Abstract:
Electrical field interaction caused by current spread in a cochlear implant was modeled in an explicit
way in an acoustic model (the SPREAD model) presented to six listeners with normal hearing. The
typical processing of cochlear implants was modeled more closely than in traditional acoustic models
by careful selection of parameters related to current spread or parameters that could amplify the electrical
field interactions caused by current spread. These parameters were the insertion depth, electrode
spacing, electrical dynamic range, and dynamic range compression function. The hypothesis was that
current spread could account for the asymptote in performance in speech intelligibility experiments
observed at around seven stimulation channels in a number of cochlear implant studies. Speech intelligibility
for sentences, vowels, and consonants at three noise levels (SNR of +15 dB, +10 dB, and +5
dB) was measured as a function of the number of spectral channels (4, 7, and 16). The SPREAD
model appears to explain the asymptote in speech intelligibility at seven channels for all noise levels
for all speech material used in this study. It is shown that the compressive amplitude mapping used in
cochlear implants can have a detrimental effect on the number of effective channels.
