Temporal coding in auditory-nerve fibers following noise-induced hearing loss
Abstract
It is estimated that 26 million people in the United States alone aged 20 to 70 suffer from hearing loss of a sensorineural nature due to exposure to loud sounds at work or leisurely activities. People suffering from sensorineural hearing loss (SNHL) face difficulty in perceiving speech when competing sounds are present (e.g. in a restaurant or a bar). SNHL is often treated with prosthetic devices such as hearing aids and/or cochlear implants. Despite recent advances in these prosthetic devices, listeners continue to face difficulty perceiving speech in noise. Speech and other complex sounds can be mathematically separated into rapidly varying temporal fine structure (TFS) and slowly varying envelope components. Recent perceptual studies have shown that poor speech intelligibility experienced by hearing-impaired listeners in degraded listening conditions is associated with their reduced ability to use TFS cues. These results have fueled an active debate about the role of TFS and envelope coding in normal and impaired hearing and have important implications for improving the ability of hearing aids and cochlear implants to restore speech perception in noise. However, these implications depend critically on the underlying physiological (neural) bases for these perceptual deficits. The present study thoroughly characterized neural coding of envelope and TFS in normal and impaired auditory systems. Spike trains were recorded from auditory-nerve (AN) fibers in chinchillas with either normal-hearing or a noise-induced SNHL. Within- and across-fiber temporal coding (i.e., phase locking) to a broad range of stimuli was quantified. In contrast to common assumptions, our data suggest that SNHL does not degrade the fundamental ability of AN fibers to phase lock to either TFS or envelope. Rather, several other effects of SNHL were observed that may contribute to perceptual deficits in the temporal processing of complex stimuli like speech. For example, (1) envelope coding was enhanced following SNHL, which may over-emphasize fluctuating background sounds, e.g., competing talkers, (2) across-fiber estimates of traveling-wave delays were reduced significantly, and (3) larger than expected shifts in the best frequency of excitation were observed, resulting in a disruption of the normal tonotopic map. Overall, this work demonstrates that perceptual TFS deficits do not result from a simple reduction in the temporal-coding ability of AN fibers, but rather are more likely due to neural response properties that are relevant for complex stimuli and that are not currently accounted for in hearing-aid and cochlear-implant signal processing strategies.
Degree
Ph.D.
Advisors
Heinz, Purdue University.
Subject Area
Neurosciences|Biomedical engineering
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