Déjà vu: is it real, and why does it happen?

You walk into a café you have never visited. You sit down, look around, and suddenly a strange little shiver runs through you. I have been here before. Except you know you haven’t. A few seconds later, the feeling melts away.

That is déjà vu, French for “already seen.” So is it real, or just a trick? The short answer: the feeling is very real and very common. Scientists are beginning to understand it as a memory phenomenon, a small hiccup in how we judge whether something is familiar. Let’s look at what they have found.

A very common feeling

In 2003, Alan Brown reviewed more than a century of déjà vu research, from surveys to case studies. About 60% of people have had it. It shows up less often as people get older. It seems to go along with stress and tiredness, and it is reported more by people with more education and a higher socioeconomic level.

A 2012 review by Nathan Illman and colleagues quotes a typical definition, written by Neppe in 1983. It calls déjà vu “any subjectively inappropriate impression of familiarity of a present experience with an undefined past.” In plainer words: something feels familiar when it shouldn’t, and you can’t say where that familiarity comes from.

The key part is the clash. As Akira O’Connor and Chris Moulin explain, in a typical déjà vu moment you feel recognition and at the same time know that this recognition is wrong. Usually the “wrong” side wins. You don’t act on the feeling. You just notice how odd it is.

Why it is so hard to study

Déjà vu is shy. It is fleeting and arrives without warning, so you cannot easily make it happen on demand. Because it leaves no visible trace in behavior, researchers have to rely heavily on what people tell them about it.

That has given scientists plenty of room to wonder. Brown sorted the explanations into four families:

• Dual processing: two mental processes briefly fall out of step.
• Neurological: a seizure, or a brief disruption in how nerve cells pass signals.
• Memory: something in the scene is truly familiar, but you don’t recognize where from.
• Attention: you take in a scene without paying attention, then see it again with full attention.

Familiar, but from where?

Sometimes we recognize something because we consciously recall where we met it. Other times we only get a feeling of familiarity, with no source attached. In a 2008 article on recognition memory, Anne Cleary suggests that déjà vu may sometimes come from that second kind: familiarity that turns up without any identification of where it came from.

So what might be familiar? Cleary and her colleagues tested one idea, called the Gestalt familiarity hypothesis. It says déjà vu can happen when the layout of a new scene matches the layout of a scene you saw before, but the old scene does not come to mind.

To test this, the team built immersive scenes in virtual reality. Some new scenes resembled an earlier scene in the way their elements were arranged. When people failed to recall the earlier scene, they rated these look-alike scenes as more familiar than completely new ones, and they reported déjà vu more often. They also felt that odd mix of “this is new” and “this is familiar” at once. The closer the match to a scene stored in memory, the stronger the familiarity and the more déjà vu.

Here is how that might look in daily life. A friend’s new living room has its sofa, lamp and window in the same spots as a room you visited years ago and forgot. Your memory whispers “familiar,” but it can’t hand you the old room. That is the kind of moment this idea describes.

Making déjà vu in the lab

In 2014, Josephine Urquhart and Akira O’Connor at the University of St Andrews tried a clever recipe. They started with a well-known false memory task. People study a list of words such as rest, bed and blanket, which are all linked to a word that never appears: sleep. Later, sleep tends to feel familiar anyway. (We met this trick before in our story on the Mandela effect.)

Then they added a twist. While studying each list, people counted the words that began with a string of letters shown on the screen. In some lists the string was the start of the missing word, such as “SLE,” so the answer was always zero. So when sleep showed up on the test and felt familiar, people could also be sure it was new. Familiarity and newness clashed, just like in real déjà vu.

It worked, at least in part. Of the 30 people tested, 18 reported déjà vu at least once. For those “familiar but certainly new” words, people reported déjà vu about 25% of the time. When the counting task gave no such clue, déjà vu stayed at a baseline of about 10%. The authors were candid about the limits, too: the lab version captured only some of the real experience, and people’s reports may have been swayed by what they thought the study expected.

What the brain does during the clash

The team then took a version of this task into an fMRI scanner. Of 21 people, 16 reported déjà vu. The brain regions linked with memory conflict included the anterior cingulate cortex, the medial prefrontal cortex and the parietal cortex. The researchers suggest that resolving a conflict in memory may play an integral role in the déjà vu experience.

Clues from the temporal lobe

Doctors have known about déjà vu-like feelings in epilepsy for a long time. In the 19th century, J. Hughlings-Jackson described a “dreamy state” that can come with some seizures. Today, déjà vu is well known as something that can happen during seizures in temporal lobe epilepsy. Edward Wild’s 2005 review also notes that the research shows déjà vu to be a common experience consistent with normality.

Epilepsy research has given scientists a rare chance to look inside. In 2004, Bartolomei and colleagues studied 24 people with epilepsy who already had electrodes placed in their brains as part of tests before surgery. Across 280 electrical stimulations, déjà vu came up more often when the team stimulated the entorhinal cortex than when they stimulated the amygdala or the hippocampus. The entorhinal and perirhinal cortex are areas that lie beneath the hippocampus, inside the medial temporal lobe.

This fits an earlier idea from Josef Spatt. He proposed that déjà vu comes from faulty, isolated activity in a recognition memory system made of the parahippocampal gyrus and its connections, which is responsible for judgments of familiarity. Meanwhile, other structures involved in memory retrieval, the prefrontal cortex and the hippocampus itself, work normally. He argued that this combination gives déjà vu its puzzling quality.

Nathan Illman and colleagues offered a related “decoupled familiarity” idea: a feeling of familiarity that floats free of what you are actually seeing. They also note that people with temporal lobe epilepsy can tell their seizure-related déjà vu apart from the everyday kind.

What about brains of healthy people? A 2012 study led by Milan Brázdil compared healthy people who did and did not report déjà vu. Those who reported it had less gray matter in a set of regions, mostly in the inner part of the temporal lobe. The less gray matter people had in these regions, the more often they said they had déjà vu. The team suggested this may reflect differences in how the hippocampus works. Still, this was a snapshot of brain structure, so it shows a link, not a cause.

Does déjà vu predict the future?

Déjà vu has long been linked with a spooky feeling of premonition, as if you know what will happen next. In 2018, Anne Cleary and Alexander Claxton put this to the test. People went through navigational paths, and some paths resembled one they had experienced before but did not recall.

The resemblance did make déjà vu more likely. But during déjà vu, people were no better than chance at predicting the next turn. What they did have was a stronger feeling of knowing which way to go. So the sense of premonition is real as a feeling, but it can be an illusion. The authors suggest that the déjà vu state itself may tilt our sense of confidence.

So, is déjà vu real?

Yes, in the sense that matters most. It is a real, common experience that scientists can measure, model and sometimes even nudge into happening in the lab. It does not appear to be a glimpse of the future. Many of the studies here point to the same picture: a feeling of familiarity shows up when it shouldn’t, while another part of your mind calmly points out that this moment is brand new.

There is still a lot to learn. O’Connor and Moulin remind us that many of the methods used to study déjà vu are young, so findings deserve careful reading. Next time that shiver arrives, you can enjoy it for what it is: a tiny, curious moment when your memory and your sense of “now” disagree.

#déjà vu #memory #familiarity #hippocampus #everyday brain

Sources

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2. Déjà experiences in temporal lobe epilepsy. Illman NA, Butler CR, Souchay C, Moulin CJA. Epilepsy Research and Treatment 2012, 539567 (2012)
3. Recognition memory, familiarity, and déjà vu experiences. Cleary AM. Current Directions in Psychological Science 17, 353–357 (2008)
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