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Deja vu explained


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Turkey, Talk, Family: One More Reason

It All Seems Familiar

November 25, 2005; Page B1

It was just a lampshade, in your line of vision as you settled in for the predinner olives and celery, but it unleashed a powerful feeling of déjà vu. Even though this was your first Thanksgiving at this relative's home, you were sure you had been there before. Or, it was a phrase spoken when a tablemate passed the creamed onions. Even though she was a new in-law, you were positive you had celebrated Thanksgiving with her in years past.

About two-thirds of people experience at least one déjà vu in their lifetime, and if you have had one you are likely to have more. For reasons unknown, the incidence of déjà vu decreases with age, rises with education and income, and is more common among people who recall their dreams, who travel, and who hold liberal political and religious beliefs.

Déjà vu -- a sense that you have previously seen, heard or experienced something that is, in fact, new to you -- occurs mainly when people are indoors and engaged in something relaxing and among friends. So, chances are pretty good that more than a few of you felt a sense of déjà vu yesterday, especially when you add the fact that it is more likely to occur when you are tired or under stress. (Preparing dinner for 20? Eating with relatives who nag you about your failings? Did someone say stress?) Alcohol also makes déjà vu more likely. It also tends to happen in the evening and on weekends.

Past lives and precognition have failed to gain traction as explanations for déjà vu. But other approaches are doing better. Studies on memory and attention, says psychology researcher Alan S. Brown of Southern Methodist University, Dallas, are pointing to the neurological stumbles that produce déjà vu.

One clue has come from the observation that some epileptics experience déjà vu right before a seizure. Seizures result from a fierce lightning storm of electrical activity in the brain, often in the temporal lobes on the left and right sides of the brain. Among other things, these structures decide whether something is familiar. Even in nonepileptics, if temporal-lobe neurons suddenly fire they could produce an illusion of familiarity, says Prof. Brown.

Another hint comes from studying the long and winding roads followed by sensory data coming into the brain. After a sight lands in the visual cortex at the back of the head, signals carrying information about shape, size, motion and other attributes proceed to what are called association areas. But like travelers taking different routes, even though the signals set out at the same moment they may arrive slightly apart. (If a synapse is temporarily short of molecules of neurotransmitter, then the signal gets held up as if at a red light.) Because the association areas integrate signals from different streams into a single experience, a delay of even milliseconds may lead the brain to misinterpret the second arrival.

"This is probably such a rare event that the brain can't tell that the second message is actually part of the first, even though it arrived less than a second later," says Prof. Brown. "The brain thinks these are separate, independent experiences, so when the second signal arrives it gives rise to the sensation that what you are perceiving now you perceived before."

In the babel of a Thanksgiving dinner, you typically catch snippets of conversations, which your brain processes only minimally and below conscious awareness, either because they are peripheral to the conversation you're focusing on or because you have retreated into your own thoughts for a few blessed moments. But if your attention suddenly snaps back to a previously unattended current of talk, the sound of the voice and the repetition of key words (such as someone's name) "connects with the first processing, of which you were unaware," Prof. Brown wrote in a 2004 paper in Current Directions in Psychological Science. "The striking match may evoke a déjà vu illusion."

The discovery of what's called inattentional blindness lends support to this idea. People fail to see things in their field of vision all the time. But even though you are oblivious to, say, the storefront next to your destination, it still registers in memory. In lab tests, when a word flashes too quickly for someone to consciously perceive it, and then flashes long enough to be read, people say they saw it on a previous list, not realizing they saw it a second ago.

Such brief initial exposure may enable the brain to process the same word or sight faster the next time, says Prof. Brown, "and such speeded reprocessing may elicit the sense of familiarity underlying the déjà vu illusion." Keep that in mind when Uncle Joe introduces his new way with yams this year: If you glimpse the dish out of the corner of your eye but focus on the plate of drumsticks beside it, when you look at the yams for real you're liable to think you saw them a year ago rather than a minute ago. "Oh, your reliable old yams, Joe; thank you," will not get you remembered in his will.

But your sense that you'd eaten that very same turkey and stuffing and cranberries and pumpkin pie many, many times before? That, dear reader, was no illusion.

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