Déjà vu is one human experience that science has yet to nail down. That "I've totally seen this before" feeling (even though you haven't) is a bit spooky and elusive; and while researchers have their theories, what causes déjà vu is still a relative mystery to science. While perhaps the most common theory is that déjà vu happens when your mind is forming a false memory, new research suggests that déjà vu is actually your mind's way of fact-checking itself for accuracy — quite the opposite.
The study, which comes out of the University of St. Andrews and was recently covered by New Scientist , bears some similarity to previous research examining the false memory theory — and for a very good reason. For all the studies, researchers read their volunteers a list of related words: "Bed," "pillow," "night," and "duvet." In previous false memory studies, participants have been asked if they had heard the word "sleep" — and they said yes, even though they hadn't heard the word. It's example of their brains remembering something that never happened, inspired by the fact that the words they actually heard are definitely related to the word "sleep."
For the current study, though, researchers tweaked this method just a bit. After reading the list of words, they asked not if the participants heard the word "sleep," but rather, whether they heard a word starting with the letter "s." The participants said no, correctly. Then, later on (the passage of time is significant), they were asked if they'd heard the word "sleep."
They knew they couldn't have, since they hadn't heard an "s"... but still: They really felt like they had heard the word "sleep" somewhere. That's déjà vu.
To make matters even more interesting, the participants were inside an fMRI while answering these questions, allowing researchers to examine their brains. If the false memory theory were accurate, then the hippocampus would've been activated... but it wasn't. It was instead the frontal area that lit up — the area that's associated with decision-making.
This isn't the only research pointing to the fact-checking theory of déjà vu. Other studies have explored the idea that maybe déjà vu is simply our brains trying to make sense of everything, and not forming false memories. A study published in Science Daily discussed a similar theory, fueled by the fact that our brain is trying to make whole stories out of the bits and pieces of information it receives. This could lead to "mismatches" in the brain's neural pathways. The study uses the example of how just a familiar smell could resurrect a whole story or memory in your mind. Now, apply this to the fact-checking study and theory: Participants hear a series of sleep-related words. Then they're asked if they heard the letter "s" or the word sleep. Would it make sense, then, for their brains to be confused, thus creating the perfect environment for the aforementioned mismatch?
There are still other theories about what causes déjà vu — in fact, there are more than 40 of them. Yet another is that because there is a split-second delay when one side of the brain sends information to the other, one side will unavoidably receive information twice: Once directly from the source, and once from the other half of the brain.
Because déjà vu isn't an experience you can recreate in a lab, it largely remains unsolved even to the brightest of minds.