Sending signals that the brain can understand

A few years ago, a team of researchers working under the leadership of Professor Stanisa Raspopovic from the Neuroengineering Laboratory at ETH Zurich attracted worldwide attention when they announced that their prosthetic legs allowed amputees to feeling for the first time the sensations of this artificial part of the body.

Unlike commercial leg prostheses, which simply provide stability and support to amputees, the ETH researchers’ prosthesis was connected to the sciatic nerve in the test subjects’ thighs via implanted electrodes.

This electrical connection allowed the neuroprosthesis to communicate with the patient’s brain, for example by relaying information about the constant changes in pressure detected on the sole of the prosthetic foot during walking. This gave the test subjects greater confidence in their prosthesis and allowed them to walk much faster over difficult terrain.

“Our experimental leg prosthesis successfully evoked natural sensations. This is something that current neuroprosthetics are mostly incapable of doing; instead, they mainly evoke artificial, unpleasant sensations,” says Raspopovic.

This is probably because current neuroprosthetics use constant electrical impulses over time to stimulate the nervous system. “It is not only unnatural, but also ineffective,” says Raspopovic.

In an article recently published in Natural communicationshe and his team used the example of their prosthetic legs to highlight the benefits of using naturally inspired biomimetic stimulation to develop the next generation of neuroprosthetics.

The model simulates the activation of nerves in the sole

To generate these biomimetic signals, Natalija Katic, a doctoral student in Raspopovic’s research group, developed a computer model called FootSim. It is based on data collected by collaborators in Canada, who recorded the activity of natural receptors, called mechanoreceptors, in the sole of the foot by touching different points on volunteers’ feet with a vibrating rod.

The model simulates the dynamic behavior of a large number of mechanoreceptors in the sole of the foot and generates neural signals that travel up the nerves in the leg to the brain, from the moment the heel touches the ground and the weight of the body begins to relax. step toward the outside of the foot until the toes are off the ground, ready for the next step.

“Through this model, we can see how the sensory receptors in the sole and the connected nerves behave during walking or running, which is experimentally impossible to measure,” explains Katic.

Information overload in the spinal cord

To assess how well the biomimetic signals calculated by the model correspond to signals emitted by real neurons, Giacomo Valle, a postdoctoral fellow in Raspopovic’s research group, worked with colleagues from Germany, Serbia and Russia on experiments with cats, whose nervous systems process movement in a manner similar to that of humans. 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 others to the spinal cord to discover how signals are transmitted through the nervous system. When the researchers applied pressure to the bottom of the cat’s paw, evoking the natural neural response that occurs when a cat takes a step, the particular pattern of activity recorded in the spinal cord did indeed resemble the patterns caused in the spinal cord. when researchers stimulated the nerve in the leg with biomimetic signals.

In contrast, the conventional approach of constant stimulation over time of the sciatic nerve in the cat’s thigh caused a markedly different activation pattern in the spinal cord. “This clearly shows that commonly used stimulation methods cause a flood of information in the neural networks of the spine,” says Valle. “This information overload could be the cause of unpleasant sensations or paresthesias reported by certain users of neuroprosthetics,” adds Raspopovic.

Learn the language of the nervous system

In their clinical trial carried out with leg amputees, the researchers were able to show that biomimetic stimulation is superior to constant stimulation over time. Their work clearly demonstrated how cues that mimic nature produced better results: not only were test subjects able to climb steps faster, but they also made fewer errors in a task that required them to climb the same steps by spelling words backwards.

“Biomimetic neurostimulation allows subjects to focus on other things while walking,” explains Raspopovic, “so we concluded that this type of stimulation is processed more naturally and less taxing on the brain.”

Raspopovic, whose laboratory is part of the Institute for Robotics and Intelligent Systems at ETH, believes that these new findings do not only concern the prosthetic limbs that he and his team have been working on for more than five years. He argues that the need to move away from unnatural, constant stimulation over time and toward biomimetic signals also applies to a range of other aids and devices, including spinal implants and electrodes for brain stimulation.

“We need to learn the language of the nervous system,” says Raspopovic. “We will then be able to communicate with the brain in a way that it really understands.”