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Bilaterally independent (mismatched) hearing aids cannot replicate the natural timing and level cues between ears, and hence, may result in negative consequences for localization and speech perception in spatially-separated noise performance.
Five gain reduction patterns were used to evaluate the impact of bilaterally mismatched gain reduction schemes on localization and speech perception performance in noise, compared to an unaltered bilaterally linear time-invariant amplification scheme (reference scheme), in which audibility was optimized. The bilaterally mismatched gain reduction schemes were later matched (synchronized) between ears to explore the possibility of restoring the deteriorated performance due to the mismatched schemes. Sound quality and listening-effort ratings among different gain reduction patterns were assessed, as well as the relationship between self-reported localization ability in daily life and measured localization performance in a laboratory setting.
Twenty-four bilateral hearing aid users were enrolled in this study and tested in a virtual environment with insert earphones. The results indicated that bilaterally mismatched gain reduction schemes had a negative impact on localization, compared to the reference scheme; whereas matching gain reduction schemes between ears improved the deteriorated localization performance. In contrast, the use of bilaterally mismatched gain reductions did not negatively impact the speech perception performance in noise. Matching the gain reduction scheme between ears actually resulted in reduced speech perception performance, compared to the mismatched gain reductions. Self-reported localization abilities were not found to be strongly related to the measured localization performance in this study. Finally, these five different gain reduction patterns did not result in significantly different overall sound quality ratings and listening-effort ratings for hearing aid users. However, the use of gain reductions (mismatched or matched) reduced the perceived noise intrusiveness, compared to the use of reference schemes.
It is unclear why there was a discrepancy between the results of the localization and speech perception performance in the present study. It is likely that hearing-impaired listeners do not use binaural cues in the localization task in the same manner as in the speech perception task.