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Hidden hearing loss and selective attention in the brainstem
Cochlear synaptopathy or hidden hearing loss can be caused by noise exposure and ageing. It refers to the damage of higher-threshold auditory-nerve fibres and may account for the differences in the ability of normal hearing threshold listeners when communicating in challenging environments (Bharadwaj et al., 2014). However, it remains unclear if the condition actually occurs in humans, how it can be best diagnosed, and how exactly it impacts speech-in-noise processing.
Recently we proposed a method for measuring the brainstem’s response to natural non-repetitive speech and employed it to show that the auditory brainstem already plays a role in selective attention to speech (Forte et al., 2017). We thereby observed individual differences in the modulation of the brainstem response by selective attention: some subjects showed large attentional modulation while others exhibited only little modulation. We therefore wondered if the strength of the attentional modulation correlates with hearing ability and if it relates to cochlear synaptopathy.
We approached this issue through a computational model and experimental measurements. First, to explore the effects of hidden hearing loss, we developed a realistic computational model of the auditory-brainstem response (ABR) to speech based on an existing model (Zilany et al., 2014). We employed it to investigate the neural response to continuous speech at different stages in the brainstem, and to explore the effects of hidden hearing loss. We found significant responses and characteristic latencies for neural signals generated at the level of the auditory-nerve fibres, the cochlear nuclei and the inferior colliculus (IC). The latency of the response of the IC matched the latency that we found experimentally, suggesting that the scalp-recorded brainstem response to speech is dominated by the IC.
Secondly, we assessed young healthy listeners for speech-in-noise comprehension, lifetime noise exposure, the middle ear muscle reflex, binaural hearing and different brainstem measures, including the brainstem response to continuous speech and its modulation by selective attention. We found that there was considerable variability in all measures across the participants. However, only few of the objective measures were able to explain the differences in speech-in-noise comprehension between the participants. Interestingly, the modulation of the brainstem response by selective attention correlated with the performance in the speech-in-noise task. Our findings suggest that the attentional modulation in the brainstem response can inform on hearing ability and potentially on hidden hearing loss.
Bharadwaj et al. (2014). Frontiers in systems neuroscience, 8, 26.
Forte et al. (2017). eLife, 6, e27203.
Zilany et al. (2014). The Journal of the Acoustical Society of America, 135(1), 283-286.