Once more, with feeling
Prosthetic medicine

Electrodes in the brain can mimic sensations from the hand

Science & technologyOct 15th 2016 edition

ARTIFICIAL limbs are nothing new. An ersatz toe, made of wood and leather and found attached to the mummified body of an Egyptian noblewoman in Cairo, in 2000, is thought to be between 2,500 and 3,000 years old. Things have moved on since then. Modern prostheses sport things like articulated fingers that can be controlled by picking up impulses from their wearer’s remaining nerves.

But a bionic limb receiving commands is only half the picture. To be a true replacement, it should also be able to send sensations back to its wearer, to enable him to control it precisely. And, in a study just published in Science Translational Medicine, a group of researchers led by Robert Gaunt, an engineer at the University of Pittsburgh, have taken an important step towards this goal. Their paper describes a way of restoring sensation by using implanted electrodes to stimulate a patient’s brain directly.

The patient in question is a 30-year-old man whose spinal cord was damaged in a car crash. He is not an amputee, but his injury means his brain and his hands can no longer communicate. Dr Gaunt’s aim was, first, to create an illusion in the patient’s brain that this was not the case, and then to use that illusion to send to that brain signals from an artificial hand which was equipped with sensors on its fingers.

Sensations from the hands are processed in parts of the brain called the somatosensory cortices, of which there is one in each cerebral hemisphere. But these areas process input from the rest of the body too, so Dr Gaunt had first to locate the precise areas within that dealt with the hands. Choosing the left hemisphere, which controls the right-hand side of the body, and asking the patient to imagine feeling sensations in his hands while he did so, he monitored tiny fluctuations in blood flow, and in the magnetic fields generated by the brain’s electrical activity. This told him exactly where the electrodes should go.

A week after the surgery which implanted them, his team began a series of tests, sending tiny pulses of electricity through the electrodes and into the patient’s brain. For the first three weeks the only feelings he reported were tinglings in his hand and arm, often when no stimulus was being applied. These, however, faded away and after four weeks of testing he began to report sensations that correlated with the electrodes firing.

Stimulating different electrodes led to sensations that seemed to the patient to be coming from different parts of his hand. Only some of the hand was covered (see diagram), but within the areas that were, he was able to pinpoint the apparent origin of the feeling quite precisely.

The final step was to hook the patient up to a prosthetic hand equipped with sensors of its own. He was then blindfolded while members of the team touched individual fingers on the prosthesis. Accuracy was good. Even with no prior training, he identified the correct finger 84% of the time. When he did make a mistake, it was usually to attribute sensation to a finger adjacent to the one being touched.

There were snags. The patient reported that some sensations seemed to be coming from below the skin of his hand, rather than from the surface. And none of the sensations extended to any of his fingertips, despite the electrodes being placed in what should have been the correct parts of the brain. These, though, are teething troubles. The important thing is that Dr Gaunt has shown his approach works, and also that it is safe. Though the data in the paper cover just the first six months of the experiment, it has now been going on for three times that period, without any sign of safety problems.

The next step, says Dr Gaunt, is to combine his team’s work with that of another group at Pittsburgh, which is using the same sorts of implanted electrodes to listen to the brain rather than talking to it. The goal is to merge these areas of research and produce a limb that is both controlled by thought, and provides natural-feeling feedback to its user.

This article appeared in the Science & technology section of the print edition under the headline "Once more, with feeling"

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