Understanding Brain Mechanisms Involved in Autobiographical Memory

Inclusion Criteria: Cognitively-Unimpaired Younger and Older Adults * Age 18 to 35 years old (younger adults) * Age 55+ years old (older adults) * without any cognitive impairment (based on the Montreal Cognitive Assessment: MoCA) * willing and capable to give informed consent for participation in the study after it has been thoroughly explained able * willing to comply with all study requirements informed consent form was signed Mild Cognitive Impairment (MCI) patients * Age 55+ years old * Clinical Diagnosis of MCI * Confirmation of diagnosis will be made by the study MD based on a holistic consideration of the participant's cognitive evaluation and history * Mini-Mental State Examination (MMSE) ≥ 18 (Mild AD ≥ 21) * CDR ≥ .5 * Demonstration or history of autobiographical memory impairments * On a stable dose of medications for memory loss including cholinesterase inhibitors (e.g. donepezil, rivastigmine) or memantine as defined as 6 consecutive weeks of treatment at an unchanging dose * Minimum of completed 8th-grade education * willing and capable to give informed consent for participation in the study after it has been thoroughly explained - note that to ensure this in the case of the MCI participants, a cut-off of MOCA score \>18 will be applied * able and willing to comply with all study requirements informed consent form was signed Exclusion Criteria: Cognitively-Unimpaired Younger and Older Adults * any cognitive impairment captured by the Montreal Cognitive Assessment (MoCA) - score \< 26 * major psychiatric co-morbidity including major depressive disorder, schizophrenia, or psychosis * blindness or other disabilities that prevent task performance * Contraindication for undergoing MRI * Any metal in the brain or skull (excluding dental fillings) or elsewhere in the body unless cleared by the responsible covering MD (e.g. MRI compatible joint replacement) Mild Cognitive Impairment (MCI) patients * Age \< 55 years old * Any current diagnosis of a major psychiatric disorder (e.g., schizophrenia, bipolar disorder, major depressive disorder) * Other than MCI, any history of other progressive or genetic neurologic disorder (e.g. Parkinson's disease, multiple sclerosis, tubular sclerosis) or acquired neurological disease (e.g. stroke, traumatic brain injury, tumor), including intracranial lesions * History of head trauma resulting in prolonged loss of consciousness * Current history of poorly controlled headaches including chronic medication for migraine prevention * History of fainting spells of unknown or undetermined etiology that might constitute seizures * History of seizures, diagnosis of epilepsy, or immediate (1st-degree relative) family history of epilepsy with the exception of a single seizure of benign etiology (e.g. febrile seizures) in the judgment of a board-certified neurologist * Any unstable medical condition or chronic (particularly) uncontrolled medical conditions that may cause a medical emergency in case of a provoked seizure (cardiac malformation, cardiac dysrhythmia, asthma, etc.) or study complication * contraindications to tACS, as recorded on a standardized screening questionnaire, which include a reported seizure within the past two years, use of neuroactive drugs, self-reported presence of specific implanted medical devices (e.g., deep brain stimulator, medication infusion pump, cochlear implant, pacemaker, etc.), or the presence of any active dermatological condition, such as eczema, on the scalp * Contraindication for undergoing MRI or receiving tACS * Any metal in the brain or skull (excluding dental fillings) or elsewhere in the body unless cleared by the responsible covering MD (e.g. MRI compatible joint replacement) * Any devices such as a pacemaker, medication pump, nerve stimulator, TENS unit, ventriculoperitoneal shunt unless cleared by the responsible covering MD