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prostheses, it has problems that suggest some general issues in neural implantation.The surgery can produce undesirable side effects: dizzi- ness, because the inner ear is also the organ of bodily balance; infec- tion at the incision site; and occasionally, facial paralysis. Furthermore, the results dont come anywhere near the quality of natural hearing. Another, subtler, potentially troubling problem for implants in general is a hint of an isolating effect that foretells what truly extensive bodily modifications might entail. Some implantees call the quality of the sound they hear artificialor roboticand, in a surprising twist, others report that instead of feeling that they have rejoined a world from which they have been cut off, they feel alienated from both the deaf and the hearing communities, with the implants leaving them in limbo without full membership in either world.

Despite these problems, the general success of cochlear implanta- tion suggests how digital implants might correct other human defi- ciencies, and even extend normal human endowments. If a cochlear implant can turn physical sound into the sensation of sound in a deaf persons brain, could a retinal implant turn physical light into the sensation of light in a blind persons brain? Even more interesting, if the implant were sensitive to wavelengths of light that humans ordi- narily do not see, such as infrared radiation, could it give a person hypervision?

Similar intriguing questions could be asked about smartpros- thetic limbs, in which sensors would encode information about a limbs position in space and the textures it encounters.The informa- tion from the sensors would be changed into neural signals and sent to the appropriate part of the brain, which would respond by provid- ing motor signals to the hand or leg to produce fine movement con- trol. Suppose also that motors and power sources are built into the limb, or even that the neural control is extended to a device outside the body such as an exoskeleton or vehicle. The result would be a person with enhanced strength, speed, mobility, or reach.

Along similar lines are what might be called internal prosthetics; that is, replacements for organs such as the heart and the liver.Artifi- cial hearts have received the greatest attention and have steadily

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