Two thirds of us will experience déjà vu (from the French for “already seen”) at some point in our lives, the feeling that you have seen something or been somewhere before, even when you know that was impossible.
For something so common it is quite poorly understood, and the mechanism behind deja vu is still unknown. Partly this is because it is so hard to study – it tends to arise spontaneously and attempts fully recreate it in the laboratory have been largely unsuccessful. A recent case of persistent déjà vu however has shed new light on its potential causes.
In January 2015 reports were given in the media about the case of a 23 year old British man who had suffered from “chronic déjà vu” for eight years. During this time he had recurrent spells lasting minutes at a time, and in at least one instance including a sense of déjà vu about having had déjà vu in that situation.
Understandably this caused the man a great deal of distress, and he described his experience as a nightmare, and like something from the film Donnie Darko, in which all of the characters experience déjà vu. In order to manage his condition he avoided television, the radio, and reading newspapers because of their constant triggering of his having feeling he had experienced it all before.
The international team that studied the man’s case now believe that deja vu is linked to anxiety. He had a history of anxiety and depression, and perhaps unsurprisingly found that his constant deja vu only increased his level of anxiety. The extent to which this then caused any subsequent increase in his experience of déjà vu is unclear, as is the mechanism by which this happened.
One explanation of the cause of déjà vu is based on the way that sensory input is sorted by the brain. Incoming signals into the brain are dealt with in the left temporal lobe of the brain, and the temporal lobe receives two sets of signals for each event. One is received from the left hemisphere of the brain, and the other milliseconds later is received from the right hemisphere. The theory is that a failure to synchronise the two signals results in them being treated as separate events, with the second experienced as a “re-living” of the first.
Déjà vu is seen more often in people with temporal lobe epilepsy and those with dementia and it is hoped that research into déjà vu will lead to insights into those conditions. There are however other states closely linked to déjà vu which may also shed light on memory and the workings of the brain.
- Deja entendu (“already heard”) is the closest, and is the sense of having heard a conversation or piece of music before, when rationally you know that that cannot have been the case.
- Presque vu (“almost seen”) is the sense of being on the brink of an epiphany or recalling a memory, and is involuntary, unlike the feeling of a consciously sought name or fact being “on the tip of your tongue”.
- Jamais vu (“never seen”) is the opposite to déjà vu, and is the sudden feeling of finding what should be a familiar word or experience somehow novel or alien.
Whatever the final explanation for déjà vu there will always of course be conspiracy theorists who will see it as a glitch in the matrix that belies a truer reality.
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