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The
aging brain: Failure to communicate
A team of Howard Hughes Medical Institute
researchers has shown that normal aging
disrupts communication between different
regions of the brain. The new research,
which used advanced medical imaging
techniques to look at the brain function of
93 healthy individuals from 18 to 93 years
old, shows that this decline happens even in
the absence of serious pathologies like
Alzheimer's disease.
Researchers have known for quite some time
that normal aging slowly degrades bundles of
axons in the central nervous system that
transmit critical signals. “Our study now
shows that cognitive decline in aging may be
linked to disruption of communication
between different regions of the brain,”
said Buckner, who is a Howard Hughes Medical
Institute investigator at Harvard
University.
The new research, published December 6,
2007, in the journal Neuron, begins to
reveal how simply growing old can affect the
higher-level brain systems that govern
cognition. “We may have caught the failure
of communication in the act,” said Buckner.
The human brain can be divided into major
functional regions, each responsible for
different kinds of “applications,” such as
memory, sensory input and processing,
executive function or even one’s own
internal musing.
The functional regions of the brain are
linked by a network of white matter
conduits. These communication channels help
the brain coordinate and share information
from the brain’s different regions. White
matter is the tissue through which messages
pass from different regions of the brain.
Scientists have known that white matter
degrades with age, but they did not
understand how that decline contributes to
the degradation of the large-scale systems
that govern cognition.
“The crosstalk between the different parts
of the brain is like a conference call,”
said Jessica Andrews-Hanna, a graduate
student in Buckner's lab and the lead author
of the study. “We were eavesdropping on this
crosstalk and we looked at how activity in
one region of the brain correlates with
another.”
Buckner, Andrews-Hanna, and their colleagues
looked at crosstalk in the brains of 93
people aged 18 to 93, divided roughly into a
young adult group (18-34 years old) and an
old adult group (60-93 years old). The older
participants were given a battery of tests
to measure their cognitive abilities --
including memory, executive function and
processing speed. Each person was studied
using functional magnetic resonance imaging
(fMRI) exams to measure activity in
different parts of the brain. fMRI can
precisely map enhanced blood flow in
specific regions of the brain. Increased
blood flow reflects greater activity in
regions of the brain that are utilized
during mental tasks.
For the task used in the Neuron study,
subjects were presented words and were asked
to decide whether each word represented a
living (e.g., dog) or nonliving (e.g.,
house) object. “Such a task requires the
participants to meaningfully process the
words,” said Buckner.
Buckner's group explored whether aging in
the older group caused a loss of correlation
between the regions of the brain that -- at
least in young adults -- engage in robust
neural crosstalk.
They focused on the links within two
critical networks, one responsible for
processing information from the outside
world and one, known as the default network,
which is more internal and kicks in when we
muse to ourselves. For example, the default
network is presumed to depend on two regions
of the brain linked by long-range white
matter pathways. The new study revealed a
dramatic difference in these regions between
young and old subjects. “We found that in
young adults, the front of the brain was
pretty well in sync with the back of the
brain,” said Andrews-Hanna. “In older adults
this was not the case. The regions became
out of sync and they were less correlated
with each other.” Interestingly, the older
adults with normal, high correlations
performed better on cognitive tests.
According to Buckner, it is inferred that in
a young, healthy brain, signals are readily
transmitted by white-matter conduits. As we
age, those conduits are compromised.
“Measures of white matter integrity in the
older adults point to decline,” he said.
Depending on the networks at play, the
result may be impaired memory, reasoning or
other important cognitive functions.
Buckner and Andrews-Hanna emphasized that
other changes in the aging brain may
contribute to cognitive decline. For
example, cells’ ability to express chemical
neurotransmitters may also be compromised.
In general, the new work promises a better
physiological understanding of cognitive
decline in the elderly and may help explain
differences among individuals. “It may help
explain why some people are just as sharp in
their 90s as they were in their 40s,” noted
Andrews-Hanna. “We all age differently and
cognitive abilities vary considerably among
individuals.”
Typically, said Buckner, as individuals get
into their 70s and 80s, you see some degree
of change. “We can use this new approach
(correlating the activities of different
regions of the brain) as a tool to
understand variation between individuals. We
can also explore risk factors for breakdowns
(in these pathways) like cardiovascular
health.”
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