Speaker
Description
The loss of a larger percentage of low-spontaneous-rate (LSR) than high-spontaneous-rate (HSR) auditory-nerve (AN) fibers has been a focus of much of the work on cochlear synaptopathy. This talk will discuss the implications of the loss of different fiber types on hearing of complex sounds, especially in noisy settings. The average rates of LSR fibers are often assumed to be important for encoding moderate to high sound levels, and more generally, spectrum levels. However, quantitative analysis based on optimal decision theory has shown that the rates of LSR fibers cannot explain level discrimination, which improves as level increases. We have proposed an alternative model for neural coding of complex sounds at moderate sound levels based on neural fluctuations (NFs), the low-frequency fluctuations in the probability of firing of AN fibers (of all SR types), which varies along the tonotopic axis depending on spectral level. To maintain the contrasts between NF depths in AN responses tuned near spectral peaks vs. spectral valleys would require broadband adjustment of cochlear gain for complex sounds at low or high levels. The low- and medium-spontaneous-rate (MSR) AN fibers, with wide dynamic ranges, could play a role in this cochlear gain control. In some species, these fibers are the exclusive inputs to cells in the small cell cap in the anteroventral cochlear nucleus, which projects to medial olivocochlear neurons in the brainstem. Using computational models that include efferent pathways, we are exploring this potential mechanism for maintaining NF contrast over a wide dynamic range.