Implants to restore sensations in the paralyzed limbs
Researchers at the California Institute of Technology (Caltech) managed to reproduce a number of sensations in the hand of a paralyzed person. They managed to achieve this thanks to the electrodes implanted in the brain, which stimulate the neurons responsible for the different sensations depending on the type of electrical signals.
Currently, neural prosthetic devices implanted in the brain’s motor cortex can allow patients with amputations or paralysis to control the movement of a robotic limb or can be either connected to or separate from the patient’s own limb. This scientific achievement will help in creating high-tech prostheses, through which users can feel objects just like or approaching normal sensory.
To do this, the researchers implanted two tiny arrays of electrodes into the somatosensory cortex, responsible for sensations of movement, body position and skin sensations, including touch, pressure, and vibration. In the next stage, the researchers will link the stimulation system to the artificial limbs, so that patients can feel certain types of touches with their help as if it were a “living” hand. It was done in the Richard Andersen Lab. A paper describing the work published on 10 April 2018 issue of the journal eLife.
The somatosensory cortex is a strip of a brain that governs bodily sensations, both proprioceptive sensations (sensations of movement or the body’s position in space) and cutaneous sensations (those of pressure, vibration, touch, and the like). Previous to the new work, neural implants targeting similar brain areas predominantly produced sensations such as tingling or buzzing in the hand. The Andersen lab’s implant is able to produce much more natural sensation via intracortical stimulation, akin to sensations experienced by the patient prior to his injury.
The patient had become paralyzed from the shoulders down three years ago after a spinal cord injury. Two arrays of tiny electrodes were surgically inserted into his somatosensory cortex at Keck Hospital of USC, and the human studies were conducted at Rancho Los Amigos National Rehabilitation Center. Using the arrays, the researchers stimulated neurons in the region with very small pulses of electricity. The participant reported feeling different natural sensations—such as squeezing, tapping, a sense of upward motion, and several others—that would vary in type, intensity, and location depending on the frequency, amplitude, and location of stimulation from the arrays. It is the first time such natural sensations have been induced by intracortical neural stimulation.
“It was quite interesting,” the study participant says of the sensations. “It was a lot of pinching, squeezing, movements, things like that. Hopefully, it helps somebody in the future.”
Though different types of stimulation did indeed induce varying sensations, the neural codes governing specific physical sensations are still unclear. In future work, the researchers hope to determine the precise ways to place the electrodes and stimulate somatosensory brain areas in order to induce specific feelings and create a kind of dictionary of stimulations and their corresponding sensations.
The next major step, according to Andersen, is to integrate the technology with existing neural prosthetics. In 2015, Andersen’s laboratory developed brain-machine interfaces (BMIs) to connect a prosthetic robotic arm to electrodes implanted in the region of the brain that governs intentions. In this way, a paralyzed man was able to utilize the prosthetic arm to reach out, grasp a cup, and bring it to his mouth to take a drink. Connecting the device to the somatosensory cortex would create bidirectional BMIs that would enable a paralyzed person to feel as before, while using special prosthetic limbs.
“Currently the only feedback that is available for neural prosthetics is visual, meaning that participants can watch the brain-controlled operation of robotic limbs to make corrections,” -Andersen, Richard A.
The research paper is titled “Proprioceptive and Cutaneous Sensations in Humans Elicited by Intracortical Microstimulation.”