Mengchao Zhang1, Richard Stern2, Deborah Moncrieff3, Bharath Chandrasekaran4, Catherine Palmer4, Christopher Brown4
School of Psychology, Cardiff University, Cardiff, United Kingdom
2Department of Electrical and Computer Engineering, Carnegie Mellon University, Pittsburgh, United States of America

3School of Communication Sciences and Disorders, University of Memphis, Memphis, United States of America
4Department of Communication Science and Disorders, University of Pittsburgh, Pittsburgh, United States of America

Recent animal models have suggested that noise exposure can affect supra-threshold temporal envelope (TE) processing without disrupting absolute hearing thresholds due to selective auditory nerve deafferentation, a process which has been named ‘cochlear synaptopathy’. However, evidence of cochlear synaptopathy in human listeners has been inconsistent, sparking debate on the existence of the synaptopathy in humans. This study compares TE processing between young adults enrolled in a dental school and an unexposed control group, because dentistry students are exposed to drill noise that is distinctively high-frequency, and non-traumatic in nature, and the schedule of noise exposure is systematic for students across different years of enrollment. Unvoiced speech recognition in noise modulated at 16 or 32 Hz was chosen to evaluate TE processing. To limit off-frequency contributions and reduce spectral redundancy of the speech, stimuli were band-pass filtered to lower- (0.8 – 2.3 kHz) or higher-frequency regions (1.7 – 12 kHz). The results showed that the group exposed to dental noise performed more poorly than the unexposed for unvoiced speech in modulated noise. The group difference was more robust for the high-frequency speech than low-frequency speech and when the noise was modulated at 32 Hz rather than at 16 Hz. Furthermore, a small but significant difference was found between the two groups when they recognized the speech in spectrally modulated noise for lower-frequency stimuli. Meanwhile, variables such as age, years of musical training, non-dental noise exposure history, and peripheral auditory screening results did not account for a significant amount of variance of the performances. The findings suggest that human listeners with non-traumatic noise exposure could show poor TE processing as was predicted by the hypotheses related to cochlear synaptopathy and that carefully designed measures are needed when exploring cochlear synaptopathy in humans.

Acknowledgements: This study was based on part of the first author’s dissertation works and was supported through funding received from the SHRS PhD Student Award, School of Health and Rehabilitation Sciences, University of Pittsburgh.