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Restoring the sense of touch in amputees using natural signals of the nervous system
Scientists at the University of Chicago and Case Western Reserve
University have found a way to produce realistic sensations of touch in
two human amputees by directly stimulating the nervous system.
The study, published Oct. 26 in Science Translational Medicine (STM),
confirms earlier research on how the nervous system encodes the
intensity, or magnitude, of sensations. It is the second of two
groundbreaking publications by University of Chicago
neuroscientist Sliman Bensmaia,
PhD, using neuroprosthetic devices to recreate the sense of touch for
amputee or quadriplegic patients with a “biomimetic” approach that
approximates the natural, intact nervous system.
On Oct. 13, in a separate publication from STM,
Bensmaia and a team led by Robert Gaunt, PhD, from the University of
Pittsburgh, announced that for the first time, a paralyzed human patient
was able to experience the sense of touch through a robotic arm that he
controls with his brain. In that study, researchers interfaced directly
with the patient’s brain, through an electrode array implanted in the
areas of the brain responsible for hand movements and for touch, which
allowed the man to both move the robotic arm and feel objects through
it.
The new study takes a similar approach in amputees, working with two
male subjects who each lost an arm after traumatic injuries. In this
case, both subjects were implanted with neural interfaces, devices
embedded with electrodes that were attached to the median, ulnar and
radial nerves of the arm. Those are the same nerves that would carry
signals from the hand were it still intact.
“If you want to create a dexterous hand for use in an amputee or a
quadriplegic patient, you need to not only be able to move it, but have
sensory feedback from it,” said Bensmaia, who is an associate professor
of organismal biology and anatomy at the University of Chicago. “To do
this, we first need to look at how the intact hand and the intact
nervous system encodes this information, and then, to the extent that we
can, try to mimic that in a neuroprosthesis.”
Recreating different sensations of intensity
The latest research is a joint effort by Bensmaia and Dustin Tyler, PhD, the Kent H. Smith Professor of Biomedical Engineering at Case Western Reserve University, who works with a large team trying to make bionic hands clinically viable. Tyler’s team, led by doctoral student Emily Graczyk, systematically tested the subjects’ ability to distinguish the magnitude of the sensations evoked when their nerves were stimulated through the interface. They varied aspects of the signals, such as frequency and intensity of each electrical pulse. The goal was to understand if there was a systematic way to manipulate the sensory magnitude.
(Image caption:
Electrical stimulation was delivered by an external stimulator (top
left) through percutaneous leads to FINEs implanted on the median,
ulnar, and radial nerves of an upper-limb amputee (bottom left). Each
electrode contact evokes sensory percepts on small regions of the
missing hand of the subject. Credit: Graczyk et al, Sci. Transl. Med.)
Earlier research from Bensmaia’s lab predicted how the nervous system
discerns intensity of touch, for example, how hard an object is
pressing against the skin. That work suggested that the number of times
certain nerve fibers fire in response to a given stimulus, known as the
population spike rate, determines the perceived intensity of a given
stimulus.
Results from the new study verify this hypothesis: A single feature
of electrical stimulation—dubbed the activation charge rate—was found to
determine the strength of the sensation. By changing the activation
charge rate, the team could change sensory magnitude in a highly
predictable way. The team then showed that the activation charge rate
was also closely related to the evoked population spike rate.
Building neuroprosthetics that approximate the natural nervous system
While the new study furthers the development of neural interfaces for
neuroprosthetics, artificial touch will only be as good as the devices
providing input. In a separate paper published in IEEE Transactions on Haptics, Bensmaia and his team tested the sensory abilities of a robotic fingertip equipped with touch sensors.
Using the same behavioral techniques that are used to test human
sensory abilities, Bensmaia’s team, led by Benoit Delhaye and Erik
Schluter, tested the finger’s ability to distinguish different touch
locations, different pressure levels, the direction and speed of
surfaces moving across it and the identity of textures scanned across
it. The robotic finger (with the help of machine learning algorithms)
proved to be almost as good as a human at most of these sensory tasks.
By combining such high-quality input with the algorithms and data
Bensmaia and Tyler produced in the other study, researchers can begin
building neuroprosthetics that approximate natural sensations of touch.
Without realistic, natural-feeling sensations, neuroprosthetics will
never come close to achieving the dexterity of our native hands. To
illustrate the importance of touch, Bensmaia referred to a piano.
Playing the piano requires a delicate touch, and an accomplished pianist
knows how softly or forcefully to strike the keys based on sensory
signals from the fingertips. Without these signals, the sounds the piano
would make would not be very musical.
“The idea is that if we can reproduce those signals exactly, the
amputee won’t have to think about it, he can just interact with objects
naturally and automatically. Results from this study constitute a first
step towards conveying finely graded information about contact
pressure,” Bensmaia said.
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