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Soft or firm touch? Study reveals how amputee patients tell the difference

American Association for the Advancement of Science

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IMAGE: Implanted peripheral nerve electrodes deliver stimulation directly to the nerve. Electrical stimulation was delivered by an external stimulator (top left) through percutaneous leads to flat interface nerve electrodes (FINEs) implanted on the... view more

Credit: Graczyk et al., Science Translational Medicine (2016)

A new study uncovers how two men with amputations, who had electrodes implanted in their residual limbs, discern between soft and firm touch. Integrating this complex sensory intelligence about pressure could help make simulating touch in neuroprosthetics feel more natural for patients with limb loss or paralysis, the researchers say. Stimulation of nerves through electrical pulses delivered by implanted electrodes enables amputees to move and direct robotic limbs to grasp objects. Importantly, artificial limbs also need to provide sensory information about intensity of touch, allowing the user to apply just enough force to manipulate objects, without crushing them. To better understand how neurons decode touch intensity and how tactile sensory feedback is transmitted, Emily Graczyk and colleagues examined the responses of two study subjects, with long-term peripherally-placed nerve electrodes, as they performed various tasks related to deciphering touch intensity. The subjects were able to distinguish stimulations across a wide range of intensities and reliably matched the magnitudes of mechanical skin indentations applied on their intact hands to artificial sensations delivered to the nerves of their missing hands. The scientists found that the firing rate of activated neurons and the number of activated neurons, a metric they called the activation charge rate, ultimately controlled the magnitude of perceived intensity of the artificial sensations. The authors emphasize that this metric could be manipulated to improve artificial touch in next-generation neuroprosthetics.

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