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  • What can explain that eerie, unsettling feeling we sometimes get that we've experienced a new situation once before?

  • It might just be the weirdest experience you'll ever have sober, but what exactly is deja vu?

  • One thing it definitely is, is common.

  • Two-thirds of us have had it, with younger people, globetrotters, and film fans likely to get it more frequently.

  • Because of its inherent weirdness, deja vu was long thought of alongside paranormal events like clairvoyance and reincarnation.

  • In fact, it was parapsychologist Émile Boirac who first named the feeling in the 1870s using the French for "already seen."

  • The focus on the uncanny has persisted, and in films like "The Matrix," deja vu is a glitch in the computer simulation.

  • So, what's actually going on?

  • The truth is, no one is 100 percent sure, but psychologists have suggested dozens of possibilities combining theories of memory, perception, and cognition.

  • One is divided perception.

  • Maybe our brains process a situation in a quick and shallow way before we become fully aware of it, and then we get a jolt of having seen it before.

  • Another is dual processing.

  • Incoming signals enter the temporal lobe from both hemispheres of the brain, one a millisecond later than the other.

  • And it's in this moment of delay that deja vu occurs.

  • Others speculate that errors around the hippocampusthe brain's librarianare to blame.

  • The problem with studying deja vu is that neurologists can't very well wait around for it to happen.

  • One solution has been to look at people with temporal lobe damage.

  • Many find that they get chronic deja vu.

  • Another way to study deja vu is to induce it under lab conditions.

  • In 2012, one study used virtual reality to immerse people in different 3D environments, some of which were very similar in layout.

  • For instance, a doctor's waiting room and an aquarium, with furniture arranged in the same configuration.

  • People were more likely to report deja vu when they encountered environments that had a similar layout to previous, forgotten scenes, suggesting it's a memory phenomenon.

  • A 2014 study had very different results.

  • Those who took part were shown a series of words with a secret common theme, words like bed, pillow, nap, dream.

  • The linking word, "sleep" never appeared.

  • Viewers were asked to keep note of any words beginning with S.

  • Those who took part were later asked if any words began with S, and sure enough, they said no.

  • But many also felt that they had been shown the word "sleep."

  • For two thirds of people, this confusion was tantamount to deja vu.

  • Neurologists have used this method to scan the brain during deja vu.

  • They found that rather than being a memory error in the hippocampus, deja vu involved the frontal areas of the brain responsible for decision making.

  • This led some to suggest that deja vu is a sign your brain's memory-checking mechanisms are actually working well.

  • But, if you're looking for something a bit more out there to explain deja vu, try quantum entanglement.

  • Perhaps through the mysterious affinity of subatomic particles, deja vu might actually be a window into a parallel universe, or else a blip in time.

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What can explain that eerie, unsettling feeling we sometimes get that we've experienced a new situation once before?

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