Déjà vu, (i/ /; French pronunciation: [de.ʒa.vy]) from French, literally "already seen", is the phenomenon of having the strong sensation that an event or experience currently being experienced has already been experienced in the past. Déjà vu is a feeling of familiarity, and déjà vécu (the feeling of having "already lived through" something) is a feeling of recollection.
Scientific approaches reject the explanation of déjà vu as "precognition" or "prophecy", but rather explain it as an anomaly of memory, which creates a distinct impression that an experience is "being recalled". This explanation is supported 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 uncertain or believed to be impossible. Two types of déjà vu are suggested to exist: the pathological type of déjà vu usually associated with epilepsy and the non-pathological which is a characteristic of healthy people and psychological phenomenon.
A 2004 survey concluded that approximately two-thirds of the population have had déjà vu experiences. Other studies confirm that déjà vu is a common experience in healthy individuals, with between 31% and 96% of individuals reporting it. Déjà vu experiences that are unusually prolonged or frequent, or in association with other symptoms such as hallucinations, may be an indicator of neurological or psychiatric illness.
Links with disorders
Early researchers tried to establish a link between déjà vu and mental disorders such as anxiety, dissociative identity disorder and schizophrenia but failed to find correlations of any diagnostic value. There does not seem to be a special association between déjà vu and schizophrenia.
The strongest pathological association of déjà vu is with temporal lobe epilepsy. This correlation has led some researchers to speculate that the experience of déjà 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. a hypnagogic jerk, the sudden "jolt" that frequently, but not always, occurs just prior to falling asleep), it is conjectured that a similar (mild) neurological aberration occurs in the experience of déjà vu, resulting in an erroneous sensation of memory. Scientists have even looked into genetics when considering déjà vu. Although there is not currently a gene associated with déjà vu, the LGII gene on chromosome 10 is being studied for a possible link. Certain forms of the gene are associated with a mild form of epilepsy and, though by no means a certainty, déjà vu occurs often enough during seizures that researchers have reason to suspect a link.
Certain drugs increase the chances of déjà vu occurring in the user. Some pharmaceutical drugs, when taken together, have also been implicated in the cause of déjà vu. Taiminen and Jääskeläinen (2001) reported the case of an otherwise healthy male who started experiencing intense and recurrent sensations of déjà vu upon 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. Because of 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 Jääskeläinen speculate that déjà vu occurs as a result of hyperdopaminergic action in the mesial temporal areas of the brain.
Research has associated déjà vu experiences with good memory function.
The similarity between a déjà-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 déjà 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 thereafter 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 déjà vu can be reliably produced that would allow it to be tested under well-controlled experimental conditions. Cleary suggests that déjà vu may be a form of familiarity-based recognition (recognition that is based on a feeling of familiarity with a situation) and that laboratory methods of probing familiarity-based recognition hold promise for probing déjà vu in laboratory settings.
A recent[when?] study that used virtual reality technology to study reported déjà vu experiences supported this idea. This virtual reality investigation suggested that similarity between a new scene's spatial layout and the layout of a previously experienced scene in memory (but which fails to be recalled) may contribute to the déjà vu experience. When the previously experienced scene fails to come to mind in response to viewing the new scene, that previously experienced scene in memory can still exert an effect—that effect may be a feeling of familiarity with the new scene that is subjectively experienced as a feeling of déjà vu, or of having been there before despite knowing otherwise.
Another possible explanation for the phenomenon of déjà vu is the occurrence of "cryptomnesia", which is where information learned is forgotten but nevertheless stored in the brain, and similar occurrences invoke the contained knowledge, leading to a feeling of familiarity because of the situation, event or emotional/vocal content, known as "déjà vu". Some experts suggest that memory is a process of reconstruction, rather than a recall of fixed, established events. This reconstruction comes from stored components, involving elaborations, distortions and omissions. Each successive recall of an event is merely a recall of the last reconstruction. The proposed sense of recognition (déjà vu) involves achieving a good ‘match' between the present experience and our stored data. This reconstruction however, may now differ so much from the original event that we ‘know' we have never experienced it before, even though it seems similar.
Robert Efron of Boston's Veterans Hospital in 1963 proposed that déjà vu is caused by dual neurological processing caused by delayed signals in the brain. Efron found that the brain's sorting of incoming signals is done in the temporal lobe of the brain's left hemisphere. However, signals enter the temporal lobe twice before processing, once from each hemisphere of the brain, normally with a slight delay of milliseconds between them. Efron proposed that if the two signals were occasionally not synchronized properly then they would be processed as two separate experiences, with the second seeming to be a re-living of the first.
One theory of déjà vu attributes the feeling of having previously seen or experienced something to having dreamt about a similar situation or place, and forgetting about it until one seems to be mysteriously reminded of the situation or the place while awake.
B-Theory of Time
A similar explanation is linked to the conscious, human experience of time and space. 
Jamais vu (from French, meaning "never seen") is a term in psychology which is used to describe any familiar situation which is not recognized by the observer.
Often described as the opposite of déjà 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 aphasia, 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 or 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 surreality) feelings.
Times Online reports (see semantic satiation):
Chris Moulin, of the University of Leeds, asked 95 volunteers to write out "door" 30 times in 60 seconds. At the International Conference on Memory in Sydney last week he reported that 68 percent of the volunteers showed symptoms of jamais vu, such as beginning to doubt that "door" was a real word. Dr. Moulin believes that a similar brain fatigue underlies a phenomenon observed in some schizophrenia patients: that a familiar person has been replaced by an impostor. Dr. Moulin suggests they could be suffering from chronic jamais vu.
Presque vu (French pronunciation: [pʁɛsk vy], from French, meaning "almost seen") is the intense feeling of being on the very brink of a powerful epiphany, insight, or revelation, without actually achieving the revelation. The feeling is often therefore associated with a frustrating, tantalizing sense of incompleteness or near-completeness.
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