How the Brain Senses Movement: New Research Brings Prostheses Closer to Feeling Natural
Reading Time: 4 minutes
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Summary:
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Single-participant Italian study evokes hand-movement sensations by vibrating muscles through implanted magnets
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Participant, a 34-year-old man with a forearm amputation, felt his hand open and close, not individual fingers
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This study and an earlier Cleveland Clinic study suggest the brain senses movement as coordinated synergies, not isolated joint sensations
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Six-week proof of concept published in Science Advances; researchers say further investigation is needed
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People who use upper-limb prostheses tend to watch them. According to Paul Marasco, PhD, a Cleveland Clinic researcher who leads work in this area, their eyes stay locked on the hand because they can’t feel how it’s moving. Controlling a prosthesis is hard, but he says the bigger problem is knowing what the limb is doing without looking.
*The image above is a representation and does not illustrate the actual research discussed in this article. (Image: JHU/APL)
Limb loss takes away proprioception, the sense of limb position, and kinesthesia, the sense of bodily movement and weight. These two support natural motor control. The brain uses both constantly, but kinesthesia operates below conscious awareness, so its mechanisms are not well understood. Prostheses have historically been unable to replicate it for people with upper-limb loss, which has been a major obstacle to limbs that move naturally, according to Marasco.
The Italian study
Researchers at the Sant’Anna School of Advanced Studies in Pisa, Italy, tested a new device over six weeks with a single participant: a 34-year-old man with a traumatic forearm amputation. The study was published in Science Advances, with Federico Masiero, PhD, as first author and Marasco as co-senior author.
The device is a myokinetic kinesthetic interface, or MKkI. Magnets are implanted in the participant’s forearm muscles, and magnetic coils deliver targeted vibrations to those muscles remotely. The vibrations are intended to evoke the sensations of grasping movements.
The team anticipated that the participant would sense movement in each finger joint, but he noticed more complicated motions, such as his hand closing and opening. He also did not report sensations of impossible or unnatural movement, which have been noted in earlier studies that delivered vibration to muscles through the skin and may have muddied the feedback.
A match with earlier research
The Italian results closely matched those from a Cleveland Clinic-led study using a structurally different system. That approach, called targeted muscle reinnervation for kinesthesia (TMRk), pairs a prosthetic socket with a robotic hand with individually powered fingers. The user controls the hand through muscle signals (EMG), while robotic vibrations convey the sensation of hand and limb movement.
The two teams then compared their data. They generated movement data from a video of the Italian participant’s hand demonstrations using a graphics engine and set it against earlier recordings from TMRk participants made with a data glove. In both studies, muscle vibration produced coordinated movement sensations, or synergies, instead of sensations isolated to individual joints.
Why the finding matters
Marasco explains the idea this way. Even a simple act like opening a hand offers the brain countless ways to do it, and storing every possible trajectory would be impractical. The brain seems to manage this by building movements from synergies, and the same building blocks appear to be what it senses during movement.
The Italian participant had trouble describing what he felt, which is common because kinesthesia is hard to put into words. At first, the researchers thought the experiment was unsuccessful since he couldn’t feel each finger separately. When Marasco reviewed the data, he recognized the coordinated movement and saw the work as telling us something about how the brain processes sensation, not just about the device.
What the research does and doesn’t show
The Sant’Anna project has its limitations. For starters, it was a six-week proof of concept involving just one participant. Its similarities to the Cleveland Clinic study point to broader implications, though researchers stress that further investigation is needed. Still, the findings gave researchers something more concrete to build on towards developing upper-limb prostheses that function more like natural arms.
Related Reading:
Advancing Neural Prosthetic Limbs
Phantom Limb Syndrome: Unraveling the Mysteries of the Brain
The ‘Little Brain’ May Help More People Control Prosthetic Limbs
New Nerve-Reading Tech Could Make Prosthetic Legs Feel More Like the Real Thing
