Prosthetic hands have gotten increasingly sophisticated. Many can recreate the complex shape and detail of joints and fingers, while powered prostheses allow for independent, wilful movement. But a new study published this week in Science Translational Medicine offers a potential glimpse into the future of the technology: Artificial hands that actually feel like a living limb as they move.
A model of how the vibration device, developed by Paul Marasco and his team at the Cleveland Clinic, can fool a person's brain into believing a prosthetic limb is actually real. Illustration: Cleveland Clinic Center for Medical Art & Photography (Cleveland Clinic)
The researchers recruited people with amputations who had been given surgery that reconfigured certain muscle and sensory nerves surrounding the amputated limb, allowing them to control their prosthesis through intuitive brain signals (thoughts) sent to the repurposed nerves.
Across a series of experiments involving three of these patients, the researchers attached devices that generated vibrations along specific muscles near the amputation site. When the device was turned on, these vibrations created an illusionary sense of kinesthesia - an awareness of conscious self-movement - in the prosthetic hand as the person performed tasks with it, both in a virtual stimulation and in the real world. The volunteers had amputations that extended just past their elbow as well as their whole arm.
Not only did the experiment let them "feel" their hand as they opened and closed it, but the restored intuition allowed them to perform tasks without needing to constantly look at their hand. And coupled with vision, it gave them overall better motor control over their prosthesis.
"We pre-position our grip when we grab things - we move our hand and space it out when we're getting ready to grab a glass of wine without even really thinking about it," lead author Paul Marasco, a biomedical engineer at the Cleveland Clinic and head of its Laboratory for Bionic Integration, told Gizmodo. "So the individuals we worked with, we were able to provide them with that sensation that they could just move their hands into specific places without ever having looking at it and perform every bit as well as able bodied people."
Scientists have known that muscle vibrations can create this same sort of illusion in able bodied people since at least the 1970s, Marasco said. But while his team had been working on exploiting the illusion for use in prosthetic technology for the last three years, they were still blown away by how potent and consistent it was in the patients.
"We thought that if it worked like it did in an able bodied person, we'd just get one or two joints; we'd get a wrist - something basic," he said. "But what we found were these complex, multi-digit synergies where the whole hand is moving and they know where their fingers are going and what they're doing, and they're adopting these really interesting grip conformations [shapes]. We were totally floored."
Adding more support to the idea that their technology could someday be adopted widely, Marasco noted that they were able to create the illusion in patients in different labs.
The research might also offer some insight into how we perceive reality. Scientists have found that there are two related components of self-perception, brought about by complex interactions of the sensory information our brains receive. There's the feeling that our movements are being made intentionally (a sense of agency) and there's the feeling that our bodies feel like our own (a sense of embodiment). The subjects did feel like they were in charge of their hand, but they still never felt as if it was "their" hand. The findings further suggest that agency and embodiment aren't created by the same mechanisms, Marasco said.
The research is one of the first to address kinesthesia in prosthetic technology, Marasco said. And since it's started, they're already had tangible improvements.
"It takes a lot of horsepower to vibrate those big muscles to the frequency required to generate those vibrations that create the illusion," he said. "But we went from a robotic vibration system that's the size of a soda can to one that's now just a little bigger than a matchbox. And it still has the same horsepower, but this one runs on batteries and it can be mounted directly to a prosthetic socket."
Agency is only part of the recipe needed to create life-like artificial limbs, though, and Marasco's team is trying to further bridge the gap with other emergent technologies.
"We've got experiments in the process of running where we've got touch and movement and motor control all running simultaneously in the same arm," he said. "So they think about moving the arm, they feel that it's moving through space and when they grab something, they feel that touch as though it's theirs. That's the next part of this, integrating these pieces into a whole."
Today, over 10 per cent of people with upper limb amputations ultimately reject using their prosthesis, oftentimes because it simply doesn't seem comfortable or essential. And as prosthetic limbs become even more intricate and able to perform functions without the person's input, scientists such as Marasco have worried that patients could be more likely to reject them, since they would feel more artificial. But the illusion of sensation might also give these patients peace of mind.
"There's a real disconnect when people have any kind of autonomous machine or computer with them in the loop," one that doesn't exist when people have to cooperate with another person for instance, Marasco explained. "And that's the place where people with prosthetics are stuck."
"We think that if we can tap into that system and provide them a sense of agency and ownership so that their brain recognises their device as being human, it will actually overcome these barriers between the two players," he added.
Marasco and his team also plan to explore whether their research could be used to help stroke and damaged spinal cord patients with sensation problems.