The illusion of a rubber hand turned the prostheses into a part of the body

Sofia Dolotovskaya, N+1

American and Swedish researchers have shown that the use of the illusion of a rubber hand in combination with electrical stimulation of the somatosensory cortex of the brain allows the subjects to create the feeling that an artificial hand is part of their body. Article by Collins et al. Ownership of an artificial limb induced by electrical brain stimulation is published in the journal Proceedings of the National Academy of Sciences.

Recently, researchers have been constantly expanding the functionality of prosthetic limbs: bionic prostheses have already been created that provide not only motor control, but also touch due to the connection between sensors in the artificial limb and electrodes implanted in the brain. However, until now, scientists have not been able to create a feeling in the owners of artificial hands that the prosthesis is a part of their body, and not a foreign object. It is difficult to do this, in particular, because most of the experimental interfaces that provide sensitivity to the prosthetic arm are connected to peripheral nerves. During training, their stimulation leads to the formation of new neuronal connections in the brain, which translate signals from the sensors of the prosthesis into tactile sensations. However, such interfaces require undisturbed communication of peripheral nerves with the cerebral cortex and therefore are not suitable for patients with spinal cord injuries or neurodegenerative diseases.

The authors of the new article decided to use electrical stimulation of the somatosensory cortex in combination with the illusion of a rubber hand to create a feeling of having a real hand in the owners of prostheses, bypassing the peripheral nerves. A classic experiment demonstrating the illusion of a rubber hand was described in 1998. In this experiment, one of the subject's hands was hidden behind the screen, leaving the rubber hand "replacing" it in sight. When touching both hands simultaneously – the rubber and the real one – the volunteer had the feeling that the rubber hand belonged to him, and tactile sensations were localized in it. Interestingly, this illusion persists even when using other objects instead of a rubber hand – for example, a smartphone.

Two volunteers with epilepsy participated in the tests described in the new article, who were assigned electrocorticographic mapping of the brain, requiring electrodes to be brought directly to the cerebral cortex, to identify foci of epileptic activity in the brain. The electrodes were implanted into the somatosensory cortex: in one patient in the representation (the area responsible for sensitivity) of the right hand, and in another – in the representation of the left hand.

The real hand of the subjects was covered from them by a screen, and instead of it, an artificial rubber hand was placed on the table in front of them – so that they could see it well. The electric generating device touched this rubber hand with a special probe and simultaneously generated an electric pulse with each touch, stimulating the representation of the corresponding parts of the hand in the somatosensory cortex. At the same time, the volunteers were asked to tell how much they feel the rubber hand as their own, as well as to localize touching it.

Drawing from an article in PNAS

It turned out that the subjects easily localized the touches with accuracy to the phalanx of the finger. At the same time, both volunteers described the sensations that the stimulation of the somatosensory cortex causes in them as "unnatural" and unlike anything they had experienced before. So, one subject described his sensations as resembling vibration, and the other as similar to light pressure. Despite this, both volunteers reported that they felt the rubber hand as their own.

Interestingly, the illusion of "one's" hand was preserved only when touching it was synchronized with electrical impulses stimulating the somatosensory cortex. When electrical impulses were generated late, this illusion disappeared. It also disappeared if the researchers stimulated areas of the somatosensory cortex responsible not for the hands, but for other parts of the body. This suggests that in order to feel an artificial hand as one's own, a simultaneous combination of a visual stimulus (the subject sees how the device touches the rubber hand) with stimulation of the corresponding area of the somatosensory cortex is necessary. As the authors conclude, the results obtained confirm that with the help of electrical stimulation of the brain, it is possible to create in the owners of prostheses the feeling that an artificial arm is part of their body.

Recently, American scientists have developed a neurointerface that allowed a completely paralyzed person to touch objects with a prosthesis. The operation of this interface connected to the sensors on the prosthesis is also based on direct stimulation of the somatosensory cortex bypassing the peripheral nerves and provides sensations close to natural.

Portal "Eternal youth" http://vechnayamolodost.ru  21.12.2016