Signals from tactile sensors under the sole of the prosthetic foot and from angle sensors in the electronic prosthetic knee joint are passed on to the nervous system.
Image courtesy of ETH Zurich..
Research conducted at ETH Zurich, Switzerland, found that transmitting sensory signals from a lower-limb prosthesis user’s nervous system helped him perceive the prosthesis as part of his body. The feedback helped the participant perceive the prostheses as significantly lighter, which can improve acceptance of the devices.
Even though prosthetic legs are usually less than half the weight of a natural limb, users can have the perception that the weight is too high.
Stanisa Raspopovic, Prof. Dr., a professor in the university’s Department of Health Sciences and Technology, led the research. The nervous system feedback is done via electrodes implanted in the thigh, which are connected to the nerves present there. Information from tactile sensors under the sole of the prosthetic foot and from angle sensors in the electronic prosthetic knee joint are converted into pulses of current and passed into the nerves.
“To trick an above-knee amputee’s brain into the belief that the prosthetic leg was similar to his own leg, we artificially restored the lost sensory feedback,” said Raspopovic.
In order to determine how heavy a transfemoral amputee perceives their prosthetic leg to be, the study participant completed gait exercises with either neurofeedback switched on or off. The researchers weighed down the healthy foot with additional weights and asked for a rating of how heavy the participant felt the two legs were in relation to each other. Neurofeedback was found to reduce the perceived weight of the prosthesis by 23 percent, or almost 500 grams.
The scientists also confirmed a beneficial involvement of the brain by a motor-cognitive task, during which the volunteer had to spell five-letter words backwards while walking. The sensory feedback allowed him to have a faster gait and a higher spelling accuracy.
“Neurofeedback not only enables faster and safer walking and positively influences weight perception,” said Raspopovic. “Our results also suggest that, quite fundamentally, it can take the experience of patients with an artificial device closer to that with a natural limb.”
The article, “Lightening the perceived prosthesis weight with neural embodiment promoted by sensory feedback,” was published in Current Biology.
Editor’s note: This story was adapted from materials provided by ETH Zurich/Fabio Bergamin.
