Several studies describe how peripheral processing of auditory information changes after synaptopathy, but only for basic acoustic sounds (e.g., transients, noise, tones). Studies rarely consider speech stimuli, even though this is necessary to ultimately relate speech intelligibility deficits to synaptopathy. We used computer-generated phonemes that enabled the differentiation of temporal fine structure (TFS) and temporal envelope (TENV) coding as stimuli to record responses from individual neurons in the inferior colliculus of gerbils, both controls and animals with noise-induced cochlear synaptopathy (CoSy). These stimuli, including maskers, are identical to those used in collaborative parallel studies in humans and other animal labs.

CoSy has been quantified using a variety of non-invasive measures, in parallel with human studies, including auditory brainstem responses (ABR), distortion product otoacoustic emissions (DPOAE), cortical auditory evoked potentials (CAEP), and validated by immunohistochemistry to identify the number of sensory cells and synapses in the cochlea. Awake, freely moving gerbils were exposed to an octave band of noise (2.6–5.2 kHz) at 100 dB SPL, for 2h, in an antiparallel chamber. During exposures, animals were unrestrained within small cells in a subdivided cage (1 animal/cell).

Here, we present findings categorizing the responses to different speech stimuli from individual neurons in the inferior colliculus of anaesthetized gerbils and relate it to the frequency response areas of each neuron. We further present how various masking conditions affect these responses and present the efficacy of the coding to represent the acoustic stimuli. Finally, we present how noise induced cochlear synaptopathy affects these coding mechanisms and their efficacy.

Funding: Work supported ERA-NET NEURON JTC 2020 (CoSySpeech Project) and the Consejería de Educación y Cultura de la Junta de Castilla y León (grants SA252P20).