Déjà vu is a sense of having already seen something you're currently seeing or experiencing—coupled with knowing you haven’t actually seen it, which is why it catches many people so off guard. It is thought to be the equivalent of a small brain “glitch,” with two streams of thought colliding. The phrase literally translated from French to mean “already seen.”

It is an incredibly common experience; something upwards of 97% of people are thought to have experienced déjà vu at least once, with more than two-thirds of people experiencing it with some regularity.1

St. Augustine, an ancient philosopher, first referred to the concept of déjà vu in 400 AD as “false memoriae,” but French philosopher Emile Boirac was the first to use the term déjà vu in 1890. The first use of the phrase in the scientific world was from F.L. Arnaud, a neurologist who proposed to use it at a meeting of the Societe Medico-Psychologique. 2

Early research showed promise for déjà vu to be a sign that helped doctors diagnose epilepsy,3 but more recent research has shown it may be a matter of perception or memory.

How Does Déjà Vu Happen?

It is believed that déjà vu may be the result of two different streams of awareness colliding: the experience of recognizing a current situation, alongside the feeling that this is an inaccurate recollection. A key feature is that the person realizes that they have not actually seen this before.

Sometimes, what happens is really a matter of split perception4 and someone is processing a sight twice because they may have been distracted or their vision was obstructed for some reason. 

The second perception, immediately after the first one, becomes the one that is consciously experienced—but it feels unfamiliar because we are not cognizant of the first experience, which we only partially processed.

Types of Déjà Vu

Though the actual feeling of déjà vu is the same across people with healthy brains and those with neurological conditions, different things are happening in the brain during each of these types.

Those who do experience it more regularly show less grey matter than those who don’t.5 Grey matter is the outermost layer of the brain, and it is responsible for controlling movement, memory, and emotions. Typically, the more grey matter a brain has, the more effective it is.

In those with neurological conditions, three parts of the brain are impacted: the hippocampus, parahippocampal gyrus, and temporal neocortex—areas that are associated with forming and retaining memories.

For those with epilepsy and déjà vu, alterations in memory circuitry have been observed, meanwhile, alterations in emotional circuitry are seen in “healthy” individuals experiencing déjà vu.3 Déjà vu is thought to perhaps be abnormal signaling within the medial temporal lobe, which governs memory processing, particularly visual memories.

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