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Frequency-lowering signal processing in hearing aids has re-emerged as an option to improve audibility of the high frequencies by expanding the input bandwidth. However, few studies have investigated the usefulness of the scheme as a bimodal option for cochlear implant users. In this study, that question was posed. It was hypothesized that, following fitting and a period of adjustment to a frequency-compression hearing aid, sound localization and speech perception would be improved compared to conventional amplification. More specifically, more high-frequency cues would be perceived in the hearing aid ear using frequency compression, thereby providing better sensitivity to interaural level differences when a cochlear implant is used contralaterally.
There were two experiments in this study. In the first experiment, the goal was to determine if frequency compression was a better bimodal option than conventional amplification. Performance was assessed on tests of sound localization, speech perception in a background of noise, and using questionnaires. Ten subjects with a cochlear implant plus hearing aid participated in experiment one. In the second experiment, the goal was to determine the impact of frequency compression on speech perception in quiet. Consonant and vowel perception in quiet was assessed using the frequency-compression and conventional hearing aid. Seventeen adult subjects participated in the second experiment.
In both experiments, subjects alternated daily between a frequency-compression and conventional hearing aid for two months. The parameters of frequency compression were set individually for each subject and audibility was measured for the frequency compression and conventional hearing aid programs by comparing estimations of the Speech Intelligibility Index (SII) using a modified algorithm (Bentler, R., Cole, B., Wu, Y-H. (2011, March). Deriving an audibility index for frequency-lowered hearing aids. Poster session presented at the meeting of the American Auditory Society, Scottsdale, AZ). In both experiments, the outcome measures were administered following the hearing aid fitting to establish baseline performance and after two months of use.
Results revealed no significant difference between the frequency-compression and conventional hearing aid on tests of localization and consonant recognition. Spondee-in-noise and vowel perception scores were significantly higher with the conventional hearing aid compared to the frequency-compression hearing aid after two months of use. These results suggest that, for the subjects in this study, frequency compression is not a better bimodal option than conventional amplification. In addition, speech perception may be negatively influenced by frequency compression because formant frequencies are too severely compressed and can no longer be distinguished.