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
Adult Results
Scott N.'s ability to understand speech demonstrates that an implant
can restore a normal level of speech recognition in quiet
environments. However, Scott's case is exceptional. Speech is
neither as clear nor as easy to understand for most patients.
Although average scores range between 80 and 100 percent correct on
tests of sentence understanding, the comprehension of isolated words
lies between 45 and 55 percent. The gap between scores shows that
average patients fail to hear the details of many spoken words.
Sentence context allows the missing elements to be reconstructed.
What is the difference between Scott's auditory system and that of
a patient with average or below–average speech understanding?
A patient's performance probably depends on many factors, including
the number and location of surviving cells in the spiral ganglion,
the spatial pattern of current flow from the electrodes, and the
degree to which neurons in the brainstem and cortex can encode
frequency by phase–locking their firing patterns. When only a
few cells survive in the spiral ganglion—for example, after a
long period of deafness—the electrode stimulation is less able
to convey frequency–specific information to the cochlear
nucleus and cortex. And if the surviving cells are clustered at one
end of the ganglion, then the signal that does arrive at the cortex
will lack the range of frequencies needed to understand speech. Even
if there are neurons along the length of the cochlea, individual
electrode currents need to be highly focused to provide independent
channels of stimulation (and therefore, information). If these
currents overlap, either because the signal spreads too far through
the conductive cochlear fluid or because of individual differences
in cochlear anatomy, then the number of functional channels will be
less than the number of electrodes.
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