gms | German Medical Science

Background: Individuals with a hearing aid servicing one ear and a cochlear implant (CI) servicing the other ear have very poor sound localization abilities. A combination of level, tonotopy, and latency mismatches prevents the exploitation of binaural cues. Compensating for the estimated latency mismatch by adding a fixed delay to the CI stimulation has been shown to improve sound localization (Angermeier et al., Trends in Hearing, 2023). Here, we measure sound localization error and bias as a function of the additional delay.

Methods: 13 loudspeakers were placed semicircular with a 15° spacing at ear level in the frontal azimuthal half-plane with a radius of 1 meter. Bimodal MED-EL CI users that have functional hearing up to at least 4 kHz performed a loudspeaker identification experiment. Each stimulus consisted of three 70-ms long broadband noise bursts each gated with a 5-ms Hanning windows, separated by two 30-ms long silent intervals. Stimuli were presented 5 times from each of the 11 most central speakers and each CI latency (0, 2, 4, 6, 8, and 10 ms). For a subset of CI-latencies and subjects the test was repeated with the German word “Doris” as the stimulus.

Results: In line with Angermeier et al. (2023) the localization bias shifted towards the hearing aid side with increasing CI latency. The amount of bias shift varied from 1° to 9° per millisecond latency. When using speech instead of noise the trends remained similar, but the bias shift was smaller. A bias-free latency could usually be identified within the tested latency interval. This latency also resulted in one of the smallest RMS localization errors. However, the bias-free CI latency was usually smaller than the latency compensation suggested by Angermeier et al. (2023).

Conclusions: The results emphasize the importance of latency optimization when fitting single-sided deaf and bimodal CI users. The difference between the estimated latency mismatch and the bias-free best localization latency may originate from latency estimation errors, from a level bias towards the hearing aid side that can be compensated for by a CI-favoring latency mismatch, or it originates from an additional tonotopic mismatch and latencies being larger at lower frequencies. The findings underline the relevance of latency as a free fitting parameter in bimodal CI users.