Originally published 2020-03-03.
Scientists at MIT have shown that exposing mice to light flickering and sound pulsing 40 times a second can reduce the amyloid plaques linked to Alzheimer's disease and improve the animals' memory. The work, led by neuroscientist Li-Huei Tsai, made headlines in 2019 when sound and light were combined for the first time. Small human studies have since found the approach safe and hinted at benefits, and a large phase III trial is under way, but it is not yet a proven treatment for people.
This article explains what the mouse experiments found, why 40 Hz matters, what has happened in human trials, and what the research does and does not mean for families living with dementia today.
Why researchers are looking for new approaches
Alzheimer's is the most common cause of dementia. According to the Alzheimer's Association's 2026 report, an estimated 7.4 million Americans aged 65 and over live with it, and it was the fifth-leading cause of death in that age group in 2024. Alzheimer's and other dementias kill more older Americans than breast and prostate cancer combined.
For decades, available drugs only eased symptoms for a while. Two antibody drugs approved in the US in recent years, lecanemab and donanemab, can modestly slow decline in early-stage disease by removing amyloid, but they require regular infusions, carry a risk of brain swelling and bleeding, and do not stop the illness. That leaves plenty of room for safer, simpler approaches, and a non-invasive treatment using only light and sound would be very attractive if it worked.
The science of gamma rhythms
Brain cells fire in coordinated rhythms. Gamma waves, which oscillate at roughly 30 to 100 cycles per second, are associated with attention, perception and memory. Studies in both people and animals have found that gamma activity is disrupted in Alzheimer's disease.
Tsai's team wondered whether restoring that rhythm from the outside could help. The idea is called sensory entrainment: when you see a light flickering at 40 Hz or hear clicks at 40 Hz, neurons in the visual or auditory parts of the brain start firing in step with it. The question was whether that activity would do anything to the disease itself.
What the mouse experiments found
2016: light alone
The first results, published in Nature in 2016, came from mice genetically engineered to develop Alzheimer's-like changes. One hour a day of 40 Hz flickering light reduced amyloid beta and a harmful form of the tau protein, and it activated microglia, the brain's immune cells that clear debris. The catch was that the effect was largely confined to the visual cortex.
2019: adding sound
In a study published in Cell in March 2019, the team added 40 Hz tones. One hour a day for a week of sound alone reduced amyloid in the auditory cortex and in the nearby hippocampus, a structure that is central to memory. The treated mice did better at finding their way through a maze using landmarks and at recognising objects they had seen before.
Combining light and sound worked best. Only the combined treatment reduced plaques in the prefrontal cortex, and it cleared amyloid across a much larger part of the brain. Tsai described the outcome as a very dramatic reduction of amyloid
. Microglia clustered around plaques, and blood vessels also changed in ways that may help with clearance.
Sections of mouse brain from the MIT study: with 40 Hz treatment (left) and without (right). Image: Gabrielle Drummond, MIT.
There was an important limit: when the researchers checked a week after treatment stopped, many of the benefits had faded. That suggests any real-world therapy would need to be used continuously, not as a one-off cure.
2024: how the brain clears the plaques
Later work explained part of the mechanism. A 2024 MIT study in Nature found that 40 Hz stimulation prompts certain interneurons to release a peptide called VIP, which increases the flow of fluid through the brain's glymphatic "drainage" system and helps wash amyloid away. A separate team in China reported a similar effect on glymphatic flow the same year.
What happened in human studies
In December 2022 the MIT group published two small studies in PLOS ONE. In the first, 43 volunteers, including 16 with early Alzheimer's, received brief 40 Hz light and sound exposure while their brain activity was recorded. The rhythm entrained the brain as expected, and the most common side effect was drowsiness.
In the second, 15 people with early Alzheimer's used a home device with a light panel and speaker for an hour a day. Eight received 40 Hz stimulation and seven received a sham version. After about four months, the treated group showed less shrinkage of the hippocampus, more regular sleep and better scores on a face-name memory test, but most other cognitive tests showed no difference. Tsai was clear that These results are not sufficient evidence of efficacy
, though they supported larger trials.
Cognito Therapeutics, a company spun off from the research and co-founded by Tsai, reported slower brain atrophy and some cognitive benefit in its own phase II work, and by 2025 it had been running a nationwide phase III trial for more than a year. Results from that trial are what doctors are waiting for. It is worth remembering that the company has a commercial interest in the outcome, which is why independent replication matters.
Myths and realistic expectations
"Scientists have cured Alzheimer's with light and sound." Not in people. The striking results are in mice, and many mouse treatments for Alzheimer's have failed when tested in humans.
"Any flickering light or 40 Hz video on the internet will do the same." The trials use calibrated devices and monitored schedules. Home-made versions have not been tested, and flickering light can trigger seizures in people with photosensitive epilepsy.
"Clearing amyloid is the same as restoring memory." Amyloid is only part of the story. Tau, inflammation, blood vessels and the loss of neurons all contribute, which is one reason the human results so far are modest.
Anyone worried about memory changes in themselves or a relative should talk to a doctor, who can discuss assessment and, where suitable, the clinical trials that are recruiting.
Why this line of research stands out
Most Alzheimer's treatments target one molecule. The 40 Hz approach is different because it seems to recruit several of the brain's own systems at once: immune cells, blood vessels, fluid drainage and the protective coating around nerve fibres. Researchers are also testing whether it might help in other conditions, including Down syndrome and the memory problems that can follow chemotherapy. If the phase III trial confirms meaningful benefits, a simple device used at home could become part of care. If it does not, the work will still have taught scientists a great deal about how brain rhythms and brain health are linked.
Sources: Mouse brain image credited to Gabrielle Drummond, MIT Picower Institute; MIT News, 14 March 2019: Brain wave stimulation may improve Alzheimer's symptoms; MIT News, 13 December 2022: Small studies of 40Hz sensory stimulation confirm safety, suggest Alzheimer's benefits; MIT News, 14 March 2025: Evidence that 40Hz gamma stimulation promotes brain health is expanding; Alzheimer's Association: Alzheimer's Disease Facts and Figures.
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