Prosthetic arm with feedback

Tactile stimulation has simplified the management of the prosthetic arm

Elizaveta Ivtushok, N+1

American scientists have managed to improve the operation of a motor cortex-controlled prosthetic arm using simultaneous tactile stimulation. To do this, they connected the prosthesis to two plates with microelectrodes: one was located in the motor, and the other in the somatosensory cortex of the left hemisphere of the brain of a paralyzed patient. Additional tactile stimulation of the prosthesis helped the patient to pick up and carry objects twice as quickly, and also with the help of it it was possible to pour cubes from one glass into another without dropping any on the table. The article was published in the journal Science (Flesher et al., A brain-computer interface that evokes tactile sensations improves robotic arm control).

For fine motor skills, not only movements are important, but also the sensory response received, in particular, touch, which helps to determine the boundaries of an object, the density and texture of its surface, and vision, which helps to determine the position of an object in space. With complete or partial paralysis, touch is not available, and feedback can often be obtained only with the help of visual information. Of course, to fully restore the working capacity of the limbs (primarily with the help of a prosthesis controlled by brain activity), only vision as a sensory response channel is not enough, so developers are actively looking for solutions that would help transfer not only motor, but also tactile functions to the prosthesis.

One of these prostheses was shown in 2016 by scientists led by Robert Gaunt from the University of Pittsburgh. Then they implanted a plate of 32 microelectrodes into the brain of Nathan Copeland, a 28-year-old patient with limbs paralyzed as a result of a cervical spine injury: they were placed on the somatosensory cortex of the left hemisphere. Stimulation of areas of the cortex was able to partially restore the sensations of touch (Copeland described them as tingling, pressure or electric shocks). Connecting the electrodes to the prosthesis helped to set up biofeedback: thanks to the stimulation of the somatosensory cortex, Copeland was able to touch them when the sensors on the prosthesis came into contact with the object. 

Now Gaunt and his colleagues have decided to combine stimulation of the somatosensory cortex with stimulation of the motor cortex – also in the left hemisphere. 80 additional microelectrodes were placed in the cortex, which allowed Copeland to freely move the prosthesis in space, as well as grab objects. As a result, the prosthesis was connected to both the motor cortex and the somatosensory cortex–and Copeland could simultaneously move it and feel what he was touching.

To test the effectiveness of using such a prosthesis, Copeland was asked to move several objects from one place to another – either with tactile stimulation turned on or off. So, the participant had to lift, move and lower a ball, a stone, a pen from a stand and several cubes of different sizes. With the help of a prosthesis with connected tactile stimulation, Copeland managed to complete tasks twice as fast on average (10.2 versus 20.9 seconds), and he also dropped objects less often. 

Video from the press release Sense of Touch Improves Control of Robotic Arm.

In an additional task, Copeland was asked to raise a glass with cubes, pour them into another and lower the glass – so the researchers tried to simulate the transfusion of water. With tactile stimulation turned on, the task was completed in 24 seconds; in the same case, when the stimulation was turned off, it took more than a minute, and several cubes spilled out of the glass onto the table.

According to the authors of the work, the effectiveness of using a prosthesis with tactile stimulation is explained by the fact that it reduces the time required to capture an object, which once again confirms the importance of touch for fine motor skills. Despite the fact that the use of such a prosthesis allowed Copeland to perform simple actions with sufficient speed and accuracy, the researchers clarify that the development of their prosthesis is still at the experimental stage: it is unknown, at least, whether it will be possible to achieve similar results in other patients.

Biofeedback also helps when using a prosthetic leg: at the beginning of this year, Swiss scientists found out that such a prosthesis seems easier, and it is also easier to walk and count in your mind at the same time.

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