What is déjà vu?
What is déjà vu and why does it occur?
I could have sworn I'd answered this question before. However, having scoured the files, I guess it just seems like I did. Is this a déjà vu experience? No, this is an out-to-lunch experience. I feel it's important to make these fine distinctions lest the meaning of this too casually flung about term become even more muddled in the popular mind.
The definition of déjà vu commonly cited in the medical literature these days is "subjectively inappropriate impressions of familiarity of the present with an undefined past." This definition unfortunately sucks, since it requires you to understand the thing being defined before you can understand the definition.
A better take on it is that déjà vu is the uncanny sensation that you are reliving some unknown past experience. I throw the word uncanny in there because it exudes the musty air of cheap paperbacks we like to cultivate in this column and also because an essential feature of déjà vu is that it seems intensely strange at the time.
The other essential feature is that the relived past experience is unknown — you cannot recall having previously had the experience, and indeed you may realize that it's impossible for you to have had it. You just somehow feel that you have.
The déjà vu phenomenon is a favorite of creative types. Proust mentions it, fittingly, in Remembrance of Things Past. In David Copperfield Dickens has his title character say, "He seemed to swell and grow before my eyes; the room seemed full of the echoes of his voice; and the strange feeling (to which no one is quite a stranger) that all this had occurred before, at some indefinite time, and that I knew what he was going to say next, took possession of me."
Depending on the survey, anywhere from 30 to 96 percent of respondents report having experienced déjà vu. But one suspects the high-end figures are a function of having worded the question too vaguely. Déjà vu doesn't mean merely going through the same situation twice, as many journalists seem to think. Nor should it be confused with other mental hiccups such as flashbacks, precognition (the sense that the present situation has been foretold), and so on.
Déjà vu is said to occur more frequently in those under 30. The experience is usually brief, lasting from a few seconds to a few minutes, but in pathological cases may be prolonged. Although the term déjà vu (French for "already seen") suggests it's primarily a visual phenomenon, it can involve all the senses, which is why some prefer the term déjà vecu, "already experienced." The opposite of déjà vu is jamais vu, ("never seen"), the sensation that a familiar situation is completely strange.
What causes déjà vu? Almost all who've studied the subject have come up with their own explanations, and hey, why not? Our knowledge of the brain is so fragmentary that no explanation can be definitely discounted. Still, the chances that déjà vu is a sign of telepathy, reincarnation, or visitations by one's astral body, as some have suggested, seem pretty slim.
Among the quasi-scientific explanations, what might be called the split-image school holds that two parts of the brain participate simultaneously in the process of perception. If for some reason the impression from part A arrives in one's consciousness out of sync with the impression from part B, one has the sensation of experiencing the thing twice.
Others explain déjà vu by analogy to a tape recorder. They propose that memory storage is accomplished by means of a "recording head" and memory recall by a "playback head." During déjà vu the two heads are erroneously situated above the same bit of mental blank tape. An experience is thus recorded and remembered simultaneously, with the result that the present is experienced as the past.
There are lots more theories, but you get the idea.
Déjà vu was a hot topic in the 1890s among French psychiatrists, who came up with the name. But later researchers dismissed it as a curiosity. The Dutch psychiatrist Herman Sno sparked a revival of interest in the 1990s, arguing that déjà vu provided insight into the functioning of both the normal and abnormal brain.
It's long been known that prolonged or frequent episodes of déjà vu are associated with various psychiatric or neurological disorders. Some now consider déjà vu, in conjunction with other symptoms, to be diagnostic of a type of epilepsy. Researchers have found that electrical stimulation of the brains of epileptic patients in some cases can trigger the déjà vu phenomenon.
Nothing you need to worry about. On the contrary, it seems pretty clear that what some consider a glimpse of the supernatural is more than likely just a cognitive burp.