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Amputees "Feel" the Movement of Their Prosthetic Hands

With a vibrating gadget and some nerve rewiring, prostheses users get sensation in their plastic arms

3 min read

Close-up of a prosthetic hand
Photo: Nordicphotos/Alamy Stock Photo

The name of Paul Marasco’s lab says it all: the Laboratory for Bionic Integration. Here, technology and human biology come together. And the lab’s latest project shows the potential of this approach: Marasco’s team used a vibrating gadget and a neat perceptual illusion to give amputees a natural feeling of their plastic-and-metal arms moving through space. 

Marasco, a bioengineer at the Cleveland Clinic’s Lerner Research Institute, worked on this study with amputees sporting a certain type of advanced prosthetic arm. These amputees had undergone a procedure to reroute and reassign some of the nerves originally designated to control their missing limbs.

For example, in these prosthetic systems, a nerve that originally controlled elbow movements is often rerouted to a chest muscle. Then a sensor is placed on the chest to pick up the muscle’s electrical activity. When the amputee tries to move that missing elbow, the nerve signal triggers a muscle movement in the chest. That muscle movement is translated into a command for the prosthetic elbow joint, giving the amputee an intuitive way to move the prosthetic arm.  

But “these sophisticated limbs are still frustrating to use,” Marasco tells IEEE Spectrum. Despite the intuitive way of sending motion commands, these limbs don’t provide any feedback about how they’re moving through space. The users “have to watch their arms constantly, and use vision to make up for the fact that they can’t feel them,” he says.

The new research, published in the journal Science Translational Medicine, provides that feeling of movement, which is called the kinesthetic sense (it’s distinct from the sense of touch). 

Marasco’s team made use of a well-known tactile illusion: In able-bodied people, vibrating the various tendons of the arm at a certain frequency produces the illusory feeling that specific joints are moving.

For the amputees, the researchers used a vibrating gadget on the muscles containing the rerouted arm nerves. First the researchers built out a library of which vibrations created which illusory sensations of movement: making a fist, rotating the wrist, pinching the fingers together, etc. Then the amputees were asked to send particular movement commands to their prosthetic limbs. Each movement command was split, going both to the prosthetic limb to move the elbow, for example, and also to the vibrating gadget positioned to give the illusion of elbow movement. 

With this restored sense of limb movement, the amputees could perform movement tasks while blindfolded. “A normal amputee is completely lost with a blindfold,” says Marasco, “and can’t even do the test.”

Diagram shows an amputee sending a command to a prosthetic limb and receiving feedback about its movement.

Image: P.D. Marasco
With Marasco's system, an amputee can both send movement commands (shown in red) to a prosthetic arm and receive sensory feedback about the arm's movement (shown in blue).

Bioengineer Dustin Tyler, who wasn’t involved in the current research, says he finds it fascinating that an amputee’s muscles containing rerouted arm nerves can be used to create a sense of movement. “If you’d asked me before, I would have said it’s unlikely to work,” Tyler says.

Tyler does related research at the Louis Stokes Cleveland Veterans Affairs Medical Center, which he has written about for IEEE Spectrum. His team has given amputees the sense of touch by putting force sensors on their prosthetic hands, and sending the signals to implanted electrodes wrapped around the residual nerves in their arms. He’s now working on prosthetic systems that provide both the sense of touch and of movement. 

Tyler notes that there are pros and cons to Marasco’s system, which doesn’t require implanted electrodes. “A pro for the external system is that you don’t have to go through surgery,” he says. “But the trick is to have external components that are robust, reliable, and stable during everyday use.” 

Both Marasco and Tyler are involved in DARPA’s HAPTIX program, which aims to give amputees precision control and sensory feedback from their prosthetic devices. The program funds both engineering experiments and basic neuroscience to understand the biological systems at work. 

Marasco says part of his team’s research is focused on figuring out how the brain interacts with the introduced technology. “As these systems become more integrated with the people themselves, how are their internal models [of their bodies in space] functioning?” he asks. “Are their brains changing and relating to these devices more effectively? How are we blurring the lines between human and machine?”

For the answers to those questions, stay tuned for future updates from the Lab for Bionic Integration. 

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