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
Figure of Speech
The timing and frequency of consonants and vowels in a spoken word
determine its acoustics. For example, slow changes in overall
amplitude indicate the timing of syllables, phonetic transitions
within syllables and boundaries between silence and sound. In terms
of frequency, the vocal tract produces multiple concentrations of
energy between 300 and 5,000 hertz as it produces speech sounds.
The slow amplitude variations of speech are referred to as the
speech envelope, an aspect that conveys a surprising amount of
information. Victor Zue at the Massachusetts Institute of Technology
classified the envelope shapes of 126,000 words by applying a series
of only six shape variations. He found that, on average, only 2.4
word candidates matched a given sequence. This observation suggests
that implant patients could understand speech much better if their
implants conveyed the shape of the envelope, thereby constraining
the number of word possibilities. However, envelope shape by itself
does not provide enough information to understand speech. To
identify specific words, frequencies in the 300–5,000 hertz
range must be extracted from the signal.
In any vowel or consonant, the frequencies of the first two energy
concentrations comprise the essential signature of the sound. For
example, in Dorman's voice the vowel in bat has energy peaks at 624
and 904 hertz. The vowel in bought has peaks at 620 and 1,055 hertz.
Because a very small difference in the acoustic pattern—150
hertz in this case—can significantly alter the meaning of the
word, investigators initially assumed that a neural prosthesis for
hearing would need a very large number of channels. As we have seen,
this did not turn out to be the case, at least for low–noise environments.
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