The goal of this study is to explore the efficacy and safety of 4Hz transcranial alternating current stimulation (tACS) in patients with mild Alzheimer's disease (AD). The study will recruit 40 individuals with mild AD, and the participants will be randomized to either a 4 Hz tACS group or a sham stimulation group. Both groups will undergo 30 one-hour sessions across 3 weeks (21 days). At the end of the intervention and 3-month follow-up, all subjects will repeat the baseline assessments.

Background: Recently, transcranial alternating current stimulation (tACS), which could interact with ongoing neuronal activity, has emerged as a potentially effective and promising treatment for Alzheimer's disease (AD), and the 4Hz frequency was suggested to improve working memory Methods: The proposed study is a double-blinded, randomized controlled trial that will include 40 individuals with mild AD, and the participants will be randomized to either a 4Hz tACS group or a sham stimulation group. Both groups will undergo 30 one-hour sessions in 3 weeks (21 days). The outcome measures will be assessed at baseline, end of intervention and 3 months after the first session. Primary outcome is global cognitive function, assessed by the 11-item cognitive subscale of the Alzheimer's Disease Assessment Scale (ADAS-cog), and the second outcomes include changes in other neuropsychological assessments, structural MRI, resting electroencephalogram (EEG) and simultaneous EEG-functional MRI (fMRI).

Inclusion Criteria:

1. Subjects with informed consent;
2. 45-75 years of age;
3. At least 6 years of education;
4. AD according to the National Institute on Aging and the Alzheimer's Association (NIA-AA) guidelines;
5. Clinical Dementia Rating Scale (CDR)=1.0;
6. Positive findings in amyloid PET imaging or amyloid protein levels in CSF;
7. On a stable dose of cholinesterase inhibitors (e.g. donepezil or rivastigmine) as defined as 6 consecutive weeks of treatment at an unchanging dose, and without any intentions to modify the dosage during the observation period.

Exclusion Criteria:

1. Current or past history of any neurological disorder other than dementia, such as epilepsy, stroke, progressive neurologic disease (e.g. multiple sclerosis), poorly controlled migraines or intracranial brain lesions; and history of previous neurosurgery or head trauma that resulted in residual neurologic impairment;
2. Contraindication for undergoing MRI or receiving tACS;
3. Eczema or sensitive skin;
4. Familial AD;
5. Depression or other psychiatric disorders;
6. Abnormal brain structural magnetic resonance imaging (MRI) scan, including hydrocephalus, stroke, structural lesions, etc. that would potentially confound the outcome;
7. Severe cardiovascular/pulmonary disorders;
8. Other conditions, in the investigator's opinion, might not be suitable for the study.