Imagine suffering from chronic déjà vu. You don't even go to the doctor because you feel like you've already been there.
"We had a peculiar referral from a man who said there was no point visiting the clinic because he'd already been there, although this would have been impossible," said psychologist Chris Moulin, who runs a memory clinic at the University of Leeds in the UK.
So Moulin has started the first known study of the condition.
Déjà vu hits most of us now and then. We're struck by the sensation that we have experienced an event before, even though we can't fully remember it or perhaps know it didn't really happen. The sensation is fleeting, so researchers can't study it.
But Moulin figures chronic déjà vu sufferers offer an opportunity to do research that might unlock the secrets of the everyday variety.
The man who thinks he's been to Moulin's clinic even gave details of the visit that never occurred. He has déjà vu so bad that he doesn't watch TV news because he feels like he's seen it all before, Moulin said. Things get tricky when the man is asked to predict what's ahead, however.
"When this particular patient's wife asked what was going to happen next on a TV program he'd claimed to have already seen, he said, 'How should I know? I have a memory problem!'"
Moulin and colleagues have since found other patients, now that they know what to look for.
The condition can cause depression and is sometimes diagnosed as a state of delusion. But Moulin's team believes it to be a dysfunction of memory.
"The exciting thing about these people is that they can 'recall' specific details about an event or meeting that never actually occurred," Moulin said. "It suggests that the sensations associated with remembering are separate to the contents of memory, that there are two different systems in the brain at work."
The problem might involve a memory circuit that is overactive or stuck in the "on" position.
The researchers plan now to use brain scans in an effort to pinpoint the problem.
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Robert is an independent health and science journalist and writer based in Phoenix, Arizona. He is a former editor-in-chief of Live Science with over 20 years of experience as a reporter and editor. He has worked on websites such as Space.com and Tom's Guide, and is a contributor on Medium, covering how we age and how to optimize the mind and body through time. He has a journalism degree from Humboldt State University in California.