Saturday, June 9, 2007

The latest on deja vu

Déjà vu (French for "already seen"); an uncanny feeling or illusion of having already seen or experienced something that is being experienced for the first time.

If we assume that the experience is actually of a remembered event, then déjà vu probably occurs because an original experience was neither fully attended to nor elaborately encoded in memory. If so, then it would seem most likely that the present situation triggers the recollection of a fragment from one's past. The experience may seem uncanny if the memory is so fragmented that no strong connections can be made between the fragment and other memories.

Thus, the feeling that one has been there before is often due to the fact that one has been there before. One has simply forgotten most of the original experience because one was not paying close attention the first time. The original experience may even have occurred only seconds or minutes earlier.

The brain cranks out memories near its center, in a looped wishbone of tissue called the hippocampus. But an article from reports on a new study suggesting that only a small chunk of it, called the dentate gyrus, is responsible for “episodic” memories—information that allows us to tell similar places and situations apart.

The finding helps explain where déjà vu originates in the brain, and why it happens more frequently with increasing age and with brain-disease patients, said MIT neuroscientist Susumu Tonegawa. The study is detailed today in the online version of the journal Science.

Like a computer logging its programs’ activities, the dentate gyrus notes a situation’s pattern—it’s visual, audio, smell, time and other cues for the body’s future reference. So what happens when its abilities are jammed?

When Tonegawa and his team bred mice without a fully-functional dentate gyrus, the rodents struggled to tell the difference between two similar but different situations.

“These animals normally have a distinct ability to distinguish between situations,” Tonegawa said, like humans. “But without the dentate gyrus they were very mixed up.”

Déjà vu is a memory problem, Tonegawa explained, occurring when our brains struggle to tell the difference between two extremely similar situations. As people age, Tonegawa said déjà-vu-like confusion happens more often—and it also happens in people suffering from brain diseases like Alzheimer’s. “It’s not surprising,” he said, “when you consider the fact that there’s a loss of or damage to cells in the dentate gyrus.”

As an aging neuroscientist, Tonegawa admitted it’s a typical phenomenon with him. “I do a lot of traveling so I show up in brand new airports, and my brain tells me it’s been here before,” he said. “But the rest of my brain knows better.”

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