What causes déjà vu? What scientists think so far

Déjà vu is the sense that a new situation is familiar even though you know it has not happened before. The BBC's science explainer says there is no definitive answer to what causes it, only several possible explanations. Most of them describe the memory system briefly going out of step with itself, so that a feeling of familiarity appears without a real memory behind it. Roughly 60 percent of people are reported to experience it at least once. [1] [2]

Memory is thought to rely on two cooperating processes. Familiarity is quick and depends on regions of the medial temporal cortex. Conscious recollection depends on the hippocampus and prefrontal cortex. When these fall out of sync, déjà vu can result. [2]

Cognitive psychologist Akira Robert O'Connor of the University of St Andrews describes a sequence. The medial temporal lobe fires and produces a sense of familiarity. The frontal cortex, which handles reasoning and fact-checking, then compares that feeling with what you actually remember. When nothing matches, the feeling is flagged as an error. O'Connor suggests this may really be the avoidance of an error rather than a fault. [3] [1]

One view is that a new scene resembles something you have experienced but cannot recall. Émile Boirac, who possibly coined the term in 1876, speculated about residues of forgotten perceptions. Anne Cleary of Colorado State University tested a version called the Gestalt Familiarity Hypothesis, in which a scene shares a layout with an earlier one, such as furniture placed in the same arrangement. In a 2009 study, scenes with subtle similarities produced more déjà vu than dissimilar scenes. [3] [4]

O'Connor is cautious about this. He says the ideas make sense, but people are good at telling similar things apart, so true familiarity may not be needed. He suggests that in spontaneous cases the medial temporal lobe may simply fire overenthusiastically for no particular reason. He points out that younger people, whose brains are more excitable, report more déjà vu than older people. [3]

Chris Moulin of Grenoble Alpes University proposes that older adults may still generate false familiarity but lose the certainty that it is false, because the frontal fact-checker may brake less effectively. In some people with dementia, familiarity arises without the error being recognised, so they may act as if they have seen everything before. [3]

Studies of patients with persistent déjà vu, including two described by Leeds researchers in 2005, offer the best evidence for a neural mechanism. Damage to relevant regions likely made familiarity circuits fire even in novel situations. The Cleveland Clinic also lists fatigue, stress, migraines, frontotemporal dementia and temporal lobe seizures as conditions linked to déjà vu. [2] [5]