Déjà vu: "already seen"; also called paramnesia or promnesia, 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 happened in the near past), although the exact circumstances of the previous encounter are uncertain. The term was coined by a French psychic researcher, Émile Boirac. The experience of déjà 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 déjà vu seems to be quite common among adults and children alike. References to the experience of déjà vu are also found in literature of the past,indicating it is not a new phenomenon. It has been extremely difficult to evoke the déjà vu experience in laboratory settings, therefore making it a subject of few empirical studies. Recently, researchers have found ways to recreate this sensation using hypnosis.
Scientific research on déjà vu:
Since the last years of the 20th century, déjà vu has been subject to serious psychological and neurophysiological research. Scientifically speaking, the most likely explanation of déjà vu is not that it is an act of "precognition" or "prophecy," but rather that it is an anomaly of memory giving 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 déjà vu itself, but little or no recollection of the specifics of the event(s) or circumstance(s) they were "remembering" when they had the déjà 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). The events would be stored into memory before the conscious part of the brain even receives the information and processes 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 déjà vu of someone slapping the fingers on his left hand, then the déjà vu feeling is certainly not due to his right hand experiencing the same sensation later than his left hand considering that his right hand would never receive the same sensory input. Also, persons with only one eye still report experiencing déjà vu or déjà vécu (a rare disorder of memory, similar to persistent déjà vu). The global phenomenon must therefore be narrowed down to the brain itself (say, one hemisphere would be late compared to the other one).