Electrodes dupe brain into feeling touch

Experiments in monkeys could lead to sensitive replacement limbs for humans

A TOUCHING PROSTHETIC Touch-sensitive prosthetic limbs like the one shown may one day restore the sense of touch in people who have lost limbs or are paralyzed.

G. Tabot et al/PNAS 2013

Tickling the brain in just the right spot can create the sensation of touch. By zapping monkeys’ brains with electrodes, scientists tricked the animals into feeling a poke.

The feat, described October 14 in the Proceedings of the National Academy of Sciences, offers a blueprint for creating advanced prosthetics that meld with the brain. Restoring touch to people who have lost limbs or are paralyzed is one of the biggest goals in prosthetic design, says study coauthor Sliman Bensmaia of the University of Chicago.

Without the sense of touch, everyday tasks like cracking an egg, holding a coffee cup or folding a newspaper would be nearly impossible. Touch is also important for forging emotional connections and for the sensation of body ownership.

Bensmaia and his team first trained rhesus macaques to report by looking in a certain direction which of their fingers was being touched. Electrodes implanted in the animals’ somatosensory cortex — a brain area that handles tactile feedback from the body — simultaneously detected which neurons were active when an animal felt a precisely placed touch.

Then the scientists bypassed the finger and instead went straight to the brain. By zapping the finger-sensing neurons, the scientists tricked the monkeys into thinking they were being touched. “We are trying to mimic natural signals in the brain,” Bensmaia says.

The animals couldn’t describe the quality of the sensation, but the directions they looked in suggested that they experienced the electrode-induced artificial touch in the same way as the real thing. “That’s really important,” says neuroscientist Paul Marasco of the Department of Biomedical Engineering at the Cleveland Clinic. “There’s a nice clear pathway for implementation with the amputee or the spinal cord–injured population in mind.”

In another experiment, a prosthetic finger equipped with pressure sensors could also deliver these touch signals to the brain. The monkeys could not see the prosthetic; it was not attached to their bodies except through the electrodes. Monkeys were equally good at detecting pressures with a prosthetic finger and one of their real fingers, the team found.

The details about how the brain senses location, pressure and timing of touch could help create super sensitive, realistic prosthetic systems that convey touch information, Bensmaia says. “I think the foundation is laid for human trials,” he adds.

But work remains before the system is fully ready for people. The method requires surgery to implant electrodes into the brain, and it’s not clear how long the electrodes would last.

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