Assessing Cochlear-Place Specific Temporal Coding Using Multi-Band Complex Tones to Measure Envelope-Following Responses.

Previous studies suggest that envelope-following responses (EFRs) reveal important differences in temporal coding fidelity amongst listeners who have normal hearing thresholds, consistent with these listeners differing in the degree to which they suffer from cochlear synaptopathy. Like conventional hearing loss, the severity of cochlear synaptopathy may vary along the cochlea. A number of earlier studies have suggested methods for estimating EFRs driven by specific frequency regions of the cochlea, which would allow synaptopathy to be estimated as a function of cochlear place. Here, we tested a method for measuring EFRs from multiple locations along the cochlea simultaneously, using narrowband stimuli. We compared responses to multiple simultaneous narrowband complex harmonic tones in three non-overlapping frequency bands, each having a unique fundamental frequency, to responses to the individual narrowband stimuli alone, and to responses when noise was added to different combinations of the frequency bands. Our results suggest that simultaneous presentation of multiple tone complexes with different fundamental frequencies leads to repeatable measures of temporal coding fidelity at the cochlear frequency regions corresponding to the narrowband carrier frequencies. Other results suggested that while off-frequency contributions to EFRs driven by narrowband signals (due to spread of excitation) can add destructively to the on frequency response, these interactions were small compared to EFR magnitude. Overall, our results point to the utility of using multi-band complex tone stimuli to estimate the profile of temporal coding fidelity, and thus the degree of synaptopathy, as a function of cochlear place. This article is part of a Special Issue entitled: Hearing Loss, Tinnitus, Hyperacusis, Central Gain.