A research team led by Sant’Anna School of Advanced Studies in collaboration with the Cleveland Clinic uncovered new insight into how the brain senses movement. Their findings could help improve sensation and movement for prosthetic limbs.
By combining data from the world’s only two neural-machine interfaces designed to restore kinesthetic sensation in upper-limb prosthetic devices, the researchers found that the brain appears to process this information not as isolated signals, but as coordinated hand grasp movement patterns.
Kinesthesia, the sense of muscle movement, is essential for natural motor control. It is lost after amputation, making prostheses harder to use intuitively. Muscle vibration can be used to generate perceptions of movement, but these vibrations typically stimulate both skin and muscle at the same time, which can confuse the brain while using prosthetic devices.
To address this, Sant’Anna developed the myokinetic kinesthetic interface, a new bidirectional interface for hand prostheses that uses vibrations generated by small magnets implanted in the residual forearm muscles to restore natural sensations of movement. The system was integrated with the Mia Hand robotic hand developed by the Sant’Anna spin-off company Prensilia.
The team tested the interface between the hand and the brain for six weeks in a 34-year-old patient in Italy, who perceived hand opening and closing with coordinated movements, very similar to real ones.
“The myokinetic kinesthetic interface is unique because it uses a simple, minimally invasive implant to stimulate muscles without touching the skin,” said Federico Masiero, PhD, first author of the related study. “This approach may be the key to better understanding how human motor control works, but also how to restore movement sensation after amputation.”
Masiero was a doctoral student at Sant’Anna, and is currently a postdoctoral researcher at the Munich Institute of Robotics and Machine Intelligence of the Technical University of Munich.
The coordinated hand movements felt by the patient appeared similar to those felt by participants with a different kinesthetic feedback system built by researchers at the Cleveland Clinic. The two prosthetic interface systems were structurally different, with the one developed at Sant’Anna using implanted magnets and the other at Cleveland Clinic using surgical nerve redirection and robotics.
Both kinesthetic interfaces, which function by specifically vibrating the deep muscles, produced similar perceptual results: Induced movement sensations were perceived as coordinated finger movements rather than separate signals. Both research teams also observed that some sensations transmitted through their respective interfaces were perceived by the patient without their users being immediately aware of them.
Together, the teams’ findings suggest that the brain may organize movement sensation from the muscles in a more coordinated and more subconscious fashion than previously understood.
“The ability to compare independently generated data from two very different interfaces makes these findings especially compelling,” said Paul Marasco, PhD, coordinator of the study at the Cleveland Clinic. “It gives us a stronger foundation for designing therapies and devices that work with the nervous system in a more natural way, with the ultimate goal of improving outcomes for patients.”
The team’s next goal is to use prior work reading out the position of implanted magnets to control the prosthesis while simultaneously writing in to the magnets with superimposed vibration to restore natural sensory perceptions. The longer-term aim is to develop a permanent implant. The current projects and the earlier collaborative studies lay the groundwork for combining natural grasp sensation with intuitive motor control in people with hand loss, potentially elevating the function of an emerging generation of more human-like prosthetic devices, the researchers said.
“Our solution was implemented as a preliminary demonstrator: The implant was designed to last six weeks, a period we considered sufficient for an initial verification of the interface’s usefulness and effectiveness. The results were very promising and prompted us to explore a permanent implantable solution, which will allow us to study the interface over much longer periods and with a larger number of participants,” said Christian Cipriani, a professor and dean of the faculty of experimental sciences at Sant’Anna and head of the artificial hands area at the BioRobotics Institute, the creator of the interface, and coordinator of the study.
Editor’s note: This story was adapted from materials provided by the Cleveland Clinic.
The open-access study, “Coordinated hand movement sensation revealed through an implanted magnetic prosthetic kinesthetic interface,” was published in Science Advances.
