Glasses, wigs and even artificial joints – all this is easily perceived by us as an extension of the human body. Next in line are neuroprostheses: prostheses capable of transmitting tactile sensations to the brain.
Neurophysiologists at the University of Chicago (USA) found that in order to create artificial tactile sensations, especially those that differ in intensity, it is important to take into account the strength and frequency of electrical signals that transmit this information to the brain.
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“We have come close to creating neuroprostheses capable of touch. We understand how to create these sensations in the brain with artificial stimulation,” says study co-author Sliman Bensmaia, associate professor in the Division of Organism Biology and Anatomy at the University of Chicago.
The research by Bensmayi and his colleagues is part of a multi-year Prosthetic Revolution project run under the auspices of the Defense Advanced Research Projects Agency (DARPA). The aim of the project is to create modular upper limb prostheses capable, in particular, of transmitting full-fledged tactile sensations. The project involves a large interdisciplinary team of experts from government agencies, private companies and universities.
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The team of Slimane Bensmayi and colleagues at the University of Chicago is looking specifically at the sensory capabilities of prostheses. For their study, they conducted experiments on monkeys, whose sensory system is very similar to that of humans. Experimental animals were implanted with electrodes in the area of the brain that processes information about touching the hands. The monkeys were trained to sense electrical stimulation, as well as to show the researchers which of two consecutive electrical signals felt stronger.
The researchers varied various parameters of the signals, such as amplitude, frequency, and duration, to understand how these factors affect the animals’ perception of the signals. They were particularly interested in the so-called “Slightly perceptible difference” (EOP) – the minimum change in the parameters of the signal, in which the sensation created by stimulating the brain with this signal was already perceived as different. For example, at a certain frequency, the effect of a signal begins to be felt at a current strength of 20 microamps, and at a strength of 50 microamps, it already creates a different sensation. In this case, the EOP is 30 microamps.
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The sense of touch is a complex mixture of various sensations – contact, pressure, surface texture, vibrations, movement, and so on. Bensmayi’s group has documented the various parameters of the signals and the sensations they evoke when the brain is stimulated. Based on these data, the researchers compiled a kind of “notes” with which it will be possible to play a “concert” of tactile sensations.
“When you take an object in your hand, you can adjust the grip strength of your fingers. In order to realistically convey tactile sensations, we need to know how many “nuances” of pressure can be conveyed by electrical stimulation. Ideally, the range of possible “artificial” sensations should be no less wide than for “natural” ones. Our research is helping to move closer to prototyping such prostheses ready for human trials,” says Slimane Bensmaja.
Подробнее см. S. Kim et al. «Behavioral assessment of sensitivity to intracortical microstimulation of primate somatosensory cortex», PNAS, October 2015.