neurocognitive disorder 3. Schizophrenia spectrum disorders 4. Autism spectrum disorder 5. Depression which has failed to respond to ECT, with treatment failure as determined by evaluating physician 6. Failure of greater than 6 antidepressant trials in the current depressive episode 2. Current evidence of the following, defined by present symptomology as per study physician: 1. Active psychotic symptoms 2. Bipolar disorder with a current manic, hypomanic, or mixed episode as defined by DSM-IV criteria 3. Depression secondary to a general medical illness 4. Dysphoria better explained by a personality disorder 5. Dysphoria associated with a primary anxiety disorder or PTSD 6. Active suicidal intention (inability to contract for safety) 7. Any change in psychotropic medication within 30 days of the initiation of tDCS 8. History of neurological disorders (e.g., epilepsy) and/or off any treatment medication for a neurological disorder for minimum 3 years (36 months) 9. Any open wounds (e.g., blisters, burns including sun burns, cuts or irritation) under or near the scalp regions where electrodes are placed 10. Metal implants (e.g., Deep Brain Stimulator, Vagus Nerve Stimulator) 11. Any skin disorders (e.g., eczema, severe rashes, sensitive skin, any communicable skin disorder or treatment for a communicable skin disorder in the past 12 months) or other skin defects which compromise the integrity of the skin at or near stimulation locations (where electrodes are placed) 12. Any history of head injury resulting in unconsciousness for more than 5 minutes (e.g., head trauma, brain surgery) 13. Metallic face or scalp tattoos 14. Any chronic headaches or migraines (occurring for consecutive days and are longer than an hour) 15. If a participant has a change in the rate or severity of head pressure, headache, or migraine in the past two weeks, they will also be excluded as per study clinician as we are not aiming to assess the treatment of headache with tDCS 16. Any personal history of seizures. 3. Currently pregnant or lactating, as determined in part from urine pregnancy test at Screening/Baseline appointment and medical history. 4. Any other factor that in the investigators' judgment may affect patient safety or compliance (e.g. distance greater than 100 miles from clinic)