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.

噪声性听力损失是最常见的听觉病理之一，由人类耳蜗（cochlea）过度刺激引发——耳蜗是一种极其灵敏的微机械装置。然而，在极低频（<250赫兹）环境下，人类听觉的灵敏度以及由此产生的感知响度均较差。感知响度由耳蜗内毛细胞（inner hair cells）介导，而内毛细胞在低频环境下的激活程度极不充分。为评估低频声音的影响，我们利用了外毛细胞（outer hair cells）执行声音主动放大过程时产生的副产物——即所谓的自发性耳声发射（spontaneous otoacoustic emissions）。这类信号是内耳产生的微弱声音，可用于检测耳蜗生理状态的变化。我们的研究表明，短暂暴露于感知上不易察觉的低频声音，会对外毛细胞产生显著影响：时长90秒、声级为80分贝A计权（80 dB(A)）的低频声音，可诱发自发性耳声发射出现缓慢、协调一致且呈正相关的频率与声级振荡，该振荡会在低频声音停止后持续约两分钟。与人们感知上不易察觉的特点相反，低频声音会强烈刺激人类耳蜗，并影响人类听觉最灵敏、最重要的频率范围内的放大过程。