Déjà vu, French for already seen, strikes roughly two thirds of people at least once. French philosopher Émile Boirac coined the term in 1876 in The Future of the Psychic Sciences. This episode explains who experiences it most, including younger people, frequent travelers, heavy film watchers and people who remember their dreams, and why a 2008 study found no diagnostic link to schizophrenia. It does accompany temporal lobe epilepsy, often as a seizure aura, with links to variants of the LGI1 gene on chromosome 10, and documented cases from 2001 and 2007 show drug combinations that raised dopamine or serotonin triggering repeated episodes.

Robert Efron at the Boston Veterans Hospital proposed in 1965 that a tiny delay between signals arriving from the two hemispheres makes the brain register one moment twice. Other theories include split perception and cryptomnesia, tested in a 2010 hypnosis study by O’Connor, Moulin and Conway using the puzzle game Railroad Rush Hour, and a 2012 virtual reality study showing that familiar room layouts can trigger the feeling. The episode covers Carl Jung’s collective unconscious, jamais vu, presque vu and déjà vécu, and a 2018 fMRI study showing that frontal regions, not memory centers, light up during the experience.

• The computer mouse double-click analogy for Efron’s dual processing theory
• Hypnotized volunteers told to feel familiar with a puzzle they had never played
• Chris Moulin’s study in which 68 percent of 95 volunteers writing door 30 times developed jamais vu
• Déjà vécu, in which patients lose the ability to tell false familiarity from reality
• The word list built around window, used to induce déjà vu inside an fMRI scanner