A Week of 40-Hz Sound Triggers Long-Lasting Amyloid Changes in Monkeys

By Pooja Toshniwal PahariaReviewed by Lauren HardakerJan 15 2026

A week of precisely timed sound doubled cerebrospinal amyloid-β in aged monkeys for more than a month, offering rare primate evidence that non-invasive brain rhythms can reshape Alzheimer’s-linked biology.

Study: Long-term effects of forty-hertz auditory stimulation as a treatment of Alzheimer’s disease: Insights from an aged monkey model study. Image credit: AtlasStudio/Shutterstock.com

In a recent study published in Proceedings of the National Academy of Sciences, researchers explored gamma-frequency auditory stimulation at 40 Hz as a treatment for Alzheimer’s disease (AD) in monkeys.

Forty-Hertz Sound Produces Sustained CSF Amyloid Shift

One week of stimulation more than doubled amyloid-beta (Aβ) protein levels (a ~200 % increase) in cerebrospinal fluid (CSF), consistent with enhanced interstitial-to-CSF Aβ efflux, potentially mediated by glymphatic or meningeal lymphatic mechanisms. However, elevated CSF Aβ does not necessarily indicate a reduction in brain amyloid burden and may also reflect altered Aβ redistribution or production dynamics.

The increase persisted for at least five weeks (35 days) after stimulation ended, an observation not previously reported in rodents. Pathological examinations revealed age-associated amyloid plaque pathology in the temporal cortex, with little to no tau pathology. The findings suggest that 40-Hz stimulation may be a potential, non-invasive strategy to expand the therapeutic landscape of AD.

Why New Alzheimer’s Treatments Are Still Needed

Alzheimer’s disease is the leading cause of dementia and a major neurodegenerative disorder. Characterized by gradual cognitive decline, particularly in memory and learning functions, the disease involves the accumulation of phosphorylated tau-comprising neurofibrillary tangles (NFTs) and Aβ protein-comprising senile plaques (SPs) in parenchymal cells of the brain.

The United States Food and Drug Administration (US FDA) has authorized a few monoclonal antibodies to manage AD. However, these drugs have moderate efficacy in the initial stage of AD, do not halt disease progression, and are associated with multiple side effects. These limitations warrant the development of safer and more effective treatments.

Auditory stimulation of 40-Hz has shown therapeutic promise for AD in rodents. However, therapeutic validation in non-human primates (NHPs) is crucial for clinical translation, particularly given conflicting findings reported in some prior rodent studies.

Testing Gamma-Frequency Sound in Aged Monkeys

In the present study, researchers administered gamma-frequency auditory stimulation to rhesus monkeys. Subsequently, they monitored changes in CSF tau and Aβ protein levels as biomarkers of brain Aβ metabolism. No behavioral or cognitive outcomes were assessed in this study. Additionally, they conducted post-mortem investigations to examine the temporal cortices of a subset of animals at the end of the experiment.

The study included nine aged rhesus monkeys (26–31 years), equally divided into three groups: A, B, and C. Group A received one round of gamma-frequency auditory stimulation followed by another round of random stimulation after a 21-day interval. Group B received only one round of random stimulation. Group C (control) had speakers placed 42 cm in front of them, without any auditory stimuli.

Each round consisted of one hour of auditory stimulation at the same time, daily, for seven consecutive days. The gamma-frequency auditory stimuli consisted of 1.0-kHz pure tones, each lasting 1.0 ms at a 40-Hz frequency (i.e., one sound produced every 25 ms) at an intensity of 60 dB. The random stimuli consisted of pure tones of the same duration and intensity, but presented at random intervals with an average interstimulus interval of approximately 25 ms.

The team maintained the animals in individual cages, exposing them to alternating light-dark cycles, a 21°C room temperature, and 60 % humidity conditions. They fed them monkey chow twice a day and a fruit daily, with water available ad libitum. Four monkeys from the experimental groups were humanely anesthetized and euthanized for postmortem analysis, while one animal died unexpectedly from non-neurological causes and another entered a separate study.

The team collected CSF samples via lumbar puncture at baseline and after two weeks in all groups. Additionally, Groups A and C were sampled at five and seven weeks to assess the durability of the effect. 6E10 and AT180 immunohistochemical stains identified Aβ plaques and phosphorylated tau aggregates, respectively, in the temporal cortices.

Forty-Hertz Stimulation Doubles CSF Amyloid Levels

Within a week of consecutive 40-Hz stimulation, Aβ40 and Aβ42 levels significantly increased in CSF by two-fold (over 200 % relative to baseline). These elevated levels were maintained for at least 35 days (~five weeks) following treatment cessation, an effect not previously reported in rodent studies.

The controls showed no changes in Aβ levels without the auditory stimulation. A postmortem investigation of four monkeys revealed pre-existing senile plaques in the temporal cortex. At the same time, phosphorylated tau pathology was absent or very limited, consistent with the lack of significant changes in CSF tau biomarkers.

Studies suggest that 40-Hz auditory stimulation can enhance astrocyte aquaporin-4 polarization, arterial pulsatility, and dilation of meningeal lymphatic vessels. These effects may promote the flow of CSF into the brain parenchyma, facilitating the movement of metabolic waste, including Aβ, from the brain into the CSF. However, these mechanisms were inferred from prior work and were not directly measured in the present study.

Non-Invasive Brain Rhythms Open New Alzheimer’s Questions

The study provides the first primate evidence of persistent changes in CSF Aβ dynamics following brief 40-Hz auditory stimulation, highlighting its potential as a non-invasive therapeutic strategy for Alzheimer’s disease. Whether these changes translate into reduced brain amyloid burden or clinical benefit remains unknown.

Although the findings suggest that the auditory stimuli may promote the clearance or redistribution of Aβ from brain tissue into CSF, this study did not directly measure interstitial Aβ efflux, glymphatic flow, or lymphatic vessel function.

Future studies could directly observe 40-Hz-induced Aβ efflux by injecting isotope-labeled Aβ into monkey brain parenchyma to measure CSF Aβ concentrations post-stimulation. An alternate strategy would be to use implanted lumbar puncture needles for continuous collection and measurement of Aβ levels in CSF during and after the 40-Hz auditory stimulation.

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Journal Reference

Wang, W. et al. (2026). Long-term effects of forty-hertz auditory stimulation as a treatment of Alzheimer’s disease: Insights from an aged monkey model study. Proceedings of the National Academy of Sciences, 123(2), e2529565123. DOI: 10.1073/pnas.2529565123. https://www.pnas.org/doi/10.1073/pnas.2529565123