In a study published in the journal Proceedings of the National Academy of Sciences, researchers have shown that it is possible to re-create the sense of touch in prosthetic limbs by using a direct interface to the brain.

Showing that artificial touch depends on several electrical stimuli features, like the frequency and strength of signals, the researchers were able to describe specific signal characteristics, including how each can be adjusted to create different sensations, in the study. Now that they have a clear idea of what goes into producing stimulation and have the tools needed to create them, the researchers are now ready to apply their findings in building neuroprosthetics sensitive to the touch.

From movement, vibration and texture to pressure and contact, the sense of touch is made up of a nuanced and complex set of sensations. By noting the specific increments, range and composition of signals that allow for each sensation to be different, Sliman Bensmaia, Ph.D. and colleagues were able to provide the "notes" that need to be played to produce "music" in the brain that is the sense of touch.

"When you grasp an object, for example, you can hold it with different grades of pressure. To recreate a realistic sense of touch, you need to know how many grades of pressure you can convey through electrical stimulation," said Bensmaia, adding that artificial and natural touch should ideally have the same dynamic range.

For the study, the researchers worked with monkeys with sensory systems closely resembling those of people. Electrodes were implanted in the monkeys' brains where touch information from the hand is processed and the animals were trained to do two tasks: one showing that they were able to detect electrical stimulus and the other indicating which of the two successive stimuli they received was more intense.

The researchers also manipulated different electrical pulse train features, like duration, frequency and amplitude, noting how interactions between these features affected the ability of the monkeys to detect signals.

The study is part of a multi-year project from the Defense Advanced Research Projects Agency called Revolutionizing Prosthetics. The project seeks to create modular artificial upper limbs to restore natural sensation and motor control for amputees and was brought together by various experts from academic institutions, private companies and government agencies.

Photo: Lisa Williams | Flickr