Adapted from Edward Willet's Déjà Vu

'It seems that I already visited this place.' Ever get this feeling when in fact you know very well that you never actually came near this particular area before? About two-thirds of adults had this feeling at least once. It's called 'déjà vu', French for 'already seen'.

How déjà vu occurs and why déjà vu occurs are questions that scientists are now trying to clear up.

Alan S Brown, professor of psychology at Southern Methodist University and writer of the book The Déjà Vu Experience: Essays in Cognitive Psychology, categorizes déjà vu theories into four families.

One group of theories proposes that there is a neurological cause behind déjà vu. Before seizures, people suffering from epilepsy often report déjà vu. This may mean that déjà vu is in fact a small seizure. It should also be noted that déjà vu can be triggered by electrically stimulating certain brain areas, probably proving that déjà vu is indeed linked to neurology.

Another group of theories places memory as the answer to the why of déjà vu. The latter might be awoken by a scene we have witnessed but only consciously forgotten. According to researchers, a teacup with the same pattern as a teacup you have seen in a movie is enough to start déjà vu!

Yet another group of theories involves dual processing. Memory apparently involves two distinct systems: familiarity and retrieval. Dual processing theories propose that déjà vu occurs when the familiarity system is activated but the retrieval system is not.

The fourth group suggests that déjà vu arises due to an interruption in the normal process of perception. This group of theories is backed by an experiment. Subjects were shown a long list of words on a screen. A week later, the same subjects were shown another long list of words. This second list, however, only contained half of the words found in the first list. The subjects were then asked to identify the words they had read on the first list in this second list.

The scientists who were monitoring this experiment, however, also flashed a word for about 20 milliseconds before it made its 'true' appearance. This often caused the subjects to say that they had seen that particular word in the first list even though the word had never been present there. The experiment shows that we do remember things that we do not notice consciously. Such 'consciously unnoticed' things may even appear familiar later on.

Dr Brown thinks that several mechanisms are at work during déjà vu, perhaps different ones for different people in different situations.

Man, was that déjà vu?

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