Research ArticleNEUROTECHNOLOGY

The neural basis of perceived intensity in natural and artificial touch

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Science Translational Medicine  26 Oct 2016:
Vol. 8, Issue 362, pp. 362ra142
DOI: 10.1126/scitranslmed.aaf5187

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Perceived intensity: A touchy subject for neuroprostheses

Without tactile sensory input, amputees discern a firm handshake from a bone-crushing grip by visual cues and learned behavior. Next-generation prostheses aim to lend a more natural feel to artificial touch by transmitting nuanced sensory feedback. Graczyk et al. looked at direct stimulation of the radial, ulnar, and median nerves via implanted electrodes in two amputees to understand how levels of intensity are perceived and how tactile sensory feedback is transmitted. By modulating the number of nerve fibers stimulated and the frequency of stimulation, sensory information could be transmitted such that the amputees could distinguish distinct levels of tactile intensity, that is, the difference between a 7 and a 10 on a scale of intensity.

Abstract

Electrical stimulation of sensory nerves is a powerful tool for studying neural coding because it can activate neural populations in ways that natural stimulation cannot. Electrical stimulation of the nerve has also been used to restore sensation to patients who have suffered the loss of a limb. We have used long-term implanted electrical interfaces to elucidate the neural basis of perceived intensity in the sense of touch. To this end, we assessed the sensory correlates of neural firing rate and neuronal population recruitment independently by varying two parameters of nerve stimulation: pulse frequency and pulse width. Specifically, two amputees, chronically implanted with peripheral nerve electrodes, performed each of three psychophysical tasks—intensity discrimination, magnitude scaling, and intensity matching—in response to electrical stimulation of their somatosensory nerves. 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.

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