You’re walking down a street, chatting with your friends when suddenly you come to the alarming conclusion that this exact same moment, down to every last detail has happened to you before, but you know that's impossible. Yet the similarities are shocking, as if history is repeating itself or as if you have just glimpsed into your future. And then poof, a few seconds later – it's over.
This feeling is Déjà vu, a French term, meaning 'already seen'. It is an experience a person has, which they are sure they have encountered before. Many of us report having had such an experience at some point during our life, but there are still many conflicting theories about why this occurs.
One of the original theories about déjà vu was that it was caused by out of sync sensory signals from the eye to the brain, so that the image from one eye arrived before the image of the other eye, making it seem like the same image is being seen twice. However, lately Chris Mulin and researchers from Leeds reported cases of blind people having experiences of déjà vu, which contradicts the original view and, as a result, has reinvented the way we view déjà vu. They explained that people can have such an experience in non-visual ways, such as when you overhear a conversation whilst waiting for the bus or hearing a particular song whilst getting dressed.
In addition to Mulin, Anne Cleary and other researchers from Colorado State University used the video game 'The Sims 2' to create a virtual reality in which they could test their theories about déjà vu. They created over 100 different scenes, with the exact same layout used in each one, with certain objects or colour schemes changed and the researchers found that different locations with identical layouts did trigger a reaction similar to déjà vu.
Cleary summarises her results, by suggesting déjà vu could be due to a person failing to retrieve something from their memory. Often we can only remember certain aspects of a past experience, and déjà vu could be caused by a misplaced familiarity. For example, one night you walked down an alley wafting very peculiar smells. Ten years later you walk down another alley with the same scent and you recall the sensory information that is responsible for that scent, and all of a sudden certain parts of a detailed, ten year old memory comes back to you, but not all of it, leaving your brain to fill in the gaps. As a result you believe you’ve had the experience before, when actually, you have never been there, but the similarities mislead your brain.
However, those who have experienced déjà vu before will recognise the eerie and unsettling sensation that comes with it. Cleary suggested that this may be due to “the contrast between the sense of newness and the simultaneous sense of oldness—something unfamiliar should not also feel familiar."
Another related theory suggests that a brain malfunction, affecting the circuits of the brain responsible for long term and short term memory could result in déjà vu. Some researchers suggest a form of short circuit, where information from our short term memory is bypassed into our long term memory, making it seem as though an experience we are encountering now, occurred months, or even years, ago.
There are also theories that attempt to explain the mechanism behind the infamous déjà vu. Researchers have suggested a disturbance of the medial temporal lobe could be responsible for causing déjà vu. They concluded that stimulation of the rhinal cortex (a structure involved in sensory processing) could induce déjà vu in epileptic patients. They monitored patterns of electroencephalography signals from the rhinal cortex and other regions of the brain (such as the hippocampus and amygdala) in the patients. It seemed that synchronized neural firing between these areas were increased when patients were experiencing déjà vu. This increased activity could be responsible for causing the recollection of a previous event to occur.
Indeed, there have been multiple investigations into the origins of déjà vu and they all contradict each other, leaving the scientific mechanism of the process a mystery to us all. This is because it occurs randomly and is over promptly, making it very difficult to study or isolate in a lab. It's important to realise that there hasn't been any conclusive evidence to explain the true origin of déjà vu, its details are still up for debate, although there have been multiple correlations present and it is definitely an exciting area of research that could provide brilliant results in the future.