Déjà vu

Déjà vu (French for "already seen") is a quirk of memory in which a new experience seems familiar.

This phenomenon of the mind is extremely common, but its nature makes it difficult to test in an empirical setting. Accordingly, there has been very little solid scientific research done on déjà vu, and neither its trigger nor its mechanism is fully understood.

Déjà vu has often been linked with various kinds of woo because of its ubiquitous yet mysterious nature: nearly everyone experiences the phenomenon, but no one can satisfactorily explain either what causes it or how it works. This allows for all kinds of conjectures about experiences in "past lives" or a psychic ability to "remember" the future. (Insert rolled eyes here.)

One of the most popular fictional explanations of déjà vu occurred in the hit 1999 film The Matrix, where it is explained as programming glitches in the simulated reality. The concept was also parodied by Monty Python.

Certain drugs have been shown to increase the chances of déjà vu occurring in users. Specifically, some pharmaceutical drugs when taken together have been implicated as a cause which could indicate a possible mechanism of action for the phenomenon. Taiminen and Jääskeläinen reported the case of an otherwise healthy male who started experiencing intense and recurrent sensations of déjà vu upon taking the drugs and  together to relieve flu symptoms. He found the experience so interesting that he completed the full course of his treatment and reported it to the psychologists to write up as a case study. Because of the action of the drugs and previous findings from electrode stimulation of the brain, Taiminen and Jääskeläinen speculate that déjà vu occurs as a result of hyperdopaminergic action in the  areas of the brain.

Jamais vu
Often described as the counterpart or "opposite" to déjà vu, "jamais vu" (French for "never seen") is the phenomenon in which an often-repeated experience feels as if it is new to the observer. The most common example can be induced by repeatedly saying a common word - "girls" is often used in this exercise - many times in a row. At some point in the repetition, most people find that the word ceases to make immediate sense to them as a communicating symbol, leaving them free to experience the word as wholly new before the effect fades. This is known as.