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Bypassing the spinal cord

Reading the brain, moving the muscles

About 450,000 Americans are living with damaged spinal cords, and controlling the limbs by thought alone remains a Holy Grail for those with severe paralysis. To date, scientists have tried to detect the brain's intentions by monitoring groups of nerve cells that normally control movement.

It's complicated, but the idea has worked in preliminary tests.

Now, a University of Washington research group has taught monkeys to control one muscle using one neuron -- through an external circuit that bypasses the spinal cord.

In brief, the researchers:

taught two monkeys to move their wrists to play a simple video game

implanted several electrodes in the monkey's brain and others in muscles in its arm, then connected both electrodes to a control device

paralyzed the arm for several hours with a reversible anesthetic and

allowed the monkeys to play the video game by activating individual neurons in the brain

Bypassing the spinal cord

Through a feedback process, the monkeys learned that firing a particular neuron could stimulate the paralyzed muscle, says co-author Chet Moritz, a postdoctoral fellow at the University of Washington. "The only way they could move the wrist was to change the neural activity in their brains," Moritz explains.

It's not clear how the monkeys figured out that firing the specific neuron would move the wrist, but doing so took them only two to 60 minutes, Moritz says. "It was very rapidly adopted," he says. "We don't know what goes on in the brain, but the technique we use is based on operant conditioning. They get applesauce or fruit juice every time they do something close to what we want them to do."

You don't have to force the monkeys to play the games, Moritz emphasizes. "These are teenage male monkeys playing video games; it's the favorite part of their day, they enjoy the challenge, they enjoy going to the next level," Moritz says. Temporarily paralyzing the arm, he says, makes the game harder, and "that increases the motivation to learn."

At night a spot lit Macaca walks on all fours
The Macaca nemestrina monkey, which learned to play a video game using a few neurons in the brain, normally lives in trees and on the ground in Southeast Asia. Most reside in groups ranging from 15 to 40 animals. These macaques eat fruits, and small vertebrate and invertebrate animals.

Going it alone

The new technique has yet to be replicated, but its emphasis on watching one neuron confers some theoretical advantages. The technique was "distinctly different from the conventional approach, which is typically based on recording the activity of populations of cells," says co-author Eberhard Fetz, a professor of physiology and biophysics at Washington.

Reading individual neurons reduces the need for signal processing, says Moritz. "We use a very simple conversion that does not require a complicated computer algorithm or a large amount of processing power," he says, "so we may be one step closer to a low-power device than previous studies that required several desktop computers."

Small is big, Moritz adds. The control box is only as big as a cell phone, and further miniaturization is possible. Smaller devices, in turn, allow smaller batteries, a key constraint for prosthetics -- especially for brain implants.

Spatula-shaped clear plastic piece with inlayed circuits
A prototype implantable neuroprosthesis gathers neural signals from a sensor implanted in the skull, and sends them down a fiber optic cable to a wireless transmitter in a patient's chest. Such prostheses could play a key role in overcoming paralysis or other neurological conditions.

Got those "how long?" blues

Now we reach the bummer paragraph. Although promising, nerve damage may put patients with multiple sclerosis or Lou Gehrig's disease, also known as ALS, out of reach of the new approach.

And even those with an injured spinal cord must be patient, Moritz says. "This was an initial demonstration that this type of technology is possible," he says. "It will take several years -- even several decades -- before it's ready for a clinical application." Among the important obstacles: Creating a system that avoids the infection-prone step of passing wires through the skin, and making an electrode that can record for years on end.

After a few weeks, many existing brain electrodes begin to lose their sensitivity, Moritz adds.

Nonetheless, the new approach may be simpler to learn and to implement, and it takes advantage of the brain's ability to alter itself to satisfy a changing environment.

 Array of electrodes on man looking at EEG feedback screen
Photo: NASA
NASA is working on a brain-computer interface that will read brain waves and muscles, and operate alongside standard controls, such as keyboards, mice and speech.

Perhaps the most remarkable result of the new experiment was the finding that neurons outside the motor cortex were also able to play video games. "Even neurons that were unrelated to the movement of the wrist could be used to control the wrist muscles," says Moritz, "so it dramatically expands the potential pool of control signals available for brain-computer interfaces."

- David J. Tenenbaum

Related Why Files

• Nanotubes make nifty electrodes.

Overcoming paralysis.

• The benefits of exercise.

Neural Prosthesis

The Spinal Cord


• Direct control of paralysed muscles by cortical neurons, Chet T. Moritz et al, Nature, 16-Oct-2008 (Vol. 455, No. 7215).

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