Using Neuroprostheses for Sensory Loss
The loss of a major sense such as vision or hearing is one of the most disabling of all nervous system disorders. Most vision and hearing losses occur from damage to the peripheral receptors or receptor organs — the eye in the case of vision and the inner ear in the case of hearing.
While many people are familiar with the use of mechanical prosthetics for limb loss, not many are aware of neuroprostheses, prosthetics designed to address nervous system disorders.
One of the most successful applications of neuroprosthetics is the artificial cochlear stimulator, used to address hearing loss. Cochlear stimulators consist of a microphone, frequency analyzer, and transmitter outside the head, and a receiver and cochlear stimulator inside. Worldwide, the number of cochlear implants probably exceeds 200,000. In many cases, these implants allow the wearer to converse normally in person or on the telephone.
The situation for artificial vision is much more difficult. One reason is that the visual system is a more complex system (one million ganglion cell axons versus 30,000 auditory nerve fibers). Another is that the cochlea provides a uniquely appropriate physical interface for relatively simple, 20-electrode arrays in a cochlear stimulator. The visual system doesn’t have a similar interface.
Early artificial vision devices have been located in either the retina or visual cortex.
- Retinal devices: Devices located in the retina have been used when the vision loss is caused by photoreceptor death but the rest of the retina, including the output retinal ganglion cells, is intact. Current devices have allowed some blind patients to see a few blobs of light, but they haven’t yielded high enough acuity for reading.
- Cortex devices: Some visual prostheses have been implanted in the cortex. Some patients with these devices have been able to resolve 20 or 30 pixels, which is enough to detect, but not really identify, complex objects.