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By
Michael Purdy
June
29, 2007 -- Stepping out of your own head might seem
like the last thing one would want to do to avoid tripping
and falling, but neuroscientists who study the brain's
navigation and orientation systems recognize this change
of perspective as a necessity. To successfully orient
yourself and move about the environment, you have to
look at the world both from the viewpoint of your own
sensory organs, which are fixed in your head and body,
and from the viewpoint of your relationship to the space
around you and to the force of gravity.
Now
researchers at Washington University School of Medicine
in St. Louis have identified the brain cells that mathematically
massage sensory data to allow us to perform this tricky
perspective shift. Using an animal model, they directly
recorded Purkinje cells in the cerebellar cortex changing
sensory data to transform it to a point of view that
encompasses the broader world around us.
"If
we couldn't consider this non-egocentric point of view,
we'd be clueless as to how to approach or avoid objects
in our environment," says study coauthor J. David
Dickman, Ph.D., professor of neurobiology. "We
would, for example, be unable to separate whether we
are approaching a train, or a train is approaching us.
Obviously it is to our advantage to be able to tell
the difference."
Loss
of orientation is associated with several disorders,
including brain damage from stroke, cancer and Alzheimer's
disease. By studying how the healthy brain handles the
challenges of orientation and navigation, scientists
hope to lay the groundwork that will one day permit
better understanding of how those systems go awry and
what medical science can do about it.
The
paper appears in Neuron.
Senior
author Dora Angelaki, Ph.D., Alumni Endowed Professor
of Neurobiology, notes that the brain unconsciously
performs many complex mathematical calculations. Confronting
some of those same math problems with our conscious
brain would leave many of us stumped, but the subconscious
brain easily handles them on a daily basis to enable
such basic tasks as walking across a room or lying down
in bed.
"We
don't appreciate that the brain's navigation system
does all of this math, but we know right away when the
system stops working," Angelaki says. "We
get disoriented, we get motion sickness and we know
very quickly that something is wrong."
In
a primate model, Angelaki's lab studies how the brain
uses sensory inputs from the vestibular system in the
inner ear to establish balance and orientation. Using
implanted electrodes in the brain, they have tracked
these signals through the brainstem at the base of the
brain and into the cerebellum, a region just above the
brain stem that handles sensory inputs and motor controls.
As
researchers tracked the signals' paths, they looked
for signs that cells were transforming the data from
a self-centered point of view to the broader perspective.
In various regions, they saw signs that some brain cells
were doing the calculations. But they had never previously
identified a region where the data was uniformly transformed.
For
the new study, Angelaki and her colleagues looked at
Purkinje cells, a special group responsible for conveying
all output signals from the cerebellum to the rest of
the brain. They found the cells uniformly and "elegantly"
transformed the sensory data to put it in the context
of the broader point of view.
For
follow-up, scientists plan to study the Purkinje cells
and their connections in the cerebellum in closer detail.
They hope to learn more about how the brain circuits
that handle these transformations are engineered.
"Another
way we'd like to continue our studies is to look at
how visual stimuli affect the processing of these signals,"
Angelaki says. "We've been doing our tests in the
dark to isolate the signal from the vestibular organs,
but a more natural way to do it will be to include visual
stimuli as well."
Yakusheva
TA, Shaikh AG, Green AM, Blazquez PM, Dickman JD, Angelaki
DE. Purkinje cells in posterior cerebellar vermis encode
motion in an inertial reference frame. Neuron, June
21, 2007.
Funding
from NASA and the National Institutes of Health supported
this research.
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