Ryan, Marie Siobhan; (2001) The influence of neural activity on cochlear mechanics in humans. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This study concerns the influence of neural activity on cochlear mechanics in alert humans. Cochlear mechanical activity is responsible for amplifying sound within the cochlea thus providing hearing sensitivity and sharp tuning. This is an energy requiring process involving the outer hair cells (OHCs). The OHCs are innervated by efferent neurons and it has been postulated that this efferent innervation is an integral part of the specialization of OHCs for their role in cochlear amplification. The function of the efferent neurons therefore is considered to provide a means of optimising OHC activity. In order to contribute to elucidating the role of cochlear efferent activity in hearing, some attributes of the effects of efferent activity on cochlear mechanics were explored in this study. Non-invasive means are used to monitor cochlear mechanical activity and to evoke efferent neural activity. Transient evoked otoacoustic emissions (TEOAEs), recorded in the ear canal and evoked by clicks, were used to monitor cochlear mechanical activity while efferent activity was evoked acoustically by wide band noise presented either contralaterally or ipsilaterally to the test ear. The results of the study show that the general effect of efferent activity is a suppression of cochlear mechanical activity. As shown in chapter 6 suppression occurred when efferent activity was stimulated via the contralateral ear and the ipsilateral ear. This showed that, not only is there is a functional afferent/efferent neural pathway between the ears, but also a functional afferent/efferent loop to an ear. Moreover the strength of the efferent effects on cochlear mechanics was of the same magnitude whether their activity was evoked ipsilaterally or contralaterally. This conclusion was drawn from the finding that cochlear mechanical activity was suppressed by similar amounts and the recovery times from the suppression were similar following both ipsilateral and contralateral activation of efferent activity. The first part of the study explores the characteristics of the effects of efferent neural activity on cochlear mechanics. Chapter 4 shows that some ears are more susceptible to suppression than others. The results show that those ears that are the most mechanically active are also the most susceptible to suppression. In chapter 5 the effects of efferent activity are shown to decline as the levels of stimuli evoking mechanical activity are raised. These results contribute to the conclusion that the role of efferent activity is to prevent over oscillation of the OHCs. Chapter 7 explores the effects of changes in the middle ear impedance on TEOAEs. The results show that middle ear impedance does indeed change under experimental conditions similar to those used to detect efferent effects. These middle ear changes however, cannot account for the imputed effects of efferent activity.

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