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Have you experienced déjà vu?

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It's that shadowy feeling you get

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when a situation seems familiar.

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A scene in a restaurant plays out

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exactly as you remember.

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The world moves like a ballet

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you've choreographed,

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

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because you've never eaten here before.

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This is the first time you've had clams,

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so what's going on?

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

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The experience is brief

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and occurs without notice,

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making it nearly impossible

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for scientists to record and study it.

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Scientists can't simply sit around

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and wait for it to happen to them --

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this could take years.

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It has no physical manifestations

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and in studies, it's described by the subject

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as a sensation or feeling.

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Because of this lack of hard evidence,

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

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Since Emile Boirac introduced déjà vu

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as a French term meaning already seen,

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more than 40 theories attempt

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to explain this phenomenon.

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Still, recent advancements in neuroimaging

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and cognitive psychology narrow down

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the field of prospects.

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Let's walk through

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three of today's more prevalent theories,

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using the same restaurant setting for each.

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First up is dual processing.

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We'll need an action.

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

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As the scene unfolds,

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your brain's hemispheres process

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a flurry of information:

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the waiter's flailing arms,

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his cry for help,

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the smell of pasta.

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Within milliseconds,

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this information zips through pathways

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and is processed into a single moment.

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

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However, this theory asserts

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that déjà vu occurs when there's a slight delay

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in information from one of these pathways.

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The difference in arrival times

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causes the brain to interpret the late information

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as a separate event.

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When it plays over the already-recorded moment,

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it feels as if it's happened before

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because, in a sense, it has.

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Our next theory deals with a confusion of the past

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rather than a mistake in the present.

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This is the hologram theory,

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and we'll use that tablecloth to examine it.

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As you scan its squares,

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a distant memory swims up

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from deep within your brain.

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According to the theory,

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this is because memories are stored

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in the form of holograms,

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and in holograms,

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you only need one fragment

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to see the whole picture.

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Your brain has identified the tablecloth

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with one from the past,

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maybe from your grandmother's house.

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However, instead of remembering

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

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your brain has summoned up the old memory

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without identifying it.

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This leaves you stuck with familiarity

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but no recollection.

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Although you've never been in this restaurant,

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you've seen that tablecloth

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but are just failing to identify it.

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Now, look at this fork.

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Are you paying attention?

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Our last theory is divided attention,

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and it states that déjà vu occurs

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when our brain subliminally takes in an environment

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while we're distracted by one particular object.

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When our attention returns,

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we feel as if we've been here before.

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For example, just now you focused on the fork

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and didn't observe the tablecloth

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or the falling waiter.

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Although your brain has been recording everything

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in your peripheral vision,

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it's been doing so below conscious awareness.

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When you finally pull yourself

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away from the fork,

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you think you've been here before

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because you have,

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you just weren't paying attention.

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While all three of these theories

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share the common features of déjà vu,

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none of them propose to be the conclusive source

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of the phenomenon.

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Still, while we wait for researchers and inventers

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to come up with new ways

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to capture this fleeting moment,

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we can study the moment ourselves.

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After all, most studies of déjà vu

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are based on first-hand accounts,

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so why can't one be yours?

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The next time you get déjà vu,

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take a moment to think about it.

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Have you been distracted?

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Is there a familiar object somewhere?

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Is your brain just acting slow?

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Or is it something else?