Date of Award

Spring 2015

Document Type

Dissertation

Degree Name

Doctor of Audiology (AuD)

Department

Department of Communication Sciences and Disorders

Advisor(s)

Christopher G. Clinard

Abstract

Understanding speech in the presence of background noise is a common complaint of middle-aged and older listeners with clinically normal audiograms. There is great interest in understanding how age-related changes in auditory physiology make it harder for older adults to understand speech in difficult listening situations, compared to young listeners. It was recently reported that middle-aged and older normal-hearing listeners showed frequency-dependent, age-related declines in the behavioral and physiological detection of interaural phase differences (Grose & Mamo, 2010; Ross et al, 2007). There is also evidence of an age-related, frequency-dependent decline in the frequency-following response (FFR) (Clinard et al., 2010), an auditory evoked potential dependent on phase-locked neural activity (Worden & Marsh, 1968). Age-related declines in binaural processing may be related to age-related declines in phase locking.

This study used the frequency-following response (FFR) to examine monaural and binaural phase locking in subjects of two groups; younger and middle-aged. Responses were obtained from 300 ms toneburst stimuli at four frequencies (500, 750, 1000, and 1125 Hz) at an intensity of 80 dB SPL. FFRs were analyzed for response amplitude, binaural amplitude differences, and stimulus-to-response cross-correlations.

Results showed FFR amplitude decreased as frequency increases and, at 500 Hz, the summed left and right monaural FFR amplitudes were smaller than the binaural FFR amplitude, which is in contrast to previous literature (Clinard, 2010; Fowler, 2004; Krishnan & McDaniel, 1998). Results further indicated that the stimulus-to-response correlation coefficient is greatest for 500 Hz and declines as frequency increases.

There was no significant difference between the age groups but perhaps a broader age range including older adults would show the hypothesized amplitude differences between groups. Further, results may be different than expected due to minimal difference between summed monaural and binaural processing at 750, 1000, and 1125 Hz. When looking at the data there is a larger difference between groups at 1125 Hz and, while it is not significantly different, it may be that a broader frequency range (e.g., 1250 Hz or above) and age range (e.g. 0 to 100), may be more effective at revealing a group x frequency interaction.

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