Deja vu
Deja vu (pronounced Template:Audio-IPA; French Template:Audio-IPA "already seen"; also called paramnesia, from Greek  para, "near" +  mneme, "memory") is the experience of feeling sure that one has witnessed or experienced a new situation previously (an individual feels as though an event has already happened or has repeated itself).  The term was coined by a France|French psychic researcher, Emile Boirac (18511917) in his book L'Avenir des sciences psychiques (The Future of Psychic Sciences), which expanded upon an essay he wrote while an undergraduate.  The experience of deja vu is usually accompanied by a compelling sense of familiarity, and also a sense of "eeriness", "strangeness", or "weirdness".  The "previous" experience is most frequently attributed to a dream, although in some cases there is a firm sense that the experience "genuinely happened" in the past.
The experience of deja vu seems to be quite common among adults and children alike; in formal studies 70% of people report having experienced it at least once. References to the experience of deja vu are also found in literature of the past, indicating it is not a new phenomenon. It has been extremely difficult to evoke the deja vu experience in Experiment|laboratory settings, therefore making it a subject of few empirical studies.  Recently, researchers have found ways to recreate this sensation using hypnosis.

# Types of deja vu
According to Arthur Funkhouser there are three major types of deja vu.

## Deja vecu
Deja vecu refers to an experience involving more than just sight, which is why labeling such "deja vu" is usually inaccurate. The sense involves a great amount of detail, sensing that everything is just as it was before and a weird knowledge of what is going to be said or happen next.
Translated literally as 'already lived,' deja vecu is described in a quotation from Charles Dickens:
When most people speak of deja vu, they are actually experiencing deja vecu.  Surveys have revealed that as much as 70% of the population have had these experiences, usually between ages 15 to 25, when the mind is still subject to noticing the change in environment.  The experience is usually related to a very ordinary event, but it is so striking that it is remembered for several years afterwards.
More recently, the term deja vecu has been used to describe very intense and persistent feelings of a deja vu type, which occur as part of a memory disorder.

## Deja senti
This phenomenon specifies something 'already felt.' Unlike the implied precognition of deja vecu, deja senti is primarily or even exclusively a mental happening, has no precognitive aspects, and rarely if ever remains in the afflicted person's memory afterwards.
Dr. John Hughlings Jackson recorded the words of one of his patients who suffered from temporal lobe or psychomotor epilepsy in an 1888 paper:
As with Dr. Jackson's patient, some temporal-lobe epileptics may experience this phenomenon.

## Deja visite
This experience is less common and involves an uncanny knowledge of a new place. The translation is "already visited." Here one may know his or her way around in a new town or landscape while at the same time knowing that this should not be possible.
Theories involving dreams, reincarnation and also Out-of-body experience|out-of-body travel have been used to explain this phenomenon. Additionally, some suggest that reading a detailed account of a place can result in this feeling when the locale is later visited.  Two famous examples of such a situation were described by Nathaniel Hawthorne in his book Our Old Home and Sir Walter Scott in Guy Mannering.  Hawthorne recognized the ruins of a castle in England and later was able to trace the sensation to a piece written about the castle by Alexander Pope nearly a century earlier.
C. G. Jung published an account of deja visite in his 1952 paper On synchronicity.
In order to distinguish deja visite from deja vecu, it is important to identify the source of the feeling. Deja vecu is in reference to the temporal occurrences and processes, while deja visite has more to do with geography and spatial relations.

# Scientific research
In recent years, deja vu has been subjected to serious psychological and neurophysiological research. Scientifically speaking, the most likely explanation of deja vu is not that it is an act of "precognition" or "prophecy", but rather that it is an anomaly of memory; it is the impression that an experience is "being recalled". This explanation is substantiated by the fact that the sense of "recollection" at the time is strong in most cases, but that the circumstances of the "previous" experience (when, where and how the earlier experience occurred) are quite uncertain. Likewise, as time passes, subjects can exhibit a strong recollection of having the "unsettling" experience of deja vu itself, but little to no recollection of the specifics of the event(s) or circumstance(s) they were "remembering" when they had the deja vu experience. In particular, this may result from an overlap between the neurological systems responsible for short-term memory (events which are perceived as being in the present) and those responsible for long-term memory (events which are perceived as being in the past).  In other words, the events would be stored into memory before the conscious part of the brain even receives the information and processes it. This would explain why one is, if it ever comes to mind, powerless trying to twist the outcome of the event in order to create a paradox.  The delay is only of a few milliseconds, and besides, already happened at the time the consciousness of the individual is experiencing it.
Another theory being explored is that of vision. As the theory suggests, one eye may record what is seen fractionally faster than the other, creating that "strong recollection" sensation upon the "same" scene being viewed milliseconds later by the opposite eye. However, this one fails to explain the phenomenon when other sensory inputs are involved, such as the auditive part, and especially the digital part. If one, for instance, experiences deja vu of someone slapping the fingers on his/her left hand, then the deja vu feeling is certainly not due to his/her right hand to be late on the left one.  Also, persons with only one eye still report experiencing deja vu or deja vecu.  The global phenomenon must therefore be narrowed down to the brain itself (say, one hemisphere would be late compared to the other one).

## Links with disorders
Early researchers tried to establish a link between deja vu and serious psychopathology such as schizophrenia, anxiety, and dissociative identity disorder, with hopes of finding the experience of some diagnostic value. However, there does not seem to be any special association between deja vu and schizophrenia or other neurotic conditions. The strongest pathological association of deja vu is with temporal lobe epilepsy. This correlation has led some researchers to speculate that the experience of deja vu is possibly a neurological anomaly related to improper electrical discharge in the brain. As most people suffer a mild (i.e. non-pathological) epileptic episode regularly (e.g. the sudden "jolt", a hypnagogic jerk, that frequently occurs just prior to falling asleep), it is conjectured that a similar (mild) neurological aberration occurs in the experience of deja vu, resulting in an erroneous sensation of memory.

## Pharmacology
It has been reported that certain drugs increase the chances of deja vu occurring in the user. Some pharmaceutical drugs, when taken together, have also been implicated in the cause of deja vu.  Taiminen and Jaaskelainen (2001) reported the case of an otherwise healthy male who started experiencing intense and recurrent sensations of deja vu on taking the drugs amantadine and phenylpropanolamine 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. Due to the dopaminergic action of the drugs and previous findings from electrode stimulation of the brain (e.g. Bancaud, Brunet-Bourgin, Chauvel, & Halgren, 1994), Taiminen and Jaaskelainen speculate that deja vu occurs as a result of hyperdopaminergic action in the mesial temporal areas of the brain.

## Memory-based explanations
The similarity between a deja vu-eliciting stimulus and an existing, but different, memory trace may lead to the sensation.  Thus, encountering something which evokes the implicit associations of an experience or sensation that cannot be remembered may lead to deja vu.  In an effort to experimentally reproduce the sensation, Banister and Zangwill (1941) used hypnosis to give participants posthypnotic amnesia  for material they had already seen. When this was later re-encountered, the restricted activation caused by the posthypnotic amnesia resulted in three of the 10 participants reporting what the authors termed paramnesias.  Memory-based explanations may lead to the development of a number of non-invasive experimental methods by which a long sought-after analogue of deja vu can be reliably produced that would allow it to be tested under well-controlled experimental conditions.
Another possible explanation for the phenomenon of deja vu is the occurrence of "cryptamnesia", which is where information learned is forgotten but nevertheless stored in the brain, and occurrence of similarities invokes the contained knowledge, leading to a feeling of familiarity because of the situation, event or emotional/vocal content, known as "deja vu".

## Neural theories
In the late 20th and early 21st Centuries, it was widely believed that deja vu could be caused by the mis-timing of neuronal firing. This timing error was thought to lead the brain to believe that it was encountering a stimulus for the second time, when in fact, it was simply re-experiencing the same event from a slightly delayed source. A number of variations of these theories exist, with miscommunication of the two cerebral hemispheres and abnormally fast neuronal firing also given as explanations for the sensation.

## Parapsychology
Deja vu is associated with precognition, clairvoyance or extra-sensory perceptions, and it is frequently cited as evidence for "psychic" abilities in the general population. Non-scientific explanations attribute the experience to prophecy, visions (such as received in dreams) or past-life memories.

## Dreams
Some believe deja vu is the memory of dreams. Though the majority of dreams are never remembered, a dreaming person can display activity in the areas of the brain that process long-term memory. It has been speculated that dreams read directly into long-term memory, bypassing short-term memory entirely. In this case, deja vu might be a memory of a forgotten dream with elements in common with the current waking experience. This may be similar to another phenomenon known as deja reve, or "already dreamed." However, later studies on mice indicate that long-term memories must be first established as short-term memories.
Not only is the link to dreams as they pertain to deja vu the subject of scientific and psychological studies, it is also a subject of spiritual texts, as is found, for example, in the writings of the Baha'i Faith with quotes like "... perchance when ten years are gone, thou wilt witness in the outer world the very things thou hast dreamed tonight." and "Behold how the thing which thou hast seen in thy dream is, after a considerable lapse of time, fully realized."
Daniel Heady suggested that a feeling of remembering occurs in a sense that a he might realize that what he had dreamt is now a relevant present action that is taking place right here right now.
"I was once sitting down in the kitchen noticing that my plate seemed well too familiar, it seemed as if my head motions were foreseen, and that every move would trigger a continuation to happen or so, I had many deja vu's as a child but this was extraordinary, I knew from the bottom of my heart that I had dreamed this situation years ago, as a little boy, that amazingly an entire piece of memory was regained and I finally understood when and where I was dreaming and how long this dream was, and most importantly how many years ago did I dream."Template:Who

## Reincarnation
Those believing in reincarnation theorize that deja vu is caused by fragments of past-life memories being jarred to the surface of the mind by familiar surroundings or people. Others theorize that the phenomenon is caused by astral projection, or out-of-body experiences (OBEs), where it is possible that individuals have visited places while in their astral bodies during sleep. The sensation may also be interpreted as connected to the fulfillment of a condition as seen or felt in a premonition. For further cases of remembering information from past lives, see Ian Stevenson.

# Related phenomena

## Jamais vu
Jamais vu is a term in psychology (from the French, meaning "never seen") which is used to describe any familiar situation which is not recognized by the observer.
Often described as the opposite of deja vu, jamais vu involves a sense of eeriness and the observer's impression of seeing the situation for the first time, despite rationally knowing that he or she has been in the situation before.
Jamais vu is more commonly explained as when a person momentarily does not recognize a word, person, or place that they already know.
Jamais vu is sometimes associated with certain types of amnesia and epilepsy.
Theoretically, as seen below, a jamais vu feeling in a sufferer of a delirious disorder or intoxication could result in a delirious explanation of it, such as in the Capgras delusion, in which the patient takes a person known by him/her for a false double or impostor. If the impostor is himself, the clinical setting would be the same as the one described as depersonalisation, hence jamais vus of oneself or of the very "reality of reality", are termed depersonalisation (or irreality) feelings.
Times Online reports:

## Tip of Tongue (Presque vu)
Deja vu is similar to, but distinct from, the phenomenon called tip of the tongue which is when one cannot recall a familiar word or name or situation, but with effort one eventually recall the elusive memory. In contrast, deja vu is a feeling that the present situation has occurred before, but the details are elusive because the situation never happened before.
Presque vu (from French, meaning "almost seen") is the sensation of being on the brink of an epiphany (feeling)|epiphany. Often very disorienting and distracting, presque vu rarely leads to an actual breakthrough. Frequently, one experiencing presque vu will say that they have something "on the tip of their tongue."
Presque vu is often cited by people who suffer from epilepsy or other seizure-related brain conditions, such as temporal lobe lability.

## L'esprit de l'escalier
L'esprit de l'escalier (from French, "staircase wit") is remembering something when it is too late. For example, a clever come-back to a remark, thought of after the conversation has ended.
An example of L'esprit de l'escalier in popular culture can be seen in the TV sitcom Seinfeld.  In one episode, the character George Costanza thinks of a perfect comeback-line to an insult he received from a co-worker, but too late to deliver it.  While George is rapidly consuming a bowl of shrimp during an office meeting, a co-worker teases him, saying "Hey George, the ocean called.  It's running out of shrimp".  Only later in the day does George conceive of the perfect comeback: "Well, the jerk store called.  They're running out of you."