Sound coding in the auditory nerve of gerbils

• The high-spontaneous rate (SR) fibers show a very good synchronized response in quiet but saturate rapidly under noisy conditions.
• Inversely, the synchronized response of low-SR fibers is weaker compared to high-SR fibers, but more robust in noise.
• Patients with a specific degeneration of low-SR fibers may have poor speech intelligibility in noisy environments.

Gerbils possess a very specialized cochlea in which the low-frequency inner hair cells (IHCs) are contacted by auditory nerve fibers (ANFs) having a high spontaneous rate (SR), whereas high frequency IHCs are innervated by ANFs with a greater SR-based diversity. This specificity makes this animal a unique model to investigate, in the same cochlea, the functional role of different pools of ANFs. The distribution of the characteristic frequencies of fibers shows a clear bimodal shape (with a first mode around 1.5 kHz and a second around 12 kHz) and a notch in the histogram near 3.5 kHz. Whereas the mean thresholds did not significantly differ in the two frequency regions, the shape of the rate-intensity functions does vary 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. Finally, the greater vulnerability of low-SR fibers to various injuries including noise- and age-related hearing loss is discussed with regard to patients with poor speech intelligibility in noisy environments. Together, these results emphasize the need to perform relevant clinical tests to probe the distribution of ANFs in humans, and develop appropriate techniques of rehabilitation.This article is part of a Special Issue entitled .