Slow modulation of cochlear response by the olivocochlear efferent system elicited by sustained noise or threshold elevation in the contralateral ear

Abstract

This thesis describes results from two projects related to the efferent innervation of the cochlea. First, we investigated peripheral olivocochlear effects of sustained contralateral broadband noise in anesthetized guinea pig. We found evidence of medial olivocochlear (MOC) effects on two timescales: the classic MOC 'fast effect', followed by a gradually increasing suppression, which we call the MOC 'delayed effect'. Delayed suppression typically takes 2-3 minutes to build up, occurs at all frequencies of guinea pig hearing, and suppresses distortion product otoacoustic emissions (DPOAEs), compound action potentials (CAPs), and round window noise. In contrast to the MOC slow effect, which has been reported for sustained shockevoked MOC activity, MOC delayed suppression does not build up in the outer hair cells but is due to a central modulation (enhancement) of MOC responsiveness and can thus be viewed as a gradual increase in the strength of the MOC fast effect. We found that, on average, the magnitude of the delayed suppression is comparable to that of the MOC fast effect, but that there is an overall negative correlation between fast and delayed effect magnitudes. Thus, it may have significant implications for the functional roles of the MOC system, such as protection against acoustic trauma, anti-masking, and dynamic range extension. Second, we investigated the LOC bilateral balancing model, which proposes that the LOC system acts to balance long-term average neural output from both cochleae, which would be important for binaural processing of sounds. For this, we tested various cohorts of mice by repeatedly measuring bilateral auditory brainstem responses (ABR) and "DPOAE growth functions across a wide range of frequencies and levels for periods of about 1-2 months. About halfway through the period, a unilateral reduction in neural output was created, either by acoustic overexposure or conductive impairment. Although the LOC balancing model predicts that the unilateral reduction in neural output should be matched contralaterally, we found no evidence for short-term or long-term efferent-induced contralateral response changes in any of the cohorts, either for DPOAE or ABR metrics.

(cont.) In view of these results, a revision of the LOC bilateral balancing model is called for.