The project is a placebo-controlled study that aims to use closed-loop transcranial alternating current stimulation (tACS) to study patients with symptoms of mild cognitive impairment which is likely due to Alzheimer's disease or another form of dementia (AD-MCI). Patients will undergo an EEG and complete some questionnaires and computer tasks during each study visit. The project has the following aims and hypotheses: 1.) To determine the impact of closed-loop 40 Hz tACS on the entrainment of natural gamma rhythms in patients with AD-MCI, 2.) To determine the impact of closed-loop 40 Hz tACS on cognitive performance in patients with AD-MCI, and 3.) To assess the relationship between baseline neurodegenerative burden and impact of tACS. [exploratory]

Inclusion Criteria:

• Overall cognitive functional status consistent with amnesic MCI (CDR 0.5) likely due to AD (AD biomarker supported)
• 50-80 years of age
• English native speakers

Exclusion Criteria:

• Known presence of a structural brain lesion (e.g., tumor, cortical infarct)
• Acute or decompensated active medical conditions, including cancer, cardiovascular disease, stroke, congestive heart failure
• Active hematological, renal, pulmonary, endocrine or hepatic disorders
• Longstanding premorbid history (i.e., longer than 10 years) of alcohol or substance abuse with continuous abuse up to and including the time that the symptoms leading to clinical presentation developed
• tACS contraindications (lesions in the scalp, history of seizures)