Restoring natural sensory feedback results in functional and cognitive benefits for leg prosthesis users. The team at ETH Zurich applied this in neuroprosthetics. [Image courtesy of Pietro Comaschi/ETH Zurich]
A team of researchers at ETH Zurich found evidence that neuroprosthetics work better when they use signals that are inspired by nature.
The team, working under Professor Stanisa Raspopovic at the ETH Zurich Neuroengineering Lab, garnered attention years ago with prosthetic legs that enabled amputees to feel sensations from the artificial body part for the first time, according to ETH. This device connected to the sciatic nerve in the thigh through implanted electrodes. The electrical connection enabled the neuroprosthesis to communicate with the patient’s brain.
With this connection, the neuroprosthetics could relay information on the constant changes in pressure detected on the prosthetic foot’s sole when walking. That gave the test subjects more confidence in their prosthesis and enabled them to walk faster on challenging terrains.
In a recently published paper, Raspopovic and his team used the example of their leg prostheses to highlight the benefits of using naturally inspired, biomimetic stimulation to develop the next generation of neuroprosthetics.
Using neuroprosthetics to simulate the activation of nerves in the sole
To generate biomimetic signals, Natalija Katic – a doctoral student in Raspopovic’s research group – developed a computer model called FootSim. Katic used data collected by collaborators in Canada to develop this model. They recorded the activity of natural receptors — mechanoreceptors — in the sole of the foot while touching different points on the feet of volunteers with a vibrating rod.
The model simulates the behavior of those receptors on the sole of the foot. It then generates neural signals that shoot up the nerves in the leg toward the brain. This occurs from the moment the heel hits the ground and the body’s weight begins to shift toward the outside of the foot before the toes push off the ground, ready for the next step.
“Thanks to this model, we can see how semsory receptors from the sole, and the connected nerves, behave during walking or running, which is experimentally impossible to measure” Katic said.
Giacomo Valle – a postdoc in Raspopovic’s research group – worked with colleagues in Germany, Serbia and Russia on experiments with cats, whose nervous system processes movement in a similar way to that of humans, in an effort to understand how biomimetic signals correspond to the ones emitted by real neurons. The experiments took place in 2019 at the Pavlov Institute of Physiology in St. Petersburg and were carried out in accordance with the relevant European Union guidelines.
Researchers implanted electrodes, connecting some to the nerve in the leg and some to the spinal cord. When applying pressure to the bottom of the cat’s paw, they evoked the natural neural response that occurs when a cat takes a step. They saw the peculiar pattern of activity in the spinal cord that resembled patterns elicited when the researchers stimulated the leg nerve with biomimetic signals.
However, they saw stimulation of the sciatic nerve in the cat elicit a much different pattern compared to the spine.
“This clearly shows that the commonly used stimulation methods cause the neural networks in the spine to be flooded with information,” Valle said. “This information overload could be the reason for the unpleasant sensations or paraesthesia reported by some users of neuroprosthetics,” Raspopovic adds.
Learning how the nervous system works here
In a clinical trial with leg amputees, the researchers showed the superiority of biomimetic stimulation compared to time-constant stimulation. Their work showed how the signals that mimicked nature produced better results.
Not only could test subjects climb steps faster, but they made fewer mistakes in a task requiring them to climb the same steps while spelling words backward.
“Biomimetic neurostimulation allows subjects to concentrate on other things while walking,” Raspopovic said, “so we concluded that this type of stimulation is more naturally processed and less taxing on the brain.”
Raspopovic believes the new findings highlight the need to move away from unnatural, time-constant stimulation and toward biomimetic signals. This applies to a number of other aids and devices, he says, including spinal implants and electrodes for brain stimulation.
“We need to learn the language of the nervous system,” he said. “Then we’ll be able to communicate with the brain in ways it really understands.”