Scientists have found, in a breakthrough, a novel way to successfully produce realistic sensations of touch in human amputees by directly stimulating the nervous system.
Scientists at the University of Chicago and Case Western Reserve University in the US worked with two male subjects who each lost an arm after traumatic injuries.
Both men were implanted with neural interfaces; devices embedded with electrodes that were attached to those nerves of the arm 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 Sliman Bensmaia, neuroscientist at 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," said Bensmaia.
Researchers systematically tested the subject's 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.
Previous research by Bensmaia had predicted how the nervous system discerns intensity of touch, for example, how hard an object is pressing against the skin. It suggested that the number of times certain nerve fibres 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.
While the new study furthers the development of neural interfaces for neuroprosthetics, artificial touch will only be as good as the devices providing input.
Using the same behavioural techniques that are used to test human sensory abilities, researchers tested the finger's ability to distinguish different touch locations, different pressure levels, the direction and speed of surfaces moving across it and 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. The study was published in the journal Science Translational Medicine.
