What causes déjà vu? The quirky neuroscience behind the memory illusion
Why déjà vu is far from a memory fault – and how some people experience it constantly.
Sorry if you think we’ve already asked, but do you know exactly what déjà vu is? If you’re like most sensible people, you’ll likely say it’s the bizarre feeling that you’ve experienced something that’s happened before.
However, many neuroscientists would say this answer lacks a little je ne sais quoi. According to experts like Dr Akira O’Connor, senior psychology lecturer at the University of St Andrews, déjà vu – the French for 'already seen' – is not only a feeling of familiarity, but also the metacognitive recognition that these feelings are misplaced.
“Déjà vu is basically a conflict between the sensation of familiarity and the awareness that the familiarity is incorrect. And it’s the awareness that you're being tricked that makes déjà vu so unique compared to other memory events,” he explains.
“Most healthy people don’t tend to believe the sensation of familiarity and change their behaviour – like Neo in The Matrix, they logically know something isn’t right.”
So what happens in the brain during déjà vu? And why do some people experience this phenomenon more than others? In case you’ve drawn a blank, you can familiarise yourself with our full guide below.
What is déjà vu, according to neuroscience?
Unfortunately, as far as we know, the 60 per cent of people who report feeling déjà vu in their lives haven’t just experienced a glitch in the Matrix.
However, neuroscientists have determined that this memory illusion isn’t a sign of an unhealthy brain – it’s by no means a memory error. In fact, almost the opposite. As O’Connor argues, déjà vu occurs when the frontal regions of the brain attempt to correct an inaccurate memory.
“For the vast majority of people, experiencing déjà vu is probably a good thing. It's a sign that the fact-checking brain regions are working well, preventing you from misremembering events.
"In a healthy person, such misremembering is going to happen every day. This is to be expected because your memory involves millions and billions of neurones. It’s very messy,” he says.
Unfortunately, there is no single agreed model that explains exactly what happens in the brain during déjà vu. However, most of the main competing theories share the same idea: déjà vu occurs when areas of the brain (such as the temporal lobe) feed the mind's frontal regions signals that a past experience is repeating itself.
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“After this, the frontal decision-making areas of the brain effectively checks to see whether or not this signal is consistent with what is possible. It will ask ‘have I been here before?’
"If you have actually been in that place before, you may try harder to retrieve more memories. If not, a déjà vu realisation can occur.”
What makes somebody more likely to experience déjà vu?
Although O’Connor estimates a healthy person will experience déjà vu once a month on average, several factors can raise your chance of feeling the sensation.
Firstly: how tired and stressed you are. “When your brain is fatigued like this, your internal neuronal systems haven't had the chance to kind of recuperate and really regulate themselves. And so your neuronal firing is more likely to be a bit off and result in déjà vu,” he explains.
Research has also highlighted a possible link between the neurotransmitter dopamine (a known mood-booster) and déjà vu.
“Dopamine is what’s called an excitatory neurotransmitter. And when we talk about the brain regions that are signalling familiarity, there would be dopaminergic action in those neurones – this basically means dopamine is involved,” O’Connor explains.
“This could be why dopaminergic drugs, recreational or otherwise, tend to often cause elevated reports of déjà vu.”
There’s also another factor. Wondered why you don’t experience déjà vu as much as you used to? This is because, according to rigorous scientific research, you’re getting on a bit.
“Unfortunately, like many memory issues, it's just a natural part of ageing. You become less able to notice errors,” says O’Connor.
“It’s really interesting that younger people get more déjà vu – older people are normally expected to have more memory quirks. However, this happens as younger people generally have a greater kind of excitatory activity in their brains – they’re generally more active. And they have a real healthy fact-checking frontal part of the brain.
“When I first started researching déjà vu nearly 20 years ago I had it all the time, but a lot less so now!”
Can déjà vu ever be unhealthy?
If memory serves you right, you’ll now know déjà vu is a healthy mind mechanism – something far from dangerous. But what would happen if you felt déjà vu constantly? What if every new experience felt familiar?
Strangely enough, this can happen to some people. “There’s an extraordinary case of a man in Finland who took a mixture of flu medications which are known to be particularly excitatory to certain dopamine neurons. And this caused him to have persistent déjà vu,” says O’Connor.
“He found it so interesting he kept on taking them for a while – it did stop eventually!”
Unfortunately, however, some people don’t have the luxury of halting this mental Groundhog Day. People suffering from ‘déjà vécu’ (French for 'already lived') have the constant sensation of having already experienced the present situation. In short, nothing feels new to them.
“One of the things that makes people with déjà vécu so interesting is that they tend to lose the ability to fact check these sensations. Often these people will simply stop watching TV as it feels like they’ve seen every episode previously – everything will feel like a repeat,” says O’Connor.
“Although it sounds interesting and novel, it’s heartbreaking to see as it can often happen in dementia patients, and it can be a sign of a worsening development.”
Read more about the science of memory:
- My déjà vu is so extreme I can’t tell what’s real any more
- Where do memories form and how do we know?
- Memory and the brain – the key discovery
- What happens in your brain when you make a memory?
There’s also another strange parallel to déjà vu: jamais vu, 'never seen' en Francais. This essentially means failing to recognise a situation that, logically, should be familiar. Although often associated with amnesia, this is more than a mere momentary memory lapse.
“This isn’t standard forgetting,” explains O’Connor. “It’s a disorientating feeling that you don’t recognise something when you know you should. The really important thing is that awareness element – you know this feeling is factually wrong. Unless you notice it, you’re not experiencing jamais vu.
“It happens less frequently than déjà vu, but like déjà vu, it happens when we’re tired and happens more to young people than older people.”
Some lab experiments have seemingly been able to prompt jamais vu in participants. For instance, one University of Leeds study tasked 93 participants to write down a single familiar word (‘door’ in this case) as many times as possible within two minutes.
After this time, over 70 per cent of subjects were seen to doubt if ‘door’ was spelt correctly or even a real word at all – despite logically knowing it was.
The most intriguing thing about this experiment? It can be repeated anywhere. So, if you have two minutes and a pen, simply repeat after us: door, door, door, door…
About our expert – Dr Akira O’Connor
Akira O’Connor is a Senior Lecturer at the School of Psychology and Neuroscience, University of St Andrews. He mainly investigates the way in which we make decisions about our memories, and how we experience memory.
Read more about memory and the brain:
Thomas is Digital editor at BBC Science Focus. Writing about everything from cosmology to anthropology, he specialises in the latest psychology, health and neuroscience discoveries. Thomas has a Masters degree (distinction) in Magazine Journalism from the University of Sheffield and has written for Men’s Health, Vice and Radio Times. He has been shortlisted as the New Digital Talent of the Year at the national magazine Professional Publishers Association (PPA) awards. Also working in academia, Thomas has lectured on the topic of journalism to undergraduate and postgraduate students at The University of Sheffield.
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