The mentality behind the popular adage "seeing is believing" is helping some amputees find a new connection with their prosthetic limbs.
Researchers from Ecole polytechnique fédérale de Lausanne (EPFL) created a way to overcome the "phantom limb" problem experienced by millions of amputees, helping their brains identify their prosthetics as part of their bodies.
In the United States along, nearly 2 million people have amputations, according to nonprofit group the Amputee Coalition. Some 185,000 amputations are performed each year in the U.S., and a majority of those limbs are replaced by prosthetics.
However, issues arise when an amputee's prosthetic doesnt fit their brain's perception of their missing limb. Many amputees still feel their missing limb in a sense, called a phantom or ghost limb, and those phantom limbs are often perceived by the brain to be smaller than the lost limb. And, despite a rapidly-improving prosthetics industry, most prosthetic limbs are not equipped to provide any sense of touch back into a patient's body. This means amputees have to consistently make sure their limb is operating how they need it to operate -- something they wouldn't do if the limb was still present.
The EPFL team wanted to analyze just how much an amputee "seeing" his or her limb affected perception of a prosthetic. Rather than relying only on sight, however, the team combined sight and touch.
"The brain regularly uses its senses to evaluate what belongs to the body and what is external to the body. We showed exactly how vision and touch can be combined to trick the amputee's brain into feeling what it sees, inducing embodiment of the prosthetic hand with an additional effect that the phantom limb grows into the prosthetic one," explained Giulio Rognini of EPFL's Laboratory of Cognitive Neuroprosthetics led by Olaf Blanke, in a collaboration with Silvestro Micera of EPFL and Scuola Superiore Sant'Anna in Italy.
In two-hand amputees, the researchers gave the test subjects tactile sensations at the tip of their phantom limbs by stimulating the nerves found in their stump. The patients wore virtual reality goggles that showed an index finger within their prosthetic limb. Whenever the sensation was applied, the index of the finger glowed in response.
The patients reported they felt like the prosthetic hand belonged to their own body. At the end of the study, the amputees also reported they felt like their phantom limb now fit the prosthetic. Prior to the experiment, the patients told researchers they felt like their ghost hands didn't fit into the prosthetic. However, during the experiment, patients reported a sensation that their phantom limb "stretched" into the prosthetic -- letting it fit like a glove.
This is the first time multisensory information has been used to create compelling experiences in human subjects and gives researchers a better understanding of a sensation known as "telescoping" (when a phantom limb stretches into a more 'standard' size).
This technology could give amputees a better connection to their prosthetics while prosthetics continue to become more like limbs themselves.
"The setup is portable and could one day be turned into a therapy to help patients embody their prosthetic limb permanently," Rogini said.
This study and more insights from the EPFL team can be found in a recent edition of the Journal of Neurology, Neurosurgery & Psychiatry.