Deja vu describes the experience of being familiar with particularly new situations or settings. This phenomenon was at the tip of the tongues of many psychologists, whom used the French term for "already seen" as a label.

A team of neuroscientists, who worked at the Picower Institute for Learning and Memory at MIT (Massachusetts Institute of Technology), discovered a specific memory circuit in the brain that causes this weird sensation. It is said from this study that a person or a thing being seen as either identical or comparable to another thing is based on our memory, in which we tend to see or perceive things other than what they appear. An example of a scenario of this: a boy is raking leaves in his backyard during the autumn season. He suddenly develops a questionable feeling of familiarity just from the way he holds his rake, and from the way he bends his body down while raking. The boy gets this familiar feeling of being in that same body position as when he had played for his ice hockey team during the winter ("What is Deja Vu?" by Teale and Akira, 2015).

It is suggested that deja vu can be an analogue of an optical illusion. Thomas McHugh and several colleagues from the PILM (Picower Institute for Learning and Memory) realized that memory is made up of different components: short-term and long-term memories, memories of events (episodes), and fact-based memories. Though, one of their main focuses on the brain was the hippocampus, a region where new memories are formed. McHugh and his team try to untangle the neurological circuitry of the hippocampus. Since memories are formed by a group of brain cells linked by a strong chemical bond, finding and recalling memories can involve activating a specific group of these brain cells.

The brain has an essential ability to know the similarities of certain memories, but also being able to detect these memories as being similar and not identical. This ability, known as pattern separation, and the ability to retrieve memories based on a single cue (pattern completion) are regulated by a specific gene, according to McHugh and Susumu Tonegawa's (his senior colleague) study. The results from their studies have shown that every so often, the pattern-separation circuit misfires. As a result, the person will merely experience a sensation of a current thing or situation being seemingly identical to another thing or situation within their memory. People who have epilepsy, unlike others, experience this feeling constantly. Tonegawa discovered the random firings of neurons in the temporal lobes, in which defines an epileptic seizure. He and McHugh also theorized that the strange feeling of deja vu derive from the conflict between two parts of the brain when the circuit misfires; the neocortex (knowing you've never been in the situation before) and the hippocampus (telling you that you have been in the situation before) ("Explaining Deja Vu" by Michael D. Lemonick, 2007).