For the Brain, Remembering Is Like Reliving
By BENEDICT CAREY
Excerpts from The New York Times September 4, 2008
Scientists have for the first time recorded individual brain cells in the act of summoning a spontaneous memory, …
The recordings, taken from the brains of epilepsy patients being prepared for surgery, demonstrate that these spontaneous memories reside in some of the same neurons that fired most furiously when the recalled event had been experienced. …
Experts said the study had all but closed the case: For the brain, remembering is a lot like doing (at least in the short term, as the research says nothing about more distant memories).
The experiment, being reported Friday in the journal Science, is likely to open a new avenue in the investigation of Alzheimer’s disease and other forms of dementia, some experts said, as well as help explain how some memories seemingly come out of nowhere. The researchers were even able to identify specific memories in subjects a second or two before the people themselves reported having them….
In the study, a team of American and Israeli researchers threaded tiny electrodes into the brains of 13 people with severe epilepsy. The electrode implants are standard procedure in such cases, allowing doctors to pinpoint the location of the mini-storms of brain activity that cause epileptic seizures.
The patients watched a series of 5- to 10-second film clips, some from popular television shows like “Seinfeld” and others depicting animals or landmarks like the Eiffel Tower. The researchers recorded the firing activity of about 100 neurons per person; the recorded neurons were concentrated in and around the hippocampus, …
In each person, the researchers identified single cells that became highly active during some videos and quiet during others. More than half the recorded cells hummed with activity in response to at least one film clip; many of them also responded weakly to others.
After briefly distracting the patients, the researchers then asked them to think about the clips for a minute and to report “what comes to mind.” The patients remembered almost all of the clips. And 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, which signaled to researchers the memory to come….
Dr. Fried said in a phone interview that the single neurons recorded firing most furiously during the film clips were not acting on their own; they were, like all such cells, part of a circuit responding to the videos, including thousands, perhaps millions, of other cells…
Single-cell recordings cannot capture the entire array of circuitry involved in memory, which may be widely distributed beyond the hippocampus area, experts said. And as time passes, memories are consolidated, submerged, perhaps retooled and often entirely reshaped when retrieved later…
“The exciting thing about this,” said Dr. Kahana, the University of Pennsylvania professor, “is that it gives us direct biological evidence of what before was almost entirely theoretical.”