Department of Biomedical Engineering, University of Rochester, United States; Department of Neuroscience, University of Rochester Medical Center, United States.
Department of Biomedical Engineering, University of Rochester, United States.
Hear Res. 2023 Aug;435:108788. doi: 10.1016/j.heares.2023.108788. Epub 2023 May 8.
This study concerns the effect of hearing loss on discrimination of formant frequencies in vowels. In the response of the healthy ear to a harmonic sound, auditory-nerve (AN) rate functions fluctuate at the fundamental frequency, F0. Responses of inner-hair-cells (IHCs) tuned near spectral peaks are captured (or dominated) by a single harmonic, resulting in lower fluctuation depths than responses of IHCs tuned between spectral peaks. Therefore, the depth of neural fluctuations (NFs) varies along the tonotopic axis and encodes spectral peaks, including formant frequencies of vowels. This NF code is robust across a wide range of sound levels and in background noise. The NF profile is converted into a rate-place representation in the auditory midbrain, wherein neurons are sensitive to low-frequency fluctuations. The NF code is vulnerable to sensorineural hearing loss (SNHL) because capture depends upon saturation of IHCs, and thus the interaction of cochlear gain with IHC transduction. In this study, formant-frequency discrimination limens (DLs) were estimated for listeners with normal hearing or mild to moderate SNHL. The F0 was fixed at 100 Hz, and formant peaks were either aligned with harmonic frequencies or placed between harmonics. Formant peak frequencies were 600 and 2000 Hz, in the range of first and second formants of several vowels. The difficulty of the task was varied by changing formant bandwidth to modulate the contrast in the NF profile. Results were compared to predictions from model auditory-nerve and inferior colliculus (IC) neurons, with listeners' audiograms used to individualize the AN model. Correlations between DLs, audiometric thresholds near the formant frequencies, age, and scores on the Quick speech-in-noise test are reported. SNHL had a strong effect on DL for the second formant frequency (F2), but relatively small effect on DL for the first formant (F1). The IC model appropriately predicted substantial threshold elevations for changes in F2 as a function of SNHL and little effect of SNHL on thresholds for changes in F1.
这项研究关注的是听力损失对元音共振峰频率辨别能力的影响。在健康耳朵对谐波声音的反应中，听神经（AN）率函数在基频（F0）处波动。调谐到频谱峰附近的内毛细胞（IHC）的响应被单个谐波捕获（或主导），导致比调谐在频谱峰之间的 IHC 的响应波动深度更小。因此，神经波动（NF）的深度沿着音调轴变化，并编码包括元音共振峰频率在内的频谱峰。这种 NF 编码在广泛的声级和背景噪声中是稳健的。NF 谱在听觉中脑被转换为率-位代表，其中神经元对低频波动敏感。NF 编码易受感音神经性听力损失（SNHL）的影响，因为捕获取决于 IHC 的饱和，因此耳蜗增益与 IHC 转导的相互作用。在这项研究中，对具有正常听力或轻度至中度 SNHL 的听力损失者进行了共振峰频率辨别限（DL）估计。F0 固定在 100 Hz，并且共振峰峰值要么与谐波频率对齐，要么置于谐波之间。共振峰频率为 600 和 2000 Hz，处于几个元音的第一和第二共振峰的范围内。通过改变共振峰带宽来调制 NF 谱的对比度来改变任务的难度。结果与模型听觉神经和下丘（IC）神经元的预测进行了比较，使用听力图对 AN 模型进行了个体化。报告了 DL 与共振峰频率附近的听阈、年龄和快速语音噪声测试得分之间的相关性。SNHL 对第二共振峰（F2）的 DL 有很大影响，但对第一共振峰（F1）的 DL 影响相对较小。IC 模型适当地预测了 F2 变化时 SNHL 引起的阈值显著升高，而 SNHL 对 F1 变化时阈值的影响很小。
