Déja vu is a French term that literally means "already seen" and is reported to occur in 60-70% of people, most commonly between the ages of 15 and 25. The fact that déja  vu occurs so randomly and rapidly—and in individuals without a medical condition—makes it difficult to study, and why and how the phenomenon occurs is up to much speculation. Psychoanalysts may attribute it to wishful thinking; some psychiatrists cite mismatching in the brain causing us to mistake the present for the past. Still, parapsychologists may even believe it is related to a past-life experience. So what do we know for certain about what happens during an episode of déja vu?

Some researchers speculate that déja vu occurs when there is a mismatch in the brain during its constant attempt to create whole perceptions of our world with very limited input. Think about your memory: it only takes small bits of sensory information (a familiar smell, for instance) to bring forth a very detailed recollection. Déja vu is suggested to be some sort of "mix-up" between sensory input and memory-recalling output. This vague theory, however, does not explain why the episode we experience is not necessarily from a true past event.

Ventral (bottom) view of the brain, showing the perirhinal (red) and entorhinal (yellow) cortices.

Bartolomei F, Barbeau EJ, Nguyen T, McGonigal A, Régis J, Chauvel P, & Wendling F (2012). Rhinal-hippocampal interactions during déja vu. Clinical neurophysiology : official journal of the International Federation of Clinical Neurophysiology, 123 (3), 489-95 PMID: 21924679