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Have you ever walked into a room and been overwhelmed by a feeling of familiarity—even though it’s your first time being there? Or been in the middle of a conversation when it dawns on you: I’ve had this conversation with this person before, despite their insisting that you haven’t?

Déjà vu refers to the feeling that you have already experienced your present situation. It is remarkably common, with two out of three people—both men and women—experiencing the phenomenon at some point in their lives. Further, in those who experience déjà vu, the phenomenon happens about once per year, although its frequency decreases with age.

However, déjà vu doesn’t happen all the time: If we experienced the feeling every time we retraced our steps or went through our routine, it’d cease to be an anomaly. Instead, it’s seen as a bit of a psychiatric oddity that’s extremely common, occurring in about 60 percent of the population. And understanding how déjà vu works might shed light on the functions of human memory and our complex brains.

The déjà vu sensation has always been associated with a slight air of strangeness and mystery. It first entered scientific circles in 1876, when the French philosopher and investigator of the paranormal Émile Boirac coined the term in a letter. It’s fascinated psychologists, scientists and artists ever since; Freud thought it was prompted by repressed desires (because of course he did), Marcel Proust based the most famous segment in his masterpiece In Search Of Lost Time on déjà vu sparked by a madeleine cookie, and the makers of the Matrix trilogy made the feeling a sign of a “glitch” in the artificial world.

Despite déjà vu being relatively common, relatively limited research has been done on the subject. What we know so far is that in people without psychosis or temporal lobe epilepsy, the causes of déjà vu fall into four categories—attentional, memory, dual processing, and neurological.

  1. Attentional explanations of déjà vu involve an initial perception that is made under degraded attention, which is then followed by a second take under full attention. For example, if you are about to unlock the front door of your house, and you’re momentarily distracted by a noise in the distance, when you return to the task of unlocking the door, the first perception may seem further off in the past. The distraction that separates these two perceptions could be as fleeting as an eye blink.
  2. Memory explanations make the assumption that some detail of the new experience is familiar but the source of this familiarity has been forgotten. The premise of this explanation is that people encounter countless things during the course of a day but don’t pay attention to all of the information. Later reprocessing of the information may occasionally induce familiarity and déjà vu.
  3. Dual-processing explanations of déjà vu suggest that two usually synchronous cognitive processes become momentarily asynchronous. For example, familiarity and retrieval could become out of sync. Alternatively, perception and memory could become asynchronous.
  4. Neurological explanations of déjà vu attribute the phenomenon to either a small temporal lobe seizure in a person without epilepsy, or to a delay in neuronal transmission between the eyes, ears, or other perceptual organs and higher-order processing centers in the brain.

Dual-processing explanations have received a lot of attention—they are much more philosophical and theoretical, and less mechanistic, but dual-processing explanations can’t be tested in a lab. Similarly, neurological explanations are appealing in their neurological basis and seem logical, but again, we lack the advanced technology to test them. Thus, dual-processing and neurological explanations are less germane to researchers. Instead, attentional and memory explanations are best supported by what we know about cognition and can be empirically tested.

The aforementioned theories are only that—theories that attempt to explain the mystery that is déjà vu. However, what we do know is that these strange feelings of familiarity are harmless and probably not signs that you’re psychic or flashbacks from a past life. So the next time you’re transported back in time due to a simple smell, sound, or experience, know that you have nothing to worry about and bask in the mystery of it all.

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