It sounds like science fiction — a chip implanted in the brain that allows a paralyzed person to operate a computer using only his thoughts — but it's true.

Twenty-five year-old Matthew Nagle has been paralyzed from the neck down since suffering a spinal chord injury five years ago. After having a sensor implanted on the surface of his brain at Rhode Island Hospital in June 2004, however, he learned how to move a computer cursor simply by thinking about moving it.

Nagle can open simulated e-mail, draw circular shapes using a paint program on the computer and play a simple video game, "neural Pong," using only his thoughts. He can change the channel and adjust the volume on a television, even while talking to someone. He eventually learned to open and close the fingers of a prosthetic hand and use a robotic limb to grasp and move objects.

Nagle's sensor is part of the BrainGate Neural Interface System, a "neuromotor prosthesis" developed by Cyberkinetics Neurotechnology Systems, Inc., of Foxborough, Mass.

In the 13 July 2006 issue of Nature, researchers present the results from the first clinical trial of the BrainGate Neural Interface System, using Nagle and another paralyzed man.

The second subject, a 55-year-old man with a similar injury, had the sensor implanted by surgeons at the University of Chicago in April 2005 and was followed by researchers from the Rehabilitation Institute of Chicago and Cyberkinetics. He was able to learn to control the cursor until a technical issue caused signal loss.

"The results," according to John Donoghue, professor and director of the brain science program at Brown University and chief scientific officer of Cyberkinetics, "hold promise one day to be able to activate limb muscles with these brain signals, effectively restoring brain-to-muscle control via a physical nervous system."

The BrainGate System uses a sensor about the size of an aspirin tablet. It is implanted on the surface of the area of the brain responsible for voluntary movement, the motor cortex. Here the sensor picks up electrical signals from nearby neurons and transmits them to a titanium pedestal on the patient's scalp. A cable connects the pedestal to computers, signal processors and monitors.

Despite the exciting results of this initial trial, the BrainGate System is still in its infancy and is far from perfect. It is bulky and cumbersome. The quality of the signal can vary from patient to patient and from day to day. The second subject never developed as much control as Nagle, and even Nagle's level of control, the authors note, "is considerably less than that of an able-bodied person using a manually controlled computer cursor."