Déjà vu translates literally as “already seen“ and is the feeling that one has lived through the present situation before. Now, before we jump into the possible scientific explanations of this phenomena, as well as a couple of crazy theories people came up with, lets state some facts about it:
There are two types of deja vu: the pathological deja vu usually associated with epilepsy, and the non-pathological type characteristic of healthy people
About 2/3 of people have reported experiencing déjà vu
People who travel more frequently are more likely to experience déjà vu. A study carried out in 1967 found that only 11% of people who never traveled experienced déjà vu, compared to 41% of those who made 1-4 trips per year, and 44% of those traveling 5+ times per year.
Studies demonstrated that déjà vu is more common when the person is under high stress, is tired, or both.
Certain drugs may increase the likelihood of experiencing déjà vu. For example, there was a 2001 case study that recounts a mentally healthy 39-year-old male who experienced recurrent déjà vu when taking amantadine and phenylpropanolamine together to treat flu (please do not try this at home. Even if it seems tempting it is NOT a good idea)
Déjà vu occurs more commonly in younger people, steadily becoming less common as we age
Despite of how common and well-known it is in our society, there is still a lot we do not know about the phenomena. Our understanding is limited due to how hard it is to study in a laboratory setting, thus leaving a lot to the imagination.
From what is known so far on the subject, we can conclude that in people without psychosis or temporal lobe epilepsy, the causes of deja vu can be divided into four categories-attentional, memory, dual processing, and neurological.
Attentional explanations involve an initial perception made under degraded attention, followed by a second one made under full attention. For example, if you are about to turn off the TV and are momentarily distracted by a noise in the distance, when you return to your previous task of turning off the TV, the first perception/memory may seem further off in the past. The likeliness of such a thing happening is greatly increased if you are under high stress or are tired, since your perception of the reality is not as sharp.
Memory explanations are based on the assumption that some detail of the new experience is familiar but the source of this familiarity has been forgotten. The premise of this explanation is that we as people encounter countless objects during the course of a day but don’t pay enough attention to remember every single one. Reprocessing the information can act as trigger and induce familiarity and déjà vu
Dual-processing theory holds that déjà vu occurs when two usually synchronous cognitive processes are momentarily out of sync. It is worth keeping in mind that this is one of the oldest déjà vu theories and has no empirical evidence to back it up. It is fascinating to think about, however, as it takes you on a journey of questioning the human brain, consciousness and the great mystery that is life in itself.
Neurological explanations of déjà vu attribute the phenomenon to either a small temporal lobe seizure in a person without epilepsy, or to a delay in neuronal transmission between perceptual organs such as the eyes or ears and the processing centers in the brain. Their appeal lies in their neurological basis and seemingly logical functioning, however, due to the lack of advanced technology required to test them, they are less suitable to researchers compared to attention and memory based explanations.
Where in the brain does it occur?
This may come as a surprise, but déjà vu has no correlation to any particular mental disorder. The only condition that is reliably associated with the phenomenon is temporal lobe epilepsy (TLE).
Temporal lobe (shown here in yellow) appears to be the occuring place of déjà vu
In this particular form of epilepsy, as with other types, there are certain warning signs commonly described as an “aura”. For some people with TLE déjà vu is considered one of them.
As far as where exactly in the brain it occurs, a study conducted back in 2012 narrowed down the search when they found that stimulating the entorhinal cortex (EC) can produce experiences similar to déjà vu. EC is an area of the brain located in the medial temporal lobe, and it plays a role in spatial memory, memory consolidation and time perception.
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