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Artificial Connections Restore Movement to Paralyzed Limbs

Researchers have shown for the first time that a direct artificial connection from the brain to muscles can restore movement in monkeys whose arms have been temporarily anesthetized. The results have promising implications for future prosthetic design.

A quarter of a million Americans are affected by spinal cord injuries, and thousands of others have paralyzing neurological diseases. Researchers have made progress over the past few years in developing strategies to reestablish broken connections between the brain and muscles. One approach aims to bridge spinal cord damage using stem cells. Other studies have used computers to decode brain activity and control external devices. Monkeys, for example, could control a robotic arm to feed themselves.

A team led by Dr. Eberhard E. Fetz at the University of Washington in Seattle tried another strategy: to use brain cell activity to directly control the stimulation of paralyzed muscles. Their work was funded by NIH's National Institute of Neurological Disorders and Stroke (NINDS) and performed at the Washington National Primate Research Center, which is supported by NIH's National Center for Research Resources (NCRR).

The researchers reported in the online edition of Nature on October 15, 2008, that they first trained monkeys to control the activity of single nerve cells in the motor cortex, an area of the brain that controls voluntary movements. The researchers detected nerve cell activity using a brain-computer interface, with electrodes implanted in the motor cortex and connected to a computer. They trained the monkeys by connecting the nerve activity to cursor movements and having the animals play a target practice game.

After the nerve cells were trained to move the cursor, the researchers temporarily paralyzed the monkey's wrist muscles using a local anesthetic. Then, instead of having the monkey's nerve activity control a cursor, the activity was converted into electrical stimulation to the paralyzed wrist muscles—a technique called functional electrical stimulation. The monkeys continued to play the target practice game, but now their actual wrist torque controlled the cursor movements.

The researchers found that the monkeys could successfully control the otherwise paralyzed wrist to play the game. Practice time was limited by the duration of the nerve block, but the monkeys' control improved significantly with practice. They successfully hit the target 3 times more often and with less error during a 2-minute peak performance period than during an initial 2-minute practice. The monkeys also achieved independent control of both the wrist flexor and extensor muscles.

Until now, brain-computer interfaces were designed to decode the activity of neurons known to be associated with movement of specific body parts. In this study, any motor cortex cell, regardless of whether it had been previously associated with wrist movement, proved capable of controlling muscle activity.

“This study demonstrates a novel approach to restoring movement through neuroprosthetic devices, one that would link a person's brain to the activation of individual muscles in a paralyzed limb to produce natural control and movements,” said Dr. Joseph Pancrazio, a program director at NINDS.

Clinical applications are still probably at least a decade away, according to Dr. Fetz. Better methods for recording neuron activity and for controlling multiple muscles must be developed, along with implantable circuitry that could be used reliably and safely.