Elsevier

Journal of Communication Disorders

Volume 31, Issue 6, November–December 1998, Pages 471-483
Journal of Communication Disorders

Feedback control of the auditory periphery: Anti-masking effects of middle ear muscles vs. olivocochlear efferents

https://doi.org/10.1016/S0021-9924(98)00019-7Get rights and content

Introduction

There are two major efferent feedback pathways to the auditory periphery: the middle ear muscle (MEM) reflex and the olivocochlear (OC) efferent reflex. Although the peripheral effects of these two sound-evoked reflexes have been known for many years, there is still significant controversy as to their functional significance: that is, why is it important to have sound-evoked negative-feedback reflexes which can elevate thresholds in the cochlear periphery? Over the years, a number of hypotheses have been advanced, including (1) that the reflexes help to protect the ear from acoustic injury and (2) that the reflexes aid in the control of masking. This article considers only the anti-masking effects of the MEM and MOC reflexes. It is not meant to be a review of the literature [Reviews of MEM and MOC effects can be found elsewhere Borg and Zakrisson 1975, Guinan 1996.], rather it is meant to explain, in broad outline, how negative feedback circuits in the auditory periphery can have anti-masking properties, under what stimulus situations their effects will be largest, and how these anti-masking effects should be affected by hearing impairment. Original research reports on MEM and MOC anti-masking effects are cited in Pang and Guinan 1997, Kawase et al. 1993.

Section snippets

Reflex overview

Both the MEM and OC reflex pathways can be divided into two subsystems with different central origins and peripheral targets. The MEM system consists of two muscles: the tensor tympani, which inserts onto the malleus, and the stapedius, which is attached to the head of the stapes. This article focuses on the stapedius muscle reflex, because in humans it may be the only MEM that responds to sound, and its functional significance is better understood. As illustrated in Figure 1, the stapedius

Physiological basis of masking

How can feedback systems which suppress responses in the auditory periphery aid in the control of masking? The anti-masking properties of the MEM and MOC systems are based on different mechanisms, and complement each other in the sense that the MEM system helps control masking from low-frequency noise while the MOC system helps with mid- and high-frequency noise. To understand how anti-masking properties arise, we must first consider the physiological bases for masking at the level of the

MEM Reflex

Activation of the stapedius reflex and the selective attenuation of low-frequency sounds which results, can significantly reduce the upward spread of masking. In other words, the MEM reflex can decrease the suppressive making of high-CF fibers in the presence of low-frequency masking noise. This anti-masking effect is illustrated in Figure 5. Figure 5A shows the response of a high-CF fiber to a high-frequency signal in the absence of noise, as was illustrated in Figure 4B. When the noise is

Anti-masking effects in impaired ears

Sensorineural hearing loss does not necessarily increase the threshold for the MEM reflex. The same may be true for the MOC system, although it has not been as well studied. In general, we would predict that sensorineural hearing loss would reduce the types of anti-masking effects described here. This is expected because sensorineural hearing loss so often involves damage to the OHCs and the associated cochlear amplifier. Although different mechanisms underlie the anti-masking effects of the

Summary

  • Both MEM and MOC systems are sound-evoked reflexes to the auditory periphery which can be elicited by sound in either ear.

  • Both MEM and MOC systems can increase thresholds in the auditory periphery: the MEM system acts by stiffening the ossicular chain, the MOC system by decreasing outer hair cell amplification of sound-induced motion in the inner ear.

  • MEM-induced attenuations are largest for low frequency stimuli, MOC-induced attenuations are largest for mid- to high-frequency sounds.

  • Both MEM

Acknowledgements

The authors’ research described here has been supported by grants from the NIDCD: RO1 DC00188, PO1 DC00119 and RO1 DC00235.

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References (7)

  • Borg, E., & Zakrisson, J.-E. (1975). The stapedius muscle and speech perception. Symposium of the Zoological Society of...
  • B.A.R. Delgutte

    Physiological mechanisms of masking

  • J.J. Guinan

    The physiology of olivocochlear efferents

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  • On the importance of interaural noise coherence and the medial olivocochlear reflex for binaural unmasking in free-field listening

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