For the Brain, Remembering Is Like Reliving
By BENEDICT CAREY
Published: September 4, 2008

http://www.nytimes.com/2008/09/05/science/05brain.html?
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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 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 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.

"This is what I would call a foundational finding," said Michael J.
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 new 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.

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, a sliver of
tissue deep in the brain known to be critical to forming memories.

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."