It's like déjà vu all over again.
- Lawrence 'Yogi' Berra

Translated directly from the French, déjà vu means 'already seen', which sums up the experience nicely. Anyone can experience it; it is the feeling one gets when doing something for the first time: that it has happened before in exactly the same manner - the same people are present, the same atmosphere surrounds, the same words and motions are used. It is also called paramnesia.

Many people have experienced déjà vu at some point in their lives. In fact, 70% of the population report having experienced déjà vu at some point. A higher rate of occurrence appears between the ages of 15 and 25, but psychologists are unsure of why this is exactly. (Some attribute it to a late stage of brain development.)

One Researcher describes it thus:

Ironically, I usually experience déjà vu in situations so similar that they could have been experienced before. For three or four years in a row on Thanksgiving I'd have a bout of déjà vu sitting around the dinner table with my family. I always knew what was happening - that it was déjà vu, I mean - but that didn't help. Nobody else noticed anything strange. It's sort of like walking around in a dream; you think you know what's going to happen next, and you might be able to change it, but you can't, and what good would that really do? Weird.

	The Causes

Because of its transitory and unpredictable nature, no one knows the causes - mental, chemical, or physical - of déjà vu. It is a very difficult phenomenon to study.

Some psychoanalysts believe it to be simple fantasy or wish fulfilment - trying to repeat a past experience. Others suspect it is a temporarily mismatched connection in the brain. Information from sensory organs is routed through the memory portion of the brain before being interpreted; by the time the sensations are processed, the brain notices them already in memory and believes them to have previously occurred. Since this happens rapidly at the chemical rate of the brain, the individual notices them almost simultaneously.

Déjà vu might also be brought about as part of another memory. A current sight or smell or taste might evoke an entire past memory. The present is then associated with the feeling of remembering and, therefore, the past.

Scientists have, however, associated déjà vu closely with the brain's temporal lobe¹. This attachment was based on accounts from persons with temporal-lobe epilepsy. Individuals experiencing seizures caused by the condition have a tendency to experience déjà vu immediately before and during the convulsions.

On a slightly less scientific bent, parapsychologists think déjà vu is a glimpse into a past life. The event did happen similarly before, you just happened to die between the occurrences.

	Other Kinds of Vu

The subtle, recurring confusion between illusion and reality that was characteristic of paramnesia fascinated the chaplain, and he knew a number of things about it. He knew, for example, that it was called paramnesia, and he was interested as well in such corollary optical phenomena as jamais vu, never seen, and presque vu, almost seen.
- Joseph Heller, Catch-22

Psychoanalysts have, in fact, officially named jamais vu. Meaning 'never seen', it refers to any familiar situation that seems strange because the individual thinks it is being experienced for the first time.

Presque vu means 'almost seen'. Not scientifically recognized, it is the sense that one is on the verge of a large mental breakthrough, almost seeing the absolute truth about something but not quite getting it.

Arthur Funkhouser, a Swiss psychologist, divides déjà vu itself into smaller, more specific areas to study them more closely. For example, if an individual feels he is in a place he has been before, he is experiencing déjà visité, 'already visited'. If a person feels she is experiencing something just as before - everything is exactly the same - it is déjà vecu, 'already lived through'.