Where the Brain Monitors the Body
As any klutz will attest, coordination is complicated. Just to keep
track of their limbs, for example, people and animals use information
from several senses, such as vision, touch, and proprioception, which
tells them their body's position. Indeed, large portions of the brain
are devoted to keeping track of these sensations and dictating the
body's movements. "As you interact with the world, you need constant
information about where the body is," says neurophysiologist Lawrence
Snyder of Washington University in St. Louis. Researchers haven't
known exactly where all those signals are integrated, but now a team
may have located some of the neurons that first make these
On page 1782, a team led by psychologist Michael Graziano of Princeton
University reports evidence that a small region of the parietal cortex
of the monkey brain known as area 5 may enable the monkey to integrate
many sources of information about its body and thereby update its
mental model of what the body is doing. The researchers based this
conclusion on their finding that some area 5 neurons fire at their
fastest rates when the visual feedback from a monkey's arm matches the
sensory feedback, an indication that the neurons are sensitive to both
streams of information.
Neuroscientists had suspected for some time that parts of the parietal
cortex, located below the crown of the head, might be involved in
maintaining a coherent representation of the body. One indication of
this came from instances in which people with damage in the parietal
cortex fail to recognize one of their limbs. Such patients might wake
up startled, thinking someone put a fake leg in the bed.
Mismatched. Neurons in area 5 aren't fooled by flipped arms.
CREDIT: M. GRAZIANO ET AL.
Graziano and his colleagues were inspired to look for multisensory
neurons a few years ago when they uncovered "roundabout evidence" that
neurons in another movement area, called the premotor cortex, are
sensitive to both vision and proprioception. If neurons in areas that
process the body's movement and sensations also respond directly to
vision, they reasoned, such neurons might be key to integrating the
different kinds of signals that provide a coherent model of the
body. Graziano and colleagues then decided to track down where this
integration starts--where in the brain's body-sensory system vision
first makes an appearance.
To do this, the researchers devised a technique for giving a monkey
information from both vision and proprioception; this would enable the
researchers to identify neurons that are sensitive to whether the
information matches. After fitting the monkey with a long collar that
restricts its near-body vision, the researchers hide one of the
animal's arms beneath a shallow ledge. They then place a realistic,
stuffed monkey arm or other objects on top of the ledge, either in the
same position as the hidden arm or on the other side of the
body. Because of the collar, the fake arm might appear, from the
monkey's perspective, to be coming from its own body.
When the researchers recorded the responses of single neurons in area
5 of the monkey's brain, they found cells that are sensitive to
whether the sight of a fake arm matches the feel of its real
arm. Neurons that respond to one arm didn't change their firing rate
when the researchers placed apple slices on the ledge or lined the
fake arm up with the monkey's other, also hidden, arm. But when the
fake arm was aligned in the same position as the real, hidden, arm,
29% of the neurons changed their firing rate.
What's more, these neurons weren't fooled by mismatched arms:
Right-arm-sensitive neurons didn't fire strongly when a fake left arm
was put in the right arm's place; likewise, no neurons ramped up their
firing if the fake arm was placed in a different position than the
real arm, say, with the palm near the animal's body rather than the
shoulder. And neurons upstream of area 5--those that participate in
earlier stages of body-sensation processing--didn't respond to the
fake arm at all, suggesting that area 5 is the first to integrate
different streams of input.
Earlier research had shown that area 5 responds to proprioceptive
signals, says Snyder, but this new result suggests that "the
information processed by area 5 is more multisensory, more abstract"
than simple proprioception. And if area 5 neurons integrate signals
from many channels, Snyder says, they might be the first stages of a
"representation of where the body is in space."
Related articles in Science:
Coding the Location of the Arm by Sight.
Michael S. A. Graziano, Dylan F. Cooke, and Charlotte S. R. Taylor
Science 2000 290: 1782-1786. (in Reports) [Abstract] [Full Text]