Prevention of Alzheimer's Disease With CR Plus tDCS in Mild Cognitive Impairment and Depression (PACt-MD)

MCI Group Inclusion: * Age \> 60 (on day of randomization) * DSM 5 criteria for Mild Neurocognitive Disorder ("MCI") * Willingness to provide informed consent * MADRS score of 10 or below * Availability of a study partner who has regular contact with the participant * Ability to read and communicate in English (with corrected vision and hearing, if needed) Exclusion: * Met DSM 5 criteria for Major Depressive Episode in past 10 years * Lifetime DSM 5 diagnosis of schizophrenia, bipolar disorder, or OCD * DSM 5 diagnosis of alcohol or other substances use disorder within the past 12 months * High risk for suicide * Significant neurological condition (e.g., stroke, seizure disorder, MS) * Unstable medical illness, (e.g., uncontrolled diabetes mellitus or hypertension) * Having taken a cognitive enhancer (acetylcholinesterase inhibitor or memantine) within the past 6 weeks. * Participants taking anticonvulsants, and other psychotropic medication (see exceptions below) that cannot be safely tapered and discontinued. The following psychotropic medications are allowed: i) any antidepressant; ii) zopiclone, trazadone, or a benzodiazepine if they have been taken at a stable dose for at least 4 weeks prior to study entry and; iii) gabapentin and pregabalin if they have been taken at a stable dose for at least 4 weeks prior to study entry AND if prescribed for chronic pain. * A pace-maker or other metal implants that would preclude safe use of tDCS. MDD Group Inclusion: * Age ≥ 65 (on day of randomization) * Meets DSM 5 criteria for one or more MDE(s)with: 1. an offset of 2 months to 5 years from the screening visit date. It is not necessary for this (these) episode(s) to have received medical attention OR 2. an offset of 5 years or more from the screening visit date. It is necessary that at least one MDE received medical attention (e.g., previously been on one or more antidepressant(s), saw a psychiatrist, primary care physician, or had a previous hospitalization). Also, the MDE must have occurred during the participant's adult life (i.e., at 18 years of age or older). * MADRS score of 10 or below * Willingness to provide informed consent * Availability of a study partner who has regular contact with the participant * Ability to read and communicate in English (with corrected vision and hearing, if needed) Exclusion: * Meets DSM 5 criteria for Major Neurocognitive Disorder ("dementia") * Lifetime DSM 5 diagnosis of schizophrenia, bipolar disorder, or OCD * DSM 5 diagnosis of alcohol or other substances use disorder within the past 12 months. * High risk for suicide. * Significant neurological condition (e.g., stroke, seizure disorder, MS) * Unstable medical illness (e.g., uncontrolled diabetes mellitus or hypertension) * Participants taking anticonvulsants, and other psychotropic medication (see exception below) that cannot be safely tapered and discontinued. In addition to any antidepressant, the following psychotropic medications are allowed if they have been taken at a stable dose for at least 4 weeks prior to study entry: zopiclone, trazodone, or a benzodiazepine; and gabapentin or pregabalin if prescribed for chronic pain. * Having taken a cognitive enhancer (acetylcholinesterase inhibitor or memantine) within the past 6 weeks. * A pace-maker or other metal implants that would preclude safe use of tDCS. * Received electroconvulsive therapy (ECT) within 6 months of baseline neruopsychological testing. Control group Inclusion: * Age \> 60 * MADRS score of 10 or below * Willingness to provide informed consent * Ability to read and communicate in English (with corrected vision and hearing, if needed) Exclusion: * Meets DSM 5 criteria for Minor or Major Neurocognitive Disorder * Any other lifetime DSM 5 diagnosis except for simple/specific phobias * Significant neurological condition (e.g., stroke, seizure disorder, MS) * Unstable medical illness, (e.g., uncontrolled diabetes mellitus or hypertension) * Participants taking anticonvulsants, and other psychotropic medication (see exception below) that cannot be safely tapered and discontinued. The following psychotropic medications are allowed if they have been taken at a stable dose for at least 4 weeks: zopiclone up to 15 mg/day; trazadone up to 150 mg/day; benzodiazepine at a dose of up to 3 mg/day lorazepam-equivalents; gabapentin and pregabalin (if prescribed for pain). * A pace-maker or other metal implants * Neuropsychological testing within the past 12 months