While the Air Force project calls for one-dimensional control -- simple movement of a cursor back and forth -- the Wadsworth Center project, which is financed by the National Institutes of Health, is far more demanding of both technology and subject training.

Because the left and right hemispheres of the brain do different things, Dr. Wolpaw's group is looking for differences that can be used by a disabled subject to communicate with the outside world without moving or speaking. The Wadsworth Center gets the best results so far, he said, by positioning a pair of electrodes over the left hemisphere and another pair over the right hemisphere, with both pairs near the top of the head over the central sulcus of the brain.

The system does not use flickering lights but instead looks at natural mu brain wave emissions, fluctuations from a part of the brain called the sensorimotor cortex, which occur at a natural rate of 8 to 12 times a second. A digital computer applies a mathematical technique called the fast Fourier transform to detect and measure the signal a subject attempts to transmit along with these fluctuations, a signal so faint, it is almost obscured by other brain emissions.

After teasing out the needed signal, the computer then applies a method that adds the amplitudes of selected signals picked up from the two hemispheres, using their sum to make a cursor move up or down. The computer simultaneously calculates the difference between the right- and left-hemisphere signals, and uses this value to move the cursor right or left.

Somehow, a brain can train itself to exploit this two-channel computer algorithm, or method, to move a cursor. To do so, the brain must simultaneously control both the sum and the difference between emissions from the right and left sides of the brain. Amazingly, an average person can quite quickly learn to do this and move a cursor around, although movements are crude; Dr. Wolpaw says that subjects cannot easily moderate the speed or distance of cursor movements, but must use a "ballistic" approach, pushing as hard as possible to move a cursor in a selected direction.

A subject is presented with a computer screen divided into quadrants, with the cursor starting out at the center. The experimenter randomly picks one of four general directions, and by force of will, the subject tries to move the cursor in that direction to one of the four corners of the screen. If the cursor reaches the perimeter of the screen within the correct quadrant, the trial is counted as a hit.

The technique is not infallible, but two of the laboratory's better subjects scored hits 70 percent of the time in more than 100 trials, Dr. Wolpaw reported. (A 25 percent hit rate would be no better than chance.)