You can probably complete an incredible number of tasks with your hands without looking at them. But if you put on gloves that muffle your sense of touch, many of those simple tasks become frustrating. If we eliminate proprioception (the ability to sense the relative position and movement of our body), we could even end up breaking an object or injuring ourselves.
“Most people don’t realize how often they rely on touch instead of vision: writing, walking, picking up a flimsy glass of water,” said Charles Greenspon, PhD, a neuroscientist at the University of Chicago. “If you can’t feel, you have to constantly watch your hand while doing anything, and you still run the risk of spilling, crushing, or dropping objects.”
Greenspon and his research collaborators recently published papers in Nature Biomedical Engineering and Science documenting important advances in a technology designed to address precisely this problem: direct, carefully timed electrical stimulation of the brain that can recreate tactile feedback to give a nuanced “feel” to prosthetic hands.
The science of restoring sensation.
These new studies build on years of collaboration between scientists and engineers at UChicago, the University of Pittsburgh, Northwestern University, Case Western Reserve University, and Blackrock Neurotech. Together they are designing, building, implementing and perfecting brain-computer interfaces (BCIs) and robotic prosthetic arms aimed at restoring both motor control and sensation in people who have lost important limb function.
On the UChicago side, the research was led by neuroscientist Sliman Bensmaia, PhD, until his unexpected passing in 2023.
The researchers’ approach to prosthetic sensation involves placing small arrays of electrodes in the parts of the brain responsible for moving and feeling the hand. On the one hand, a participant can move a robotic arm simply by thinking about the movement, and on the other hand, sensors on that robotic limb can trigger pulses of electrical activity called intracortical microstimulation (ICMS) in the part of the brain dedicated to touch.
For about a decade, Greenspon explained, this stimulation of the tactile center could only provide a simple sensation of contact in different places on the hand.
“We could evoke the sensation that you were touching something, but it was mostly an on/off signal and was often quite weak and difficult to know where on the hand the contact occurred,” he said.
The recently published results mark important milestones in overcoming these limitations.
Advancing the understanding of artificial touch
In the first study, published in Nature Biomedical EngineeringGreenspon and his colleagues focused on ensuring that electrically evoked tactile sensations are stable, precisely localized, and strong enough to be useful in everyday tasks.
By sending short pulses to individual electrodes in participants’ touch centers and asking them to report where and how strongly they felt each sensation, the researchers created detailed “maps” of brain areas that corresponded to specific parts of the hand. The test revealed that when two widely spaced electrodes are stimulated together, participants feel a stronger and clearer touch, which can improve their ability to locate and measure pressure in the correct part of the hand.
The researchers also conducted extensive testing to confirm that the same electrode consistently creates a sensation corresponding to a specific location.
“If I stimulate an electrode on the first day and a participant feels it on their thumb, we can test that same electrode on day 100, day 1,000, even many years later, and they still feel it in roughly the same place,” Greenspon said. , who was the lead author of this article.
From a practical point of view, any clinical device would have to be stable enough that the patient could rely on it in everyday life. An electrode that continually changes its “contact location” or produces inconsistent sensations would be frustrating and require frequent recalibration. In contrast, the long-term consistency revealed by this study could allow prosthetic users to develop confidence in their motor control and sense of touch, much as they would in their natural limbs.
Add sensations of movement and shapes.
the complementary Science The paper went a step further to make artificial contact even more immersive and intuitive. The project was led by first author Giacomo Valle, PhD, a former postdoctoral fellow at UChicago who now continues his research in bionics at Chalmers University of Technology in Sweden.
“Two electrodes next to each other in the brain do not create sensations that ‘cover’ the hand in small patches with one-to-one correspondence; instead, the sensory locations overlap,” explained Greenspon, who shared lead authorship on this study. . role with Bensmaia.
The researchers decided to test whether they could use this overlapping nature to create sensations that would allow users to feel the boundaries of an object or the movement of something sliding across their skin. After identifying pairs or groups of electrodes whose “contact zones” overlapped, the scientists activated them in carefully orchestrated patterns to generate sensations that progressed along the sensory map.
Participants described feeling a soft, gliding touch gently passing over their fingers, even though the stimulus was delivered in small, discrete steps. Scientists attribute this result to the brain’s remarkable ability to stitch together sensory inputs and interpret them as coherent, moving experiences by “filling” gaps in perception.
The approach of activating electrodes sequentially also significantly improved participants’ ability to distinguish complex tactile shapes and respond to changes in the objects they touched. They could sometimes identify letters of the alphabet electrically “traced” on their fingertips, and could use a bionic arm to stabilize a steering wheel when it began to slip between their hands.
These advances help bring bionic feedback closer to the precise, complex and adaptive capabilities of natural touch, paving the way for prostheses that enable safe handling of everyday objects and responses to changing stimuli.
The future of neuroprostheses
Researchers hope that as electrode designs and surgical methods continue to improve, the coverage on the hand will become even thinner, allowing for a more realistic response.
“We hope to integrate the results of these two studies into our robotic systems, where we have already shown that even simple stimulation strategies can improve people’s ability to control robotic arms with their brains,” said co-author Robert Gaunt, PhD, associate professor . of physical medicine and rehabilitation and leader of stimulation work at the University of Pittsburgh.
Greenspon emphasized that the motivation behind this work is to improve the independence and quality of life of people living with paralysis or limb loss.
“We all care about the people in our lives who are injured and lose the use of a limb – this research is for them,” he said. “This is how we give touch back to people. It’s the cutting edge of restorative neurotechnology and we’re working to expand the approach to other regions of the brain.”
The approach also holds promise for people with other types of sensory loss. In fact, the group has also collaborated with UChicago surgeons and obstetricians on the Bionic Breast Project, which aims to produce an implantable device that can restore the sense of touch after a mastectomy.
Although many challenges remain, these latest studies offer evidence that the path to restoring touch is increasingly clear. With each new set of findings, researchers move closer to a future in which a body prosthesis is not just a functional tool, but a way of experiencing the world.