A dual neural bypass restored hand movement and touch in a person with complete tetraplegia
A neural interface restored arm movement and touch in a person with complete tetraplegia, and some of the improvement remained after the system was switched off
A study published in Nature Medicine on 16 July describes one person with a complete cervical spinal cord injury. The implants linked his intention to move his hand to stimulation of the muscles and spinal cord, while signals from the hand were relayed to the sensory cortex. After a course of training, he could once again eat independently and pick up fragile objects.
A spinal cord injury disrupts two communication pathways at once. The motor cortex sends commands to the hand, while the skin and joints return information about touch, force, and finger position to the brain. Without this feedback, holding a glass is difficult because the hand must not only close around it, but also relax its grip at the right moment.
Previous neural interfaces addressed these functions separately. One implant could detect an attempt to move and activate electrical stimulation of the paralyzed muscles. Another could stimulate the sensory cortex to produce the sensation of touch on a finger. A study of sensory implants published the previous day showed that these sensations can persist for years. Santosh Chandrasekaran, Chad Bouton, and their colleagues combined movement and touch in a dual neural bypass.
Electrodes in the motor cortex detected the participant's intention to open or close his hand. Software translated that signal into stimulation of the muscles and spinal cord, while sensors on the hand relayed touch signals to the cortical region that contains the brain's map of the body. The system helped the hand move in real time, but the authors aimed to do more. They sought to reconnect the attempted movement, sensory feedback from the skin, and the remaining neural pathways.
To achieve this, they used a “cortical mirror”. At the same time, they prompted the participant to imagine being touched, stimulated his skin, and activated selected electrodes in his spinal cord and brain. After repeated training sessions, his elbow flexion and sensation of touch at the wrist improved. These changes remained when the stimulators were switched off.
A neuroprosthesis can temporarily replace a damaged pathway. The brain issues a command, and the electronics deliver it to the muscles. This study tests whether such an artificial bridge can preserve some movement and sensation after the device has been switched off.
A single case cannot establish whether the effect can be reproduced in other people with spinal cord injuries or how long it will persist. However, the study defines a more precise, testable question for neurotechnology: after the device is switched off, can the person still bring his hand to his face, hold an object, and feel his hand?