What causes déjà vu?

Déjà vu is the sensation that a new experience has happened before, and the leading explanations treat it as a processing glitch rather than a memory. Familiarity and recollection are handled by partly separate systems in the brain, and the dominant theory holds that déjà vu occurs when the familiarity signal fires without any corresponding retrieved memory, leaving you with a strong sense of recognition and nothing to attach it to. Related hypotheses point to a brief delay or duplication in how sensory information reaches processing regions, so an experience is registered twice a fraction of a second apart, or to genuine partial overlap with a forgotten past scene whose layout resembles the current one.

Supporting evidence comes partly from epilepsy research, since déjà vu is a recognized aura preceding some temporal lobe seizures, and stimulation of the temporal lobe during surgical mapping can induce it, which localizes the phenomenon to memory-related structures. In healthy people it is entirely normal, reported by around two-thirds of the population, and interestingly it becomes less frequent with age rather than more, peaking in adolescence and early adulthood. It is also associated with travel, tiredness, and stress. The version that warrants medical attention is déjà vu that is frequent, prolonged, or accompanied by other symptoms such as loss of awareness, unusual smells, or confusion, which can indicate seizure activity and is worth describing to a neurologist rather than dismissing.