Measuring and Modeling Loudness Adaptation in Normal and Impaired Hearing

• Main Author: Wynne, Dwight Paul
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01.01.2013
• Subjects: Audiology; Biomedical engineering
• ISBN: 9781267998668; 1267998660

Senses of sight and touch readily adapt to continued stimulation, but the sense of hearing rarely does. Only under certain pathological conditions, such as tumors of the auditory nerve, does the auditory system adapt to constant sound. Adaptation of loudness was first explored in auditory neuropathy, a disorder of the auditory nerve, inner hair cells, and/or ribbon synapse between them. Significant loudness adaptation observed in auditory neuropathy subjects could not be fully explained by adaptation of auditory brainstem responses to trains of clicks, suggesting that loudness adaptation involves both peripheral and central auditory structures. Loudness adaptation was next explored in normal hearing using high-frequency, amplitude-modulated tones. Loudness adaptation decreased (i.e., loudness was more constant) with increasing modulation depth, which was inconsistent with previous mathematical models of loudness adaptation. Two different models were developed to explain loudness adaptation: (1) A "tonic-phasic" model consisting of a "tonic" exponential rise to a maximum in parallel with a "phasic" exponential decay to zero, and (2) a second-order differential equation describing a driven, damped harmonic oscillator. Both of these models well fit loudness adaptation to amplitude modulated tones in normal hearing. While the critically damped oscillator model better explained observed loudness changes in auditory neuropathy, previous results in normal hearing were better explained by the tonic-phasic model. Biological analogs of the model parameters are suggested and might be revealed in future experiments measuring auditory cortical responses to sustained stimuli.