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Size of brain structure could signal
vulnerability to anxiety disorders
Area appears smaller in those that
continue to react to images associated with discomfort
BOSTON - July 11, 2005 - The size of a particular structure
in the brain may be associated with the ability to recover emotionally
from traumatic events. A new study by researchers from Massachusetts
General Hospital (MGH) finds that an area called the ventromedial
prefrontal cortex is thicker in volunteers who appear better able
to modify their anxious response to memories of discomfort. The
report will appear in the Proceedings of the National Academy
of Science and has received early online release on the PNAS
website.
"We've always wondered why some people who are exposed to traumatic
experiences go on to develop anxiety disorders like post-traumatic
stress disorder and others do not," says Mohammed Milad, PhD,
a research fellow in the MGH Department of Psychiatry, the study's
lead author. "We think this study provides some potential answers."
In the classical model of conditioned fear, individuals respond
with physical and emotional distress to situations that bring back
memories of traumatic events. Such responses are normal and usually
diminish over time, as those situations are repeated without unpleasant
occurrences. But some people continue to respond with what can be
overwhelming fear and may develop post-traumatic stress disorder
(PTSD).
For example, it would not be unusual for a soldier who experienced
a traumatic battlefield situation to become distressed when hearing
noises that bring back those memories, such as the sound of a helicopter.
Most commonly, repeated exposure to such sounds without additional
trauma reduces or extinguishes the fearful response - a phenomenon
called "extinction memory." But some individuals continue
to experience anxiety, along with other symptoms characteristic
of PTSD, when hearing the sounds.
Prior studies in animals have suggested that the ventromedial prefrontal
cortex (vmPFC) - an area on the lower surface of the brain - may
be involved in extinction memory. The vmPFC may help to quell potential
fears by inhibiting the activity of the amygdala, an area known
to be involved with fear. The current study was designed to see
if the structure of the vmPFC is related to the ability to modify
response to an unpleasant memory.
Over a period of two days, 14 volunteer study participants viewed
a series of digital photos of two different rooms. Each room contained
a lamp that was turned on - sometimes with a red light, sometimes
a blue light. On the first day, participants viewed the photos several
times, and then viewed them again with a mild electric shock - described
as annoying but not painful - delivered to their hands right after
a lamp with a blue light appeared. They then viewed a series of
the photos with no shocks administered.
On the second day, measurements of skin conductance were taken while
the volunteers once again viewed the photos with both colors of
lights displayed but no shocks given. A measurement of anxiety level,
skin conductance is determined by the amount of perspiration on
the palm of the hand. After that part of the experiment, the volunteers
had structural magnetic resonance (MR) images taken of their entire
brains.
The MR studies showed that those participants who appeared to have
less anxiety response upon viewing the blue lights the second day,
as measured by skin conductance, also had a thicker vmPFC. "That
was the only area of the brain that correlated with extinction memory,"
says Milad. "So, these results suggest that a bigger vmPFC
may be protective against anxiety disorders or that a smaller one
may be a predisposing factor. But exactly how that might work we
just don't know."
Scott Rauch, MD, the senior author of the paper and director of
the Psychiatric Neuroscience Research Division in MGH Psychiatry,
notes that future research could look at genetic or environmental
factors that may underlie these differences in brain structure and
also investigate whether vmPFC size predicts the success of exposure-based
therapies for anxiety disorders. Another factor to study would be
whether vmPFC measurement should be used to screen those likely
to be exposed to traumatic situations or to develop preventive strategies.
Rauch is an associate professor of Psychiatry at Harvard Medical
School.
The report's co-authors are Roger Pitman, MD, of MGH Psychiatry;
Brian Quinn and Bruce Fischl, PhD, of MGH Radiology, and Scott Orr,
PhD, of the VA Medical Center in Manchester, N.H. The study was
supported by grants from the National Institute of Mental Health
and the MGH Tosteson Fellowship.
Massachusetts General Hospital, established in 1811, is the original
and largest teaching hospital of Harvard Medical School. The MGH
conducts the largest hospital-based research program in the United
States, with an annual research budget of more than $450 million
and major research centers in AIDS, cardiovascular research, cancer,
cutaneous biology, medical imaging, neurodegenerative disorders,
transplantation biology and photomedicine. In 1994, MGH and Brigham
and Women's Hospital joined to form Partners HealthCare System,
an integrated health care delivery system comprising the two academic
medical centers, specialty and community hospitals, a network of
physician groups, and nonacute and home health services.
Media Contact: Sue
McGreevey, MGH Public Affairs
Physician Referral Service: 1-800-388-4644
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