How the monkey brain assimilates a virtual limb into its body map | Mo Costandi | ...
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How the monkey brain assimilates a
virtual limb into its body map
New research reveals surprising details about the cellular basis of
the rubber hand illusion
Monkeys can
incorporate a computer-generated limb into their body map. Image: Katie Zhuang/
Duke University
About a hundred years ago, the great neurologist Henry Head suggested that the brain
contains maps of the body, and that these maps – which he referred to as 'schemata' –
can expand to incorporate clothes, tools and other objects. "Anything which participates
in the conscious movement of our bodies," he wrote in a classic 1911 paper, "is added to
the model of ourselves and becomes part of these schemata: a woman's [schemata] may
extend to the feather in her hat."
Head's idea was very prescient; it has been confirmed by modern research which shows
that the brain's representation of the body is indeed highly malleable. This work shows
that the body map can be temporarily altered by tool use, distorted by various drugs and
diseases, and deliberately manipulated in illusions of bodily awareness. A study
published today now reveals how the activity of individual brain cells changes as
external objects are assimilated into the body map.
One of the most striking examples of body map's malleability is the so-called rubber
hand illusion, which was first demonstrated in 1998. As the film clip below shows, the
illusion can be induced by simple manipulations of the sensory information entering the
brain, which create a discrepancy between vision and touch that tricks the brain into
treating an artificial hand as a real body part, or taking 'ownership' of it.
The new study, led by Solaiman Shokur of L'Ecole Polytechnique Fédérale de Lausanne
(EPFL) in Switzerland, involved inducing a virtual reality version of the rubber hand
illusion in two macaque monkeys. Working in collaboration with Miguel Nicolelis and
his colleagues at Duke University, Shokur strapped the monkeys into a customized
chair, with their arms were hidden from view under a large screen. The screen displayed
a pair of computer-generated arms, whose size and position closely resembled those of
the monkeys' real limbs.
http://www.theguardian.com/science/neurophilosophy/2013/aug/26/monkey-rubber-h... 26/08/2013
How the monkey brain assimilates a virtual limb into its body map | Mo Costandi | ...
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Thus, the researchers attempted to induce the illusion in the animals, by using a brush
to stroke their real arms in a way that corresponded exactly with the 'virtual' touches of
a ball on the computer-generated limbs. During some trials, the brush strokes did not
correspond to the virtual touches – a condition that would not induce the illusion – and
during yet others, they applied the virtual touch, or the physical touch, alone.
Throughout the experiments, they also recorded the activity of dozens of individual
neurons, using microelectrode arrays implanted into the somatosensory cortex, which
receives touch information from the body surface, and the motor cortex, which sends
movement signals to the muscles.
The researchers found that neurons in both brain regions eventually responded to the
virtual touch alone, after several minutes of synchronous virtual and physical touch.
After a period of asynchronous virtual and physical touch, however, far fewer of the cells
responded, and of those that did, their responses were much smaller. This suggests that
the monkeys did in fact experience something like the rubber hand illusion, although it's
impossible to know if they experienced it in exactly the same way that a human would.
It's not entirely surprising that the animals experienced this virtual rubber hand
illusion. Two years ago, the same researchers reported that monkeys can use a brainmachine interface to control a virtual arm, which suggests that they readily incorporated
the computer-generated limb into their body map. What is surprising is the finding that
neurons in the somatosensory and motor cortices responded to the visual stimuli of the
virtual touch. These responses occurred 50 – 70 thousandths of a second later than
those to physical touch alone, a timescale that is consistent with information from the
visual areas at the back of the brain being transmitted to the somatosensory and motor
areas via multiple connections.
Previous work on the rubber hand illusion and related phenomena clearly shows that
the brain generates its body map by a process called multisensory integration, by which
visual information is combined with touch and proprioceptive (or muscle sense)
information. The rubber hand illusion also shows that visual information is far more
important than information from these other types of senses, and can override them to
alter the state of the body map.
This study suggests a potential mechanism by which it does so, as it provides evidence
that vision can directly modulate the activity of neurons that are specialized to process
information about touch and vision. As well as contributing to our understanding of the
mechanisms of body map formation and alteration, the findings will also be useful to
researchers who are developing advanced prosthetic limbs which feel more realistic
because they are fully incorporated into the body by the brain.
Reference: Shokur, S., et al. (2013) Expanding the primate body schema in
sensorimotor cortex by virtual touches of an avatar. PNAS, doi:
10.1073/pnas.1308459110
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