Déjà vu, pronounced ‘day-zhaa voo’, is French for ‘already seen.’ It describes the eerie sensation of familiarity in a seemingly new setting and environment. For instance, you may have found yourself in the middle of a conversation thinking to yourself that this exact exchange has taken place before. Or you may have walked into a totally new environment, and said to yourself “I’ve been here before!”. It’s all strangely familiar, but you can’t quite wrap your mind around the whole thing. In a sense, it’s a bit like seeing into the future.

Déjà vu is commonly featured in pop culture. In the movie The Matrix, a déjà vu is usually a glitch in the Matrix —  a computer-generated dream world, built to keep us under control while a cybernetic organism farms our bodies. But despite most people (60-80%) reporting that they have experienced the unsettling feeling at least once in their lifetimes, déjà vu has been startling understudied. To researchers’ credit, the fleeting, uncontrollable nature of déjà vu has made it difficult to study.

Scientists have proposed various hypotheses for déjà vu formation in the brain. According to one theory, the phenomenon might arise due to some sort of “mismatch” in how we’re simultaneously sensing and perceiving the world around us. Another theory suggests that the brain may occasionally take a short-cut to long-term memory storage, bypassing short-term working memory, evoking the sensation that of experiencing something in the distant past. The mainstream line of thinking is that déjà vu is a product of false memories — glitches in our neural matrix.

“[Déjà vu] is certainly related to false memory in the sense that it is a memory dissociation kind of effect. It dissociates reality from your memory,” said Valerie F. Reyna, a psychologist and Professor of Human Development at Cornell University and an expert on false memory and risky decision making.

“There’s all kinds of different dissociative experiences that can happen. Sometimes you cannot be sure, for example, if you dreamed something or experienced it, if you saw it in a movie or it happened in real life.”

But in 2016, Akira O’Connor, a psychologist at the University of St. Andrews in Britain, found that the phenomenon might be the result of front regions of the brain ‘fact checking’ our memories and sending signals when it encounters a sort of error. This error generates a conflict between what we’ve actually experienced and what we think we’ve experienced. By this explanation, déjà vu represents a mechanism that ensures that we don’t form false memories, or at least keeps them to a minimum, and is not the product of false memories themselves. This may also explain why the phenomenon is far more common in younger people, since memory deteriorates with age.

“Déjà vu is characterized by having a false sensation of familiarity, alongside the awareness that that sense of familiarity cannot possibly be correct,” O’Connor said. “It’s a memory clash where there’s false familiarity, together with an objective awareness that it can’t be real.”

O’Connor and colleagues enlisted 21 volunteers, who each were exposed to a list of words related to sleep — such as bed, pillow, night, or dream — but not the keyword itself that links all of them together. The participants were then asked whether they had heard the word ‘sleep’. Those who responded affirmatively were then asked whether they had heard any word from the previous list start with the letter “s”, to which they responded negatively. The resulting confusion triggered déjà vu in two-thirds of participants.

The next step was to scan the brains of the participants with a functional magnetic resonance imaging (fMRI) machine to see which brain regions lit up when the experience took place.

“What we found was that it wasn’t memory-linked regions that are driving déjà vu,” he said. “Traditionally, researchers thought déjà vu was being driven by false memories. What it actually is is that the cognitive control, error-monitoring conflict-checking frontal brain regions are the ones which show greater activity in people reporting the experience.”

What does this mean for people who have never had déjà vu? Well, it may mean that these people don’t reflect on their memory system. On the other hand, people who don’t have déjà vu might simply have a better memory than the general population, so there’s no risk of triggering memory errors.

As to why people have déjà vu in the first place, the jury isn’t out yet. It may be an evolutionary product that makes people more cautious when memory is playing tricks on us, but there’s no evidence backing this idea up yet.