Brain regions
influence our memories, scientists say
BY KATHLEEN O'TOOLE
Neuroscientists at
Stanford and Harvard have been able to show that neural
activity in certain brain regions predicts what
experiences will be remembered later. The study at
Stanford involved memory for scenic photos while the
study at Harvard involved memory for words. Both are
reported in the Aug. 21 issue of the journal Science,
along with a commentary article about them.
A view of
the front one-third of the brain (from the back of the
head) shows the areas where the level of brain activity
is greater when a person is looking at a scenic
photograph that will be remembered well than when looking
at a photograph that will be forgotten or remembered
vaguely. The activated areas are the right frontal lobe
on the upper right and the parahippocampal cortex on the
lower left.
Graphic
by James Brewer
Scientists have long
suspected that people remember some things better than
others partly because of differences in the way the
initial experience is encoded into the brain. These are
the first studies to show which parts of the brain
determine whether a specific, current experience is fated
to be remembered or forgotten.
Both studies took
advantage of new developments in functional magnetic
resonance imaging to obtain measures of the neural
activity of volunteers as they viewed words or
photographs, one at a time. (Only recently has the
temporal resolution of imaging been sufficient to allow
brain activity to be measured separately for each picture
or word.) Previous studies had indicated more neural
activity when volunteers were presented with novel photos
than when they were presented repeatedly with the same
ones, but the earlier work had not determined the quality
of their later memories for the photos.
The research at Stanford
was conducted by James Brewer, a graduate student in
neurosciences and in medicine; associate psychology
Professor John Gabrieli; radiology Professor Gary Glover;
and research associates Zuo Zhao and John Desmond.
Gabrieli said that people
with memory difficulties as a result of brain injuries or
diseases like Alzheimer often have vivid memories for
events early in their lives but do not remember recent
experiences.
"This gives us the
tool to ask whether such selective problems in memory for
recent events occurs due to poor initial encoding the
memory is never formed in the first place or later
difficulties of storing or using the memories," he
said.
In the Stanford study
healthy volunteers were shown a group of photos of indoor
and outdoor scenes and then a larger group of photographs
about a half hour later, Brewer said. They were asked to
identify which photos they clearly remembered seeing
earlier, which ones looked familiar but that they weren't
certain they had seen earlier, and which ones they did
not recall seeing at all.
"Several regions of
the brain are active when one processes a photograph,
such as visual areas and areas that allow a person to
make sense of the picture, but the level of activity in
most of those regions does not predict whether the
photograph will be remembered or not," Brewer said.
Activity in one region of
the right prefrontal lobe and several areas on the right
and left inner aspects of the temporal lobes called
the medial temporal lobe system did predict whether
the individuals would later remember the picture very
well, somewhat or not at all, he said. The area in the
right frontal lobe is one that is known to be associated
with making sense of spatial relationships.
The largest region of
activation occurred in the parahippocampal cortex, a
major structure in the medial temporal lobe that was
known previously to play a role in memory, based on
studies of animals and of people who suffer brain lesions
or Alzheimer disease, Brewer said. Scientists did not
know before, however, if this region was involved in
encoding information into the memory, or if it was
involved in storing it or retrieving it later.
The individuals in this
study varied greatly on which pictures they remembered
well, but the activity levels in each person's brain at
the time a picture was viewed predicted if he or she
would remember that picture later, Gabrieli said.
"It's interesting to
speculate about what could be driving the difference in
the activation and the correlating differences in
memory," Brewer said. "Perhaps one person would
see a photograph of Zion National Park and think, 'Hey, I
just visited that place on my way to California!' while
another would think, 'Outdoor desert scene. . . What's
the next scene going to be?' " The first person's
memory of the picture would likely be stronger, he said.
In the Harvard study, led
by Anthony Wagner, a recent Stanford graduate who worked
in Gabrieli's lab, activity in several regions of the
left prefrontal cortex predicted subsequent memory for
words. In their Science report, the Harvard
researchers said they found some of these areas also
showed greater activity when people analyzed words on the
basis of their meaning rather than their appearance, such
as whether the letters were in upper or lower case. This
suggests that part of the brain is encoding for meaning
at the time of the experience.
The research might lead
someday to better treatment of Alzheimer disease, Brewer
said. "Alzheimer disease ravages the medial temporal
lobe. We hope that these findings might give us a tool to
examine the very earliest effects of Alzheimer disease so
that any treatment that is employed will merely have to
spare the neurons that are at risk, rather than replace
the ones that have been destroyed."
Gabrieli said one of the
next steps is to "apply these new techniques to
older people at risk for Alzheimer disease and see if
they predict who will and will not get the disease."
The research might be
applied in other ways, he said. "This may give us a
new tool to objectively measure what is memorable for the
brain, and that could inform everything from curriculum
development in schools to advertising." SR
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