Assessment of the noise-protective action of the olivocochlear efferents in humans.

It has been demonstrated in different mammals that the medial olivocochlear efferents (MOC) exert a noise-protective effect on the cochlea. In humans such an effect has not unambiguously been shown as of yet. The objective of this study was to assess the relationship between MOC activity and susceptibility of the cochlea to noise-induced hearing loss in humans. In 40 normally hearing human subjects, we measured the following: (1) magnitude of temporary threshold shift (TTS) after exposure to 60 min broadband noise of 94 dB SPL and (2) contralateral suppression (CS) of distortion product otoacoustic emissions (which reflects MOC activity) using two different measurement paradigms. CS was measured in duplicate on 2 measurement days. The relationship between TTS and CS was assessed. Individual TTS in the most affected frequencies (4 > 3 > 8 kHz) ranged from 9 to 28 dB HL, with an average maximum TTS of 18.4 dB HL. The amount of CS ranged between 0.3 and 3 dB. The repeatability of CS, evaluated by Cronbach's α value, ranged from 0.76 (acceptable repeatability) to 0.86 (good repeatability). One of the two different measurement paradigms showed a statistically significant inverse correlation between CS magnitude and amount of TTS, which was hypothesized. This is the first study on the relationship between TTS and CS in humans employing TTS induced under controlled laboratory conditions and two different MOC paradigms. The findings are compatible with the hypothesis that MOC activity is noise protective in humans. Future perspectives include modified CS paradigms, longitudinal cohort studies or efforts to also monitor lateral efferent effects in humans.