Not long ago, while watching TV with a few friends, Akira O’Connor was overcome by a bizarre sensation. “For a split second, I noticed that the geometry being formed by the individuals on-screen seemed extremely familiar,” he says. “Instantaneously, this feeling became not just about what I was looking at but about everything I was experiencing: The company I was in, the position in which I sat, the exact distances between myself and everything around me. It all seemed a carbon copy of a moment that I felt I must have experienced before.” In those few fleeting moments, he had a distinct feeling of déjà vu, a sensation that psychologists once considered too unpredictable and ephemeral to be studied in any systematic way.

O’Connor, a graduate student in psychology at the University of Leeds in England, is one of a small group of researchers who are giving déjà vu a second look. For the past two years, he’s been trying to induce it in the laboratory. In his study, he places subjects in a soundproof cubicle and puts them under hypnosis. Once their eyes have drifted shut, he has a computer read a list of 20 words aloud. Some are common, like penny. Others, like synod, are supposed to be unfamiliar. After several minutes, O’Connor brings his subjects out of their trance and asks them to gauge the familiarity of a new list of words—including some they’ve just heard under hypnosis. At this point something remarkable often happens: Around 40 percent of the subjects report feeling a sense of déjà vu.

The most common technical definition of déjà vu (French for “already seen”) is “any subjectively inappropriate impression of familiarity of a present experience with an undefined past.” The term is often used, incorrectly, to describe anything that happens twice: As Yogi Berra joked, “It’s déjà vu all over again.” But most people know it as the uncanny feeling of having experienced something before. In the past, psychologists spoke not only of déjà vu but of déjà entendu (already heard), déjà senti (already smelled), déjà lu (already read), and déjà vécu (already lived). Freud traced the feeling, somewhat predictably, to the mother’s genitals: “There is indeed no other place about which one can assert with such conviction that one has been there before.”

To those who have never experienced it—roughly a third of the population, according to recent surveys—déjà vu may sound like an outlandish phenomenon, akin to seeing ghosts. But some researchers believe it may help answer basic questions about how memories are recalled and how the mind registers familiarity.

In the late 19th century, dozens of hypotheses were put forward to explain déjà vu, and more than 20 different terms were coined to describe it, including “paramnesia” and “been-here-before feeling.” Just as interest in the field was peaking, however, Pavlov and his dogs arrived, turning everyone’s attention to behaviorism. Déjà vu offered no behaviors to observe, so mainstream psychologists gradually lost interest. Until the 1980s, most articles about it were published in journals of parapsychology.

“The field has been contaminated with paranormal theories. It’s been a hot potato for scientists,” says psychologist Alan Brown, author of the recent book The Déjà Vu Experience. “We’re trying to extinguish that bad rap. We’re trying to bring it into a more legitimate framework.” Any time the brain behaves strangely, there’s an opportunity to learn something about how it behaves normally, Brown says. “If we can get a handle on it, we’ve got a gold mine.”

Until recently, much of what was known to science about déjà vu came from a single extraordinary study conducted in the 1940s by Morton Leeds, an undergraduate at the College of the City of New York. Leeds had unusually frequent bouts of déjà vu and decided to keep a detailed diary of his experiences. He logged the time, circumstances, duration, and intensity of each spell. For example, at 12:25 p.m. on January 31, 1942, he wrote: “Awake, active. Extremely intense. Stood still for a moment in the shop. Then the feeling grew and grew. One of the most complete I have ever had. As the awareness grew, the feeling of being able to predict the next scene also came. It was so strong it almost nauseated me.” Over the course of 12 months, Leeds chronicled 144 episodes—one almost every two days.

When Leeds analyzed his records, he found that his déjà vu usually occurred in mundane settings. He also discovered that it was more likely to occur during periods of stress and fatigue and that it tended to occur late in the day and late in the week. Other surveys have since shown that a number of factors influence the frequency of déjà vu. The more educated, well traveled, wealthy, and liberal a person is, the more likely he or she is to experience déjà vu. As people grow older, the frequency of déjà vu trails off dramatically. The average twentysomething experiences it about three times a year; middle-aged people rarely experience it more than once a decade.

“Déjà vu is a daunting phenomenon to capture in the laboratory,” Brown says. Although he has spent his career examining what he calls “the cute gremlins out on the cognitive horizon,” Brown has found déjà vu particularly challenging. Because it occurs rarely and without warning, he can’t wait around to watch it happen. And because it is so fleeting, his subjects tend to forget the details of their experiences, making retrospective surveys unreliable.

Over the past two years, Brown, who teaches at Southern Methodist University, has been collaborating with Duke University psychologist Elizabeth Marsh on a study using students at both schools. Each professor shows his or her other students photographs of locations at the other professor’s campus, then asks the students to quickly locate small cross-shaped markers superimposed on the background. Three weeks later, the students are shown another batch of photographs, including some they’ve glimpsed before. Brown and Marsh have found that a startling 89 percent of the students feel certain they’ve visited the other campus, though they’ve never been there before. Around half feel a sensation akin to déjà vu.

Brown and Marsh’s work suggests that déjà vu is more than just a hallucination—a misfiring of neurons—as many psychologists have long believed. One explanation for their results is the “double perception” theory, which has been around since the late 19th century. According to the theory, people sometimes see things twice in quick succession: the first time superficially or peripherally; the second time with full awareness. You might glance at a building while talking on a cell phone, for instance, and not really register it, then give it a second look a little while later after you get off the phone. You might not remember the first glance, but your brain has registered it subliminally, so the second glance may seem oddly familiar.

Some epileptics suffer from repeated episodes of déjà vu, which typically occur immediately before a seizure. They describe them as intensely discomforting. “A wave would sweep over me, and I had the distinct sensation that I knew what was going to happen in my immediate environment,” one epileptic says. “It is like I know that my arm will move before it does.” By studying these patients, neurologists have traced the phenomenon to the temporal lobe and its surroundings. They’ve even triggered déjà vu by stimulating those areas with electrodes. It remains to be seen whether common déjà vu stems from the same source, but the similarities are striking.

If repeated bouts of déjà vu sound unsettling, imagine one that lasts all day. Christopher Moulin, O’Connor’s adviser at the University of Leeds, has been studying four patients with damaged temporal lobes who suffer from chronic déjà vu. They greet strangers like old acquaintances. They have no interest in watching television or reading the newspaper because they’re convinced they’ve seen everything before. When asked how they could possibly know events that have yet to occur, they often invent elaborate lies. One patient claims to sneak out at night and read newspapers in advance. When the wife of another asked him what was about to happen on a television show he claimed to have seen already, he replied, “How should I know? I have a memory problem!”

These cases suggest that déjà vu may be the result of a small seizure in the part of the temporal lobe that governs our sense of familiarity. But if that’s the case, how are Brown and O’Connor able to induce the phenomenon in the lab? They’re not sure. “It’s a real puzzle. We don’t know what causes it, what triggers it, who has it and who doesn’t, and why. We don’t even understand why it dissipates with age,” Brown says. “But the more we can understand about how this illusion occurs, the more we’ll understand our normal memory processes.”