The results of a new study indicate that long-term prosthetic use may not rewire the brain as once thought. If true, this could have implications for creating prosthetic devices indistinguishable from a human limb.
Close but no cigar
A new study seems to indicate that despite long-term prosthetic use by amputees, their nervous system has not adapted to the new limbs as anticipated. This could overthrow the idea that the human nervous system isn't quite as plastic as is widely believed.
The study involved three amputees whose lost limbs had been replaced with neuromusculoskeletal prosthetic limbs. After a full year of using the devices, it was discovered that participants' sensations never shifted to match the location of the touch sensors in their prosthetic devices.
This, the researchers believe, highlights the limits of the ability of the human nervous system to adapt to different, non-organic, sensory input.
The three study subjects all had above-elbow amputations and were equipped with high-tech neuroprosthetic devices that were affixed directly to their humerus bone. Users could control the prosthetics using signals derived from electrodes implanted in the residual arm muscles and receive sensory feedback via another set of implanted electrodes.
One sensor, located in the prosthetic "thumb", triggered stimulation of the nerve which would, in turn, replicate a touch sensation. All well and good, but there was a problem.
It turns out that the organization of the nerve is effectively arbitrary in life. This makes it difficult for surgeons to be sure whether their placement of the electrodes will accurately replicate the sensation of touch in the patient's lost thumb.
When asked where the patient felt "touch" from the prosthetic thumb, users reported that they could "feel" touch in other locations, like the middle finger or the palm of the prosthetic.
The human brain might not be as plastic as previously thought
The same users then wore the prosthetic for around 12 hours a day, every single day, and used it to manipulate objects during their daily routine over the course of 12 months. Even with this level of interaction with their new prosthetics, it appears that their nervous systems did not adapt.
“One problem with current neural electrodes is that you can’t tell during the implantation surgery which part of the nerve corresponds to what sensation, so the electrodes don’t always land in exactly the location in the nerve that would match the location of the sensors in the prosthetic hand,” explained lead author and developer of the neuromusculoskeletal prostheses Max Ortiz Catalan, an associate professor of bionics at the Chalmers University of Technology and Director of the Center for Bionics and Pain Research in Gothenburg, Sweden.
“We hoped that because the patients were grabbing objects and feeling the sensation somewhere else in the hand, all day and every day for several months, the brain would resolve the mismatch by shifting the perceived sensation to the thumb,” he added.
This was frustrating for the patients as although they could observe the hand while manipulating objects, none of them reported the feeling of touch in the thumb of the device.
“Every day, for a year, these subjects saw their prosthetic thumb touching things and felt it in a different location—sometimes close to the thumb, but not on it—and the sensation never budged. Not even a smidge,” explained senior author Sliman Bensmaia, professor of organismal biology and anatomy at the University of Chicago.
The results clearly challenge the accepted view that the brain is plastic and can readily adapt following the loss of a limb. Many had previously hough that the brain had a great capacity to reorganize itself following the loss of sensory input by co-opting existing, and unused for the same purpose, brain tissue.
“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,” added Bensmaia.
“For example, if you sew two fingers together and look at how that’s represented in the brain, they seem to have merged," he added.
This appears to not be the case if the results of this study are anything to go by.
“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,” Bensmaia added.
“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,” he said.
These unexpected results show the importance of knowing precisely where to place electrodes when implanting sensory arrays in patients using similar neuroprosthetic devices. This is because it appears, that the human brain and nervous system are not as able to rewire themselves as previously thought.
“This means that you really have to get it right,” concludes Bensmaia. “There are no do-overs here.”
You can find the original study in the journal Cell Reports.