If you've ever had that fleeting, mysterious sense that something new—a city or person you're seeing for the first time—is somehow familiar, that you've been there or known them before, then you can count yourself among those who have experienced déjà vu. It's typically a brief sensation, lasting no more than ten to thirty seconds, but ninety-six percent of the population claims to have experienced at least one occurrence.

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"Déjàvu, a French term meaning 'already seen,' is considered a disconnect or clash between objective unfamiliarity and a subject sense of familiarity," says Claire Flaherty-Craig, a consulting and treating neuropsychologist at Hershey Medical Center. "It's been most closely studied in epilepsy, where patients often experience it before a seizure. The brain regions for memory are in the temporal lobes, and there's an area for monitoring memory accuracy in the middle frontal lobe. Those patients reporting déjà vu are temporal lobe seizure patients. The actual trigger for it in healthy individuals is not exactly known, but we do know those same regions of memory and memory monitoring are involved."

The concept of déjà vu has been around since French philosopher and researcher Émile Boirac coined the term in 1876. Proponents of psychic phenomenon quickly latched onto it as evidence of past lives, while early psychiatrists and psychologists bandied about various theories to explain its occurrence: Sigmund Freud attributed it to repressed desires. Carl Jung suggested it arose from tapping the collective unconscious. Dozens of "causes" of déjà vu have been proposed over many decades, remarks Flaherty-Craig, but most fall by the wayside as researchers learn more about the human brain and cognitive processes.

"There was a long-standing theory about a visual disconnect," she explains. "It was thought that one hemisphere of the brain would process the visual information first and so the delayed information reaching the other hemisphere was processed like a memory." However, recent studies done on the blind have challenged this idea, and Flaherty-Craig notes at least one case where the blind individual reported déjà vu involving hearing, touch, and smell.

One popular belief is that déjà vu might result from an accumulation of life experience, but science says otherwise, explains Flaherty-Craig. "Statistically it occurs more in late adolescence and frequency of episodes declines with age."

She recalls a déjà vu experience of her own from early adulthood as an example: "After college graduation, I took five weeks and went on a hiking trip in Ireland. When I was on the west coast hiking one day, I had a very distinct feeling, a real strange sense that I'd been there already. I could even envision what the landscape looked like around the bend in the road, and about an hour later when I reached there, that's what it looked like. I had discounted the idea of previous lives and things like that by my late teens and was already looking for more scientific explanations for the world, so when I had the experience, it kind of made me a believer that things like déjà vu even existed."

Claire Flaherty-Craig, Ph.D., is currently a consulting and treating neuropsychologist at The Milton S. Hershey Medical Center in both the Comprehensive Epilepsy Program and the Affiliated Rehabilitation Program of Pinnacle Health. She is also an assistant professor in the Department of Neurology at the College of Medicine. She can be reached at cflahertycraig@hmc.psu.edu.