A study from neuroscientists from the University of Chicago hints at the possibility of someday building prosthetics that re-create a sense of touch via a direct interface with the brain.
Led by Sliman Bensmaia, PhD, associate professor in the Department of Organismal Biology and Anatomy at the University of Chicago and senior author of the study, the research team studied the various nuances of touch, according to a medial release from the University of Chicago Medical Center.
The study, published recently in the Proceedings of the National Academy of Sciences, suggests that artificial touch is highly dependent on several features of electrical stimuli, such as the strength and frequency of signals. They also describe the specific characteristics of these signals, including how much each feature needs to be adjusted to produce a different sensation.
This research is part of Revolutionizing Prosthetics, a multi-year Defense Advanced Research Projects Agency (DARPA) project that seeks to create a modular, artificial upper limb that will restore natural motor control and sensation in amputees. The project has brought together an interdisciplinary team of experts from government agencies, private companies and academic institutions, including the Johns Hopkins University Applied Physics Laboratory and the University of Pittsburgh, the release explains.
“This is where the rubber meets the road in building touch-senstive neuroprosthetics,” Bensmaia says in the release.
“Now we understand the nuts and bolts of stimulation, and what tools are at our disposal to create artificial sensations by stimulating the brain,” he adds.
In their study, the research team implanted electrodes into areas of monkeys’ brains that process touch information from the hand. They then trained the monkeys to perform two tasks: one in which they detected the presence of an electrical stimulus, and another in which they indicated which of two successive stimuli was more intense.
During this time, the research team manipulated various features of the electrical pulses, such as their amplitude, frequency, and duration, and noted how the interaction of these features affected the monkeys’ 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 microamps of electricity. If the signal has to be increased to 50 microamps to notice a difference, the JND in that case is 30 microamps, the release explains.
According to the release, a principle known as Weber’s Law states that the just-noticeable difference between two stimuli is proportional to the size of the stimulus. However, the release continues, this study suggests that, with electrical stimulation of the brain, Weber’s Law does not apply—the JND remains nearly constant, no matter the size of the stimulus. This means that the brain responds to electrical stimulation in a much more repeatable, consistent way than through natural stimulation.
“It shows that there is something fundamentally different about the way the brain responds to electrical stimulation than it does to natural stimulation,” Bensmaia explains.
“This study gets us to the point where we can actually create real algorithms that work. It gives us the parameters as to what we can achieve with artificial touch, and brings us one step closer to having human-ready algorithms,” he states.
[Source(s): University of Chicago Medical Center, Science Daily]