Quantifying the effects of the cochlear amplifier on temporal and average-rate information in the auditory nerve

An active mechanism referred to as the cochlear amplifier is believed to be responsible for sharp tuning and excellent sensitivity in the normal auditory system, and is impaired or absent in many common forms of sensorineural hearing loss. The cochlear amplifier is thought to benefit normal-hearing listeners, especially in complex listening environments in which hearing-impaired listeners have difficulty even with hearing aids. A modeling approach was developed to relate nonlinear physiological response properties associated with the cochlear amplifier to human psychophysical performance. Quantitative methods combined analytical and computational population models of the auditory-nerve (AN) with statistical decision theory to evaluate performance limits imposed by the random nature of AN discharges (modeled by a nonstationary Poisson process). A new theoretical approach was developed to predict performance for psychophysical tasks that use random-noise stimuli to mask signal information. The ability of temporal and average-rate information in the AN to account for human performance was evaluated for several listening tasks for which the cochlear amplifier has been suggested to be significant. The benefit of the cochlear amplifier for extending the auditory system’s dynamic range was evaluated in terms of AN information available for encoding changes in stimulus level. An analytical model included nonlinear gain, level-dependent phase, and high-, mediumand low-spontaneous-rate AN fibers. Both nonlinear-phase and average-rate information were required to encode levels up to 120 dB SPL based on a narrow range of AN characteristic frequencies. A physiologically realistic mechanism to decode nonlinear gain and phase cues is monaural, across-frequency coincidence detection. Level-discrimination performance of a coincidence-counter population matched human performance across the entire dynamic range of hearing at both low and high frequencies. Results suggest that the cochlear amplifier is beneficial for encoding sound level within narrow frequency regions, and has only a small influence on simple listening tasks in quiet. The cochlear amplifier alters tuning within the normal auditory system based on the spectral and temporal configuration of the stimulus. The influences of compression and suppression on psychophysical measures of auditory frequency selectivity were evaluated. Implications for the interpretation of psychophysical methods for estimating auditory filters are discussed. Thesis Supervisor: Laurel H. Carney, Ph.D. Title: Associate Professor of Biomedical Engineering, Boston University