Robo : Amputees Feel Sense of Touch Through Missing Arms

A collaboration between scientists at University of Chicago and Case Western Reserve University has led to two amputees perceiving a sense of touch in their missing hands. Last month we reported on a man who was able to feel via the fingers of a robotic arm that were equipped with pressure sensors. In that study, the volunteer had a brain implant placed into the primary somatosensory cortex of the brain, but the newly published research relies on activating existing median, ulnar, and radial nerves within the arm, avoiding having to access the brain directly.
The researchers from Case Western and UChicago worked extensively with the two volunteers in the latest study, mimiking how a normal hand would encode signals and trying different stimulation frequencies and signal widths. By working through various combinations of different settings, the investigators were able to identify which resulted in tactile sensations most similar to natural feelings. In the process they developed an understanding of the programming language the body uses to create signals in the nervous system, which will help to further prepare for a future of prosthetic devices with an advanced sense of touch.

Electrical stimulation was delivered by an external stimulator (top left) through percutaneous leads to FINEs implanted on the median, ulnar, and radial nerves of an upper-limb amputee (bottom left). Each electrode contact evokes sensory percepts on small regions of the missing hand of the subject. (Image: Graczyk et al, Sci. Transl. Med.)

From the study abstract in Science Translational Medicine:

We found that stimulation pulse width and pulse frequency had systematic, cooperative effects on perceived tactile intensity and that the artificial tactile sensations could be reliably matched to skin indentations on the intact limb. We identified a quantity we termed the activation charge rate (ACR), derived from stimulation parameters, that predicted the magnitude of artificial tactile percepts across all testing conditions. On the basis of principles of nerve fiber recruitment, the ACR represents the total population spike count in the activated neural population. Our findings support the hypothesis that population spike count drives the magnitude of tactile percepts and indicate that sensory magnitude can be manipulated systematically by varying a single stimulation quantity.