 The investigators design is informed by their experience conducting randomized controlled trials RCTs in older participants with dementia, MCI, or MDD over more than two decades. In the investigators recent donepezil prevention trial, combining donepezil with standard antidepressant maintenance prevented cognitive decline and the incidence of dementia in participants who had had both MDD and MCI. Building on this prevention trial, the investigators conceptualize the proposed study as a high-risk, high-gain RCT aimed at enhancing cognitive reserve and preventing cognitive decline and dementia in a high risk population. If the investigators are successful in this high risk population, then tDCS CR can be tested in, and extended to, the general population i.e., for universal prevention or other groups at high risk for AD i.e., for selective or indicated prevention. 