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

Abstract

Many cochlear implant users with binaural residual (acoustic) hearing benefit from combining electric and acoustic stimulation (EAS) in the implanted ear with acoustic amplification in the other. These bimodal EAS listeners can potentially use low-frequency binaural cues to localize sounds. However, their hearing is generally asymmetric for mid- and high-frequency sounds, perturbing or even abolishing binaural cues. Here, we investigated the effect of a frequency-dependent binaural asymmetry in hearing thresholds on sound localization by seven bimodal EAS listeners. Frequency dependence was probed by presenting sounds with power in low-, mid-, high-, or mid-to-high-frequency bands. Frequency-dependent hearing asymmetry was present in the bimodal EAS listening condition (when using both devices) but was also induced by independently switching devices on or off. Using both devices, hearing was near symmetric for low frequencies, asymmetric for mid frequencies with better hearing thresholds in the implanted ear, and monaural for high frequencies with no hearing in the non-implanted ear. Results show that sound-localization performance was poor in general. Typically, localization was strongly biased toward the better hearing ear. We observed that hearing asymmetry was a good predictor for these biases. Notably, even when hearing was symmetric a preferential bias toward the ear using the hearing aid was revealed. We discuss how frequency dependence of any hearing asymmetry may lead to binaural cues that are spatially inconsistent as the spectrum of a sound changes. We speculate that this inconsistency may prevent accurate sound-localization even after long-term exposure to the hearing asymmetry.

Towards personalized and optimized fitting of cochlear implants

“…It is important to randomly vary the absolute sound levels of the stimuli over a sufficient range (i.e., 20-25 dB) to reliably identify the contribution of the monaural head-shadow effect to their localization estimates (Van Wanrooij and Van Opstal, 2004;Agterberg et al, 2011Agterberg et al, , 2014. Our experience with patient sound-localization experiments typically has yielded consistent and reproducible (multiple) regression results (Van Wanrooij and Van Opstal, 2004;Agterberg et al, 2011Agterberg et al, , 2012Agterberg et al, , 2014Veugen et al, , 2022Ausili et al, 2020;Sharma et al, 2023). In the data analysis, the localization response is described as a bi-linear function of the actual stimulus location (T AZI ; true soundlocalization sensitivity), and the stimulus level (L SND ; head-shadow sensitivity; Van Wanrooij and Van Opstal, 2004;Agterberg et al, 2012…”