Do you know that strange sensation of walking into a room and feeling like you've been there before, even though you know you haven't? Or when you hear someone say something for the first time, but there's a certain familiarity to it that gives you pause? That's déjà vu–a phenomenon that's not well understood, but scientists have some ideas. 

Déjà vu is the eerie feeling that you have had the same novel experience before. It's a spontaneous, elusive sensation that reveals the workings of consciousness, allowing us to see the separation between what we feel and what we know to be true, explains Akira O'Connor, a psychologist and senior lecturer at the University of St. Andrews School of Psychology and Neuroscience. The experience occurs when certain brain regions, particularly those responsible for recognizing familiarity, "twitch" or send false familiarity signals. This causes a brief mix-up that triggers a sense of recognition and creates a conflict with your current perception. A self-aware déjà vu is your brain's way of letting you know that the memory you're experiencing is inaccurate—and that's a good thing because it means your frontal lobes are working as they should be. 

"Déjà vu is the process of correcting that error and making sure you don't act as though you remember that thing," said O'Connor, one of the few experts on déjà vu. "There are all sorts of reasons why I think that's the case, but one of them is this paradox that for a memory error, if that's what it is, déjà vu happens when people's brains are at their healthiest." (He added that, for people with certain conditions, like dementia, the frontal lobes may fail to fact-check properly, resulting in repeated sensations of familiarity. This can become disruptive, as everything begins to feel familiar even if those memories aren't real. It can delay receiving a proper diagnosis since they appear to have regained their memories, even though they haven't.)