Someone with an amputated right arm showing where they feel a certain temperature in their phantom hand
ALAIN HERZOG
People with amputated arms could be made to sense temperature or material changes in their phantom hand, a discovery that could help equip prosthetics with a more heightened sense of touch.
After an amputation, some people experience the feeling that their missing arm or leg is still attached, known as phantom limbs. For more information on these limbs, Solaiman Shokur at the Swiss Federal Institute of Technology in Lausanne and colleagues looked at 26 people who lost at least part of their arm in an accident and reported having phantom hands.
The researchers placed a device with a variable temperature on the stump of these participants. They then applied three temperatures: 25°C (77°F), 32°C (89.6°F), and 37°C (98.6°F), and the participants reported whether they could feel heat in their phantom hands and, if so, if they could tell the temperatures apart.
Seventeen said they felt a temperature change in their phantom hand when the device was used, what the researchers called phantom thermal sensation. Of these, 15 were able to distinguish between the three temperatures. “Our hypothesis is that nerves continue to grow into the skin after amputation,” says Shokur. “By targeting those nerves precisely, we produce this phantom sensation.”
It’s unclear why only 17 of the participants reported feeling the temperatures. “What we’ve noticed is that a lot of the people who didn’t respond had accidents related to fire and so burned their skin, and so maybe lost a lot of sensitivity in their skin,” he says.
In another part of the experiment, the researchers applied a sensor to three materials: glass, copper and plastic. This sensor was connected to the residual arms of nine of the participants with phantom thermal sensation, who were blindfolded. It was initially set at 32°C, the approximate temperature of the skin, and then cooled down at about the same rate as the temperature of the skin on our hands when it touches copper, glass, or plastic.
The participants identified which of the materials touched the sensor with a 66 percent success rate, compared to a 67 percent success rate when their intact hand touched the materials.
The researchers hope to develop their sensor so that it can be applied to the fingertips of prosthetics, allowing people with amputated arms or hands to detect temperatures. This will help amputees avoid burns and make touch feel more natural, says Shokur. “One person told me they would like to wear this device while holding their child’s hand so they can feel their warmth,” he says.
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