The Why Files The Why Files --

The Substitution Solution: Restoring vision, hearing and movement


Paralysis, blindness, deafness: Technology to the rescue?
Bride and groom smile blissfully at camera, bouquet of flowers adorns walker.Neurological losses are, in many ways, the cruelest injuries. Hearing is snuffed by a flaw in the inner-ear cells that convert pressure waves into nerve impulses. Vision goes dark by the inexorable genetic destruction of a hair-thin layer of light-sensitive cells in the eye. Movement is stilled by injury to the spinal cord.

In each of these cases, assistive-device technology is starting to replace broken neurons and restore a limited amount of sensation or movement. Many of these devices are highly experimental, but one is already a routine treatment for some types of deafness.

Standing with help from an electronic device, this paralyzed woman stands for her wedding. Photo courtesy P. Hunter Peckham

In all cases, these "neural prosthetics" exploit high technology to restore some degree of normalcy to patients whose lives have been shaped by disability. This edition of The Why Files will look at these technologies:

Experimental motor signaling systems have started to reconnect limbs and brain to overcome paralysis. Some systems are controlled by movement in those muscles that the patient can still move. Others read the intent to move directly from the brain. In either case, the system can trigger movement on a computer screen, an artificial limb, or in the patient's own body.

Recent advances in artificial retinas show that certain types of blindness may be partly reversible. A television camera routes a signal to an electrode that stimulates the retina, which sends messages to the brain. The technology is still basic, but may have the potential to eventually allow some blind people to recognize faces, even to read.

Cochlear implants have already restored some hearing to about 100,000 people who are deaf due to defects in the cochlea, the organ that converts vibrations into nerve signals.

Neural prostheses take advantage of a key fact about the brain, says Michael Merzenich, a neuroscientist at the University of California at San Francisco: It's the ultimate learning machine. To use prostheses to observe or control the world, the patient must learn to make the most of a signal that is tiny compared to the usual signal.

Orb of orange has red veins running through it.
A retinal prosthesis uses an external camera and processor to capture and process image data and transmit the information to an implant, which stimulates the retina with a pattern of electrical impulses to produce a visual image. Graphic: USC

The next challenge for engineering neural prostheses, Merzenich says, is to devise a "smart prosthesis. ... We want to make a device that can expand on the capability of patients who have a very smart brain," but are limited by severe damage to the nervous system.

Paralysis: Help on the way?

moreBackground from drawings by Leonardo da Vinci

Megan Anderson, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

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