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

Abstract

Purpose Speech understanding in noise and horizontal sound localization is poor in most cochlear implant (CI) users with a hearing aid (bimodal stimulation). This study investigated the effect of static and less-extreme adaptive frequency compression in hearing aids on spatial hearing. By means of frequency compression, we aimed to restore high-frequency audibility, and thus improve sound localization and spatial speech recognition. Method Sound-detection thresholds, sound localization, and spatial speech recognition were measured in eight bimodal CI users, with and without frequency compression. We tested two compression algorithms: a static algorithm, which compressed frequencies beyond the compression knee point (160 or 480 Hz), and an adaptive algorithm, which aimed to compress only consonants leaving vowels unaffected (adaptive knee-point frequencies from 736 to 2946 Hz). Results Compression yielded a strong audibility benefit (high-frequency thresholds improved by 40 and 24 dB for static and adaptive compression, respectively), no meaningful improvement in localization performance (errors remained > 30 deg), and spatial speech recognition across all participants. Localization biases without compression (toward the hearing-aid and implant side for low- and high-frequency sounds, respectively) disappeared or reversed with compression. The audibility benefits provided to each bimodal user partially explained any individual improvements in localization performance; shifts in bias; and, for six out of eight participants, benefits in spatial speech recognition. Conclusions We speculate that limiting factors such as a persistent hearing asymmetry and mismatch in spectral overlap prevent compression in bimodal users from improving sound localization. Therefore, the benefit in spatial release from masking by compression is likely due to a shift of attention to the ear with the better signal-to-noise ratio facilitated by compression, rather than an improved spatial selectivity. Supplemental Material https://doi.org/10.23641/asha.16869485

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

“…Finally, hearing asymmetry led to a consistent bias, which was not reduced even with long-term exposure to this hearing asymmetry. This is in stark contrast to our expectation that bimodal EAS users could potentially use perturbed low-frequency ILDs in the absence of ITDs (i.e., for mid-frequency sounds; Veugen Chalupper et al, 2016 ; Loiselle et al, 2015 ; Gifford & Stercker 2020 ; Sharma et al, 2021 ). Evidence from other studies suggests that a bias induced by acute asymmetric hearing may be overcome by long-term exposure or adaptation ( Van Wanrooij & Van Opstal, 2007 ; Kumpik et al, 2010, in acutely plugged normal-hearing: Agterberg et al, 2018 ; Agterberg, Hol, et al, 2011 ; Agterberg, Snik, et al, 2011 , in participants with bone-conduction devices; Van Wanrooij & Van Opstal, 2004, participants with single-sided deafness).…”

“…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.…”

“…For mid-frequency sounds the implant does not provide temporal fine structure information and hearing loss in the hearing-aid ear is often substantial (exceeding 30 dB HL in Figure 1a ). In this case, bimodal EAS users might be able to utilize low-frequency ILDs (as suggested for bimodal users: Veugen et al, 2016 ; Sharma et al, 2021 ; and for bimodal EAS users: Gifford & Stecker 2020 ). If the hearing asymmetry (favoring the implanted ear) is not accounted for, this might yield a bias toward the implanted ear ( Figure 1c ).…”

Reaction Time Sensitivity to Spectrotemporal Modulations of Sound

“…We assessed the extent of separability of joint spectrotemporal sensitivity and investigated how binaural listening affected modulation sensitivity compared to monaural listening conditions by comparing the data with the race-model prediction. As a validation of our reaction-time paradigm, we also collected data under more challenging impaired-hearing simulations that are known to affect temporal and spectral sensitivity (Bacon & Viemeister, 1985;Golub et al, 2012;Henry et al, 2005;Moore & Glasberg, 2001) and binaural integration (Ausili et al, 2019;Sharma et al, 2019Sharma et al, , 2021Veugen et al, 2016aVeugen et al, , 2016b.…”