Subtitles section Play video Print subtitles Hey there, welcome back to Life Noggin. Have you ever had that eerie feeling that you're reliving a past experience? Maybe you're traveling to New York City for the first time and it feels strangely familiar. Or maybe you're experiencing it right now, when you think you've already watched this video before. Ooh. Spooky. This phenomenon is called "déjà vu", which is French for "already seen". Historically, déjà vu has been linked with epilepsy and seizures in the brain. But that doesn't explain why people without epilepsy experience déjà vu. After all, about two thirds of the population have experience it at least once in their lives. So there has to be something else going on here. The exact cause of déjà vu is unknown, but there are a lot of theories attempting to explain it. For simplicity's sake, let's narrow it down to three main categories. The first of these is called "Dual Processing". This is when two normally coordinated cognitive processes become momentarily out of sync. For example, if I feel familiar with something, it might be because my brain has retrieved a memory similar to that one that I'm currently experiencing. Retrieving a memory always comes before the familiarity. Well... except in déjà vu. In that case, I would have a feeling of recalling something without my brain actually doing any retrieving. I could feel like I recognize something on that street corner in New York City without ever having been there before. The next theory is related to the nervous system not functioning properly. Déjà vu can either be caused by a spontaneously firing of a neuron, or from a delay in the neuron's transmission speed. To demonstrate the second one, let's take a look at this alpaca walking in high heels. Now you've more than likely never seen an alpaca do that before, if you had, I'm very jealous of you. But if you were experiencing déjà vu, one explanation is that the information passing through your eyes into your brain is reaching in a different time. Let's say the information gets from you left eye to your visual cortex just before the information from your right eye gets there. These two events happen in such a short amount of time that the left eye's information has not stored it's memory yet. But since your brain received the information twice, it feels familar nontheless. The last theory's a bit more intuitive and states that if you have a false sense of familiarity with something new, it's because some elements of the situation are familiar. For example, if you were to go to a new friend's home for the first time, and they have the same kitchen table as your great aunt, you may have a strong feeling of familiarity without realizing where you've seen it before. In your confusion, you mistakenly think that you must have experienced the entire situation already, instead of just the table. Well, then why don't we experience déjà vu every time we subconsciously recognize something? Some theories say that there may have to be multiple elements that are familiar to you. Or maybe the room just has to be organized in a similar way as the one you recognize. Either way, something is familiar and you just haven't realized it yet. Have you ever experienced déjà vu? And tell us out of all these, which theory sounds the most convincing to you. If you want even more Life Noggin, check out this great video we did on why our hair turns gray, and make sure you're following us on Facebook and Twitter. We post some pretty cool stuff there, too! I'm Blocko. This has been Life Noggin. Don't forget to keep on thinking.