Scientists have developed many possible explanations for déjà vu. In a review of the science in 2003, the Psychological Bulletin outlined four major schools of thought about why might happen. The first is the simplest: that the event has in fact already happened, and that for some reason you had forgotten this and are being reminded. "This can give it a spooky sort of feeling, because you are so certain that you have never been to this place before or done this thing before, even though it really is just a normal memory process at work," Cleary says. The second is that it's brought about by a processing error in the brain, where two elements are trying to operate simultaneously and something gets out of step.

The third idea is what's called the "disruption" theory, where neural firings are somehow interrupted or go awry. This, it's argued, is why people with epilepsy experience déjà vu as part of the auras of their seizures. In people without epilepsy, it's proposed that, when déjà vu happens, there's an accidental delay or repetition in the transferral of sensory stimuli information the brain, causing a kind of overlap — and the sensation that the event being registered has happened already.

The fourth, meanwhile, is the "attentional" explanation. While you might be paying attention to what's happening around you, it suggests, you might be distracted for a split second, and when you re-focus on the event at hand, it may seem oddly familiar in a "past" way. All of these ideas could actually be true and contribute to various kinds of déjà vu.

Neuropsychologist Sanam Hafeez Psy.D. tells Bustle that in déjà vu, the hippocampus, the brain's seat of memory, appears to activate two neural circuits at the same time, trying to line up the present experience with past memories. And other parts of the brain seem to be be involved too: a study in Memory in 2017 found that people with déjà vu showed activity in their anterior cingulate cortex, medial prefrontal cortex and parietal cortex, all of which are related to memory conflicts. A study in Neurology in 2004 even found that déjà vu can be induced by stimulating another area of the brain, the entorhinal cortex, which connects the hippocampus to other parts of the brain and is necessary to memory.

Epileptic déjà vu can give us insights, too. "People with extremely frequent déjà vu often have seizure activity emanating from the medial temporal lobe region of the brain (roughly behind your ears)," Cleary says. Medial temporal lobe epilepsy affects the brain's hippocampus. Epileptic and non-epileptic déjà vu seem to differ; in one 2013 study in Epilepsy & Behavior Case Reports, the EEG patterns of people with and without epilepsy experiencing déjà vu are very different. But all these studies give a good boost to the theory that memory mis-filing is behind the experience of déjà vu itself. The mechanism behind it, though, remains mysterious.

A new theory about déjà vu emerged in 2016 that may be one of the best solutions of the mystery. Scientists at the University of St. Andrews managed to induce déjà vu in non-epileptic people, by doing a word experiment.

The St. Andrews scientists examined the brains of the subjects as they experienced this induced déjà vu, and discovered that instead of activity in the hippocampus or other areas of the brain to do with memory, they noted that the frontal areas of the brain were active instead. They suggest that this might mean our frontal lobes are actually "checking" our memory input and waking up to raise an alarm when something doesn't fit with our other memories.

Cleary thinks that many experiences of déjà vu may actually correspond to real memories, and that the feeling of déjà vu represents a memory that's hidden. "Familiarity-based recognition occurs when you have a sense of having experienced something before but cannot pinpoint why, such as when you recognize a person’s face as familiar but cannot place where you have seen them before," she says. "Normal experiences of déjà vu can be a special case of this familiarity experience." Her lab has induced this experience using virtual reality, where they put people in spaces that resemble places they've previously seen, but forgotten. People seeing those scenes were more likely to experience déjà vu than people who were shown completely new scenes — even if they couldn't remember ever seeing anything like it before.

Other mechanisms may be at work, too. A study published in Epilepsy & Behavior in 2019 found that people who were more prone to déjà vu actually used different parts of their brain when they were retrieving memories to those who didn't get déjà vu often at all. Their hippocampi were slightly less active, for one thing. It's not known why this happened: maybe, as the lead author of the Edinburgh study suggested to New Scientist, people who don't get déjà vu simply have better memories.