Cochlear dead regions constrain the benefit of combining acoustic stimulation with electric stimulation

The aims of this study were to (1) detect the presence and edge frequency (fe) of a cochlear dead region in the ear with residual acoustic hearing for bimodal cochlear implant users, and (2) determine whether amplification based on the presence or absence of a dead region would improve speech understanding and sound quality.

Twenty-two listeners with a cochlear implant in one ear and residual acoustic hearing in the nonimplanted ear were tested. Eleven listeners had a cochlear dead region in the acoustic-hearing ear and 11 did not. Dead regions were assessed with the threshold-equalizing noise (TEN) and the sweeping noise, psychophysical tuning curve tests. Speech understanding was assessed with monosyllabic words and the AzBio sentences at +10 dB signal-to-noise ratio. Speech- and music-quality judgments were obtained with the Judgment of Sound Quality questionnaire.

Using shifted tips of the psychophysical tuning curve as a basis for diagnosis, the TEN had high sensitivity (0.91) and poor specificity (0.55) for this population. The value of fe was lower when estimated with the sweeping noise, psychophysical tuning curve test than with the TEN test. For the listeners with cochlear dead regions, speech understanding, speech quality and music quality were best when no amplification was applied for frequencies within the dead region. For listeners without dead regions, speech understanding was best with full-bandwidth amplification and was reduced when amplification was not applied when the audiometric threshold exceeded 80 dB HL.

The data from this study suggest that, to improve bimodal benefit for listeners who combine electric and acoustic stimulation, audiologists should routinely test for the presence of cochlear dead regions and determine amplification bandwidth accordingly.