Low-frequency sound affects active micromechanics in the human inner ear

Noise-induced hearing loss is one of the most common auditory pathologies, resulting from overstimulation of the human cochlea, an exquisitely sensitive micromechanical device. At very low frequencies (<250 Hz), however, the sensitivity of human hearing, and therefore the perceived loudness is poor. The perceived loudness is mediated by the inner hair cells of the cochlea, which are driven very inadequately at low frequencies. To assess the impact of low-frequency sound, we exploited a by-product of the active amplification of sound outer hair cells perform, so-called spontaneous otoacoustic emissions. These are faint sounds produced by the inner ear that can be used to detect changes of cochlear physiology. We show that a short exposure to perceptually unobtrusive, low-frequency sounds significantly affects outer hair cells: A 90 s, 80 dB(A) low-frequency sound induced slow, concordant and positively correlated frequency- and level oscillations of spontaneous otoacoustic emissions which lasted for about two minutes after low-frequency sound offset. Low-frequency sounds, contrary to their unobtrusive perception, strongly stimulate the human cochlea and affect amplification processes in the most sensitive and important frequency range of human hearing.噪声性听力损失（noise-induced hearing loss）是最常见的听觉病理之一，其诱因是人类耳蜗（cochlea）——一种极其灵敏的微机械装置——受到过度刺激。然而，在极低频率（<250 Hz）下，人类听觉的敏感度以及由此产生的主观响度感知均较差。主观响度感知由耳蜗内毛细胞（inner hair cells）介导，而这类细胞在低频环境下的激活程度严重不足。为评估低频声波的影响，我们利用了外毛细胞（outer hair cells）主动放大声波过程中产生的副产物——即所谓的自发性耳声发射（spontaneous otoacoustic emissions）。这类发射是内耳产生的微弱声响，可用于检测耳蜗生理状态的变化。本研究证实，短暂暴露于主观上不易察觉的低频声波，会对外毛细胞产生显著影响：90秒、80 dB(A)的低频声波可诱发自发性耳声发射出现缓慢、协调且呈正相关的频率与幅度振荡，该振荡在低频声波结束后可持续约2分钟。与主观感知上的不明显特性相反，低频声波会强烈刺激人类耳蜗，并影响人类听觉最敏感、最关键的频率范围内的声波放大过程。