Memories Reside In Same Cells That Fire When An Experience Is Registered Times of India, September 6, 2008
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 also, in part, how the brain is able to recreate it.
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. Researchers had long theorized as much but until now had only indirect evidence.
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 in the journal Science, is likely to open a new avenue in the investigation of Alzheimer's disease and other forms of dementia, as well as help explain how some memories seemingly come out of nowhere. The scientists were even able to identify memories in subjects a second or two before the people themselves reported having them.
“This is what I would call a foundational finding,” said Michael Kahana, a professor of psychology at the University of Pennsylvania, who was not involved in the research. “I cannot think of any recent study that's comparable.
“It's a really central piece of the memory puzzle and an important step in helping us fill in the detail of what exactly is happening when the brain performs this mental time travel” of summoning past experiences.
The study moved beyond most previous memory research in that it focused not on recognition or recollection of specific symbols but on free recall - whatever popped into people's heads when, in this case, they were asked to remember short film clips they had just seen.
This ability to richly reconstitute past experience often quickly deteriorates in people with Alzheimer's and other forms of dementia, and it is fundamental to so-called episodic memory - the catalog of vignettes that together form our remembered past.
The team of researchers from US and Israel threaded tiny electrodes into the brains of 13 people with epilepsy. The electrode implants are standard procedure in such cases, allowing doctors to pinpoint the location of the ministorms of brain activity.
The patients watched a series of 5- to 10-second film clips, some from TV shows like “Seinfeld” and others depicting animals or landmarks. The researchers recorded the firing activity of about 100 neurons per person; the recorded neurons were concentrated in and around the hippocampus, a sliver of tissue deep in the brain critical to forming memories.
In each person, the researchers identified 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. After briefly distracting the patients, the researchers asked them to think about the clips 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 the Simpsons - the same cells that were active during the Homer clip reignited. The cells became active a second or two before people were conscious of the memory.
The study suggests that at least some of the neurons that fire when a distant memory comes to mind are those that were most active back when it happened, however long ago that was.
