Evoked Potentials Reveal Noise Exposure-Related Central Auditory Changes Despite Normal Audiograms

Area of Science:

• Auditory Neuroscience
• Audiology
• Neuroscience

Background:

• Auditory perceptual deficits like tinnitus and difficulty understanding speech in noise can occur despite normal audiograms, a condition termed "hidden" hearing loss.
• Cochlear synaptopathy, the loss of synaptic connections between inner hair cells and auditory nerve fibers, is a potential cause of hidden hearing loss.
• Animal models show cochlear synaptopathy linked to aging, noise, or ototoxic drugs, manifesting as reduced auditory brainstem response (ABR) wave I amplitudes.

Purpose of the Study:

• To investigate the impact of noise-induced reductions in ABR wave I amplitude on the central auditory system.
• To examine auditory brainstem response (ABR), middle latency response (MLR), and late latency response (LLR) in individuals with normal audiograms.

Main Methods:

• Measured ABR, MLR, and LLR in 65 young Veterans and non-Veterans with normal audiograms.
• Analyzed the relationship between ABR wave I amplitudes and MLR/LLR.
• Compared responses between Veterans and non-Veterans, considering tinnitus and other factors.

Main Results:

• Veterans showed a weaker MLR compared to non-Veterans in response to click stimuli; LLR was not reduced.
• Lower ABR wave I amplitudes correlated with reduced MLR but increased LLR.
• Veterans reporting tinnitus exhibited the largest mean LLRs.

Conclusions:

• Reduced peripheral auditory input can induce compensatory gain in the central auditory system, even with normal audiograms.
• This central gain may influence auditory perception.
• The observed pattern of reduced MLR and preserved LLR in Veterans suggests a role for synaptic loss in central auditory changes.