Abstract
Recent evidence has shown that noise-induced damage to the synapse between inner hair cells (IHCs) and type I afferent auditory nerve fibers (ANFs) may occur in the absence of permanent threshold shift (PTS), and that synapses connecting IHCs with low spontaneous rate (SR) ANFs are disproportionately affected. Due to the functional importance of low-SR ANF units for temporal processing and signal coding in noisy backgrounds, deficits in cochlear coding associated with noise-induced damage may result in significant difficulties with temporal processing and hearing in noise (i.e., “hidden hearing loss”). However, significant noise-induced coding deficits have not been reported at the single unit level following the loss of low-SR units. We have found evidence to suggest that some aspects of neural coding are not significantly changed with the initial loss of low-SR ANFs, and that further coding deficits arise in association with the subsequent reestablishment of the synapses. This suggests that synaptopathy in hidden hearing loss may be the result of insufficient repair of disrupted synapses, and not simply due to the loss of low-SR units. These coding deficits include decreases in driven spike rate for intensity coding as well as several aspects of temporal coding: spike latency, peak-to-sustained spike ratio and the recovery of spike rate as a function of click-interval.
Highlights
Reviewed by: Peter Cariani, Harvard Medical School, USA Sarah Theodoroff, VA RR&D National Center for Rehabilitative Auditory Research and VAPORHCS, USA
Recent evidence has shown that noise-induced damage to the synapse between inner hair cells (IHCs) and type I afferent auditory nerve fibers (ANFs) may occur in the absence of permanent threshold shift (PTS), and that synapses connecting Inner hair cell (IHC) with low spontaneous rate (SR) Auditory nerve fiber (ANF) are disproportionately affected
Due to the functional importance of low-SR ANF units for temporal processing and signal coding in noisy backgrounds, deficits in cochlear coding associated with noise-induced damage may result in significant difficulties with temporal processing and hearing in noise (i.e., “hidden hearing loss”)
Summary
Noise-induced hearing loss has traditionally been quantified by changes in auditory sensitivity evidenced by shifts in hearing threshold (Borg et al, 1995). Given the reported differences between mice and guinea pigs as summarized above, there may be a species difference in the ability to regenerate synapses after noise-induced damage Another recent study in C57 mice reported a reversible loss of ribbon synapses after a similar non PTS-inducing noise (Shi et al, 2015). It is reasonable to conclude that noise-induced synaptic disruption is, at least partially, repairable
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