Auditory central gain compensates for changes in cochlear output after prolonged low-level noise exposure

Area of Science:

• Auditory Neuroscience
• Sensory System Plasticity
• Acoustic Trauma Research

Background:

• The central auditory system exhibits plasticity, adjusting neural response strength (central gain) based on acoustic experience.
• While noise-induced gain changes are known after loud noise, effects of prolonged low-level noise exposure are less understood.
• Previous studies showed cochlear output reduction and inferior colliculus (IC) gain enhancement after 75 dB noise exposure.

Purpose of the Study:

• To investigate central auditory gain changes following prolonged exposure to lower-intensity (65 dB) noise.
• To determine if 65 dB noise exposure affects cochlear output and IC neural responses.
• To assess the recovery of auditory function and central gain 1 week post-exposure.

Main Methods:

• Rats were exposed to 65 dB noise for an extended period.
• Auditory function was assessed using distortion product otoacoustic emissions (DPOAEs) and compound action potentials (CAPs).
• Neural responses in the inferior colliculus (IC) were recorded to evaluate central gain changes.

Main Results:

• 65 dB noise exposure did not affect DPOAEs or cause hair cell loss.
• Auditory nerve output (CAP amplitude) was significantly depressed (50-75%) across frequencies.
• Inferior colliculus (IC) neural responses showed frequency-specific enhancements (up to 70%) and later, complex up- and down-regulation of gain.

Conclusions:

• Prolonged exposure to 65 dB noise alters auditory nerve output and induces complex, frequency-dependent gain changes in the central auditory system.
• The central auditory system dynamically regulates gain to maintain consistent neural signaling and prevent perception of muffled or overly loud sounds.
• These findings highlight the auditory system's homeostatic mechanisms in response to common environmental noise levels.