Enabling someone with paralyzed legs to rise to their feet and walk again has long been considered impossible, the kind of bogus miracle promised by faith healers. But who needs faith healers when you have clever scientists and electricity? In the new field of bioelectronic medicine, doctors may soon make the miraculous a reality. A new experiment using paralyzed monkeys has shown the way toward that goal.
Researchers conducted a proof-of-concept study using two monkeys with partial spinal cord injuries, which prevented brain commands from reaching a back leg. The researchers used electrodes implanted in the monkeys’ brains to record electrical signals from the motor cortex, the part of the brain that controls movement. They used a computer to decode those signals and translate them into commands sent to other electrodes implanted in the monkeys’ lumbar spines; those electrodes stimulated the spinal cord. This brain-spine interface (BSI) bypassed the injured part of the spinal cord, allowing the monkeys’ natural movement commands to reach their injured legs.
Study coauthor David Borton, a neuroengineer at Brown University, says he was surprised by how effortlessly the animals took to the technology. “Their behavior did not make us think that they were bothered by it at all,” he tells IEEE Spectrum. “They didn’t turn around and look at their legs—they just walked.”
Much research remains to be done before humans can benefit from this technology, says study coauthor Gregoire Courtine, a professor at the Swiss Federal Institute of Technology Lausanne, where he focuses on spinal cord repair. “We’re not going to see people walking in the street with brain-spine interfaces tomorrow,” he says. But Courtine and his colleagues are working toward that goal, and are striving to improve the hardware to make it suitable for paraplegic humans.
Other research groups are working toward the same goal, including Susan Harkema of the University of Louisville in Kentucky. IEEE Spectrum has covered her success in using electrodes implanted in the spine to get paraplegic people back on their feet. Harkema’s research, however, uses an external computer to generate the commands that are sent to the implanted electrodes, rather than tapping into the brain’s natural commands.
Both research projects are exciting examples of bioelectronic medicine, a new field that leverages neuroscientists’ growing ability to understand the electrical signals neurons use to communicate. Neurons in the brain “fire” with electrical impulses that control every aspect of our bodies and behavior, and electrodes can pick up these patterns of pulses as they arise in the brain and course through the nervous system.
The brain-spine interface used in the monkeys didn’t directly stimulate specific neurons in the spinal cord to send commands down the leg nerves to the muscles. That type of micromanagement would be akin to a person trying to walk by consciously flexing each leg muscle in turn. Instead, the researchers sent the brain commands to what Courtine calls the “spinal brain,” a network of neurons in the lumbar spine that automatically controls the basic mechanism of walking. “This spinal brain is very smart, and is able to make a lot of decisions,” he says. “But it needs some instructions, and that’s what we’ve been able to provide with this interface.”
There were engineering challenges aplenty in the effort to build a brain-spine interface that worked for freely moving monkeys. The researchers didn’t want any entangling wires, so the brain implant had to wirelessly send its data to the external computer. And there was a lot of data: The 96-electrode array, implanted in the part of the motor cortex that controls the back legs, sent out 40 MB of data per second.
Decoding the signal recorded in the brain was another enormous challenge, this one for the software team. They further developed decoders created by the BrainGate research consortium, which has made headlines over the last decade by using brain-computer interfaces to let paralyzed people control robotic arms and computer cursors. Study coauthor Borton explains that they decided to pull all the data from the neurons—to get a “full bandwidth recording” rather than some filtered data set. “We don’t yet know what the most useful parts of the signal are,” he says. “Later, we can simplify.”
The final challenge was to get the movement command to the monkeys’ spinal cords. Again, the researchers wanted a wireless system, so the monkeys wore little vests containing transmitters that sent the data through skin and tissue to a small pulse generator implanted in the muscles between the ribs. The pulse generator then sent the electrical signal through wires that connected to the electrodes sitting on top of the spinal cord.
[A note on animal testing: The monkeys recovered from their partial spinal cord injuries in due time.]
All the hardware deployed has already been used in humans; both the electrode array implanted in the brain and the pulse generator are commercial products. Courtine says the research team chose to use this gear to facilitate the move from monkey experiments to human clinical trials. The pulse generator comes from medical device company Medtronic, which helped support the research. Courtine is now working with Medtronic on a human feasibility study, which just enrolled its first patient in Lausanne, Switzerland. This preliminary study will only test the spinal stimulation protocol that the researchers developed.
That stimulation protocol is a source of pride to the research team. Most bioelectronic medical treatments use a steady and unvarying sequence of electric pulses; that’s how current treatments such as spinal stimulation for chronic pain work. To get a response from the body, these treatments use blasts of electricity. In contrast, the researchers’ stimulation sequence varied according to the monkey’s brain signals, providing more subtle instructions to the spinal cord. It’s an improvement over prior methods that essentially shouted at the nervous system, Borton says. “Now, we’re learning how to really communicate with it.”
