Deja vu: What really happens to your body when you experience this feeling

Déjà vu is a French term that literally means "already seen" and is used to describe the feeling that something being witnessed has already happened. The term was first coined in 1876 by Émile Boirac, a philosophy professor who described his own experience with it in a letter published in the Revue Philosophique. But "déjà vu" wouldn't be an accepted scientific term until two decades later, when French neurologist F.L. Arnaud officially proposed its use at a meeting of the Societe medico-psychologique.

The concept of déjà vu has fascinated psychiatrists and laypeople alike for decades, with examples ranging from Sigmund Freud's A Disturbance of Memory on the Acropolis to Charles Dickens' David Copperfield. In a 1991 study titled "The déjà vu experience: Remembrance of things past?" it's noted that somewhere between 30 to 96 percent of people have experienced déjà vu at some point in their lives — a wide variation that probably has something to do with differing definitions for the experience.

So what exactly is déjà vu? The scientific community is split on its causes, and there are studies that back a number of different explanations. This is what's really going on when you experience déjà vu.

The deja vu experience

In 1983, Dr. Vernon M. Neppe published The Psychology of Déjà Vu, the first of many books what he refers to as the "déjà experience." When it was first released, Neppe listed 20 different ways in which the experience can manifest, including déjà entendu ("already heard"), déjà pensé ("already thought"), and déjà raconté ("already recounted"). Neppe's first list accounted for what he defined as "any subjectively inappropriate impression of familiarity of the present experience with an undefined past" — basically, any instance where something feels like it's happened before.

But Neppe went further with his description of the déjà experience, listing an additional number of terms in his later work that hadn't yet been described. Between Déjà Vu: A Second Look and Déjà Vu Revisited, Neppe added nine more terms to the list that were more abstract by definition. Déjà paradoxe, for example, is meant to describe how "déjà differentness feels familiar," while déjà halluciné is a descriptor for the feeling of having hallucinated something before. By the time Neppe had finished categorizing déjà terms in 2009, there were 34 terms to describe feelings of having previously experienced something — 31 of them are still used.

Distracted people experience deja vu more

The experience of déjà vu can often feel like some sort of supernatural occurrence. But as incredible as it may be to think that you've suddenly tapped into a precognitive ability, it's more likely that your brain is just firing a little slower than usual. Alan Brown, a professor in the psychology department at Southern Methodist University shared that you may experience déjà vu when you're only half paying attention to our surroundings.

"Because we often navigate the world on autopilot, we take in much of our surroundings on an unconscious level," he wrote for Scientific American in 2014, noting, "When we emerge into full awareness, we might do a perceptual double take. We are struck by a strange sense of familiarity because we saw the scene just moments before, unconsciously." So, for example, when you're texting and walking (if you're well-coordinated enough to be able to multi-task, that is), your brain is picking up on the people and places you pass, even if you're not consciously making note of them. That kind of "autopilot" behavior makes it seem as though we've experienced déjà vu as soon as we snap back into regular thinking.

You may experience deja vu when you're anxious

In 2014, a 23-year-old man became the subject of a déjà vu study conducted by Dr. Christine Wells, a psychology lecturer at Sheffield Hallam University. According to the study, the man had started suffering from "persistent déjà vu" three years earlier, which became so severe that he described it as being "trapped in a time loop." The man's experience was so bad that he had stopped watching TV and reading magazines because he was convinced he already knew the content.

During the study, it was discovered that the man had a history of anxiety, but was otherwise healthy and normal. What Wells learned is that the man's anxiety and his déjà vu seemed to be intertwined. "In relation to our case, distress caused by the déjà vu experience may itself lead to increased levels of déjà vu: similar feedback loops in positive symptoms are reported in other anxiety states (e.g. panic attacks)," the study noted. While the man's anxiety may have triggered his déjà vu, it also worked the other way around. The déjà vu was so stressful for the man that the experience of it led to higher anxiety.

Deja vu may be a memory you've half-forgotten

Have you ever traveled somewhere new and experienced the feeling of having been there before? That particular kind of déjà experience is called déjà visité, and it's one of Dr. Vernon M. Neppe's more common types. According to Alan Brown, a psychology professor at Southern Methodist University, it might be that you've just forgotten about a prior trip. He explained in Scientific American that sometimes when people are "overwhelmed by an uncanny sense" of having visited a place already, it's because they actually have, but it was as a very young child.

Similarly, Brown wrote that the media can have an effect on our sense of memory as well. "Television and photographs can breed a false sense of familiarity later on," he said. While seeing a place on TV is completely different than actually going there, it can still affect our perception of that place if we ever do visit, making it possible to experience déjà vu somewhere totally new. "Our brain is always searching for connections," Brown explained. "As a result, we can sometimes make links that simply aren't there."

Deja vu could be your brain making sure your memories are real

Narrowing down the exact cause of déjà vu has been tricky because it's an experience that happens suddenly, so there's no real way to study it in a lab setting. In 2016, however, Akira Robert O'Connor, a senior lecturer of psychology and neuroscience at the University of St Andrews, figured out a way to trigger déjà vu through the use of false memories.

As reported in New Scientist, O'Connor's study involved telling a subject a series of related words, but not the word linking them together — so dream, pillow, and bed triggered déjà vu for someone when they thought of the word sleep. "This meant that when they were later asked if they had heard the word sleep, they were able to remember that they couldn't have, but at the same time, the word felt familiar," New Scientist noted.

O'Connor discovered that the part of the brain  used during the process had to do with decision making — not memory. According to the study, "the frontal regions of the brain are probably checking through our memories, and sending signals if there's some kind of memory error…"

Is this happening in your brain when you have deja vu?

Some of the oldest déjà vu theories have to do with dual processing. Basically, it's when two things that are supposed to work together in your brain temporarily stop functioning properly. According to Medical News Today, although dual processing theories have been around for a long time, there isn't much in the way of empirical evidence to support them.

The idea of familiarity and recall is one such dual processing theory, which works under the assumption that the parts of the brain in control of familiarity and recollection are sometimes thrown out of whack and a false sense of familiarity can be triggered instead of actual recall. 

Then there's dual consciousness. According to Medical News Today, the idea of dual consciousness has been around since the late 19th century, and it suggests that our brain works simultaneously to process both the outside world and our own internal monologue. According to the theory of dual consciousness, when we get tired, we tend to allow our internal consciousness to take over, which is when new experiences start to feel like memories.

Deja vu could be neurological

The most likely cause of déjà vu is neurological, and there's better evidence to support neurological theories than dual processing ones. According to Psychology Today, déjà vu may be triggered when different circuits in the brain malfunction or fire at the wrong time. Most of the time, it may have to do with short-term vs. long-term memory — information we take in may reroute directly to long-term memory, giving us the sense that what we're experiencing happened long before a particular moment.

Likewise, it could have something to do with our brain's dominant hemisphere working faster than our non-dominant hemisphere. In The Déjà Vu Illusion, Alan Brown, a psychology professor at Southern Methodist University, wrote that the déjà experience can happen when the brain accidentally processes the same piece of information twice. He noted that "because the brain routinely integrates information received from separate pathways into a unitary experience, a slight delay (or acceleration) in the speed of one pathway relative to another could cause the brain to interpret the data from the two as independent and separate copies of the same experience, even though the two impressions are only milliseconds off."

People who travel more experience deja vu more than others

Those who see the world are opening up their minds to different perceptions of that world, it seems. Turns out, if you've been bitten by the travel bug, you're more likely to experience déjà vu than those who are more comfortable sticking around at home. According to a déjà vu study published in 2013, 32 percent of people who travel up to five times a year have experienced some version of déjà vu, whereas only 11 percent of those who don't travel have experienced it. 

But it isn't just travel that affects one's chances of a déjà experience. Southern Methodist University psychology professor Alan Brown listed a number of other contributing factors in The Déjà Vu Illusion. He wrote, "Déjà vu incidence decreases with age, increases with education and income, and is more common in persons who travel, remember their dreams, and have liberal beliefs (political and religious) compared with those who do not travel, do not remember their dreams, and have conservative beliefs." While the reasons behind why these groups of people experience a higher rate of déjà vu aren't known, it is an interesting correlation regardless.

Certain drugs may contribute to deja vu

If you've ever watched a prescription medication commercial, then you know that most of them come with a laundry list of side effects that get rattled off in the final three seconds of the ad. While stomach issues, headache, and dizziness are all fairly common things to hear, déjà vu generally doesn't make the list. Turns out, though, that the strange experience can actually happen because of certain drugs.

In 2001, an article was published in the Journal of Clinical Neuroscience that described an otherwise healthy 39-year-old man who began experiencing intense déjà vu episodes when he started taking a flu medication that increased his dopamine levels. A second study in 2007 found a correlation between recurring déjà experiences and serotonin-producing medication like 5-HTP, as reported by Vice. "Since the doctor did not think it was the pills, I decided to try [them] again," the subject said. "I had the same feeling of having seen and done all of this before. I did not have any eerie type of feeling, I knew I couldn't know these things but I felt like I did."

When deja vu is cause for concern

Experiencing déjà vu somewhat frequently could mean it's time to see a doctor, especially if the feeling is followed by a loss of consciousness. Dr. Patrick Chauvel, a neurologist at Cleveland Clinic's Epilepsy Center, notes that déjà vu is common in epileptic patients but has also been observed in dementia patients.

Pat Long detailed his experience with déjà vu after having a tumor removed from the right side of his brain, as reported by the Independent. Long began suffering from epileptic seizures after the surgery, which began with an "aura." Epileptic auras are described as "a sort of minor foreshock" that signal when a seizure is about to begin. They vary from person to person, and, for Long, they presented as déjà vu. "During my most intense seizures, and for a week or so afterwards, this feeling of precognition becomes so pervasive that I routinely struggle to discern the difference between lived events and dreams, between memories, hallucinations and the products of my imagination," he said.

If you experience frequent déjà vu, be sure to tell your doctor.