Top-down influences of the medial olivocochlear efferent system in speech perception in noise

Abstract

One of the putative functions of the medial olivocochlear (MOC) system is to enhance signal detection in noise. The objective of this study was to elucidate the role of the MOC system in speech perception in noise. In normal-hearing human listeners, we examined (1) the association between magnitude of MOC inhibition and speech-in-noise performance, and (2) the association between MOC inhibition and the amount of contralateral acoustic stimulation (CAS)-induced shift in speech-in-noise acuity. MOC reflex measurements in this study considered critical measurement issues overlooked in past work by: recording relatively low-level, linear click-evoked otoacoustic emissions (CEOAEs), adopting 6 dB signal-to-noise ratio (SNR) criteria, and computing normalized CEOAE differences. We found normalized index to be a stable measure of MOC inhibition (mean = 17.21%). MOC inhibition was not related to speech-in-noise performance measured without CAS. However, CAS in a speech-in-noise task caused an SNRSP enhancement (mean = 2.45 dB), and this improvement in speech-in-noise acuity was directly related to their MOC reflex assayed by CEOAEs. Individuals do not necessarily use the available MOC-unmasking characteristic while listening to speech in noise, or do not utilize unmasking to the extent that can be shown by artificial MOC activation. It may be the case that the MOC is not actually used under natural listening conditions and the higher auditory centers recruit MOC-mediated mechanisms only in specific listening conditions-those conditions remain to be investigated.

Figure 1. Bland and Altman plot for normalized index (ΔCEOAEn).

Bland-Altman plot showing bias (solid line), 95% limits of agreement (heavy-dashed lines), and 95% confidence intervals (CIs) of bias (light-dashed lines). CEOAE indicates click-evoked otoacoustic emission; circles indicate repeated data (n = 13). The bias line shows average discrepancy between trials; the zero lying within 95% CIs of bias suggests no significant systematic error between trials; the limits of agreement represent the range of values in which agreement between trials may lie for approximately 95% of the sample. Note that intra-subject SD on replication for ΔCEOAE was 4.64%.

Figure 2. MOC inhibition and speech perception in noise.

Bivariate scatterplot depicting the relationship between MOC reflex magnitude (ΔCEOAEn) and speech recognition threshold without CAS (SRT). Pearson’s correlation coefficient (r) is inserted on top left corner of the plot.

Figure 3. MOC inhibition and CAS-induced SNR_SP enhancement.

The CAS-induced shift in SRT is plotted as a function of MOC reflex magnitude (ΔCEOAEn). A linear regression line is inserted to show the predictive relationship. Pearson’s correlation coefficient (r) is inserted on top left corner of the plot.