Date of Award

12-2023

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Program

Speech and Hearing Science

Track

Hearing Science

Research Advisor

James D. Lewis, AuD, PhD

Committee

Ashley W. Harkrider, PhD; Mark S. Hedrick, PhD; Patti Johnstone, PhD; Hua Ou, PhD

Keywords

Cortical auditory-evoked potentials;Late latency response;Medial olivocochlear reflex;Signal-to-noise ratios;Unmasking

Abstract

Purpose. All human beings, regardless of whether they have hearing impairments or not, face difficulties in detecting signals in the presence of background noise. To cope with such situations, people can enhance their performance by either increasing the signal levels or reducing the noise levels, which in turn increases the signal-to-noise ratios (SNRs). SNRs play a crucial role in determining one's speech perception abilities. For instance, if the noise levels are high, individuals may raise their voices or decrease their distance from the source to amplify the signal levels, thereby increasing the SNR. However, in real-life situations, individuals may not be able to control these variables, leading to variations in their ability to detect signals in noise. Such variations may be attributed to differences in the auditory efferent system among individuals. The auditory efferent system can potentially improve SNRs through Medial Olivocochlear Reflex (MOCR)-mediated unmasking, as supported by animal studies. Nevertheless, it is not clear to what extent the efferent system can enhance SNRs in humans. Cortical Auditory Evoked Potentials (CAEPs) can serve as a useful tool to assess the impact of the auditory efferent system on humans, as they are highly sensitive to changes in SNRs.

Methods. Out of the total 50 participants, data from 47 participants were included in the analysis. The primary goal of this study was to investigate the auditory efferent system in humans: aim 1) measures the effect of MOCR activation on CAEPs in response to a tone presented in quiet. A 1-kHz tone at 60 dB SPL was presented to the ipsilateral ear with and without contralateral noise at 60 dB SPL. aim 2) the effect of MOCR activation on CAEPs in response to a tone presented in noise. A 1-kHz tone in noise was presented to the ipsilateral ear at different SNRs (25 dB, 15 dB, and 5 dB). Otoacoustic emissions (OAEs) and tone-detection tests were additionally tested to verify the MOCR effect.

Results. In Aim 1, it was expected that the MOCR effect would decrease SNR, resulting in increased latency and decrease inter-amplitude. However, no noticeable changes in latency and inter-amplitude were observed as a result of the MOCR effect. In Aim 2, it was expected that the MOCR effect would enhance SNR, resulting in decreased latency and increased inter-amplitude. The unmasking effect on latency and inter-amplitude was observed only at specific SNR levels at 5 dB and 15 dB SNR. Furthermore, the correlation between the shift in OAE level and tone-detection level was found to be unrelated in Aim 1. However, in Aim 2, a significant correlation was observed between these shifts at a specific SNR, specifically at 5 dB or 15 dB SNR.

Conclusions. In aim 1 (MOCR effect in a quiet environment), the activation of the MOCR does not significantly affect LLR latency and inter-amplitude. One possible factor that influences these results is the duration of the measurement session, as it may reduce the MOCR effect. Another possibility is that the reduction in cochlear output caused by MOCR activation may be compensated for by central gain. In aim 2 (MOCR effect in noise environment), LLR latency decreases during MOCR activation at 5- and 15-dB SNR, indicating that the efferent system enhances SNR through the MOCR-mediated unmasking effect at the neural level. Specifically, the unmasking effect increases sensitivity to a specific SNR. However, we were unable to identify a relationship between MOCR metrics, pre-neural assay, and behavioral assay. The lack of association in the MOCR analysis may be attributed to the use of different stimuli, diminished MOCR effect over time, and the introduction of high artifacts.

Declaration of Authorship

Declaration of Authorship is included in the supplemental files.

ORCID

0009-0008-1081-6202

DOI

10.21007/etd.cghs.2023.0635

Additional Files
2023-023-Lee-DOA.pdf (144 kB)
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