Scientists Discover How the Brain Tracks Limbs in Space

European scientists discovered the part of the brain in charge of
tracking the limbs’ position in space. The parietal cortex provides the
sensory information necessary for the brain to construct a complex projection
of the body’s position within that space.

  Patrick Haggard
  Patrick Haggard

Although the parietal cortex was first identified in the late 19th
century, scientists previously thought it was part of the vision process,
Patrick Haggard, professor at University College London’s (UCL) Institute
of Cognitive Neuroscience, told O&P Business News.

Instead, scientists from UCL, Pompeu Fabra University, ICREA and
University of Barcelona found that its connection to physical space continues
even if the person cannot see the limb. The parietal cortex is responsible for
establishing this “body model” from a combination of tactile
information from the skin and proprioceptive information about the position of
the hand or arm relative to the body, according to a press release.

Haggard and his colleagues created an experiment where they positioned
people so that they were unable to see their arm, and tapped them on both the
arm and on the face, with the arm raised and lowered to various heights. They
asked the participants to determine which of the two touches was higher.

“By moving the arm up and down relative to the face, we forced them
to base their answers not just on the places on the skin where they were
touched, but also on the location of the arm relative to the face,” he

Then, they used a brief pulse of electromagnetic stimulation —
after the arm tap and before the face tap — to disrupt processing in the
posterior parietal cortex, located in the right hemisphere of the brain. This
showed significant impairment in participants’ judgment about the spatial
relationship between their face and arms, but not about their perception of
touch or location alone.

Funded by the Biotechnology and Biological Sciences Research Council
(BBSRC), this experiment just touches the surface. Haggard said that additional
study is needed to determine how much time the brain requires to make this kind
of spatial judgment — to combine information about the touch on the skin
with information about the position of those body parts in space.

  Researchers attached tactile stimulators to study participants’ faces and arms and asked them to determine which touch was higher as their arms were raised and lowered.
  Researchers attached tactile
stimulators to study participants’ faces and arms and asked them to
determine which touch was higher as their arms were raised and lowered.
  Image: University College

He said he hopes that future prosthetic research focuses on that spatial
representation in the brain.

“Add-ons to the body, such as prostheses, need to be integrated
into the brain’s spatial representation of the body to become perceived as
‘natural’ by the user,” he said. “The brain continually
updates the position in space of every body part, so we always know where all
the bits of us are. The sensations that arise in the body are translated into
this spatial form.” — by Stephanie Z. Pavlou


The implications are the same for O&P professionals, amputees and
health care practitioners and these are that this is an interesting basic
science discovery. From the perspective of current clinical practice, it will
have little or no impact. From the perspective of future research it suggests
that peripheral nerve interfaces, as opposed to brain machine interfaces, might
be a better place to locate future prosthesis interface devices, since this
would allow such interfaces to take advantage of the sensory integration
occurring in the parietal cortex. This finding further illustrates that much
goes on in the brain below the level of the motor and sensory cortices where
arrays of electrodes are typically implanted in brain machine interfaces.

— Richard Weir, PhD
Biomechatronics Development Laboratory, Rehabilitation Institute of

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