You're walking down a street in some foreign town, taking in the sights and smells, when suddenly it washes over you: a bizarre, but familiar sensation that somehow you've done this all before. In some cases, the experience is so strong that you can almost feel what's about to happen next.
It's estimated that more than 70 percent of us experience some form of déjà vu — a French word meaning "already seen." It's a puzzling phenomenon that's been blamed on everything from parallel universes temporarily coming together to your "future self" attempting to guide you in life. Or, as Keanu Reeve's character learned in "The Matrix," it's all just a computer glitch.
Science, however, has a different take on these familiar sensations, with research all around the world narrowing down the possibilities. CNN published a comprehensive overview, with journalist Sandee LaMotte interviewing experts and highlighting studies attempting to recreate these familiar experiences. Most revealing was that those aged 15-25 tend to have the most occurrences of déjà vu. Since our brains aren't fully formed until the quarter-century mark, those growing pains may have something to do with it.
Another interesting aspect: Déjà vu is more closely associated with those who watch a lot of movies and tend to travel. What may feel, look or sound familiar, despite physically being our first encounter, may align closely with a deeply buried memory from a previous experience.
"Memory is far from perfect. We simply fail to recall everything that we encounter in day-to-day life," déjà vu researcher Anne Cleary explained to LaMotte. "However, just because something fails to be recalled doesn't mean that the memory isn't still 'in there' somewhere; often it is, and it is just failing to be accessed. These types of memories might be what drive the sense of familiarity that presumably underlies déjà vu."
As the article goes on to explain, research has also associated the phenomenon with temporal-lobe epilepsy. Patients who suffer from this condition often experience déjà vu symptoms right before experiencing a seizure. These abnormal electrical discharges may occur in the rest of us, especially when we're young, but on a much smaller scale.
But what's causing it?
In a 2016 study, researchers in the U.K. figured out how to trigger déjà vu by implanting false memories in the minds of the study participants. When they looked at their functional MRI scans, they found it wasn't the parts of their brains related to memory, but instead the part related to decision-making that lit up while they were experiencing déjà vu. This suggests we're sifting through our memories, seeing if there's some kind of error, New Scientist reports.
Another possible culprit? Stress and anxiety. Scientists published the terrifying case of a 23-year-old man who has suffered from "chronic déjà vu" for the past eight years. The sensation was so crippling that nearly everything the man did came with an overpowering feeling of familiarity.
"There was one instance where he went to get a haircut. As he walked in, he got a feeling of déjà vu. Then he had déjà vu of the déjà vu," Dr Chris Moulin, a cognitive neuropsychologist at the University of Bourgogne, told the BBC. "He couldn't think of anything else."
Brain scans turned up normal, leading the researchers to believe the problem was psychological. Because the young man had a history of depression and anxiety (something which tended to exacerbate the symptoms), the possibility exists that there may be some association.
"Although this report does not prove a link between anxiety and déjà vu, it does further support the suggestion that this area is worthy of further investigation," the researchers concluded.
This story was originally published in January 2016 and has been updated with new information.