Physiological measures and perceptual consequences of noise-induced auditory synaptopathy in humans.

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Recent studies in animals have shown that noise exposure causing temporary shifts in auditory thresholds can lead to permanent and severe damage to the synaptic connections between the inner hair cells in the cochlea and the auditory nerve. If such “auditory synaptopathy” occurs in humans, it would not be detected by current standard audiometric tests, but could potentially explain why many people complain of excessive difficulty in understanding speech in noisy backgrounds, despite having clinically normal hearing. The goal of this project is to determine whether auditory synaptopathy occurs in humans and, if so, to develop clinically feasible tests to diagnose it along with research tools to study its perceptual consequences. The project identifies three populations who are considered to be potential suffers of auditory synaptopathy: 1) People who suffer from tinnitus following noise exposure, but who have clinically normal hearing (i.e., normal audiometric thresholds between 250 and 8000 Hz); 2) People who report having a history of exposure to loud sound, but have clinically normal hearing; 3) People with mild-to-moderate hearing loss at high frequencies (> 2 kHz) but clinically normal hearing at low frequencies. Results from these three groups will be compared with those from an age- and gender-matched control group of people with normal hearing and no reported tinnitus or excessive noise exposure. The project has a three-pronged approach, involving physiological, perceptual, and speech- based measures. The first aim studies the potential effects of synaptopathy on two auditory reflexes: the middle ear muscle reflex (MEMR) and the medial olivocochlear reflex (MOCR). Pilot data suggest that potential sufferers of auditory synaptopathy have severely reduced MEMR, in line with previous animal studies. The second aim investigates potential perceptual consequences of synaptopathy by measuring sensitivity to basic acoustic attributes, such as amplitude and frequency modulation, with and without background noise. Pilot data suggest that potential suffers of auditory synaptopathy are particularly susceptible to the effects of noise, even if their performance in quiet is normal. The third aim studies speech perception in a variety of background situations, including noise and speech interference with and without added reverberation. The experiments test the prediction that performance will be affected by synaptopathy particularly in cases where the level of the background noise is variable and unpredictable, and where the target and interfering speech are distinguishable only by small differences in fundamental frequency or spatial location. The results from the project will provide important new information and clinically relevant tests of a condition that potentially affects millions of people in the US and worldwide. Reliable indicators and diagnoses of auditory synaptopathy in humans will be crucial in the development of new pharmaceutical treatments of the disorder.