Chris Moulin was visiting New York for the first time when he turned down a street and was suddenly struck by an impossible feeling: he’d been there before. “It couldn’t be right but it was so strong,” he says.

Today, the neuropsychologist is a world expert on the strange phenomenon of deja vu. In French it means “already seen.” But the feeling of familiarity where there should be none can arrive with sound, taste, smell and more. Some people even become convinced they can predict what’s about to happen next, as if they are replaying a moment in time. Moulin recalls a particularly powerful wave of deja vu hitting while on a plane leaving Canada. “And it hung on. It evolved into this sense of dread – because I also felt I knew what was coming and what was coming was the plane was about to nosedive.”

Thankfully, Moulin was not actually having a premonition that day and the plane kept on flying. But what he experienced isn’t uncommon, he says, a kind of faux prescience that extends the jolt of classic deja vu into something truly spooky.

So, what is going on when deja vu hits? Is it a glitch in our heads – or the Matrix? And how are scientists using hypnosis, puzzles and The Sims video game to find out?

What is deja vu?

Moulin, now a memory researcher at the Universite Grenoble Alpes in France, first became interested in deja vu two decades ago when a woman came to him about a strange case: her husband, known as AKP, had what she called “permanent deja vu”. Everything felt familiar to this 80-year-old – every song, every TV show, snippets of conversation, strangers in the street. Everything seemed to be replaying. In fact, the man was convinced he was living his life over. “He wouldn’t even read the newspaper because he said he’d already read it,” says Moulin. “He’d invent far-fetched stories for how he got to the newsagent that morning just as the papers were being unloaded. And he’d believe it. This got pretty frustrating for his wife.” In fact, when he was first referred to the memory clinic where Moulin worked, AKP insisted he’d already been there.

It turned out there were other cases like him – all older people (“so, late onset”), generally with some form of dementia or memory issue but otherwise very functional and engaged, as was AKP. “I’ve worked with about 13 or 14 people like that,” says Moulin, who admits he “sort of became a deja vu researcher because there wasn’t a deja vu researcher” while trying to treat them. “But we wouldn’t describe it as deja vu any more because for them the feeling of familiarity is so intense they can’t overcome it. [AKP] was seduced by it. He acted on it, justified it with false memories.”

The French actually have many names for deja vu. As well as already seen, there’s deja entendu (already heard), deja senti (already felt) and deja reve (already dreamed). AKP had persistent deja vecu (already lived). “But when we get deja vu normally, we know something’s wrong,” says Moulin. “That’s why it feels so weird.”

And of course, for most of us, deja vu is fleeting, however strong in the moment. We might stop on a street corner, trying to remember when we’ve been there before. After we realise we haven’t, the feeling generally starts to fade (unless you’re Bill Murray’s character in Groundhog Day, who at first mistakes the time loop he’s stuck in for deja vu.)

At least two-thirds of people are estimated to get deja vu at some point.

In the sci-fi thriller The Matrix, our hero Neo is told that deja vu is really a glitch in the artificial reality they are locked in (when the sudden reappearance of a black cat, now known to the internet as deja vu cat, signals coming danger.)

Neurologist Professor Sam Berkovic says you can think of deja vu as a “glitch” – but in our own brains. Our little grey cells aren’t perfect, after all. We might occasionally see a flash of something that’s not there as our visual cortex misfires or hear a phantom sound thanks to a mistake by our auditory cortex, Berkovic says. “And I think when we get deja vu, it’s our memory cortex just [briefly] malfunctioning. Really, it’s surprising that our brains don’t misfire more often.”

That might explain why people are more likely to get deja vu while tired or stressed, Moulin says, when the brain can start to “get all kinds of weird disturbances which are not unlike the disturbances you get with pathology, but you’re not ill. It’s only temporary.”

At least two-thirds of people are estimated to get deja vu at some point. And patients with epilepsy often report especially powerful bursts of deja vu before or during seizures (as do some people before a migraine hits).

“I get [deja vu] very occasionally,” says Berkovic, who is director of the Epilepsy Research Centre at Austin Health. “You kind of shake your head and say, ‘Well, my brain is playing tricks’, which is exactly what it’s doing. But with epilepsy, it’s much more intense, it can last longer and people have a pretty strong belief at the time that they actually have been there before, in that situation.”

Work by Berkovic and his colleagues has found even relatives of those with epilepsy have reported more intense deja vu bursts than usual, though, as Berkovic explains, they may have also inherited epilepsy themselves, just in a much milder form not requiring treatment.

What might be happening in the brain during a deja vu “glitch”?

Remembering, psychologist Endel Tulving once observed, is a kind of “mental time travel”. We not only call up the bare facts of our past experiences, as we do facts and figures – we relive them. But how can we remember things that never happened?

Some of our best knowledge of deja vu – and many normal brain functions for that matter – comes from studying people with epilepsy, says Berkovic. Electrodes are sometimes implanted into the brain of patients to track and analyse their seizures. (“We don’t do that with healthy people.” ) These implants not only map where the electrical currents of the brain are going haywire; they can themselves induce symptoms by sending out tiny electric signals. As Berkovic explains, “The patient doesn’t feel them but they can excite local parts of the brain”, provoking smells, memories and, in the right spot, deja vu.

It turns out the phenomenon hits deep in the temporal lobe, under your temple, he says. There lies the hippocampus, key to laying down experiences into memories, and other crucial parts of the brain’s memory hardware.

But Moulin adds, even in this research, “inducing deja vu has always been something that was so weird, nobody could really incorporate it into our understanding of how memory works.”

Of course, there are plenty of theories. Along with decidedly unscientific explanations such as the memories of past lives surfacing or clairvoyance, some have wondered if the deja vu “glitch” to blame is in our sensory processing or between our short- and long-term memories.

It was Tulving who first theorised that different systems are at work in making memories compared to retrieving them. So just because someone cannot recall something, doesn’t mean the memory doesn’t exist somewhere in their brain. Perhaps then our deja vu has happened before, or something very near it, we just can’t call up the memory?

But how would that explain Moulin’s strange moment while visiting New York for the first time? Too much television?

Perhaps neurons firing out of sync causes the glitch. In 1963, neurological researcher Robert Efron concluded that the left hemisphere of the temporal lobe sorts incoming information, but it receives it twice: once directly and once again after it takes a quick detour, milliseconds long, through the right hemisphere of the brain. If that delay is stretched out a little longer than usual, and the two signals are thrown out of sync, then the same information might get two timestamps. If the first has already been processed, the second could seem like a reliving of it.

Others theorise something similar could happen if our mind momentarily wanders as we take in a scene. When our attention shifts back to finish processing what’s before us, it feels like it’s already been seen.

Does everything just look too similar?

At the University of Melbourne, associate professor of psychology Piers Howe stumbled upon another possible explanation. He recalls a student rushing up to tell him about her strange “sixth sense” – when she’d recently run into a friend, she knew at once he’d been in an accident, even though he was completely recovered and there were no signs of it, or at least, none she realised.

Howe and his student put her feeling to the test in the lab – showing research participants a series of photos of the same people in which something tiny had changed, such as an earring going missing or a hat switching colour. People reported they felt something had changed but often couldn’t explain what. Howe says that our brains routinely take in more information than our conscious mind can keep track of, and it could help explain at least some deja vu. (Moulin agrees that the brain’s ability to suck in information outpaces our awareness.)

Our memories are not perfect, says Howe. “You summarise information just like a computer compressing a file into a JPEG. So you’ll have enough to tell you there’s been a change or to realise you’ve been in a situation before but not enough to tell you what or when. That’s what gives rise to the weird feeling.”

In his experiments, the feeling usually vanished once the change was pointed out. “And it was a real eye-opener for those who thought something strange was going on, to come in and have that feeling recreated multiple times in the lab in the space of half an hour.”

Sometimes our memory “compressions” can create false memories too, says Howe. In experiments, when people are asked to remember a list of words related to a word such as window (pane or glass, for example), they will often become convinced the word window itself was on the list too.

Moulin’s former student and colleague Dr Akira O’Connor has more specifically recreated a deja vu of sorts in a Sydney lab by giving people under hypnosis a puzzle game to play, which then felt familiar to them when they woke, though they couldn’t remember why. Professor of cognitive psychology Anne Cleary has used The Sims video game, which simulates suburban life, to trigger deja vu by showing people scenes with the same layout – say, a junkyard with rubbish piled in just the same pattern as the bushes of a previous garden.

“Everyone has that same IKEA pot now. But it doesn’t give you deja vu. You just go, ‘There’s the IKEA pot. There’s another Starbucks.’”

These studies suggest that something in the environment, not just a glitch in the brain, really can trigger deja vu. Moulin laughs that often “the more mundane the scene” the more it looks familiar. Nursing stations in hospitals can look startlingly alike. “And universities, every glooming university you go to, on the inside, they’re all the same. A seminar room is a seminar room.”

But, while Moulin says, “the most primitive forms of memory seem to be about location and space,” this cannot explain all forms of deja vu. Some can spark from conversation. Others quite literally from an electric surge in the brain during epilepsy. Dreams too could possibly trigger a bout, as more Freudian theorists have supposed, when you come across something you’ve already encountered subconsciously. “People who report more deja vu also say they are better at remembering their dreams,” says Moulin, “though perhaps it’s just the easiest comparison.” After all, nothing comes as close to the weird feeling of deja vu as the shadowy world of dreams.

Moulin suspects all these forms of deja vu are likely the same thing, just triggered in different ways.

We encounter the familiar all the time, Moulin says – a part of the brain known as the rhinal cortex is constantly on the lookout for it to help call up the matching memories when we do. “And globalisation is only making things more familiar. I mean everyone has that same IKEA pot now. But it doesn’t give you deja vu. You just go, ‘There’s the IKEA pot. There’s another Starbucks.’”

But, according to Moulin’s preferred theory, deja vu might hit when this rhinal cortex is triggered without the memories to back it up, giving us a false but powerful sense of the familiar. “And you can either explain it as a little glitch in the brain, or you can say it’s something that exists in the environment; like, it’s this similarity between this flat and my aunt’s flat or this scene in Paris and something I’ve seen on the telly. But it fires that circuit.”

Is deja vu a bad thing? And what’s jamais vu?

There’s another twist to the deja vu mystery. Though our memories decline as we age, older people seem to have less deja vu, not more. It actually peaks in our 20s and 30s and then declines, Moulin says. “So that seems funny if we’re saying deja vu is a memory error.”

Adding to the puzzle: while deja vu was once thought linked to psychosis and other mental disorders, studies now suggest the opposite. Research shows schizophrenic patients, for example, have less deja vu while unwell, and report a return of the phenomenon when their psychosis goes with treatment. Says Moulin, “Those psychotic conditions which were once thought full of little bugs and illusions and delusions like this, certainly, for deja vu, it’s not the case.” (And as for AKP, and those like him with persistent deja vecu, anti-psychotics and dementia medications did nothing.)

Moulin now thinks regular deja vu is itself a sign of a healthy brain – that eerie feeling that the memory cannot be real is a sign we are fact-checking our own recollections, picking up the false note beneath the familiar. “As I get older, I’m definitely having fewer deja vu moments and I miss it,” he says. “But it’s probably just that I don’t have the same mental energy to fact-check, I’m more confused about whether I actually did have that conversation or I already made that joke to my class.”

Or perhaps, he muses, it’s the fact that we’re more likely to get up to many of the things that bring on deja vu – such as staying up all night, drinking and taking drugs – when we’re younger. “Most of us anyway,” he laughs.

And what about that “doom deja vu” Moulin had on the plane from Canada, when he became sure, with a bone-deep dread, that he had been on board that flight before and it was about to crash? (“I don’t like flying at the best of times,” he sighs.) In epilepsy patients, there are accounts of electric discharges spreading beyond the temporal lobe and into the amygdala responsible for emotion (and our flight or fight response). “So some people have euphoria associated with deja vu, other people doom,” says Moulin. “And that would just be my luck that I’m a doom person.”

“I looked at him and thought, I know you’re my dad but it’s not how I think you look.”

Cleary, meanwhile, has found that while deja vu did make her research participants more likely to feel they could also predict what would happen next, those “premonitions” were no more accurate than random guesswork.

Moulin is now researching the opposite of deja vu, “jamais vu” (never seen), where what should be familiar suddenly feels new and strange. “I’ve had that lots,” he says, recalling his local sandwich shop suddenly seeming alien, and even once his own father’s face. “I looked at him and thought, I know you’re my dad but it’s not how I think you look. Some people even have it with themselves in the mirror.”

A shadow of this eerie effect is relatively easy to provoke in the lab. If people write the same words over and over, they are likely to report them suddenly feeling strange – losing their meaning or appearing to be spelled incorrectly.

Can something become too familiar then, provoking a glitch with more exposure? Perhaps, says Moulin. “I’ve had it playing musical instruments, [forgetting] a song I know really well. And a colleague had it driving a car. All of a sudden it was, ‘What the hell does this pedal do?’.” In the lab, Moulin found that the faster someone wrote a word, the more likely it was to become strange to them. “So maybe things get overcharged and the brain snaps back for a moment. You don’t want things to become too automatic.”

As for deja vu itself, though some research has suggested a link between brain structure and frequency, experts say more work is needed to understand what that could mean.

“Of course, often people on the street are more interested in deja vu than researchers,” says Moulin. “This research isn’t going to change the world. But it can help us understand consciousness and memory and all kinds of things. I mean, don’t you want to know what it is?”

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