Lack of association between contralateral inhibition of otoacoustic emissions and vowel formant discrimination in noise

Purpose: The medial olivocochlear (MOC) reflex enhances neural encoding of signals in noise, and measurement of its function may hold clinical utility. Previous research on how the reflex aids speech-in-noise perception has been equivocal. Motivated by animal work, we examined associations between MOC reflex activity and formant discrimination in noise in humans to better understand how the MOC reflex contributes to audition. We hypothesised that participants with stronger MOC reflex activity would have better formant discrimination in noise abilities. Method: Twenty-six normal-hearing listeners met all inclusion and exclusion criteria (mean age = 21.5 years), with data from 25 participants included in the final analysis. Transient-evoked otoacoustic emissions (TEOAEs) were measured in right ears. MOC reflex activity was assessed using a contralateral inhibition paradigm in which the change in TEOAE amplitude without versus with a contralateral MOC reflex elicitor was computed. Formant discrimination thresholds for a synthetic vowel /ɛ/ were obtained in right ears using a two-alternative forced-choice procedure that adaptively varied the second formant frequency. Discrimination thresholds were obtained at three signal-to-noise ratios (SNRs). Results: TEOAE amplitudes were significantly reduced in the presence of the reflex elicitor (p <.05). Discrimination thresholds decreased significantly with increasing SNR (p <.05 in all cases). No significant correlations were found between contralateral inhibition measures and discrimination thresholds at any SNR (p >.05 in all cases). Conclusion: Contrary to hypothesis, no significant associations were found between contralateral inhibition and formant discrimination in noise performance. It is possible that the MOC reflex contributes to formant discrimination but not in a monotonic fashion. Future work should consider investigating how the MOC reflex contributes to other perceptual properties to better characterise the functional relevance of the MOC reflex.