Effect of extreme adaptive frequency compression in bimodal listeners on sound localization and speech perception

Abstract

Objectives: This study aimed to improve access to high-frequency interaural level differences (ILD), by applying extreme frequency compression (FC) in the hearing aid (HA) of 13 bimodal listeners, using a cochlear implant (CI) and conventional HA in opposite ears. Design: An experimental signal-adaptive frequency-lowering algorithm was tested, compressing frequencies above 160 Hz into the individual audible range of residual hearing, but only for consonants (adaptive FC), thus protecting vowel formants, with the aim to preserve speech perception. In a cross-over design with at least 5 weeks of acclimatization between sessions, bimodal performance with and without adaptive FC was compared for horizontal sound localization, speech understanding in quiet and in noise, and vowel, consonant and voice-pitch perception. Results: On average, adaptive FC did not significantly affect any of the test results. Yet, two subjects who were fitted with a relatively weak frequency compression ratio, showed improved horizontal sound localization. After the study, four subjects preferred adaptive FC, four preferred standard frequency mapping, and four had no preference. Noteworthy, the subjects preferring adaptive FC were those with best performance on all tasks, both with and without adaptive FC. Conclusion: On a group level, extreme adaptive FC did not change sound localization and speech understanding in bimodal listeners. Possible reasons are too strong compression ratios, insufficient residual hearing or that the adaptive switching, although preserving vowel perception, may have been ineffective to produce consistent ILD cues. Individual results suggested that two subjects were able to integrate the frequency-compressed HA input with that of the CI, and benefitted from enhanced binaural cues for horizontal sound localization.

Hearing Asymmetry Biases Spatial Hearing in Bimodal Cochlear-Implant Users Despite Bilateral Low-Frequency Hearing Preservation

“…The observed hearing asymmetry had to be adjusted by about 19 dB in favor of the hearing-aid ear (equation ( 2 ); Figure 7 ). This preference was not observed in earlier studies ( Dunn et al, 2010 ; Veugen et al, 2016 , 2017 ; Kortje et al, 2020 ; Sharma et al, 2021 ; Gifford Dorman et al, 2014 ). Whether the preference was really absent, is hard to determine as the localization bias itself depends on individual hearing abilities and on the frequency content of the target sounds.…”

Simulations of the effect of unlinked cochlear-implant automatic gain control and head movement on interaural level differences

“…Head movements have recently received more attention in auditory science (e.g. Veugen et al , 2017; Agterberg, 2018; Archer-Boyd et al , 2018; Ege et al , 2018). They are an important cue for resolving front-back confusions in NH listeners(Brimijoin and Akeroyd, 2012), and CI listeners (Goman, 2014; Mueller et al , 2014; Pastore et al , 2018).…”

Amount of Frequency Compression in Bimodal Cochlear Implant Users Is a Poor Predictor for Audibility and Spatial Hearing

“…In this case, the transposed high-frequency sounds and the original lowfrequency sounds may effectively mask each other. This limitation is overcome by the latter method, whereby the high frequencies are compressed into a smaller bandwidth at low frequencies (McDermott & Henshall, 2010;Simpson, 2009;Simpson et al, 2005Simpson et al, , 2006Veugen et al, 2017). Both methods have been shown to improve speech recognition in hearing aid users (Simpson et al, 2018), but less is known about the effect in bimodal cochlear implant users (Vroegop et al, 2018).…”