A new study that examined people who used a neuromusculoskeletal prosthesis suggests that the brain has a more limited ability to to reorganize sensory input than previously thought. Researchers found that the perceived location of touch on a bionic hand did not change despite daily use over an extended period.
The findings appear in the journal Cell Reports.
“I focused my work on restoring sensorimotor function after traumatic events such as an amputation. Creating prosthesis that can provide sensory information intuitively to the user, as our biological hands do, is important to increase their function,” said study author Max Ortiz Catalan (@MaxOrtizCatalan), an associate professor at Chalmers University of Technology and director of the Center for Bionics and Pain Research.
“There is a new concept on artificial limbs called neuromusculoskeletal prostheses which are interfaced with the user’s nerves, muscles, and skeleton. These prostheses can provide intuitive sensory feedback by stimulating the nerves that were severed due to the amputation and used to be connected to the missing limb.”
But surgeons can’t be sure whether their placement of the electrodes will generate a sensation in the correct location. “It is difficult for surgeons to place the implanted electrodes exactly in the nerve fibers that used to be connected to specific parts of the hand. It can be done roughly, but not precisely,” Ortiz Catalan told PsyPost.
Three participants with above-elbow amputations were equipped with high-tech neuroprosthetic devices that were affixed directly to their humerus bone. The users could control the prosthetic device thanks to signals received from electrodes implanted in the residual arm muscles, and received sensory feedback via another set of implanted electrodes. A sensor located on the prosthetic thumb triggered stimulation of the nerve, which in turn elicited a touch sensation.
Even after a full year of using the devices, however, the participant’s subjective sensation never shifted to match the location of the touch sensors on their prosthetic devices. The participants did not report feeling the sensation on the thumb, but rather in other hand locations, such as their middle finger or the palm. “This fixedness is especially surprising given that the prosthesis was used on a daily basis and the sensory feedback was behaviorally relevant,” the researchers said.
“We hoped that the brain would change and adapt to the new location seen by the user in daily life use, but it didn’t — the location of the perceived sensation remained stable elsewhere in the hand since the beginning. If we want a more precise targeting of locations, we need neural interfaces with higher resolution or novel surgical methods,” Ortiz Catalan explained.
The findings suggest that sensory maps are highly stable in adulthood and the nervous system has a limited ability to adapt to different sensory input.
“There’s been this idea that the nervous system is really plastic, so if you see a mismatch between what you see and what you feel, it’s a great opportunity for neural remapping,” said co-author Sliman Bensmaia. “For example, if you sew two fingers together and look at how that’s represented in the brain, they seem to have merged.”
“But I think that this idea has been vastly overstated. It’s less like you’re reorganizing a room and more like you’re just hearing echoes bouncing around an empty chamber,” he continued. “You might get some overlapping sensation from adjacent limbs, but it’s just because the area of the brain that used to respond to sensation is empty, and activating the neurons around it leads to an echo through the emptiness.”
The study, “Chronic Use of a Sensitized Bionic Hand Does Not Remap the Sense of Touch“, was authored by Max Ortiz-Catalan, Enzo Mastinu, Charles M. Greenspon, and Sliman J. Bensmaia.