It's that shadowy feeling you get

when a situation seems familiar.

A scene in a restaurant plays out

exactly as you remember.

The world moves like a ballet

you've choreographed,

but the sequence can't be based on a past experience

because you've never eaten here before.

This is the first time you've had clams,

so what's going on?

Unfortunately there isn't one single explanation for déjà vu.

The experience is brief

and occurs without notice,

making it nearly impossible

for scientists to record and study it.

Scientists can't simply sit around

and wait for it to happen to them --

this could take years.

It has no physical manifestations

and in studies, it's described by the subject

as a sensation or feeling.

Because of this lack of hard evidence,

there's been a surplus of speculation over the years.

Since Emile Boirac introduced déjà vu

as a French term meaning already seen,

more than 40 theories attempt

to explain this phenomenon.

Still, recent advancements in neuroimaging

and cognitive psychology narrow down

the field of prospects.

Let's walk through

three of today's more prevalent theories,

using the same restaurant setting for each.

First up is dual processing.

We'll need an action.

Let's go with a waiter dropping a tray of dishes.

As the scene unfolds,

your brain's hemispheres process

a flurry of information:

the waiter's flailing arms,

his cry for help,

the smell of pasta.

Within milliseconds,

this information zips through pathways

and is processed into a single moment.

Most of the time, everything is recorded in-sync.

However, this theory asserts

that déjà vu occurs when there's a slight delay

in information from one of these pathways.

The difference in arrival times

causes the brain to interpret the late information

as a separate event.

When it plays over the already-recorded moment,

it feels as if it's happened before

because, in a sense, it has.

Our next theory deals with a confusion of the past

rather than a mistake in the present.

This is the hologram theory,

and we'll use that tablecloth to examine it.

As you scan its squares,

a distant memory swims up

from deep within your brain.

According to the theory,

this is because memories are stored

in the form of holograms,

and in holograms,

you only need one fragment

to see the whole picture.

Your brain has identified the tablecloth

with one from the past,

maybe from your grandmother's house.

However, instead of remembering

that you've seen this pattern at your grandmother's,

your brain has summoned up the old memory

without identifying it.

This leaves you stuck with familiarity

but no recollection.

Although you've never been in this restaurant,

you've seen that tablecloth

but are just failing to identify it.

Now, look at this fork.

Are you paying attention?

Our last theory is divided attention,

and it states that déjà vu occurs

when our brain subliminally takes in an environment

while we're distracted by one particular object.

When our attention returns,

we feel as if we've been here before.

For example, just now you focused on the fork

and didn't observe the tablecloth

or the falling waiter.

Although your brain has been recording everything

in your peripheral vision,

it's been doing so below conscious awareness.

When you finally pull yourself

away from the fork,

you think you've been here before

because you have,

you just weren't paying attention.

While all three of these theories

share the common features of déjà vu,

none of them propose to be the conclusive source

of the phenomenon.

Still, while we wait for researchers and inventers

to come up with new ways

to capture this fleeting moment,

we can study the moment ourselves.

After all, most studies of déjà vu

are based on first-hand accounts,

so why can't one be yours?

The next time you get déjà vu,

take a moment to think about it.

Have you been distracted?

Is there a familiar object somewhere?

Is your brain just acting slow?

Or is it something else?