Effect of Modulating Gamma Oscillations Using tACS

Inclusion Criteria: * Clinical Diagnosis of mild to moderate AD\* * Mini Mental State Examination (MMSE) \> 18 * Mild AD ≥ 21 * Moderate AD 18-20 * Demonstration or history of memory impairments. \* Confirmation of diagnosis will be made by the study MD based on a holistic consideration of the participant's cognitive evaluation and history. * Amyloid positive PET imaging * At least 45 years old * 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 * IQ\> 85 as determined by the WTAR and no history of intellectual disability Exclusion Criteria: * Current history of poorly controlled migraines including chronic medication for migraine prevention * Current or past history of any neurological disorder other than dementia, such as epilepsy, stroke, progressive neurologic disease (e.g. multiple sclerosis) or intracranial brain lesions; and history of previous neurosurgery or head trauma that resulted in residual neurologic impairment. * Past or current history of major depression, bipolar disorder or psychotic disorders, or any other major psychiatric condition. * Contraindication for undergoing MRI or receiving TMS or tACS, * \>50 mSv of radiation exposure for research within the past year (PET imaging exclusion) * Presence of the Thr/Thr polymorphism in the TSPO gene (rs6971) due to low affinity binding for the PBR 28 (microlgia) PET scan * History of fainting spells of unknown or undetermined etiology that might constitute seizures. * History of seizures, diagnosis of epilepsy, history of abnormal (epileptiform) EEG or immediate (1st degree relative) family history of epilepsy; with the exception of a single seizure of benign etiology (e.g. febrile seizure) in the judgment of the investigator. * Chronic (particularly) uncontrolled medical conditions that may cause a medical emergency in case of a provoked seizure (cardiac malformation, cardiac dysrhythmia, asthma, etc.). * Metal implants (excluding dental fillings) or devices such as pacemaker, medication pump, nerve stimulator, TENS unit, ventriculo-peritoneal shunt, cochlear implant, unless cleared by the study MD. * Substance abuse or dependence within the past six months. * Medications will be reviewed by the responsible MD and a decision about inclusion will be made based on the following: The patient's past medical history, drug dose, history of recent medication changes or duration of treatment, and combination of CNS active drugs. * All female participants that are pre-menopausal will be required to have a pregnancy test; any participant who is pregnant or breastfeeding will not be enrolled in the study. * Subjects who, in the investigator's opinion, might not be suitable for the study * A hair style or head dress that prevents electrode contact with the scalp or would interfere with the stimulation (for example: thick braids, hair weave, afro, wig)