Olivocochlear Efferents in Animals and Humans: From Anatomy to Clinical Relevance

Abstract

Olivocochlear efferents allow the central auditory system to adjust the functioning of the inner ear during active and passive listening. While many aspects of efferent anatomy, physiology and function are well established, others remain controversial. This article reviews the current knowledge on olivocochlear efferents, with emphasis on human medial efferents. The review covers (1) the anatomy and physiology of olivocochlear efferents in animals; (2) the methods used for investigating this auditory feedback system in humans, their limitations and best practices; (3) the characteristics of medial-olivocochlear efferents in humans, with a critical analysis of some discrepancies across human studies and between animal and human studies; (4) the possible roles of olivocochlear efferents in hearing, discussing the evidence in favor and against their role in facilitating the detection of signals in noise and in protecting the auditory system from excessive acoustic stimulation; and (5) the emerging association between abnormal olivocochlear efferent function and several health conditions. Finally, we summarize some open issues and introduce promising approaches for investigating the roles of efferents in human hearing using cochlear implants.

Keywords: attention; cochlear implants; learning; olivocochlear reflex; otoacoustic emissions; psychoacoustics; speech-in-noise; superior olivary complex.

Figure 1

Pathways for activation of medial (MOC) and lateral olivocochlear (LOC) efferent fibers to the right cochlea. Red and blue lines illustrate the pathways for activation of the contralateral and ipsilateral MOC reflexes, respectively. Green lines illustrate the pathways for activation of the LOC reflex. The thickness of each line roughly illustrates the density of innervation. Abbreviations: LSO, lateral superior olive; MNTB, medial nucleus of the trapezoid body; PVCN, postero-ventral cochlear nucleus; SOC, superior olivary complex; VNTB, ventral nucleus of the trapezoid body; CF, characteristic frequency.

Figure 2

Effects of electrical activation of olivocochlear efferents on various physiological responses to sounds in quiet. The left and right panels illustrate input/output and threshold tuning curves for the corresponding system, respectively. (A,B) Basilar motion. Data re-plotted, in modified form, from Cooper and Guinan (33). (C,D) Inner hair cell receptor potential. Data re-plotted, in modified form, from Brown and Nuttall (34). (E,F) Discharge of single auditory nerve afferent fibers. Data in panels (E,F) are re-plotted, in modified form, from Wiederhold (35) and Guinan and Gifford (36), respectively. (G) Auditory nerve CAP. Data re-plotted, in modified form, from Elgueda et al. (37). Abbreviations: BM, basilar membrane; CAP, compound action potential; DC, direct current; MOC, medial-olivocochlear efferents.

Figure 3

Effects of olivocochlear efferent activation on physiological responses to sound in noise. The left and right panels illustrate corresponding input/output and threshold tuning curves, respectively. (B,C) Single auditory nerve fiber responses. Data in panels (B,C) are re-plotted, in modified form, from Winslow and Sachs (54) and Kawase et al. (55), respectively. (A) CAP. Data in panel A re-plotted, in modified form, from Nieder and Nieder (52). Abbreviations: CAP, compound action potential; VDL, visual detection level.

Figure 4

Effects of CAS-evoked medial-olivocochlear (MOC) activation on human behaviorally inferred cochlear input/output curves (A) and psychoacoustical tuning curves (B). The data in panel (A) are re-plotted, in modified form, from Fig. 2 in Krull and Strickland (83), subject S1. The data in panel (B) are re-plotted, in modified form, from Fig. 3 in Aguilar et al. (82).