Why Does Déjà Vu Feel So Real?

You walk into a friend's kitchen for the first time. The kettle is by the window, a bowl of lemons sits on the table, somebody laughs in the next room, and suddenly the whole scene feels as if you have stood exactly here before, heard exactly this laugh and watched exactly this light fall on exactly these lemons. You know perfectly well that you have not. The feeling does not care. For a few seconds it is completely convincing, and then it slips away, leaving you slightly dizzy and oddly delighted.

That feeling has a French name, déjà vu, which means already seen, and it is remarkably common. The psychologist Alan Brown, reviewing decades of surveys, concluded that roughly two thirds of people experience it at some point in their lives. In a 2013 study by Akira O'Connor and Chris Moulin, 84 percent of the volunteers said they had felt it at least once in the previous six months. Brown also found that the most common trigger is a place or a scene, and the next most common is a conversation.

Travel seems to help. In the same study, people who travelled more often also reported déjà vu more often, which makes a certain amount of sense: the more new places you walk into, the more chances a new place has to remind you of an old one. And that, it turns out, may be exactly what is going on.

At Colorado State University, the psychologist Anne Cleary wanted to know whether déjà vu could be made on purpose. Her team built a virtual village, nicknamed Deja ville, out of the life simulation game The Sims 2. It held 128 scenes arranged in pairs. The two scenes in each pair looked nothing alike on the surface, but their layout was the same. In one example, a statue stood among the benches of a museum exactly where a potted plant had stood among the bushes of a courtyard seen earlier. Students wearing virtual reality headsets toured the scenes and said how familiar each one felt.

The results were striking. Déjà vu was most likely when a new scene shared the layout of an earlier one that the student could not remember seeing. The mind had noticed the resemblance, but the memory behind it never came to the surface. What was left was familiarity with no explanation attached, and that is precisely what déjà vu feels like. Cleary describes it as recognising something as new and familiar at the same time.

So why does it feel so real, and how do we know it is wrong? Researchers think two separate processes are involved. One quietly signals that you have seen this before, and another checks that signal against what you can actually remember. In déjà vu the first fires, the second finds nothing to back it up, and you are left noticing the mismatch. O'Connor and Moulin see this less as a fault than as the checking system doing its job: it caught a false alarm and told you so. They also found that people who have déjà vu often do no better and no worse on ordinary memory tests than people who rarely have it.

That makes the strange moment in the kitchen something to enjoy rather than worry about. Your brain made a quick guess, noticed that the guess did not add up, and let you watch it happen. Somewhere in your past there was probably a room with a window on the left and something round and yellow on the table. You just cannot find it, and for a moment neither can the part of your brain that went looking.