20 October 2008—For years, doctors have treated patients suffering from life-threatening heart blockages by adding new blood vessels that reroute blood around arterial traffic snarls. Researchers have been working on methods for doing an electronic bypass around a damaged spine with the aim of restoring movement to paralyzed limbs.
Though it will be years before spinal bypass surgery reaches even the clinical-experiment stage, researchers at the University of Washington (UW) and the Washington National Primate Research Center, both in Seattle, have figured out a way to get macaque monkeys in their lab to manipulate temporarily paralyzed muscles in their arms using brain-controlled electrical stimulation. In research reported last week in Nature , they describe what happened when they attached electrodes to neurons in a monkey’s motor cortex—the part of the brain that controls voluntary movement—and used fairly simple algorithms to translate activity in these cortical cells into electrical signals that tell muscles when, how much, and how forcefully to contract.
In exchange for a reward of applesauce, the monkeys had been conditioned to create just the right amount of torque in their wrists to move a cursor on a display so that it hit a target. To conduct the experiments, the researchers used anesthesia to block signals in a nerve just below the shoulder of a monkey’s arm, temporarily paralyzing the rest of the limb. The brain cells that control wrist movement were still firing in response to the monkey’s desire to hit the target and get the payoff, but with the neural connection shut down, the wrist remained limp. The scientists implanted electrodes into the monkey’s motor cortex and fed the electrical signals they received from the monkey’s brain into a computer. The computer then translated the signals into a stimulating current that was fed to electrodes implanted below the nerve block in the monkey’s wrist. The monkeys were able to learn to manipulate their own brains to get their wrists moving.
The UW researchers not only demonstrated that simple, direct pathways between the brain and muscles can be established but also showed that the monkeys could quickly learn to control their brain activity with amazing precision. After a few weeks of training, the monkeys could govern the rate and intensity of activity in a single cortical cell well enough to both flex and extend the joints; with additional practice, they learned to independently control several cells. They were able to hit the targets faster and faster, with fewer and fewer misses, suggesting the development of fine motor coordination.
Though the Seattle team found that a single cell can operate the separate muscle groups that control both flexion and extension of the wrist, the group concluded that having separate connections from two or more cells would make more clinical sense, because separate pathways allow more muscle fibers to be recruited for the task, creating more force, says Eberhard Fetz, a professor of physiology and biophysics at UW. Full-muscle force is critical if doctors are to someday implant devices that help people crippled by spinal injuries or other nerve damage to rise up and walk, open doors, or remain continent.
The most surprising outcome of their experiments is the revelation that motor cortex cells that had previously been dedicated to moving, say, the big toe on a monkey’s left foot or bending its knees could be trained to control its wrists. This flexibility, says Fetz, may allow patients with head injuries that damaged part of the cerebral cortex to still be candidates for a neuroprosthesis.
Despite the brain’s plasticity, Fetz acknowledges that it will take another 5 to 10 years to get this wrist experiment to the point where its efficacy at countering paralysis can be tested on humans in a clinical setting.