This 2019 video from MIT's Picower Institute for Learning and Memory describes GENUS, the programme in Li-Huei Tsai's lab, with the labs of Emery Brown and Ed Boyden, testing whether light flicker and sound clicks at 40 Hz can increase gamma rhythms in the brain and reduce Alzheimer's disease pathology. The description links to two 2019 papers reporting results in mouse models and notes that work with human volunteers was getting under way.
We include it because 40 Hz has become a popular number in the sound-and-wellbeing world, and this is the actual research people are pointing at. What it shows is specific: effects in mice, using precisely timed sensory stimulation. Later MIT news items describe small human studies that confirmed safety and suggested benefit, which is a long way from an established treatment.
It is a piece of institutional communication, so it presents the lab's work favourably. It is still the clearest short primary-source account of what the 40 Hz claim rests on.