Today, a startling new discovery: prodding the brain with light, a group of scientists got an unexpected surprise -- they were able to turn back on a part of the brain that had been shut down by Alzheimer’s disease. This new science is not a cure, and is far from a treatment, but it’s a finding so … simple, you won’t be able to shake it. Come join us for a lab visit, where we’ll meet some mice, stare at some light, and come face-to-face with the mystery of memory. We can promise you: by the end, you’ll never think the same way about Christmas lights again. This piece was reported by Molly Webster. It was produced by Annie McEwen, Matt Kielty, and Molly Webster, with help from Simon Adler. Special thanks to Ed Boyden, Cognito Therapeutics, Brad Dickerson, Karen Duff, Zaven Khachaturian, Michael Lutz, Kevin M. Spencer, and Peter Uhlhaas. Producer's note about the image: Those neon green things in the image are microglia, the brain’s immune cells, or, as we describe them in our episode, the janitor cells of the brain. Straight from MIT’s research files, this image shows microglia who have gotten light stimulation therapy (one can only hope in the flicker room). You can see their many, super-long tentacles, which would be used to feel out anything that didn’t belong in the brain. And then they’d eat it! Support Radiolab by becoming a member today at Radiolab.org/donate.