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Illusion could give prosthetics a sense of touch
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Brain trick makes robot hand feel real Peter Aldhous
A BIZARRE illusion that makes people believe a false hand is part of their own body could be all it takes to imbue prosthetic limbs with a sense of touch. Although sophisticated robotic prosthetics can now replace amputated hands, they don’t yet provide the brain with the sensory feedback vital to control fine movement. Without feeling pressure from the fingertips, for example, an amputee operating a robotic hand could either break a wine glass by grasping it too tightly, or let it fall to the floor by failing to apply enough grip. One potential solution is to wire sensors in robotic fingers directly into nerves in the stump, but this poses some technical challenges. So instead Henrik Ehrsson, a cognitive neuroscientist at the Karolinska Institute in Stockholm, Sweden, decided to see if a trick
ANITA STOCKSELIUS & ANN RAGNÖ
“My first reaction was: they don’t have a hand, so how can the illusion work on amputees?”
known as the “rubber hand illusion” could provide a simpler alternative. The illusion arises from our brain’s attempts to reconcile conflicting information from different senses. If you place a rubber hand in front of a volunteer and stroke it with a brush while simultaneously brushing one of their own hands, hidden from view, it feels as if the sensations are coming from the rubber hand. The volunteer also experiences the eerie feeling that the rubber hand is part of their own body. Ehrsson wondered if he could use the same illusion to “trick” amputees into interpreting strokes applied to their stump as coming from a prosthetic hand. His team recruited 18 amputees who had lost a hand and stroked their stumps, which were hidden from view, for about two minutes, at the same time as a fleshylooking rubber hand. As the rubber and real hands must normally be stroked in the same place, it wasn’t clear if this would be enough to induce the illusion. “My first reaction was: they don’t
have a hand. How can it work?” says Ehrsson. While the illusion was weaker in the amputees than in people with intact hands, tests designed to measure the extent to which people fall for the illusion showed that stroking someone’s stump still works, especially in those who had lost their hands most recently (Brain, DOI: 10.1093/ brain/awn297). The illusion also had physiological effects: once an amputee started viewing the rubber hand as part of their own body, stabbing it with a needle caused a change to their skin’s electrical conductance as they came out in a cold sweat. “They were expecting it to hurt,” Ehrsson explains. Greg Clark, whose team at the University of Utah in Salt Lake City is working on ways to provide
sensory feedback from a robotic hand says: “They got effects from very limited training. That encourages me to believe that the effects would grow larger, if the individual had more experience.” Ehrsson is now working with hand surgeon and neuroscientist Göran Lundborg of Malmö University Hospital in Sweden to apply the illusion to advanced robotic prosthetics. Their goal is to design robotic hands that create the illusory sensations automatically, by connecting sensors in the fingers to actuators that deliver touches to the stump. Still, Clark suspects that it may be difficult to transmit the full range of sensory information to the brain without some direct electrical stimulation of the nerves. Ehrsson says that the illusion could be combined with electrical nerve stimulation. ■
–The illusion works every time– 24 January 2009 | NewScientist | 15
Brain trick makes robot hand feel real Peter Aldhous
A BIZARRE illusion that makes people believe a false hand is part of their own body could be all it takes to imbue prosthetic limbs with a sense of touch. Although sophisticated robotic prosthetics can now replace amputated hands, they don’t yet provide the brain with the sensory feedback vital to control fine movement. Without feeling pressure from the fingertips, for example, an amputee operating a robotic hand could either break a wine glass by grasping it too tightly, or let it fall to the floor by failing to apply enough grip. One potential solution is to wire sensors in robotic fingers directly into nerves in the stump, but this poses some technical challenges. So instead Henrik Ehrsson, a cognitive neuroscientist at the Karolinska Institute in Stockholm, Sweden, decided to see if a trick
ANITA STOCKSELIUS & ANN RAGNÖ
“My first reaction was: they don’t have a hand, so how can the illusion work on amputees?”
known as the “rubber hand illusion” could provide a simpler alternative. The illusion arises from our brain’s attempts to reconcile conflicting information from different senses. If you place a rubber hand in front of a volunteer and stroke it with a brush while simultaneously brushing one of their own hands, hidden from view, it feels as if the sensations are coming from the rubber hand. The volunteer also experiences the eerie feeling that the rubber hand is part of their own body. Ehrsson wondered if he could use the same illusion to “trick” amputees into interpreting strokes applied to their stump as coming from a prosthetic hand. His team recruited 18 amputees who had lost a hand and stroked their stumps, which were hidden from view, for about two minutes, at the same time as a fleshylooking rubber hand. As the rubber and real hands must normally be stroked in the same place, it wasn’t clear if this would be enough to induce the illusion. “My first reaction was: they don’t
have a hand. How can it work?” says Ehrsson. While the illusion was weaker in the amputees than in people with intact hands, tests designed to measure the extent to which people fall for the illusion showed that stroking someone’s stump still works, especially in those who had lost their hands most recently (Brain, DOI: 10.1093/ brain/awn297). The illusion also had physiological effects: once an amputee started viewing the rubber hand as part of their own body, stabbing it with a needle caused a change to their skin’s electrical conductance as they came out in a cold sweat. “They were expecting it to hurt,” Ehrsson explains. Greg Clark, whose team at the University of Utah in Salt Lake City is working on ways to provide
sensory feedback from a robotic hand says: “They got effects from very limited training. That encourages me to believe that the effects would grow larger, if the individual had more experience.” Ehrsson is now working with hand surgeon and neuroscientist Göran Lundborg of Malmö University Hospital in Sweden to apply the illusion to advanced robotic prosthetics. Their goal is to design robotic hands that create the illusory sensations automatically, by connecting sensors in the fingers to actuators that deliver touches to the stump. Still, Clark suspects that it may be difficult to transmit the full range of sensory information to the brain without some direct electrical stimulation of the nerves. Ehrsson says that the illusion could be combined with electrical nerve stimulation. ■
–The illusion works every time– 24 January 2009 | NewScientist | 15