Date of Award

Spring 2016

Document Type

Dissertation

Degree Name

Doctor of Audiology (AuD)

Department

Department of Communication Sciences and Disorders

Advisor(s)

Christopher G. Clinard

Abstract

Binaural hearing offers several advantages over monaural hearing and is believed to be one factor that is involved in the ability to understand speech in background noise. Binaural hearing involves analysis of interaural timing and intensity differences in signals arriving at the two ears which provides listeners with sound localization cues as well as signal in noise detection. When sounds arrive at each ear at slightly different times, there may be a release from the effects of background noise, allowing listeners to detect softer sounds in noise. Masking Level Differences (MLDs) have been widely used to evaluate behavioral binaural processing. However, the literature inconsistently reports a release from masking in physiological responses. The purposes of this study were 1) to establish the feasibility of measuring physiological masking level differences using the frequency-following response (FFR), and 2) to characterize the relationship between behavioral and physiological measures of masking level differences (MLDs). Fourteen young adults (ages 21-26) with clinically normal hearing sensitivity participated in this study. Stimuli for behavioral and physiological conditions were 500 Hz tonebursts presented in one-third octave narrowband noise. Three phase conditions were tested: SoNo, SoNπ, and SπNo. Behavioral MLDs were assessed using an adaptive 2AFC procedure. Physiological MLDs were assessed using the frequency-following response, an auditory evoked potential reliant on phase-locked neural activity. FFR analysis focused on amplitude measures. Speech-in-noise understanding was also tested using the Words-in-Noise test (WIN). Behavioral MLDs were 8.29 dB (std. dev = 4.09) for SoNπ and 10.03 dB (std. dev = 4.96) for the SπNo condition. Physiological MLDs did not indicate a robust release from masking, especially for the SπNo condition. Correlations between behavioral and physiological MLDs were not significant. However, FFR amplitude differences between having the signal, or 500 Hz tone, in phase between the ears (e.g., SoNo) and 180° out of phase (i.e., SπNo) predicted behavioral SπNo MLDs. These findings may help to clarify which scalp-recorded auditory evoked potentials reflect binaural processing in humans and report the first brainstem auditory evoked potentials in humans that can predict behavioral masking level differences.

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