Effects of Gamma-tACS on Memory and Sleep

The purpose of this clinical trial is to investigate the feasibility and efficacy of non-invasive transcranial alternating current stimulation (tACS) at gamma frequency in enhancing memory recall and modulating sleep network dynamics measured by at-home electroencephalography (EEG) in healthy elderly people. Eligible participants will first collect sleep EEG at home for one night to acclimate to the data collection during sleep. Participants are then randomized into first undergoing either tACS at gamma band frequencies (i.e. 40Hz) or tACS at a control frequency (i.e. 21Hz). Stimulation is administered in the lab during a cognitive testing battery that includes memorizing items. After a night of sleep with EEG at home, participants return to the lab the following day to measure memory recall. Recall is performed again after five days. This sequence of encoding during stimulation in the lab, sleep EEG at home for one night, and recall is then repeated for the other stimulation condition about a week later. Participants are wearing an actigraphy wristband throughout the study period.


Eligibility Criteria:
Inclusion Criteria:

* \>50 years old

Exclusion Criteria:

* Implanted device or metal in head (including cochlear implant or other hearing aid), cardiac pacemaker or any other powered medical device
* Known neurological disease from history (epilepsy, sleep disorder (insomnia, sleep apnea, restless legs syndrome, parasomnia), stroke or transitory ischemic attack, cognitive impairment, neurodegenerative disease (for example Alzheimer's disease, Parkinson's disease or amyotrophic lateral sclerosis), immune-mediated disease of the central nervous system, chronic infectious brain disease, brain tumor, traumatic brain injury with loss of consciousness and/or intracranial bleeding, chronic pain with the need for daily analgesic use)
* Positive screening for epilepsy (questionnaire)
* Pathological Montreal Cognitive Assessment (MoCA \<26/30 points)
* Brain surgery in the past (lifetime)
* Known psychiatric disorder from history (schizophrenia (lifetime), obsessive compulsive disorder (OCD; lifetime), borderline personality disorder (lifetime), anxiety disorder (lifetime), bipolar disorder (lifetime), psychosis (lifetime), eating disorder (lifetime), depression (within the last three months)
* Positive screening for anxiety disorder (GAD-7 ≥10/21 points) or positive screening for depression (PHQ-9 ≥5/27 points)
* Known other relevant medical condition from history (moderate to severe chronic obstructive pulmonary disease (COPD), abnormal kidney function (defined as estimated Glomerular Filtration Rate \<60ml/min), current liver disease (defined as hepatitis and/or liver cirrhosis), cancer, diabetes mellitus, cardiac disease (heart failure, myocardial infarct, atrial fibrillation and revascularization - all within the last three months)
* Working in night shifts or going to bed after midnight on 3 or more nights per week
* Positive screening for sleep disorder (PSQI \>5/21 points)
* Psychotropic treatment or illegal drugs (including cannabis) within the last three months
* Indication for alcohol use disorder: AUDIT score (Alcohol Use Disorders Identification Test; screening for unhealthy alcohol use) ≥7 for females and for males ≥ 65 years or ≥8 for males \<65 years
* Not willing to abstain alcohol at least 24 hours before each study visit
* Pregnancy, planned pregnancy, fertility treatment planned or ongoing
* Having experienced an adverse event in the past after receiving Transcranial Magnetic Stimulation (TMS)
* If, after filling out a questionnaire on the participant's medical history that informs researchers of relevant events that marginally increase the risk of an adverse event occurring during TMS, researchers will exercise their expert judgement to determine whether that participant should be excluded