Researchers have been able to reverse symptoms of Alzheimer's in mice using only light and sound. It remains to be seen whether this will also work in people, but the treatment cleared out amyloid deposits in the brain. It also improved the mice's ability to navigate a maze, along with other aspects of their memory.

Mice do not get full-blown Alzheimer's disease. But researchers have been able to genetically modify mice so that they exhibit many of the symptoms of Alzheimer's, such as the buildup of amyloid plaques in the brain and memory deficits.

Levels of both amyloid plaques and tau protein, another protein that builds up in people with Alzheimer's, were reduced when researchers exposed mice to flickering light at 40 hertz (cycles per second) for one hour a day for a full week in a 2016 study. This appeared to be due to stimulation of debris-clearing cells known as microglia. The improvements were only seen in one area of the brain, the visual cortex.

Treated mice also performed much better when navigating a maze requiring them to remember key landmarks.

Researchers have now expanded on that study, this time testing the effect of sound tones, once again choosing 40 hertz as the frequency. Exposure to this sound for one hour a day, for seven days dramatically reduced the amount of beta-amyloid in the brain's auditory cortex and also in the hippocampus, a region involved in the storage of long-term memory.

Treated mice also performed much better when navigating a maze requiring them to remember key landmarks. And they were better able to recognize objects they had previously encountered.

The researchers then combined the two treatments and found that this dual treatment had a much greater effect than did either one alone. Amyloid plaques were reduced in additional regions of the brain, including the prefrontal cortex, an area where many complex mental processes take place. The stimulation of the microglia was also much stronger.

The only drawback seen so far is that when they waited an additional week to test the treated mice, many of the positive effects had faded, suggesting that the treatment would need to be repeated or even given continually.

Forty hertz is the frequency of the brain's own gamma waves, one of the five types of brain waves produced by the brain's electrical activity, they show up as repeating squiggles on an electroencephalograph (EEG). The two mouse studies with light and sound suggest a previously unknown role for gamma waves in maintenance of brain health, at least in mice.

How about people? Well, the combined treatment has already been tested for safety in healthy volunteers, and the researchers are now beginning to enroll patients with early-stage Alzheimer's to see if the treatment is helpful. Stay tuned.

The study appears in Cell.