A new study shows that artificial touch is highly dependent on several features of electrical stimuli, such as the strength and frequency of signals.
It describes the specific characteristics of these signals, including how much each feature needs to be adjusted to produce a different sensation.
"This is where the rubber meets the road in building touch-sensitive neuroprosthetics," said Sliman Bensmaia, associate professor at the University of Chicago and senior author of the study.
"Now we understand the nuts and bolts of stimulation, and what tools are at our disposal to create artificial sensations by stimulating the brain," said Bensmaia.
Bensmaia's research is part of Revolutionising Prosthetics, a multi-year Defence Advanced Research Projects Agency project that seeks to create a modular, artificial upper limb that will restore natural motor control and sensation in amputees.
Bensmaia and his UChicago colleagues are working specifically on the sensory aspects of these limbs. For this study, monkeys, whose sensory systems closely resemble those of humans, had electrodes implanted into the area of the brain that processes touch information from the hand. The animals were trained to perform two perceptual tasks: one in which they detected the presence of an electrical stimulus, and a second in which they indicated which of two successive stimuli was more intense.
During these experiments, Bensmaia and his team manipulated various features of the electrical pulse train, such as its amplitude, frequency and duration, and noted how the interaction of each of these factors affected the animals' ability to detect the signal.
Of specific interest were the "just-noticeable differences," (JND) or the incremental changes needed to produce a sensation that felt different.
For instance, at a certain frequency, the signal may be detectable first at a strength of 20 microamperes of electricity. If the signal has to be increased to 50 microamperes to notice a difference, the JND in that case is 30 microamperes.
The sense of touch is really made up of a complex and nuanced set of sensations, from contact and pressure to texture, vibration and movement. By documenting the range, composition and specific increments of signals that create sensations that feel different from each other, Bensmaia and his colleagues have provided the "notes" scientists can play to produce the "music" of the sense of touch in the brain.
"When you grasp an object, for example, you can hold it with different grades of pressure. To recreate a realistic sense of touch, you need to know how many grades of pressure you can convey through electrical stimulation," Bensmaia said.
The study was published in the journal PNAS.