Hearing Systems section, Department of Health Technology, Technical University of Denmark (DTU), Kongens Lyngby, Denmark.
Interacoustics Research Unit, Kongens Lyngby, Denmark.
J Assoc Res Otolaryngol. 2019 Aug;20(4):363-382. doi: 10.1007/s10162-019-00721-7. Epub 2019 May 17.
The healthy auditory system enables communication in challenging situations with high levels of background noise. Yet, despite normal sensitivity to pure tones, many listeners complain about having difficulties in such situations. Recent animal studies demonstrated that noise overexposure that produces temporary threshold shifts can cause the loss of auditory nerve (AN) fiber synapses (i.e., cochlear synaptopathy, CS), which appears to predominantly affect medium- and low-spontaneous rate (SR) fibers. In the present study, envelope following response (EFR) magnitude-level functions were recorded in normal hearing (NH) threshold and mildly hearing-impaired (HI) listeners with thresholds elevated above 2 kHz. EFRs were elicited by sinusoidally amplitude modulated (SAM) tones presented in quiet with a carrier frequency of 2 kHz, modulated at 93 Hz, and modulation depths of 0.85 (deep) and 0.25 (shallow). While EFR magnitude-level functions for deeply modulated tones were similar for all listeners, EFR magnitudes for shallowly modulated tones were reduced at medium stimulation levels in some NH threshold listeners and saturated in all HI listeners for the whole level range. A phenomenological model of the AN was used to investigate the extent to which hair-cell dysfunction and/or CS could explain the trends observed in the EFR data. Hair-cell dysfunction alone, including postulated elevated hearing thresholds at extended high frequencies (EHF) beyond 8 kHz, could not account for the recorded EFR data. Postulated CS led to simulations generally consistent with the recorded data, but a loss of all types of AN fibers was required within the model framework. The effects of off-frequency contributions (i.e., away from the characteristic place of the stimulus) and the differential loss of different AN fiber types on EFR magnitude-level functions were analyzed. When using SAM tones in quiet as the stimulus, model simulations suggested that (1) EFRs are dominated by the activity of high-SR fibers at all stimulus intensities, and (2) EFRs at medium-to-high stimulus levels are dominated by off-frequency contributions.
健康的听觉系统使我们能够在存在背景噪声的高噪声环境下进行交流。然而，尽管对纯音的敏感性正常，但许多听力受损的人在这种情况下仍会抱怨存在困难。最近的动物研究表明，噪声过度暴露会导致听觉神经（AN）纤维突触（即耳蜗突触病，CS）丢失，这似乎主要影响中低自发率（SR）纤维。在本研究中，在正常听力（NH）阈值和轻度听力受损（HI）听力受损者中记录了包络跟随反应（EFR）幅度-水平函数，其阈值高于 2 kHz。EFR 是通过在安静中呈现的正弦幅度调制（SAM）音调诱发的，载波频率为 2 kHz，调制频率为 93 Hz，调制深度为 0.85（深）和 0.25（浅）。虽然对于所有听众来说，深度调制音调的 EFR 幅度-水平函数相似，但在一些 NH 阈值听众的中等刺激水平下，浅调制音调的 EFR 幅度降低，并且在整个水平范围内，所有 HI 听众的 EFR 幅度饱和。使用 AN 的现象学模型来研究毛细胞功能障碍和/或 CS 可以在多大程度上解释 EFR 数据中观察到的趋势。仅毛细胞功能障碍，包括假设的在 8 kHz 以上的扩展高频（EHF）处升高的听力阈值，不能解释记录的 EFR 数据。假设的 CS 导致的模拟通常与记录的数据一致，但需要在模型框架内丢失所有类型的 AN 纤维。分析了非频率贡献（即，远离刺激的特征位置）和不同 AN 纤维类型的差异损失对 EFR 幅度-水平函数的影响。当使用安静中的 SAM 音调作为刺激时，模型模拟表明：（1）在所有刺激强度下，EFR 主要由高 SR 纤维的活动主导；（2）在中等到高刺激水平下，EFR 主要由非频率贡献主导。
