What is deja vu and why does it happen?

Deja vu is the eerie feeling that an entire event or situation has happened before, even though it is completely illogical and impossible. The term comes from French, and means "already seen." It has also been described as "familiarity without awareness." Different surveys have shown that 30 to 70 percent of adults have experienced a deja vu episode. The phenomenon is most common among 15-25 year olds.

Deja Language

In a 1994 article in American Speech, Barbara Hunt Lazerson pointed out the significance of the phrase deja vu and its derivates in American English. Some humorous examples she highlighted follow.

deja flu: the feeling you've had these symptoms before.
deja goo: Even though you try to avoid doing so, you once again select from the box of chocolates the "piece with the gooey, awful, disgusting cherry center.
de java: the feeling, as you put sugar in your coffee, that you've already put sugar in your coffee.
deja who: from a Frank & Ernest comic strip: "A class reunion isn't a case of deja vu, Earnie, it's deja who?"
dijon vu: the feeling that I've used this mustard before

There are many theories that attempt to describe déjà vu, and as least as many differing views on what deja vu represents, ranging from a paranormal episode to a clinical psychiatric event. Some of the most common ideas about why deja vu occurs can be grouped into four general categories:

We experience something in the present similar to a fantasy that exists in our deep subconscious. Sound like Freud? It was his idea. If the sensory information we receive in the present is similar enough to suppressed fantasy, the two combine and the line between fantasy and reality and past and present meld together.
A type of matching occurs in our brains. A current event is so similar to a particular event in our memory that we seem to experience the event a second time. A related theory, called "global matching" is that a current event moderately similar to a large number of different events in our memories will trigger a feeling of familiarity.
Sensory input taking a wrong turn is another possible cause of deja vu. This occurs when the information about an event skips over our short-term memory storage and heads directly to the long-term parking. Then, when we try to access the information a split second later, we get the false impression that it was stored long ago due to the delay in accessing it.
The latest theory to surface is one that describes human memory storage as analogous to holograms. Just as each section of a hologram contains all of the information needed to produce a complete image, our brains can extrapolate the big picture from a small piece of memory. Small sections of a hologram may cause a fuzzy reproduction of the image. The same may occur in our brains; if we receive sensory information slightly similar to a piece of a past event, our brain may reproduce the whole image, causing deja vu.

We may never know for sure what causes the unnerving feeling that a unique event is happening for the second time or why we gain a sudden familiarity in unknown surroundings. It could be any or all of the above. Scientist also disagree on why deja vu occurs at a given time to an individual, citing reasons such as fatigue, drugs, emotional distress, epilepsy or just a random neurological misfire.

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