RCT of Brain Longitudinal Biomarker Study (OPT-Neuro RCT)

Older adult participants with treatment-resistant depression will be randomly assigned to a Step 1 medication strategy. * Adding aripiprazole to current antidepressant medication * Adding bupropion to current antideprssant medication * Replacing current antidepressant medication with bupropion If depression is not relieved at the end of 10 weeks, or if participants do not qualify for Step 1, participants will be randomly assigned to a Step 2 medication strategy: * Adding lithium to current antidepressant medication * Replacing current antidepressant medication with nortriptyline All medication strategies will be offered in collaboration with participants' own physicians with the the research team providing support and guidance. After treatment in Step 1 and/or Step 2, participants will enter the Continuation Phase to assess long term follow-up outcomes for 12 months. Participants in the Optimizing Outcomes of Treatment-Resistant Depression in Older Adults (OPTIMUM) (NCT02960763) study, will also be asked to participate in this clinical trial to gather imaging and biomarker data. The study will test if changes in brain structure and function are associated with decreases in memory. In this study, investigators will conduct a series of assessments/tests, mainly brain imaging and assessments of participant's memory and attention, to better understand how depression is linked to memory and thinking in older persons. * Investigators will be collecting blood biomarkers as part of their study procedures. These samples will be used to look at other factors that may relate to depression or memory and attention processes. * Mechanisms of Late life depression (LLD)-dementia through functional Magnetic Resonance Imaging (fMRI): Analyzing mechanisms of the LLD-dementia relationship through fMRI acquisitions and analyses, to capture the specific brain networks implicated in executive function and episodic memory decline. * Neuropsychological Data: Including Montreal Cognitive Assessment (MoCA), Wide Range Achievement Test-4 (WRAT-4) Reading subtest, Repeatable Battery for the Assessment of Neuropsychological Status (RBANS) and Delis-Kaplan Executive Function System (D-KEFS) (Color Word Interference, Trail Making and Verbal Fluency). * Clinical Scales: Including the Everyday Cognition Scale (E-Cog), Global Clinical Dementia Rating (CDR), Performance Assessment of Selfcare Skills (PASS)-Cognitive Instrumental Activities of Daily Living(CIADL) Short version, Patient Health Questionnaire (PHQ-9), and Suicide Risk Assessments (Suicide Questions, Baseline Suicidal Ideation, Suicide Intent Scale, Beck Lethality Scale, Decision Outcome Inventory, Columbia-Suicide Severity Rating Scale, and High Suicide Risk Protocol). Investigators hope that this study will help the scientific community to understand why some people with depressive symptoms that are resistant to treatment in late-life experience declines in their memory and attention and whether effective treatment of such depression reduces that risk. Finally, investigators hope that this study will eventually lead to the development of better treatment options.