TO UNDERSTAND WHAT scientists are doing and where they're headed, you have to start in the motor-cortex zone of the brain.

This complex group of brain cells is the central command post for turning thought into action, and it works through a form of mental Morse code. Every time you move your arm or leg, neurons inside the motor cortex emit bursts of electronic signals that direct the action. By placing a miniaturized sensor that sends out 100 hair-thin electrodes into this crossroad of brain cells, scientists can tap into that communication stream. A small computer attached to a wheelchair wirelessly transmits the data to a computer that can mathematically translate the electronic pulses into the cursor movements on the screen.

"It's the rate of firing of individual cells that seems to carry this information," says Tim Surgenor, president and CEO of Cyberkinetics Neurotechnology Systems, which developed BrainGate and has been testing the neurotechnology in its first human trials.

Whatever the source of their affliction, the four people who've participated in clinical tests so far, including Nagel, had lost the ability to move their arms and legs. But their brains have retained the fully functioning signaling system that is needed to communicate with the computer, even though that system's connection with their muscles has been severed.

The first tests revolve around the simplest of tasks: playing a game of Pong by changing the position of the paddle bar on the screen, communicating by selecting and clicking on words and phrases, and operating a wireless wheelchair or a robotic device. But completing these objectives is a radical step forward for a group of people who had lost all ability to control their surroundings. And it's been a remarkably easy mental leap for the volunteers to make.

"When using the mouse on your computer, you don't think about moving your hand - just think about where the cursor is," says Surgenor. "Soon, they're just thinking of where the cursor goes."

And off it goes.

"For someone who can't speak, can't breathe, and can't move to be able to affect their environment through their thoughts is just magic," says James Allen Heywood, CEO and d'Arbeloff founding director of the ALS Therapy Development Institute. Heywood's brother, Stephen Heywood, tried the BrainGate technology, signing up early in 2006 after he, formerly an architectural designer, had been completely paralyzed and silenced by amyotrophic lateral sclerosis (ALS), also called Lou Gehrig's disease.

Stephen was able to go beyond just dreaming about designing homes again - the technology was allowing him to take real steps in that direction. But he suddenly died following a respiratory failure unconnected to BrainGate and thus was unable to further test the new technology.

For Stephen, BrainGate was a way to talk to his wife and children after all communication had gone silent. "It gives you a reason for living," says Surgenor. "If they can't communicate, many patients decide not to continue on a respirator."