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 Reeves' character learned in "The Matrix," it's all just a computer glitch.
Science, however, has a different take on this familiar feeling, and research around the world is narrowing down the possibilities. CNN published an overview in 2016, with journalist Sandee LaMotte interviewing experts and highlighting studies on déjà vu. One interesting detail is that people aged 15 to 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, a professor of cognitive psychology at Colorado State University, 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."
Research has also associated the phenomenon with temporal-lobe epilepsy. Patients with this condition often experience déjà vu symptoms right before experiencing a seizure. These abnormal electrical discharges may also occur in everyone else, especially people in their teens and twenties, but on a smaller scale.
Can déjà vu predict the future?
Déjà vu is most likely a memory phenomenon, according to Cleary, but she has received feedback from people who disagree. Their déjà vu is not merely a memory, they argue, because it also sparks a feeling that they know what will happen next.
Cleary found a study from the 1950s that also mentioned this sense of premonition from déjà vu, so she decided to investigate further. In earlier research, she and her colleagues had already triggered déjà vu in people using virtual-reality scenes they built in The Sims, a life-simulation video game. Study participants were shown a scene, then later saw a thematically unrelated scene that was spatially mapped to match the first one. The spatial similarities made them more likely to report déjà vu.
For her new study, published in March 2018, Cleary expanded this approach to test déjà vu's predictive abilities. Participants first watched video scenes in which they moved through a series of turns, then watched thematically different scenes that were spatially mapped to the previous ones. Just before the second video reached its final turn, participants were asked if they were experiencing déjà vu, and whether they felt like they knew what the last turn should be.
About half of the respondents did report a strong premonition associated with déjà vu. But they were no more likely to produce the correct answer — the turn they had seen earlier in the spatially similar scene — than if they had just chosen randomly. This suggests déjà vu can't really help us predict the future, Cleary says in a statement about the study, although it can create a convincing sense of premonition. She is now conducting follow-up experiments in hopes of revealing what causes this.
"I think the reason people come up with psychic theories about déjà vu is that they are these mysterious, subjective experiences," Cleary says. "Even scientists who don't believe in past lives have whispered to me, 'Do you have an explanation for why I have this?' People look for explanations in different places. If you're a scientist, you're looking for the logical reason for why you just had this really weird experience."
Why do we experience déjà vu?
In a 2016 study, researchers in the U.K. triggered déjà vu by encouraging false memories in the minds of the study participants. When they looked at the 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. In 2014, 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.