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Hank Green: I think the French must pay more attention to all the weird little tricks their brains play on them, because they've got all the cool terminology.  Take for instance 'jamais vu', or 'never seen', the feeling that you get when something familiar to you suddenly seems new and bizarre, like when you look at the word 'hand' and it's just, it used to be the word 'hand', but now you've written it a kabillion times but it's weird now, it's just, it's weird.  

Or then you have 'presque vu', or 'almost seen', which we call 'it's on the tip of my tongue'.  But of course, the most famous French named brain trick is 'deja vu' or 'already seen', the eerie disconcerting sensation that what's going on right now has happened to you before even though it's happening right now and hasn't happened to you before.  Scientists haven't pinpointed exactly what goes on in a person's brain when they experience deja vu, there are actually as many as 30 plausible explanations for why it happens, but they can make some good guesses based on how our memories work.  

Remembering requires two things to happen in the brain: one, the region responsible for processing memory data, the middle of the temporal lobe, about where your ears are, first recognizes a thing as familiar and two, the region that handles short and long term memory, mainly the hippocampus, which is inside the temporal lobe, recalls that the thing has happened before and pulls up that memory.  Usually, these two processes, familiarity and recall, work really well together, the brain registers familiarity before it can remember why the thing is familiar, but sometimes, they get a little bit out of sync.  Neurologists have different ideas about why this happens.  Some think that since deja vu results in a visual image seeming familiar, perhaps images traveling from one eye to the brain are delayed, arriving microseconds after images from the other eye.  This might lead to the sensation that something is being seen for the second time.  Another theory is that there's some kind of glitch in the processes of familiarity and recall, and they're activated at the wrong time.  You can think about this by imagining a tape recorder that can record as well as play music, our brains are kind of like that.  Usually, you'd record music and then play it back later, but sometimes, when this tape recorder is recording, it malfunctions so that it's also playing back at the same time, but the brain starts playing back while it's recording, the present might feel like a memory.  

Researchers have noticed that children don't experience deja vu until they're about eight or nine.  It becomes more common in our teens and our twenties, and then starts tapering off after 25.  While we can file this information under good to know, it doesn't really help us get to the bottom of it.  Fortunately, we're living in kind of a golden age of brain research right now, so we're learning new stuff about our brains pretty much every day.  It's nice to figure out where this stuff comes from, but frankly, what I want to do is fix that thing, said it just at the beginning of the episode, it was--anybody?  Presque vu!  Uh, it was on the tip of my tongue.

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