Defining Deja Vu
The term déjà vu is dervied from a French term meaning “already seen”. It is used to describe the feelings of an individual when the person visits a place or goes through an experience which bears an overwhelming similarity to something that the person cannot actually recount happening in the past. For example, a person from London might be visiting a royal palace in India and feel like he has been there before, even though he had actually never traveled to this place previously. Similarly, one might be enjoying a dinner with a group of friends at a city restaurant and feel like he has gone through the exact experience sometime back, even though that is not the case. The term déjà visite ("already visited") refers to the first type of déjà vu case, and the term déjà vecu ("already experienced or lived through”) refers to the second type of déjà vu case as mentioned above. Nearly 70% of the world’s population is suspected of experiencing the phenomenon of déjà vu at some point, and the greatest number of such experiences appear to happen between the ages of 15 and 25. Men and women of all races are equally affected by this phenomenon.
Link to Other Health Problems
Scientists have always tried to find links between déjà vu and mental health disorders, especially dissociative identity disorder, schizophrenia, and anxiety disorders. Until now, however, the have largely failed to find any conclusive proof linking such disorders to déjà vu experiences. Some links between déjà vu and epilepsy, however, do appear to exist. For example, patients of temporal lobe epilepsy have reported experiencing consistent déjà vu-like feelings at the onset of an epileptic fit. Scientists have also attempted to link genetic mutations to déjà vu, but have not yet been able to identify a gene that can be clearly associated with this phenomenon. There is some study on the LGII gene on chromosome 10 (which is also associated with a mild form of epilepsy) and its possible connections to déjà vu.
Drug-Related Deja Vu
An interesting case study in 2001 gave more insight into the relationship between déjà vu and pharmacological drugs. A healthy man who took the medications of amantadine and phenylpropanolamine together experienced frequent episodes of déjà vu. The case study on the man was published by Finnish researchers Taiminen and Jääskeläinen in 2001, who also went on to claim that the dopaminergic action of these drugs, and previous studies by other scientists hinting at links between dopamine and déjà vu, could imply that a hyperdopaminergic event in the brain may be responsible for déjà vu.
What's Going on Inside the Brain?
A section of researchers speculate that déjà vu occurs due to some form of mismatch or errors in the brain’s memory-processing activities, wherein a jumbling up of the brain’s memory recall capacity and sensory output results in such a situation. A different kind of hypothesis explains that déjà vu occurs due to a "leak out" from our short term memories into our long term memories, and our brains thus register our present encounters as a past event of some kind, giving us the typical feeling of déjà vu. Some studies in epileptic patients have indicated that disturbances in the median temporal lobe could induce a déjà vu event.
Role of Past Dreams
Another highly interesting explanation of the occurrence of déjà vu events have been linked to dreams. Precognitive dreams, where one happens to dream of a future event, could trigger déjà vu experiences in a person when the prospective future of the dream world actually comes true. Since these dreams are usually completely forgotten after waking up, the recollection of having a similar experience to what was seen during the dream at a future event could explain the déjà vu effect.