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鈥檛 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 , a cognitive neuroscientist at the Karolinska Institute in Stockholm, Sweden, decided to see if a trick known as the 鈥渞ubber hand illusion鈥� could provide a simpler alternative.
Advertisement
The illusion arises from our brain鈥檚 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 鈥渢rick鈥� 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 fleshy-looking rubber hand. As the rubber and real hands must normally be stroked in the same place, it wasn鈥檛 clear if this would be enough to induce the illusion. 鈥淢y first reaction was: they don鈥檛 have a hand. How can it work?鈥� 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鈥檚 stump still works, especially in those who had lost their hands most recently (Brain, ).
鈥淢y first reaction was: they don鈥檛 have a hand, so how can the illusion work on amputees?鈥�
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鈥檚 electrical conductance as they came out in a cold sweat. 鈥淭hey were expecting it to hurt,鈥� Ehrsson explains.
, whose team at the University of Utah in Salt Lake City is working on ways to provide sensory feedback from a robotic hand says: 鈥淭hey got effects from very limited training. That encourages me to believe that the effects would grow larger, if the individual had more experience.鈥�
Ehrsson is now working with hand surgeon and neuroscientist of Malm枚 University Hospital in Sweden to apply the illusion to . 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.
