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Where the Brain Monitors the Body

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  • Eugene.Leitl@lrz.uni-muenchen.de
    http://www.sciencemag.org/cgi/content/full/290/5497/1668a NEUROSCIENCE: Where the Brain Monitors the Body Laura Helmuth As any klutz will attest, coordination
    Message 1 of 1 , Dec 1, 2000

      Where the Brain Monitors the Body

      Laura Helmuth

      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
      multisensory connections.

      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.


      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]
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