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Scientists have for the first time recorded individual brain cells in the act of summoning a spontaneous memory, revealing not only where a remembered experience is registered but how the brain is able to re-create it. The recordings, taken from the brains of epilepsy patients being prepared for surgery, demonstrate that these spontaneous memories reside in some of the very same neurons that fired most furiously when the recalled event was first experienced. Researchers had long theorized that this was the case but until now had only indirect evidence. The new study, experts said, has all but closed the case: Remembering, for the brain, is a lot like doing. The experiment, being reported Friday in the journal Science, moved beyond most earlier memory research in that it focused not on recognition of objects or recall of specific words or symbols but on free recall - whatever popped into people's heads when, in this case, they were asked to recall a series of short film clips they had just seen. Such memory often deteriorates quickly in people with Alzheimer's disease and other forms of dementia, and it is critical to so-called episodic memory: the rich catalog of vignettes that together form our remembered past. "This is what I would call a foundational finding," said Michael Kahana, a professor of psychology at the University of Pennsylvania. "I cannot think of any recent study that's comparable. It's an important step in helping us fill in the detail of what exactly is happening when the brain performs this mental time travel" when summoning past experiences.
In the study, a team of American and Israeli researchers threaded tiny electrodes into the brains of 13 people with severe epilepsy. The electrodes allow the doctors to pinpoint the location of the mini-storms of brain activity that cause epileptic seizures.
The patients watches a series 5- to 10-second film clips, some from popular TV shows like "Seinfeld," others depicting animals or landmarks, like the Eiffel Tower. The researchers recorded the firing activity of about 100 neurons during the viewing of repeated series of videos; the cells were concentrated in and around the hippocampus, a sliver of tissue deep in the brain that is known to be critical to forming new memories.
In each individual, the researchers identified single cells that became highly active during some videos and quiet during others. About half the recorded cells hummed with activity in response to at least one film clip, and responded weakly to another.
After distracting the patients for a few minutes, the researchers then asked the subjects to think about the clips for a minute and report "what comes to mind." The patients remembered almost all of the clips. And, sure enough, when they recalled a specific one - say, a clip of Homer Simpson - the same cells that had been active during the Homer clip reignited. In fact, the cells became active a second or two before people were conscious of the memory.
In effect, the scientists could identify the specific memory before the patients could.
"There were all these distractions, these people were on a noisy ward, there's a whole lot happening all around them, but still you see this absolutely robust response in the individual neurons," said the senior author, Dr. Itzhak Fried, a professor of neurosurgery at the University of California, Los Angeles, and the University of Tel Aviv. His co-authors were Hagar Gelbard-Sagiv, Michal Harel and Rafael Malach of the Weizmann Institute of Science in Israel; and Roy Mukamel, of UCLA.