The study involved two arm amputees who had a surgery called targeted reinnervation where the remaining nerve ends from the severed arm are rerouted to an area on the arm above the amputation site. This patch of skin serves as a proxy - touching different parts of this area made the amputee feel that distinct parts of the missing arm were being touched.

To create this feeling of limb “embodiment,” a research team in Chicago led by Paul Marasco designed a pressure-sensing system for the prosthetics. Each time a sensor on the prosthetic arm detected a touch, it sent a signal to a robot that would poke a targeted area of the reinnervated skin.

Marasco and his team had each subject sit at a table with the prosthetic arm unattached but arranged naturally next to them. As the subject watched a researcher touch the prosthetic arm, the robot would simultaneously press on the reinnervated skin. Seeing and feeling the touch at the same time created a powerful illusion in both amputees that the prosthetic arm was part of their own body. Interestingly, when they saw but did not feel the researcher’s touch, the subjects didn’t feel the same sense of ownership over the prosthetic.

Also, when one subject both saw and felt a touch on her prosthetic arm, there was a rise in temperature on her arm above the amputation site. Marasco says this temperature increase may reflect her body adopting the prosthetic, because when sensory information is blocked from a limb, the limb’s temperature drops slightly.

This is a big step in creating better prosthetics. Getting sensory information incorporated into a prosthetic limb would help integrate the limb as part of the body, allowing amputees to better control it and helping them to feel more whole.

From Science News, February 26, 2011; Vol. 179 #5