23 How Brain Activity Shapes the Way Your Ear Processes Sound

Faculty Mentor: Skyler Jennings (Communication Sciences and Disorders, University of Utah)

Introduction

The neurons of the auditory nerve fire in the absence of external auditory input (Liberman, 1978). This spontaneous activity results from internal physiological noise, such as a heartbeat, that is amplified by movement of the hair cells in the cochlea (Liberman, 1978). The summed spontaneous activity of many auditory nerve fibers elicits a biophysical potential that is referred to as ensemble background activity (EBA) (Searchfield et al., 2004), which can be observed in animals through electrocochleography (ECochG). This measurement (ECochG) involves placing an electrode in or near the inner ear (Popelár et al., 2001). Animal ECochG experiments are notably invasive, so it is not typically possible to obtain a similar proximity when making measurements in human participants.

In animals, it has been demonstrated that EBA is suppressed by the medial olivocochlear (MOC) reflex by decreasing cochlear amplification of internal noise, which would otherwise evoke EBA (Popelár et al., 1997; Zhou et al., 2004; Lopez-Poveda, 2018). The MOC reflex plays a role in adjusting the sensitivity of the cochlea, which is hypothesized to protect hearing and facilitate listening in noisy environments in humans (Lopez-Poveda, 2018). Suppression of cochlear amplification as a result of the MOC reflex has been observed in humans by measuring compound action potentials (CAPs) (Chabert et al., 2002; Lichtenhan et al., 2016; Smith et al., 2017; Najem et al., 2016) and otoacoustic emissions (Mertes and Leek, 2017), but not EBA.

The purpose of this study was to 1) replicate the only experiment that has successfully measured EBA in humans (Pardo-Jaude et al., 2017), and 2) suppress EBA through the presentation of noise to the contralateral ear, which is expected to elicit the MOC reflex.

Methods

Measurements were obtained using ECochG techniques, with an electrode placed on the tympanic membrane (TM). Prior to each EBA experiment, participants underwent a data quality check, which involved measuring CAPs evoked by a click in a descending intensity series from 110-40 dB peSPL. The CAP signifies a response of the auditory nerve to a stimulus, so this quality check ensured that the TM electrode was placed correctly. Many technical aspects of the experiment were manipulated to replicate the approach of Pardo-Jaude et al. (2017). Alterations were made to the acoustic stimulus, recording duration, electrode montage, participant position, preamplifier make/model, and tympanic membrane electrode make/model over the course of this project.

Results and Discussion

Ultimately, EBA was not observed in the current study, resulting in a failure to replicate the results of Pardo-Jaude et al. (2017) and suggesting that EBA may not be consistently measured across laboratories and clinics. Given the robust auditory nerve responses observed in the data quality tests, it is unlikely that the failure to observe EBA was due to the quality of the equipment or measurements taken during this experiment. This failure to replicate EBA findings may limit clinical and research applications of EBA measurements in humans.

The ability to reliably measure EBA would give clinicians insight to the auditory nerve function of individuals, which could serve as a diagnostic tool. An increased understanding of the MOC reflex could allow clinicians to adjust hearing aids to better facilitate listening in noisy environments. This study suggests that measurements other than EBA should be used to study the MOC reflex and related aspects of human auditory physiology.

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