Hearing asymmetry biases spatial hearing in bimodal cochlear-implant users despite bilateral low-frequency hearing preservation

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. Sound-localization performance was poor in general. Typically, localization was strongly biased toward the better hearing ear with some preference to localize toward the ear using the hearing aid. Only two listeners with little hearing asymmetry could localize low- and mid-frequency sounds reasonably well with negligible bias using both devices. The results suggest that hearing asymmetry in bimodal EAS listeners leads to persistent localization biases. 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.

许多佩戴人工耳蜗（cochlear implant）且尚存双耳残余（声学）听力的使用者，可通过将植入耳的电声联合刺激（electric and acoustic stimulation, EAS）与非植入耳的声学放大相结合而获益。这类双模式电声联合刺激使用者理论上可利用低频双耳线索进行声音定位。然而，他们的中高频听力通常存在不对称性，这会干扰甚至消除双耳线索。本研究针对7名双模式电声联合刺激使用者，探究了听阈的频率依赖性双耳不对称性对声音定位的影响。研究通过呈现包含低频、中频、高频或中高频频段能量的声音，来考察频率依赖性效应。双模式电声联合刺激使用状态下（同时佩戴两台设备）存在频率依赖性听力不对称性，且该不对称性也可通过单独开启或关闭设备来诱导产生。同时佩戴两台设备时，低频听力接近对称，中频听力不对称且植入耳听阈更优，高频则为单耳聆听状态，非植入耳无听力。整体而言，受试者的声音定位表现较差。通常情况下，定位结果会强烈偏向听力更优的一侧，且一定程度上偏好朝向佩戴助听器的耳侧进行定位。仅有2名听力不对称程度较轻的受试者，在同时佩戴两台设备时，可较为准确地定位低频与中频声音，且定位偏差可忽略不计。本研究结果表明，双模式电声联合刺激使用者的听力不对称性会导致持续存在的定位偏差。本文讨论了，当声音频谱发生变化时，任何听力不对称性的频率依赖性如何导致双耳线索出现空间不一致性。本研究推测，即便受试者经过长期适应这种听力不对称性，这种空间不一致性仍可能阻碍其实现精准的声音定位。