Inclusion Criteria:  Men and women aged 60 and older Current Major Depressive Disorder (MDD) Failure to respond adequately to two or more antidepressant treatment trials of recommended dose and length Patient Health Questionnaire-9 (PHQ-9) score of 10 or higher  Exclusion Criteria:  Dementia Lifetime diagnosis of bipolar I or II disorder, schizophrenia, schizoaffective disorder, schizophreniform disorder, delusional disorder, or current psychotic symptoms. High risk for suicide and unable to be managed safely in the clinical trial Non-correctable, clinically significant sensory impairment interfering with participation Unstable medical illness, including delirium, uncontrolled diabetes mellitus, hypertension, hyperlipidemia, or cerebrovascular or cardiovascular risk factors that are not under medical management. Moderate to severe substance or alcohol use disorder Seizure disorder. Parkinson's Disease Individuals with any contraindications to MRI