Hidden Hearing Loss Impacts the Neural Representation of Speech in Background Noise

Many individuals with seemingly normal hearing abilities struggle to understand speech in noisy backgrounds. To understand why this might be the case, we investigated the neural representation of speech in the auditory midbrain of gerbils with “hidden hearing loss” through noise exposure that increased hearing thresholds only temporarily. In noise-exposed animals, we observed significantly increased neural responses to speech stimuli, with a more pronounced increase at moderate than at high sound intensities. Noise exposure reduced discriminability of neural responses to speech in background noise at high sound intensities, with impairment most severe for tokens with relatively greater spectral energy in the noise-exposure frequency range (2–4 kHz). At moderate sound intensities, discriminability was surprisingly improved, which was unrelated to spectral content. A model combining damage to high-threshold auditory nerve fibers with increased response gain of central auditory neurons reproduced these effects, demonstrating that a specific combination of peripheral damage and central compensation could explain listening difficulties despite normal hearing thresholds

The influence of envelope waveform on ITD sensitivity of neurons in the auditory midbrain

Interaural time differences (ITDs) conveyed by the modulated envelopes of high-frequency sounds can serve as a cue for localising a sound source. Klein-Hennig et al. (2011) demonstrated the envelope attack, the rate at which stimulus energy in the envelope increases, and the duration of the pause, the interval between successive envelope pulses, as important factors affecting sensitivity to envelope ITDs in human listeners. Modulated sounds with rapid attacks and long pauses produce the lowest ITD-discrimination thresholds. The duration of the envelope's sustained component (sustain) and the rate at which stimulus energy falls at the offset of the envelope (decay) are only minor factors. We assessed the responses of 71 single-neurons, recorded from the midbrains of 15 urethane anaesthetized tri-colored guinea pigs, to envelope shapes in which the four envelope components: attack, sustain, decay and pause, were systematically varied. We confirmed the importance of the attack and pause components in generating ITD-sensitive responses. Analysis of neural firing rates demonstrated more neurons (49/71) show ITD-sensitivity in response to damped stimuli (fast attack and slow decay) compared to ramped stimuli (slow attack and fast decay) (14/71). Further, the lowest threshold for the damped stimulus (91 μs) was a factor of 4 lower than for the temporally reversed ramped envelope shape (407 μs). The data confirm the importance fast attacks and optimal pause durations in generating sensitivity to ITDs conveyed in the modulated envelopes of high-frequency sounds and are incompatible with models of ITD processing based on the integration of sound energy over time

Neural network based speech enhancement applied to cochlear implant coding strategies

status: publishe