By Julia Travers

Touch-sensitive prosthetics are unarguably a life-changing development. For amputees, the prospect of being able to experience not just sensation with their prosthetics, but the level of intensity of the sensation as well, is powerful and infinitely useful. A team of scientists is making great strides in providing that very experience to amputees in a study called, The neural basis of perceived intensity in natural and artificial touch, published in October 2016 in Science Translational Medicine.

As Smithsonian.com succinctly explains, the study focused on a new “prosthetic arm that integrates touch-sensing electrodes into the body's nervous system.” The researchers worked with amputees using this prosthetic to study how the nervous system communicates the experience of tactile intensity--the intensity of touch sensations. To understand how this can serve amputees, imagine all of the activities in your life that rely on the subtle perception and application of various amounts of pressure. We interviewed one of the study’s authors, Dustin Tyler, Ph.D. He is a Professor of Biomedical Engineering at Case Western Reserve University and the Associate Director of the Cleveland Advanced Platform Technology Center of Excellence.

The system includes pressure sensors on the prosthetic hand and connected electrode cuffs, which receive information from the pressure sensors and stimulate the nerves controlling hand motion--the radial, ulnar, and median nerves. These nerves communicate with the brain about the intensity of the tactile sensation. Electrical stimulation of the nerves allows the researchers, as the study abstract explains, “to elucidate the neural basis of perceived intensity in the sense of touch,” and has also been used in the past to restore sensation to amputees. Tyler explained the psychophysical tests the amputees participated in for us:

“There were three basic tasks that we applied. These are standard tests applied to people with intact perception. We modified them as needed for artificial sensation and its comparison to intact hands.

The first is an open-ended scale response to intensity magnitude. Basically, we applied artificial sensation and asked the subject to assign a rating to the intensity of the sensation. The only requirement was that the numbers be proportional. That is, if one stimulation was rated as “1” and another stimulation felt twice as intense, then it was rated a “2”. This generated an open-ended psychometric assessment of perception of intensity.

Second, we performed a “Just Noticeable Difference” test. In this case, many pairs of artificial sensations were provided to the subject. For each pair, they were forced to choose which of the two sensations was more intense. From this data, we are able to determine the smallest change in stimulation that the subject can reliably detect or, in other words, how sensitive the user’s sense of touch is with artificial stimulation. We were able to determine that artificial sensation has the same sensitivity as a normal hand.

Finally, we provided a natural, mechanical stimulation to the subject’s intact hand and an artificial sensation to the amputated hand to see if they could match the intensity of the physical stimulation of the intact hand with artificial stimulation, which they could do reliably.

The advantage of these testing techniques, compared to other more descriptive approaches, is that they are less susceptible to biasing in the data. Sensation is a very qualitative thing and without validated tests with blinded, forced-choice experiments such as these, it is very easy for the user to provide “correct” responses due to biasing rather than actual tactile perception.”

I also asked him to describe what the experience actually feels like for the amputees. Tyler explained that while they do have conscious feeling resulting from the stimulation, “they do not fell electrical impulses, per se. They feel sensation on their hand. They perceive the sensation as though they had their hand again. Sometimes, it feels like a tingle and sometimes like a pressure. We were not trying to create a specific sensory quality but focused on intensity. The approach to providing different sensations was described in our Sci Tran Med paper in 2014 (Tan, et. al., Feb 2014). It always, however, felt like their hand.”

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Courtesy of http://engineering.case.edu/profiles/dxt23

There’s a great YouTube video from Case Western Reserve University that tells the story of Igor Spetic, an amputee, who describes using one of the touch-sensitive prosthetics in this way: “it feels like my hand, instead of a tool.” In the same video, Tyler shares this insight on helping amputees in this manner: “as Igor says, to become two handed again, to be able to provide that again for him, is really rewarding.”

From Tyler, here are some of the challenges the study faced:

“The amount of data and time with the subjects required to complete these studies is one of the greatest challenges. This required over six months of data collection and dedication by the subjects and the experimenters.

The second challenge was discovery of the relationship between the stimulation parameters and the perception of intensity. We were confident that both stimulation strength and the rate of stimulation of the nerve would influence the perceived intensity of a sensation. It was the analysis of the data and linking that analysis to computational models of the artificial stimulation that allowed the discovery and validation of the underlying connection between stimulation and naturalist perception.”

Here’s what he hopes will be the next steps or future applications of this study of neural response to tactile intensity:

“We will be combining the results of the 2014 paper and this paper to produce a wide range of texture and tactile discrimination using the prosthetic hand. Now that we know how to apply sensation with the same range and sensitivity of an intact hand, we will apply these technologies to use of the hand in normal daily activities to provide users an experience that is as close to being their own hand as possible. Essentially, we want the subject to think of the prosthesis as their hand and to forget that they ever lost a hand. We are now combining the sensory feedback with advanced approaches for controlling the movement of the prosthesis so that the movements also feel like their original hand. Combining natural sensation with natural movement, we seek to provide a replacement hand that is as close as possible to the original hand.”

In the video mentioned above, Tyler also says of this work: “this is now making an impact in the real world. This is not science that’s going to sit on the shelf…this is real.”

You can visit Dustin Tyler, Ph.D.‘s site here and can follow the Case Western Reserve University Department of Biomedical Engineering on Facebook and Twitter.

-Julia Travers

Full Study Citation:

The neural basis of perceived intensity in natural and artificial touch

BY EMILY L. GRACZYK, MATTHEW A. SCHIEFER, HANNES P. SAAL, BENOIT P. DELHAYE, SLIMAN J. BENSMAIA, DUSTIN J. TYLER

SCIENCE TRANSLATIONAL MEDICINE26 OCT 2016: 362RA142