Healthy people can hardly imagine what it is like to suddenly no longer be able to move their arms and hands.

For people who were injured to the spinal cord in a serious accident and have been completely paralyzed since then, this is a reality.

Many sufferers can already control prostheses via a brain-computer interface by converting brain activities into mechanical movements.

Manfred Lindinger

Editor in the “Nature and Science” section.

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    So far, however, patients often lack sensitivity in the artificial limbs. This could change soon. Scientists from the University of Pittsburgh have developed a robotic arm that uses sensors to register tactile information and relay it to the brain. In this way, a 28 year old man paralyzed in the legs and arms can feel something with his artificial hands as soon as they grasp something.

    Nathan Copeland is a patient at the Department of Neurology in Pittsburgh and a subject in a unique multi-year study. In the brain of the man whose spinal cord was injured in a serious accident at the age of 18, microelectrodes were permanently implanted in the area of ​​the motor cortex. They transmit the electrical signals from the nerve cells via external contacts on his head to a computer that controls a robotic arm. Only the idea of ​​moving the right arm leads to the robot moving.

    After intensive training, Nathan can now steer the fully articulated arm and hand in all directions and move each finger individually.

    In the meantime he has learned to grasp objects of different sizes, to pick them up and to place them in another place.

    The control of hand movements has so far only been carried out via the eyes.

    Faster and more precise thanks to tactile feedback

    Until now, Nathan has not been able to sense whether an object is heavy or light, hard or solid. A raw egg would surely have broken between his fingers. The researchers working with Jennifer Collinger and Robert Gaunt have now been able to remedy this deficiency with another brain-machine interface. As they

    report

    in the journal

    Science

    , they have given the robotic arm tactile capabilities through sensory feedback.

    The researchers equipped the five fingers, the palm of the hand and the arm of the robot with touch sensors. Electrodes were placed in the patient's somatosensory cortex - an area of ​​the brain that is responsible for stimulus perception. The electrical signals from the sensors stimulate the neurons located there. Thanks to the tactile information, Nathan can now feel that he is holding something in his right hand and estimate how big and heavy the object is.

    This has an impact on the movements of the robot arm, as can be seen on a video.

    Nathan can now grab hold of the item much more securely, hold the item tighter and move it to a place faster.

    The movements of the robot hand are much more precise than before, without the sensory feedback.

    This became apparent when a glass of water had to be poured into a container.

    Nathan didn't spill anything anymore and put the empty glass back in its place of origin.

    According to the researchers, no longer training was required for these skills.

    The motor skills achieved are still too rough for fragile objects.

    And so, despite the progress made, it will take time before prosthetic wearers feel as though they are wearing a real arm or hand.