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. \[exploratory\]