The short-term memory problems that accompany
normal aging are associated with an inability to filter out surrounding
distractions, not problems with focusing attention, according to a study
by researchers at the University of California, Berkeley.
Although older patients often report difficulty tuning out distractions,
this is the first hard evidence from functional magnetic resonance imaging
(fMRI) studies of the brain that memory failure owes more to interference
from irrelevant information than to an inability to focus on relevant
information.
" Difficulty filtering out distractions impacts a wide range
of daily life activities, such as driving, social interactions and
reading,
and can greatly affect quality of life," said study leader
Dr. Adam Gazzaley, adjunct assistant professor of neuroscience at UC
Berkeley
and a newly appointed assistant professor of neurology and physiology
at UC San Francisco.
" These results reveal that efficiently focusing on relevant
information is not enough to ensure successful memory," he
said. "It
is also necessary to filter distractions. Otherwise, our capacity-limited
short-term memory system will be overloaded."
The finding could mean that an inability to ignore
distracting information is at the heart of many cognitive problems accompanying
aging, Gazzaley
said, and suggests that drugs targeting that problem may be more effective
at improving memory than drugs that improve focusing ability.
He now
is exploring the therapeutic role of different medications - including
one of the main drugs to treat Alzheimer's disease - in older individuals
with suppression deficits.
Because Gazzaley and his colleagues have identified areas of the brain
that are markers for focusing and ignoring visual information, fMRI may
be a good tool for assessing the value of therapies designed to improve
memory and for diagnosing attention and memory problems in young and
old, ranging from attention deficit disorder to dementia.
"Is this a unifying mechanism that can account for broader
problems regarding attention and memory? "
asked coauthor
Dr. Mark D'Esposito, UC Berkeley professor of neuroscience and psychology
and director of
the campus's Henry Wheeler Brain Imaging Center.
"I think it explains
a lot of it. If you are unable to block out distracting information,
you can't really attend to what you are supposed to attend to, you can't
get in what you are supposed to remember, and you have a hard time retrieving
what you are supposed to remember. Rather than think of it as someone
having an attention problem and a memory problem, you can just think
of it as someone having one problem - the inability to filter out distracting
information - that's affecting other domains such as attention and memory."
Gazzaley, D'Esposito, research assistant Jeffrey W. Cooney and graduate
student Jesse Rissman will report their findings in the journal Nature
Neuroscience, to be published online Sept. 11.
Gazzaley and his colleagues compared young adults aged 19 to 30 with
older adults aged 60 to 77 using a simple memory test that introduced
irrelevant information. The tests were conducted while subjects' heads
were inside a fMRI scanner so that activity in the brain could be pinpointed.
While young subjects were easily able to suppress brain activity in
areas that process information irrelevant to the memory task, older adults
on average were unable to suppress such distracting information. Both
groups were equally able to enhance brain activity in the areas dealing
with information relevant to the task.
Interestingly, six of the 16 older adults had well-preserved short-term
memory and no problems ignoring irrelevant information, suggesting that
some people are able to avoid memory loss as they age. Gazzaley hopes
to find out what makes these people different from the average aging
adult.
"Encouragingly, a subgroup of the older population does not
experience this suppression deficit and accompanying memory impairment,
opening
the road for studies of successful aging," Gazzaley said.
Gazzaley, a neurologist who specializes in treating mild cognitive impairment
common in older adults, set out to see how attention affects short term
or "working" memory. He developed a test to distinguish two
aspects of attention: the brain's ability to focus on a visual stimulus,
and the ability to suppress or ignore other visual information. He noted
that both involve brain activity in the higher level neocortex, acting
on the visual cortex - a process he refers to as "top-down modulation."
The test involves presenting a sequence of four images, two of them
faces and two natural scenes. Subjects were asked to remember either
faces, in which case the scenes were irrelevant information; or scenes,
in which case faces were irrelevant. Subjects then were asked whether
a particular face or scene appeared among the four images. In a separate
test, subjects were asked only to observe the stimuli without attempting
to remember them.
After first identifying with the fMRI the regions in the brain attentive
to faces and scenes (they differ slightly in each individual), Gazzaley
presented his subjects with the three tests and recorded brain images
in each case.
When asked to remember faces, young adults showed enhanced activity
in the brain area dealing with faces and decreased activity in the area
dealing with scenes (the parahippocampal/lingual gyrus). Similarly, when
asked to remember scenes, they showed enhanced activity in the scene
area of the brain and suppressed activity in the area dealing with faces.
Older adults, however, while showing comparable enhancement of the face
area when asked to concentrate on faces, exhibited poor or no suppression
of the scene area, and vice versa.
"These data suggest that older individuals are able to focus
on pertinent information, but are overwhelmed by interference from
failing
to ignore distracting information, resulting in memory impairment," the
authors wrote.
D'Esposito said that the technique Gazzaley developed to probe focusing
and ignoring ability opens the door to numerous experiments that could
shed light on a popular theory today - that problems of aging have to
do with a decline in the brain's frontal lobe.
"The frontal lobes are the highest level of cognition and the
area that integrates information from all over the brain," he
said.
" If
you look at the frontal lobes over time, that is the area where there
is more decline than any other part of the brain."
To shed light on this hypothesis, Gazzaley and D'Esposito plan to look
at patients with known or presumed frontal lobe damage, to see if they
also have problems with focusing or ignoring. Also, they plan to look
at people with attention deficit disorder, addiction problems, and mild
cognitive impairment in search of evidence that these problems too are
due to dysfunction of the frontal lobe.
" There may be unknown lesions in the frontal lobe that affect
attention," Gazzaley
said.
" Aging is not a disease, but I think there likely is a
problem with top-down control that could be fixed with drugs."
" If
aging is a frontal lobe dysfunction, it is a mild form of it," D'Esposito
said.
" And if we learn something about it, then
we may be able to help and know more about patient populations that have
a more severe
form of frontal lobe damage, like traumatic brain injury and strokes
and dementia."
The work was funded by grants to Gazzaley and D'Esposito from the National
Institute on Aging of the National Institutes of Health and by the American
Federation of Aging Research.
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