That odd sensation of already having experienced something may have a new scientific explanation. MIT neuroscientists studying an area of mouse brains in which rapid recognition takes place noticed neural firing that suggested old and new experiences were running together. It's impossible to ask a mouse what it is feeling, but the overlap looks a lot like what you'd expect in a human brain experiencing déjà vu. The scientists' theory: when we encounter a new location, a set of "place" neurons fires to create a map of the experience. If we visit another place with a similar map — even if we don't consciously recognize the resemblance — the eerie overlap may occur.