14th Speech in Noise Workshop, 12-13 January 2023, Split, Croatia 14th Speech in Noise Workshop, 12-13 January 2023, Split, Croatia

Cochlear coding mechanisms: From animal to human data

Jérôme Bourien
University of Montpellier - Institute for Neurosciences of Montpellier (Inserm U1298), Montpellier, France

Auditory nerve fibers (ANFs) convey acoustic information from the sensory cells to the brainstem using an elaborated neural code based on both spike timing and rate. This code is conveyed by subtypes of ANFs differing in their characteristic frequency, spontaneous rate (SR) of discharge, and threshold of activation. Since there is a negative correlation between threshold and SR, high-SR fibers are supposed to be critical for hearing sensitivity whereas low-SR fibers are presumed to be engage in the neural encoding of supra-threshold sounds, including vocalizations. Gerbils possess a very specialized cochlea in which the low-frequency inner hair cells (IHCs) are contacted by ANFs having a high-SR fibers whereas high frequency IHCs are innervated by ANFs with a greater SR-based diversity. This specificity makes this animal a unique model to investigate the functional role of different pools of ANFs. The shape of the rate-intensity functions varies significantly with the fiber characteristic frequency. Above 3.5 kHz, the sound-driven rate is greater and the slope of the rate-intensity function is steeper. Interestingly, high-SR fibers show a very good synchronized onset response in quiet (small first-spike latency jitter) but a weak response under noisy conditions. The low-SR fibers exhibit the opposite behavior, with poor onset synchronization in quiet but a robust response in noise. In humans, a cochlear synaptopathy specifically targeting the low-SR fiber pool could explain why some listeners have poor speech intelligibility in noisy environments while their tonal audiogram is almost normal.

Last modified 2023-01-06 23:41:06