. >> reporter: dennis degray is a 64-year-old quadriplegic who is writing a sentence on the computereen in front of him using only his brain. a former volunteer firefighter, degray had a bad fall 10 years ago which severed his spinal cord. as part of an early stage clinical research study led by stanford university, degray and two other volunteer participants with a.l.s. had small sensors implanted in their brains in an area called the motor cortex which controls movement. even though degray can no longer physically move his arms, the neurons in that part of his brain, and in the brains of many other paralyzed individuals, remain active. the sensors in degray's brain listen in to those neurons which emit different electrical signals depending on the direction degray thinks about moving his hand. >> to move the pointer around. i imagine a ball lying on a table and with my hand lying on the ball. and as i roll the ball forward the pointer goes up, and as i roll the ball back toward me the pointer goes down. and of course left and right correspondingly. >> reporter: the neural signals a