Assessment of the Noise-Protective Action of the Olivocochlear Efferents in Humans

Abstract

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.

Otoacoustic-emission-based medial-olivocochlear reflex assays for humans

“…Recently, Wolpert et al (2014) showed that the difference in DPOAE growth functions with and without a contralateral elicitor was moderately predictive of temporary threshold shift (TTS) magnitude. Although promising, this finding might not generalize to permanent threshold shift (PTS) risk, because PTS and TTS do not have the same physiological underpinnings (e.g., Nordmann et al, 2000), and in its current form this test takes too long for a clinical assay.…”

Absence of Early Neuronal Death in the Olivocochlear System Following Acoustic Overstimulation

“…Inner hair cells (IHC), the primary auditory receptor cells of the mammalian cochlea, are contacted by Type I spiral ganglion cell afferents that are themselves addressed by OC efferents. Although the function of OC neurons (OCN) is not fully understood so far, a protective role against acoustic overexposure and the discrimination of sounds from noise are discussed (Kawase et al, ; Guinan, ; Wolpert et al, ). It is agreed to that OCN may exert excitatory and inhibitory influence on OHC somata and IHC afferents (Giraud et al, ; Hill et al, ; Chen et al, , cf.…”

The olivocochlear system and protection from acoustic trauma: a mini literature review

“…Recently, Wolpert et al ( 2014 ) studied a group of subjects ( n = 40) who were exposed to a 60-min. broadband noise at 94 dB SPL.…”