How Does Déjà Vu Work? There's A Scientific Explanation For This Bizarre Phenomenon

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If you've experienced a flicker of puzzling recognition as you do something  —pet a cat, watch a film, enter a house you know you've never been in before — you've experienced the puzzling psychological phenomenon that is déjà vu. It's a sudden sensation, often short-lived, that implies that you've experienced your current situation before, and are recalling it vividly, even when you haven't. However, déjà vu doesn't happen all the time: If we experienced the feeling every time we retraced our steps or went through our routine, it'd cease to be an anomaly. Instead, it's seen as a bit of a psychiatric oddity that's extremely common, occurring in about 60 percent of the population. And understanding how déjà vu works might shed light on the functions of human memory and our complex brains.

The déjà vu sensation has always been associated with a slight air of strangeness and mystery. It first entered scientific circles in 1876, when the French philosopher and investigator of the paranormal Émile Boirac coined the term in a letter. It's fascinated psychologists, scientists and artists ever since; Freud thought it was prompted by repressed desires (because of course he did), Marcel Proust based the most famous segment in his masterpiece In Search Of Lost Time on déjà vu sparked by a madeleine cookie, and the makers of the Matrix trilogy made the feeling a sign of a "glitch" in the artificial world. As it happens, the truth might actually not be much less bizarre.

The Many Theories Behind Déjà Vu

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Understanding déjà vu means delving into human memory and neurology, and over the years scientists have developed many possible explanations for the mechanism, all of them pretty plausible. In a review of the science in 2003, the Psychological Bulletin outlined four major schools of thought about why déjà vu might happen. The first is the simplest: that the event has in fact already happened, and that for some reason you had forgotten this and are being reminded. The second is that it's brought about by a processing error in the brain, in which two elements are trying to operate simultaneously and something gets out of step.

The third idea is what's called the "disruption" theory, where neural firings in the brain are somehow interrupted or go awry. This, it's argued, is why people with epilepsy experience déjà vu as part of the auras of their seizures. In people without epilepsy, it's proposed that, when déjà vu happens, there's an accidental delay or repetition in the transferral of sensory stimuli information the brain, causing a kind of overlap — and the sensation that the event being registered has happened already. The fourth, meanwhile, is the "attentional" explanation. While you might be paying attention to what's happening around you, it suggests, you might be distracted for a split second, and when you re-focus on the event at hand, it may seem oddly familiar in a "past" way.

Looking at déjà vu experienced by epileptics gives us a bit of an insight, but not as much as you think. Epileptic déjà vu is most strongly related to medial temporal lobe epilepsy, which affects the brain's hippocampus, one of the seats of memory. This is a good boost to the theory that neural misfiring is behind the experience of déjà vu itself. But epileptic and non-epileptic déjà vu seem to differ; the EEG patterns of people with and without epilepsy experiencing déjà vu are very different, for one thing. And when it comes to déjà vu that isn't related to epilepsy, scientists have also found that it can be induced by stimulating another area of the brain, the entorhinal cortex, which connects the hippocampus to other parts of the brain and is necessary to memory. So what's actually going on?  

Is Déjà Vu The Brain Fact-Checking Itself?

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A new theory about déjà vu emerged in 2016 that's being acclaimed as one of the best solutions of the mystery. Scientists at the University of St Andrews managed to induce déjà vu in non-epileptic people, not by stimulating any part of their brains, but by doing a word experiment in which they were made to feel that a word was "familiar" even though it hadn't been recently mentioned. (If you want to get other people to feel déjà vu, give them a collection of words related to something without ever mentioning the key word that connects them all. Then ask them if they've heard the key word recently. They may well, as many of the St Andrews subjects did, get a very odd sense of déjà vu.)

The St Andrews scientists examined the brains of the subjects as they experienced this induced déjà vu, and discovered something very interesting. Instead of activity in the hippocampus or other areas of the brain to do with memory, they noted that the frontal areas of the brain were active instead. They suggest that this might mean our frontal lobes are actually "checking" our memory input and waking up to raise an alarm (an odd feeling) when something doesn't fit with our other memories.

But this doesn't provide a full explanation. Some experiences of déjà vu do in fact correspond to actual memories, even if they're rare. And other elements of the brain are apparently involved in déjà vu; experiments in 2006 revealed that it might also result from a kind of memory retrieval issue, in which the brain is fooled into classifying new information among memories by some sort of issue in the temporal lobe. It's entirely possible that there are several kinds of déjà vu created by different conditions (perhaps word déjà vu is different to visual déjà vu, for instance), and that we won't fully understand the mechanisms and how they interact until a lot more experimentation is done. For the time being, though, don't worry if you experience déjà vu a few times a year. You're not being haunted or having a premonition. Bits of your brain are just ever-so-slightly, briefly, putting a memory in the wrong box.