Bio Focus: Brain-spine interface helps partially paralyzed monkeys walk again

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he spinal cord serves as a kind of information relay center, transmitting signals between the brain and the rest of the body—damage to this center can lead to various issues, including paralysis. Between 250,000 and 500,000 people suffer a spinal-cord injury each year, according to the World Health Organization. For decades, researchers have sought to find ways to restore communication between mind and body in people with damaged spinal cords, and they have made some significant advances. Brain-computer interfaces and other technologies have allowed some people to control machines (robotic arms) with their thoughts and others to regain some function to their paralyzed hands. In a leap forward for the field, an international team of researchers has created a new technology that allowed a rhesus macaque with a paralyzed leg (due to a spinal lesion) regain the ability to walk within a week, without training. The new

technology, called a “brain-spine interface” and described recently in Nature (doi:10.1038/nature20118), works by bypassing severed nerves in the spinal cord. That is, a computer decodes locomotionrelated signals from the monkey’s brain into intentions and sends instructions to an implanted pulse generator (situated below the spinal-cord site of injury), which stimulates sensory neurons involved in controlling the flexion and extension of the limbs—all in real time. “For a long time, we’ve been focused on the pharmaceutical world and have neglected the possibility of technological devices that may provide very good therapy,” says David A. Borton, a neuroengineering professor at Brown University and one of the study’s lead authors. “We’re moving into an age where technology is getting small enough and safe enough that we could use it in ways that we haven’t thought of before.” While the new work certainly builds on prior research in the field, it is distinct in significant ways. All previous demonstrations of brain interfaces have focused on the upper limbs, specifically recording signals from the hand and arm areas of

The brain-spine interface works by first decoding signals from the motor cortex into intentions and then using electrical pulses to target specific circuits of the spinal cord that ultimately drive locomotion. Shown here: a pulse generator and a silicon model of a primate’s brain with an attached microelectrode array. Credit: Alain Herzog, EPFL.

the motor cortex (the region of the brain that is responsible for voluntary movement). Additionally, other systems have not stimulated nerve cells that ultimately control muscle movement; they have focused instead on directly stimulating muscles. Movement in the legs that enables walking is controlled by neurological activity in the leg area of the motor cortex. Signals from this area travel down the spinal cord to its lumbar (lower) region where they activate motor neurons that coordinate muscle movement. If the spinal cord is damaged or severed, however, those brain signals are unable to reach their destination in the lumbar spinal cord, resul