As many as 400,000 Americans are partially or totally paralyzed from spinal cord injuries, which interrupt the nerve cell signals relaying information between the brain and the body. Others lose the ability to move and communicate because of neurodegenerative disorders such as amyotrophic lateral sclerosis, or Lou Gehrig's disease, which causes the neurons controlling muscles to die. Still half a million more Americans suffer profound sensory deficits such as blindness or deafness. For more than a century, scientists have sought some type of electrical replacement for lost motor and perceptual functions to alleviate these conditions.
Only recently, however, have researchers and doctors begun testing such neuroprostheses in humans. Existing prosthetic instruments transmit signals from areas in the body to the brain--cochlear implants in the inner ear, for example, can send signals to the auditory nerve to enable hearing. The next generation of devices, however, will move into the brain itself [see box on page 67]. Various research teams are now building so-called brain-computer interfaces (BCIs), which help to restore paralyzed patients' ability to communicate and move by translating neuron signals in their brains into commands that control computer cursors or robots. And a new wave of brain implants, including a type developed in our laboratory in Germany, is poised to transfer information into the brain, thereby reviving sensory function.