Scientists believe the extraordinary case of a 23-year-old British man with "constant deja vu" may have been triggered by anxiety. It is the first time such a link has been made. But what is deja vu - and do we really know what causes it?
Most of us know the feeling - the fleeting sensation that you have been somewhere or done something before, when common sense tells you that is not possible.
The term deja vu translates literally from French as "already seen".
According to research, about two thirds of us experience at least one deja vu in our lifetime, yet very little is known about what causes it.
The group of scientists from the UK, France and Canada who studied the strange case of the man with "chronic deja vu" think one possible cause of the phenomenon could be anxiety.
The man's condition was so persistent he avoided watching television, listening to the radio and reading newspapers because he felt he had "encountered it all before".
Dr Chris Moulin, a cognitive neuropsychologist at the University of Bourgogne who worked on the study, says the man had a history of depression and anxiety, and had once taken the drug LSD whilst at university, but was otherwise completely healthy.
"This man was striking because he was young, otherwise aware, but completely traumatised by this constant sensation that his mind was playing tricks," he says.
Frightening time loop
For minutes, and sometimes even longer, the patient would feel that he was reliving experiences.
He likened the "frightening" episodes to being in the psychological thriller film Donnie Darko.
"There was one instance where he went to get a haircut. As he walked in, he got a feeling of deja vu. Then he had deja vu of the deja vu. He couldn't think of anything else," says Dr Moulin.
For eight years, the man felt "trapped in a time loop". The more distressed he became by the experience, the worse it seemed to get.
Haven't I read this before?
The term deja vu was coined in 1876 by the French philosopher Emile Boirac. It is the overwhelming sense that you have already experienced something before. But there are other, lesser known, phenomena which are thought to be related.
Jamais vu - translated as "never seen", this is the sense that something which should be familiar is alien, for example a common word which suddenly seems strange.
Presque vu - translated as "almost seen", this is the sense of being on the edge of an epiphany or realisation, for example recalling a memory.
Déjà entendu - translated "already heard", this is the sense of feeling sure you have heard something before, like a snippet of conversation or a musical phrase.
Brain scans appeared normal, suggesting the cause was psychological rather than neurological.
Whilst this case on its own does not prove a link between anxiety and deja vu, it raises an interesting question for further study, Dr Moulin says.
Unlike many other memory problems, deja vu seems to occur more in young people.
People first experience deja vu at the age of about six or seven, and it happens most often between the ages of 15 and 25, before tailing off as people get older, according to research by Professor Alan Brown at South Methodist University in Dallas.
There are several other theories about what causes the sensation.
Dr Akira O'Connor, a psychologist from the University of St Andrews, believes that in most cases it is a momentary "misfiring" of neurons in the brain which creates false connections.
"One idea is that deja vu is a sort of 'brain twitch'. Just as we get muscle spasms, or eye twitches, it could be that the bit of your brain which sends signals to do with familiarity and memory is firing out of turn," he says.
He says this fits with evidence that deja vu is more frequently experienced by people with epilepsy and dementia.
Another theory, developed by Professor Anne Cleary at Colorado State University, is that deja vu is the natural result of seeing something genuinely familiar in our surroundings - such as the shape of a structure, or the layout of a room - sparking a false memory.
She developed a computerised virtual reality called "Deja-ville" where people navigate around similar landscapes to test the hypothesis.
But Dr O'Connor says none of the current theories definitively solves the mystery of deja vu - partly because its fleeting and spontaneous nature makes it almost impossible to reliably study in lab conditions.
"Methods of trying to induce deja vu are pretty crude," he says.
"We've used hypnosis and experiments using lists of words. Another method is called 'caloric stimulation'. It's just squirting warm water into people's ears.
"It's meant to deal with various problems like vertigo, but one of the common side effects is deja vu. People have suggested that's because the ear canal is near the temporal lobe, which may control it," he says.
It is not known how many people suffer from the "chronic" version of deja vu, but Dr Moulin has encountered cases before - with some patients even insisting they had already met him because of their deja vu.
"People greet you like an old friend, even though they've never seen you before. Some of them were on Skype on the other side of the world, but they still had that sense," he says.
Since reports of the 23-year-old man appeared in newspapers, fellow report author Dr Christine Wells, from Sheffield Hallam University, says more people are coming forward.
"I've had people from Australia and America emailing me. It appears to be something quite rare, but there are people who say they're experiencing it, or they went through a period of it, or know someone who has," she says.
She says it is an area which needs "a lot more study".
But for some, it is a phenomenon that should remain unexplained.
"I've had people say to me you don't really believe in deja vu, do you? Like it's something paranormal," says Dr O'Connor.
"I've had letters from some people who believe strongly that it is something spiritual, quoting the Bible and the Qu'ran.
"Some people say I shouldn't investigate it; that 'explaining rainbows ruins their beauty'. Personally I've always loved getting deja vu - and finding out what causes it just makes the experience more beautiful."