Supra-Threshold Hearing and Fluctuation Profiles: Implications for Sensorineural and Hidden Hearing Loss

Area of Science:

• Auditory Neuroscience
• Psychoacoustics
• Speech Perception

Background:

• Supra-threshold hearing difficulties, common in aging and noise exposure, are increasingly linked to auditory nerve (AN) synaptopathy.
• Synaptopathy involves loss of low- and medium-spontaneous rate (L/MSR) AN fibers, challenging traditional models of auditory coding.
• Existing models often assume L/MSR fiber rates are key for supra-threshold sound encoding, overlooking saturation effects.

Purpose of the Study:

• To challenge the role of average discharge rates in coding moderate to high sound levels.
• To propose an alternative neural coding mechanism for supra-threshold sounds based on fluctuation profiles.
• To explore the role of the auditory efferent system and central auditory pathways in maintaining supra-threshold hearing.

Main Methods:

• Literature review and synthesis of existing data on AN fiber function and auditory processing.
• Analysis of neural encoding mechanisms, including rate saturation and transduction saturation.
• Examination of the auditory efferent system's role and central auditory pathway processing.

Main Results:

• Average discharge rates of L/MSR AN fibers may not be critical for coding sounds at moderate to high levels.
• A cross-frequency profile of low-frequency neural fluctuation amplitudes, rather than average rates, is proposed as the primary encoding mechanism.
• This fluctuation-profile coding is supported by inner hair cell (IHC) and AN synapse saturation.

Conclusions:

• The auditory efferent system and central auditory pathways (cochlear nucleus, inferior colliculus) enhance and process neural fluctuation profiles.
• A functional loop involving the inferior colliculus and efferent system may maintain sharp supra-threshold hearing.
• Understanding fluctuation-profile coding offers new strategies for hearing aid signal processing and cochlear implant design, especially for synaptopathy-related hearing loss.