With the extension of cochlear implant candidacy, more and more cochlear-implant listeners fitted with a traditional-long electrode array or a partial-insertion electrode array have residual acoustic hearing either in the nonimplanted ear or in both ears and have shown to receive significant speech-perception benefits from the low-frequency acoustic information provided by residual acoustic hearing.

The aim of Experiment 1 was to assess the minimum amount of low-frequency acoustic information that was required to achieve speech-perception benefits both in quiet and in noise from combined electric and contralateral acoustic stimulation (EAS). Speech-recognition performance of consonant-nucleus vowel-consonant (CNC) words in quiet and AzBio sentences in a competing babble noise at +10 dB SNR was evaluated in nine cochlear-implant subjects with residual acoustic hearing in the nonimplanted ear in three listening conditions: acoustic stimulation alone, electric stimulation alone, and combined contralateral EAS. The results showed that adding low-frequency acoustic information to electrically stimulated information led to an overall improvement in speech-recognition performance for both words in quiet and sentences in noise. This improvement was observed even when the acoustic information was limited down to 125 Hz, suggesting that the benefits were primarily due to the voice-pitch information provided by residual acoustic hearing. A further improvement in speech-recognition performance was also observed for sentences in noise, suggesting that part of the improvement in performance was likely due to the improved spectral representation of the first formant.

The aims of Experiments 2 and 3 were to investigate the underlying psychophysical mechanisms of the contribution of the acoustic input to electric hearing. Temporal Modulation Transfer Functions (TMTFs) and Spectral Modulation Transfer Functions (SMTFs) were measured in three stimulation conditions: acoustic stimulation alone, electric stimulation alone, and combined contralateral EAS. The results showed that the temporal resolution of acoustic hearing was as good as that of electric hearing and the spectral resolution of acoustic hearing was better than that of electric hearing, suggesting that the speech-perception benefits were attributable to the normal temporal resolution and the better spectral resolution of residual acoustic hearing.

The present dissertation research provided important information about the benefits of low-frequency acoustic input added to electric hearing in cochlear-implant listeners with some residual hearing. The overall results reinforced the importance of preserving residual acoustic hearing in cochlear-implant listeners.