Olivocochlear efferents: Their action, effects, measurement and uses, and the impact of the new conception of cochlear mechanical responses

Abstract

The anatomy and physiology of olivocochlear (OC) efferents are reviewed. To help interpret these, recent advances in cochlear mechanics are also reviewed. Lateral OC (LOC) efferents innervate primary auditory-nerve (AN) fiber dendrites. The most important LOC function may be to reduce auditory neuropathy. Medial OC (MOC) efferents innervate the outer hair cells (OHCs) and act to turn down the gain of cochlear amplification. Cochlear amplification had been thought to act only through basilar membrane (BM) motion, but recent reports show that motion near the reticular lamina (RL) is amplified more than BM motion, and that RL-motion amplification extends to several octaves below the local characteristic frequency. Data on efferent effects on AN-fiber responses, otoacoustic emissions (OAEs) and human psychophysics are reviewed and reinterpreted in the light of the new cochlear-mechanical data. The possible origin of OAEs in RL motion is considered. MOC-effect measuring methods and MOC-induced changes in human responses are also reviewed, including that ipsilateral and contralateral sound can produce MOC effects with different patterns across frequency. MOC efferents help to reduce damage due to acoustic trauma. Many, but not all, reports show that subjects with stronger contralaterally-evoked MOC effects have better ability to detect signals (e.g. speech) in noise, and that MOC effects can be modulated by attention.

Keywords: Attention; Cochlear mechanics; Medial olivocochlear efferents; Otoacoustic emissions.

Fig. 1

A conception of frequency (top) and level (bottom) functions for responses to tones of the basilar membrane, the area near the reticular lamina, and auditory-nerve fibers, without and with MOC stimulation, in the basal cochlea. Based on: Cooper and Guinan (2006) for panels A & D; Ren et al., (2016) plus Recio-Spinoso and Oghalai, (2017) for no-MOC plots in panels B and E; Stankovic and Guinan (1999) plus Guinan and Stankovic (1996) for the (speculative) estimated change due to MOC stimuation in panels B and E; Guinan and Gifford (1988) for panel C; Guinan and Stankovic (1996) for panel F. SR=spontaneous rate.

Fig. 2

MOC-induced changes in SFOAEs averaged across human subjects, expressed as ΔSFOAE magnitude (top), SFOAEmoc magnitude (middle) and SFOAEmoc phase (bottom) as functions of noise-elicitor bandwidth (left) and as functions of elicitor frequency re. the probe frequency (right). SFOAEmoc is the SFOAE measured during MOC stimulation. Note the different scales at left and right. Left: 60 dB SPL elicitors centered at the 1 kHz probe frequency. Right: 60 dB SPL, half-octave-wide noise elicitors. All used 40 dB SPL, 1 kHz probe tones. Inset: Vector diagram showing the relationships of ΔSFOAE magnitude, SFOAEmoc magnitude and SFOAEmoc phase (ϕ). Data at left from Lilaonitkul and Guinan (2009b) and at right from Lilaonitkul and Guinan (2012).