why does it occur? You know, those moments where the current situation feels like it's

happened before? You're certain it has, but you don't know when, or how it became so familiar.

It's difficult to scientifically study Deja vu because there's no reliable way to cause

it to happen in people's heads in a laboratory.

But here's what we do know: humans don't seem to experience Deja vu until they're at least 8 or 9 years old.

It's most frequent in your teens and twenties, and then tapers off as you get older.

So, it might have something to do with brain development. In fact, we should probably go get ourselves a brain.

That was easy. Now, like a stomach ache,Deja vu may be a symptom with many possible causes.

A lot of the popular theories about what causes

it involve a disconnect that may be occurring between the deep structures of the brain that

process our experiences unconsciously, and the parts of the brain that are conscious

about what they experience, process it, and then tell us what we're seeing.

For the sake of simplicity, let's begin with the visual system. Now, the brain sits in

your head like this, and your eyeballs are over here seeing things, sending that signal

to the back of the brain, the Occipital lobe, where visual cortex is. That's where the image

is processed and we become aware of what we're seeing. But visual cortex isn't the first

stop that information makes. Instead, we know that it stops in lots of other places. For

instance, the amygdala, where it may be processed at an involuntary emotional level, and this

fun little part right here, #31, the tectum. It's involved in preliminary visual processing

and helps control eye movements.

Now, that fact is incredibly important because what it means is that people with blindness

cause by brain damage to the visual cortex cannot see anything. They don't report seeing

or recognizing anything; however, the other preliminary parts of the brain that get that

message are still healthy, and so, despite being legally blind, these people exhibit

what is known as blindsight.

They can defy all odds and avoid an obstacle course on the ground. They can also be presented

faces showing joy, or anxiety and fear, and feel, without knowing why, at all, on Earth they feel

that way, a similar emotion.

So, a lot is going on when we look at things, and if those preliminary structures in the

vision system allow certain blind people to be oddly and unconsciously aware of what they're

looking at, we may have the ingredients we need for this disconnect.

Think of it this way: If I experience event A, and then B, and then C, and the inner parts

of my brain commit it to memory and generate an emotional response, but then, a fraction

of a second later, out-of-sync, my visual cortex finally gets around to telling me,

the conscious aware part of my brain, what I'm seeing. It will say "Ok guys, we're looking

at A, and then B, and then..." and then the brain says "Whoa, whoa, whoa, wait, wait,

and then C, right? That's already happened I thought?..." and that may be Deja vu.

But what could call these processes dealing with the same information to get temporarily

out-of-sync like that? Well, it's probably a neurological abnormality, possibly an epileptic

episode where neurons all fire in sync. We also know that patients who experience chronic

and persistent Deja vu tend to have brain damage in the temporal lobes of their brain-

these lobes on either side.

Well that sounds kinda bad, right? I mean, when I experience Deja vu should I run to the doctor

right away? Well, not at all. In fact, minor epileptic events are quite common. You've

probably experienced a similar phenomenon known as a Hypnogogic Jerk. That's a fancy

name for what occurs when you're about to fall asleep and then all of the sudden you

feel like you're falling, or that you tripped, and your body jolts itself awake.

What's going on there is that as you fall asleep your muscles relax, and some part of

your brain that's still awake notices that, and feels like you're no longer supported-

you must be falling- and it sends out a huge signal that causes all of your muscles to

twitch, waking you up.

Oh, and Deja vu is not the only "vu" out there. There's also something known as Presque vu,

or "tip of the tongue," and this is what happens when you're familiar with something, you know

you know it, but, for the life of you, at that moment, you cannot recall it. It's like "what's

that actor's name from that movie? I've seen it 18 times, how can I not remember it?? Wow."

One explanation of how that happens is that other words similar to the target word are

being remembered, and to help you out your brain is actively blocking other stuff around

it, including the word you're actually looking for. Now, this explanation is really nice

cause it might actually explain why Presque vu has a shared, or social aspect. Even a

bunch of people in a group, all with special brains, if they're given the same "blocking

words" that are related, no one in the group can come up with the name until the subject

is changed and those words are no longer blocked, and then all the sudden someone goes "Oh,

Gary Sinise, that's who it was. Yup, there you go."

Ok, finally, Jamais vu. This is when something that you know, something that you're familiar

with, all the sudden seems brand new and bizarre. Schizophrenic patients will sometimes see

people they know and insist that it's not the person the know. Instead, it's an imposter.

But all of us have probably experienced Jamais vu to a certain extent, for instance, when

you repeat a word over, and over, and over, and over again, the word starts to lose its

meaning and you start to think "What? How is this even a word? It's so weird!" Well,

scientists believe that happens because continuing to excite the neurons responsible for that

word causes them to become inhibited, and further use is less intense.

It is just crazy to think that there is so much out there in the world, in the universe,

that we don't know, that we don't understand, that we haven't yet discovered. But, yet,

that very feeling of familiarity for what we do know can't always be trusted. About

the only constant seems to be: As always, thanks for watching.