Have you ever had that strange feeling of passing a person and just knowing you've seen them before? Perhaps you even think that you know what might happen in the next moment. This odd sensation is known as "déjà vu" ("already seen"). But why does it occur?

Few researchers have taken much interest in this phenomenon, but Anne Cleary—from Colorado State University in Fort Collins—is one who has.

She has been paying special attention to the brain mechanics of this experience for a few years now, and recently she has extended her project to answering the question: does the feeling of premonition often associated with déjà vu have a real basis?

The results of this study—which Cleary co-led with former graduate student Alexander Claxton—have now been published in the journal Psychological Science.

A phenomenon of frustrated recall

In their new research, Cleary and Cox induced the experience of déjà vu in study participants in order to test the co-occurrence of premonitory feelings and to see whether such feelings were consistent with the actual situation.

In other words, the researchers wanted to see whether people who had had a déjà vu experience could really predict what was coming next, or whether that sensation was just a trick of the mind.

To induce déjà vu, Cleary used a strategy that she had successfully tested in a previous study.

In 2012, she argued that the feeling of having "already seen" is a memory-related phenomenon, akin to the sensation of words that elude us—much like when we have a word "on the tip of our tongue," as it were, yet try as we might we cannot recall it, despite the fact that we know we know it.

Cleary discovered that when we experience déjà vu, it could be because the context reminds us of something we've already seen or experienced in real life, but which we can no longer properly remember.

Thus, we may have the feeling that we've already been to an utterly new place if, say, it reminds us of a place once glimpsed from a train but which we're no longer conscious of ever having seen.

"We cannot consciously remember the prior scene, but our brains recognize the similarity," explains Cleary. "That information comes through as the unsettling feeling that we've been there before, but we can't pin down when or why."

Both déjà vu and the "tip of the tongue" feeling are known as "metamemory" phenomena: when we know that we remember, or that we ought to remember, something.

"My working hypothesis is that déjà vu is a particular manifestation of familiarity. You have familiarity in a situation when you feel you shouldn't have it, and that's why it's so jarring, so striking."

Anne Cleary

Déjà vu and premonitions

In their recent study, Cleary and Cox made the participants experience déjà vu by asking them to explore 3-D virtual landscapes.

The strategy was simple: landscapes were mapped in identical fashion yet they looked completely different—for example, sometimes participants would see junkyard scene, while at other times they were shown a hedge garden.

In each case, "[M]ovement through the scene stopped before a critical turn." Therefore, all of the participants felt like they had already seen a particular landscape because they had—but in a completely different form.

Then, the researchers tested whether participants with déjà vu who thought they could predict the next turn would actually be able to do so correctly, or whether they were just being duped by their brains.

Such a trick of the mind, Cleary explains, would be explained by a particular theory of memory, which argues that we store memories so we can learn to "predict" future situations. This could allow us to ensure that we survive and thrive.

The researchers saw that approximately half of the participants who reported déjà vu also said that they had premonitory sensations. But "the probability of choosing the correct turn during [...] déjà vu" was no stronger than the probability of picking a wrong direction."

In short, while we may think that we can predict what's going to happen next in an experience of déjà vu, that impression remains ungrounded in reality.

Now, Cleary is leading follow-up experiments focused on the feeling that "you just know what's meant to happen next."

In doing so, she hopes to gain a better understanding of what causes this feeling, and whether it's really related to the sensation of familiarity.