Skip the Robotics: Paralyzed Limbs Come to Life
with New Connection to Brain
Rerouting signal from neuron to muscle allows the brain to move deadened limbs
By Sharon Guynup
From the February 2009
Scientists have forged a promising avenue in the quest to restore mobility to patients paralyzed by disease or injury. Researchers at the University of Washington devised a way to reroute signals from the brain’s motor cortex to trigger hand movement directly.
For the past decade researchers have focused on “listening to” and decoding the specific brain signals that trigger muscle movement, using a wall of computers running complex algorithms to translate that brain activity into instructions for moving a computer cursor or a robotic arm or leg.
The new approach simplifies the process. Engineers and neuroscientists restored use of a monkey’s immobilized limb by replacing the lost biological connection. “Rather than decoding intention, we’ve just established a connection and encouraged the monkey to learn how to act on it,” says Chet Moritz, a neurophysiologist, who pioneered the work with fellow Washington professor Eberhard Fetz.
They trained macaques to play a simple video game using a joystick. Then they ran a wire from a single neuron in the animals’ motor cortex to a desktop computer. The electrical impulse from that cell was amplified by the computer and transmitted along another wire to one of the primates’ arm muscles, which had been temporarily anesthetized.
Within minutes, the monkeys learned to control wrist movements with their thoughts, moving the joystick left or right to match targets on a computer screen.
The surprise, Moritz says, was that any neuron within that general region of the brain could learn to stimulate wrist muscles—regardless of whether the neuron was originally involved in that specific movement.
“Monkeys can rapidly learn to change neuron activity, in this case to generate movement, much like humans can change heart rate activity with biofeedback,” Fetz explains. This control necessitated conscious attention; making such movements subconsciously would require repetitive training, much like learning a sport.
The long-term goal is to develop a miniaturized, implantable neuroprosthetic device that would enable paralyzed patients to move their own paralyzed limbs. Fetz has already taken the next step, developing a cell phone–size neurochip that can be linked to a microprocessor, small enough for monkeys to carry implanted in their head.
Many hurdles remain. It is difficult to record from the same neuron for a long period. Within days or weeks, scar tissue walls off electrodes, interrupting transmission. Guiding electrodes to new locations with tiny motors might mitigate that problem. Providing a decades-long power supply is also a challenge. Biocompatibility is another issue; fully implanting such a system under the skin presents a huge infection risk. And crucial questions exist: Can this model be scaled up to stimulate multiple neurons that trigger multiple muscles? How flexible is the brain in reassigning new functions to neurons?
The team hopes to restore arm movements in the near term—and ultimately to restore paraplegics’ ability to walk. But clinical trials remain perhaps a decade away.