Have you ever been in a deep conversation with someone and you suddenly get the feeling that you’ve had the exact conversation before, even though you know that you haven’t. Or have you ever walked through a new city for the first time and something familiar clicked in your mind, giving you a pause? If you nodded your head in agreement to these statements, congratulations! You have experienced déjà vu.

Déjà vu really is an uncanny feeling. Emile Boirac, a French scientist, one of the first to study this strange phenomenon, gave the subject its name in 1876. The term in French literally means “already seen” and that’s exactly why it’s so nerve-wrecking. It really feels like you’ve already experienced a very specific event or been somewhere, even though you haven’t or at least you believe it hasn’t.

Further complicating the matter, there’s no consensus yet on what exactly causes this phenomenon, though there are a lot of theories. The real trouble with déjà vu is that it’s hard to replicate in a laboratory setting which makes it extremely difficult to study. There is a study that suggests that déjà vu is just an extreme reaction of your brain’s memory system when encountering things with lots of familiar objects just set up a little differently (for example, when you’re in a restaurant configured almost identically to one you’ve been in before, you can get a powerful feeling of familiarity).

DÉJÀ VU IN FRENCH LITERALLY MEANS “ALREADY SEEN” AND THAT’S EXACTLY WHY IT’S SO NERVE-WRECKING.

Besides that theory, Scientists have found other likely causes for déjà vu occurrences in the brain. It could be a few different things. One of the leading scientific theories suggests that this feeling could be linked to one’s optical processing. We have two eyes (as you surely know) and both of them are transmitting data almost all the time to your brain. Although they are transmitting simultaneously, sometimes (due to myelin fluctuations or sufficiently different images or some other biological mumbo-jumbo) one eye’s signal is processed before the other. So when the second image arrives-very similar but not the same to the first- your brain is like “wait, I’ve seen this before”; but because this is all happening on the scale of microseconds, the information hasn’t had the chance to propagate over to any associative memory areas. So your brain thinks you’ve seen this before and pings your memory, essentially asking “when did this happen before, this is weird” and gets no answer because it hasn’t arrived there yet.

Another theory suggests that déjà vu could have something in common with epilepsy. Epilepsy is caused by uncontrolled neuron activity in your brain. If you often experience it, it could be because your brain is having little mini epileptic fits causing you to have the distinct impression that you are remembering something without actually giving you any information to remember. The rest of your brain thinks that’s weird, and tries to make sense of it by assuming that the information coming is simply so similar to what you’re currently experiencing you can’t tell the difference. So, not to alarm you, but if you experience frequent déjà vu, you may want to see a doctor to check if you really are suffering through epilepsy.

In some cases, however it could have more to do with semantic overlap- you’ve done similar things in the past, and semantically you remember it, but you can’t bring the episodic memory up, which confuses your brain and causes it to see patterns where there aren’t any.

If you are worried about the side effects of having this spooky feeling, we have excellent news for you. Fortunately déjà vu appears to be harmless for majority of people with higher rates of occurrence in people age 15 to 25. It’s just a bunch of stuff going a little awry in your head, nothing to worry about.