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What's
New:
Brain's
vision center plays surprising role in movement, self-perception
June/July
2004 — Pointing at an object may not seem complicated,
but even such a simple act requires an intricate network
of brain activity. Scientists traditionally thought
this network included a one-way "information highway"
between the brain's visual system and its motor and
sensory systems, but research at Washington University
School of Medicine in St. Louis now challenges this
long-held theory.
The
study presents surprising evidence that the brain's
visual system is not only responsible for seeing, or
perceiving, objects outside the body, but also is involved
when individuals sense and manipulate their own bodies.
Such
insight may help scientists understand puzzling disorders
like anosognosia, which is characterized by unusual
perceptual experiences. For example, individuals with
this disorder may not recognize their arms as part of
their own bodies.
"Vision
apparently is far more complicated and integrated than
we suspected," says Maurizio Corbetta, M.D., associate
professor of neurology, of radiology and of anatomy
and neurobiology. "Areas thought to be exclusively
involved in perceiving the world around us apparently
are involved in integrating visual, spatial and sensory-motor
signals to help each of us develop an internal representation
of our body and its position in space."
Corbetta,
who also is head of stroke and brain injury rehabilitation
at the School of Medicine and the Rehabilitation Institute
of St. Louis, led the study, which was published in
the May 2004 issue of the journal Nature Neuroscience.
The first author is Serguei Astafiev, Ph.D., research
associate in radiology. Gordon Shulman, Ph.D., staff
scientist in neurology, also is an author.
When
a person points at an object, the brain has to see the
object, determine where the person and the object are
in space, command the arm to move in the correct direction
and coordinate muscles to execute that directive.
According
to the traditional theory, the visual system, located
in the back of the brain, perceives an object in space
and sends that visual information to the brain areas
that control movement, located toward the front of the
brain. Those areas then command the body to move and
adjust those actions based on sensory feedback.
Corbetta
and his colleagues were trying to determine which parts
of the brain are involved in planning an action in space
when they discovered unexpected activity in a peripheral
part of the visual system known as the extrastriate
body area (EBA). This region recently had been implicated
in the perception of other people's bodies, but there
was no evidence that it also was involved in an individual's
perception of self.
They
decided to investigate further. Using functional magnetic
resonance imaging (fMRI), the researchers took brain
scans of volunteers doing one of three tasks. Each task
started out the same: Participants stared at a fixation
point in the center of a diamond shape on a computer
screen. Then either the left or the right half of the
diamond briefly flashed, followed by an asterisk on
the same side of the screen.
Instructions
on what to do when the asterisk appeared were different
for the three tasks. In the first condition, participants
were asked to notice the asterisk when it appeared,
but to continue fixating on the center of the screen;
in the second task, participants were asked to look
at the asterisk when it appeared, using the flash of
light beforehand to prepare to move their eyes; in the
third task, participants pointed to the asterisk, again
using the brief flash to get ready to make their response.
The
team found that the EBA was more active during the pointing
task than when participants either looked at or noticed
the asterisk, suggesting the EBA plays a role in planning
and executing the pointing motion.
"We
were very surprised by these findings," Corbetta
says. "Visual areas are not supposed to be involved
in actions. We always thought information travels from
the back of the brain, where vision is processed, to
the front of the brain, where actions originate. Moreover,
it is quite intriguing that an area involved in coding
other people's bodies also responds when you move your
own body. Some researchers have speculated that the
EBA is where a distinction between others and ourselves
is beginning to be coded in the brain."
To
make sure this phenomenon did not result from the fact
that participants could see their hands moving during
the pointing task, a second group of volunteers performed
two of the original three tasks — either pointing to
the asterisk or simply noticing it. This time, though,
participants were situated in the scanner so that they
could not see their hands move.
Even
without visual feedback, the EBA was significantly more
active during the pointing task than during the attention
task, and the effect did not depend on whether volunteers
used their right or left hand. When participants were
asked to point to the asterisk with their feet instead
of their hands, the EBA also became more active than
during the attention task, though not as much as in
the hand-pointing task.
The
EBA also was more active when participants pointed at
the asterisk than when they prepared themselves to point
at it, demonstrating that EBA activity was modulated
specifically by action, not by mental imagery.
"Showing
that EBA activity is present even when people don't
see their hands or their feet is the key part of this
study, illustrating that this is not a perception effect
but rather is the result of a motor or sensory signal
going back to the visual system," Corbetta explains.
Activity
induced by moving a body part or the eyes also was found
in two regions, the calcarine sulcus and lingual gyrus,
which are considered part of the "primary"
and "secondary" visual areas located even
deeper in the visual system. The fact that areas at
the heart of the visual system are activated during
tasks involving movements of the arms or eyes was surprising.
"Our
paper demonstrates for the first time that visual areas
are influenced by the execution of motor actions, even
in the absence of visual feedback," Astafiev says.
"That should help us understand how the brain performs
goal-directed behavior."
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Astafiev
SV, Stanley CM, Shulman GL, Corbetta M. Extrastriate
body area in human occipital cortex responds to the
performance of motor actions. Nature Neuroscience, vol.
7(5), May 2004.
Funding
from the National Institutes of Health supported this
research.
The
full-time and volunteer faculty of Washington University
School of Medicine are the physicians and surgeons of
Barnes-Jewish and St. Louis Children's hospitals. The
School of Medicine is one of the leading medical research,
teaching and patient care institutions in the nation,
currently ranked second in the nation by U.S. News &
World Report. Through its affiliations with Barnes-Jewish
and St. Louis Children's hospitals, the School of Medicine
is linked to BJC HealthCare.
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