In practice, commercial electrical stimulation systems implement some of the experimental and theoretical findings discussed above. Cochlear implants use charge-balanced, biphasic stimulation pulses to activate the auditory nerve. One important result from cochlear implant research was the effectiveness of pulses of short time duration. Early cochlear implants produced an electrical sinusoid at the same frequency as the sound wave it was trying to replicate. If a sound wave at 200 Hz was present, then the cochlear implant stimulated with a 200-Hz sine wave. It was shown, however, that short stimulus pulses of 0.1 msec presented at 200 pulses/sec were just as effective, in most cases, in replicating the sensation of a 200-Hz sound.39-41 The advantage of using a 0.1-msec pulse vs. a 200-Hz (or 50-msec) sinusoid is the savings in electrical charge used. Because it was also noted that in some cases the sinusoid was more effective, some cochlear implants have the ability to stimulate with both pulse and sinusoid stimuli.
Another example of successful clinical application of neural stimulation is the deep brain stimulator (DBS). This device is currently being evaluated for the treatment of Parkinsonian tremor.42 Seizures in the tha-lamic region of the brain are believed to cause some types of tremor that can render the afflicted individual unable to control movements. By electrically stimulating the thalamus, it is hypothesized that the seizures are moderated or depressed, allowing other areas of the brain to function properly to allow voluntary motor control. While the exact mechanism of
DBS is not clear, the results can be dramatic. In a matter of minutes, Parkinson's patient can go from being unable to stand due to tremor to being able to easily walk across a room.
Both the cochlear implant and deep brain stimulator demonstrate what should be a guiding principle for neural prosthetic devices. The purpose of the device is not to closely replicate the tissue it is replacing; rather, the purpose of the device is to replicate the lost function, regardless of how this is accomplished. We do not know how to build a cochlea, a thalamus, or a retina, but we do have some knowledge of how to electrically activate nerves. Using the established boundaries of safe stimulation, it is possible to achieve dramatic results in patients who otherwise would have no hope of a treatment.
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