Most of us have experienced the feeling that we have been in an exact moment before, that though we know this is our first time in a place or situation, there is this nagging feeling that it is strangely familiar — as if, if we could just focus a little harder, we will know what comes next.

This phenomenon is commonly known as “déjà vu”, which is French for “already seen.” The documentation of this experience goes as far back as 400 AD, when St. Augustine referred to it as “false memories,” before French philosopher Emile Boirac cited the official term in 1876.

According to psychologist Arthur Funkhouser, there are three subtypes of déjà vu: déjà vécu, which translates to, “I have already experienced this,” déjà senti, “I have already felt this,” and déjà visite — feeling that a place is familiar though one has never been there.

About two-thirds of Americans have reported experiencing déjà vu, and it occurs more frequently in youth, particularly between age 15 to 25. But why does this experience occur and what does it mean? As a neurologist, I’m here to provide insight and share theories related to this strange phenomenon.

Theories behind déjà vu

In the medical community, it has been difficult to know how to explain this experience, mainly because it is so fleeting and occurs in diverse populations — often with no medical conditions unifying them. But researchers have a few theories as to why déjà vu happens.

One thought is that it might be misplaced scene familiarity: responding to a new experience or setting as familiar because its configuration is similar to a past setting or experience. (This theory could be useful for individuals with memory impairment.)

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The second theory is that there is a brief error in the storing of memory, in which something occurring in real time ends up accidentally filed in long-term memory, so that it actually feels familiar when it is not.

There is also the thought that there is a brief delay in the processing of sensory information by higher cognitive centers in the brain that results in our interpretation of an experience as not novel, or new.

More recently, the University of St. Andrews performed a study that was able to actually induce a feeling of déjà vu in its subjects. Study participants listened to a host of words that were related to the idea of sleep, without ever hearing the exact word. Later, when participants were requested to report words they heard, they incorrectly chose the word, “Sleep,” although that word was never mentioned.

Another theory of déjà vu is one of the “buried” or unrecalled memory that then serves as foresight for the future.

The study participants then underwent a functional magnetic resonance imaging (fMRI), and were asked if they were told any words that began with the letter “s” or the word “sleep”. If it had been a memory, one would expect the hippocampus to be activated, but it was not. Instead, frontal areas of the brain, which are involved in decision-making, were activated instead.

Although this study included a small sample size of 21 participants, the results were still fascinating: Rather than déjà vu being caused by false memories, the study demonstrates that the phenomenon is our brain’s way of checking our memory-making pathways.

Epileptic seizures and déjà vu

Though the specifics regarding déjà vu are still unclear, the source of this experience has been located: the medial temporal lobe (which is critical for long-term memory and new memory formation). Patients with epilepsy who have undergone placement of direct electrical stimulations and stimulation to that area of the brain have reported déjà vu experiences, in particular, déjà senti.

Even though there are various hypotheses about déjà vu, there still remains a great possibility for further understanding of this phenomenon — especially through individuals with epilepsy. But, for now, déjà vu remains a peculiar and fascinating anomaly, reminding us how beautifully complex the brain truly is.