Debbie O'Leary, 40, a writer in Bloomington, Ind., said that some years ago she felt sensations of déjà vu while sick at home, as if her life had become a scratched record, with the same chorus repeating.

"It was literally like my brain was stuttering, like the same tape kept replaying the same thoughts, the same motions," as if time were stuck, she said. "I think it was the fatigue, the fever."

Psychologists have long known, too, that people register impressions and images well before they are aware of what they have seen. The brain sends visual signals through at least two circuits, which move from the retina through the brain to the visual cortex via different routes.

It is an exquisitely tuned system, but common experience suggests many ways its functioning might be thrown off. The classic example, from Dr. Edward Bradford Titchener, a founder of the field, is when a person is about to cross a busy street, glances both ways and then is distracted by a shop window display: "As you cross then, you think, 'Why, I crossed this street just now'; your nervous system has severed two phases of a single experience, and the latter appears as a repetition of the earlier."

In one of the first experiments to produce this unconscious familiarity, Drs. Larry Jacoby and Kevin Whitehouse, psychologists at Washington University in St. Louis, had 30 students memorize a list of words as if in preparation for a test. Shortly afterward, the students were asked to sit in front of a computer screen and watch as another series of words appeared, one at a time, and to flag the ones they had seen the first time around. All it took for the researchers to make an unfamiliar word look suddenly familiar was to flash it subliminally on the screen for a few milliseconds. The students' brains registered the word subconsciously, and they were highly likely to say it had been on the first list.

"The punch line," Dr. Jacoby said, "was that when we slowed it down so that they were actually aware of and could read the flashed word," the rate of inaccurately identified words plunged.

Word games fall well short of full-blown déjà vu. But the point, psychologists who study memory say, is that people take in a rich banquet of information without noticing it, or noticing and simply forgetting where it came from. It is entirely possible to read an Anne Rice novel and years later, after having forgotten the book, find that a first visit to New Orleans seems like a glimpse into a former life. Or to see one of the bar scenes from the movie "L.A. Confidential" and later walk into the Formosa Cafe for the first time and catch your breath.