These experiments, Dr. Nicolelis said, are the first steps toward a brain machine interface that might permit paralyzed people to walk by directing devices with their thoughts. Electrodes in the person's brain would send signals to a device worn on the hip, like a cell phone or pager, that would relay those signals to a pair of braces, a kind of external skeleton, worn on the legs.

"When that person thinks about walking," he said, "walking happens."