FEATURE ARTICLE
The Design and Function of Cochlear Implants
Fusing medicine, neural science and engineering, these devices transform human speech into an electrical code that deafened ears can understand
Michael Dorman, Blake Wilson
The Next Verse
One advance that we will see shortly is the union of electric and
acoustic stimulation, or combined EAS. Many hearing–impaired
people have some ability to hear low frequencies but retain little
or no hearing at higher frequencies. If an electrode array can be
inserted about two–thirds of the way into the cochlea, then
hearing at 1 kilohertz and above can be restored by electrical
stimulation. And if the surgery doesn't damage the distal third of
the cochlea, then electrical and acoustic hearing can together
provide access to the range of frequencies necessary for speech understanding.
Christoph von Ilberg and his colleagues at the University Clinic at
Frankfurt were the first to demonstrate the feasibility of this
approach. Recent studies have shown that acoustic hearing can be
preserved in 75 to 90 percent of patients in whom a 20
millimeter–long electrode array is inserted into the cochlea,
which is normally 28 to 35 millimeters long. Experiments from author
Wilson's lab have shown that just a small region of acoustic hearing
below 500 hertz greatly improves the performance of electrical
hearing, even when acoustic speech comprehension is near zero. For
example, one patient who understood only 10 percent of words via
acoustic stimulation and 60 percent by electric stimulation
recognized 90 percent with the combined stimulation.
We suspect that auditory nuclei in the brainstem, which sort signals
from noise, recognize patterns of neural discharge that are unique
to acoustic stimulation. The output from even a small region of
normal hearing may engage these nuclei in a way that electrically
evoked patterns cannot, thereby allowing more of the signal to reach
higher levels of auditory processing. Thus the combination of
electric and acoustic stimuli can have a synergistic effect on
speech understanding, especially in noisy environments.
Combined EAS has produced some remarkable results for patients with
residual hearing in the low frequencies, and patients with residual
hearing up to 1,000 hertz may one day become candidates for the
procedure. The popularity of this approach as a treatment for
severe, but not total, hearing loss will depend on how reliably the
remaining hearing can be preserved. Such preservation might be
improved with shorter electrode insertions or with
pre–treatment of the cochlea with certain drugs. However,
shorter arrays also reduce the performance of electric
stimulation—leaving the patient with few options if the
remaining hearing is lost. These trade–offs—electrode
insertion depth versus preservation of unaided hearing, combined EAS
performance versus the performance of electric stimulation
alone—remain to be fully explored.
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