Consequences of cochlear synaptopathy in noise-exposed adults

Evidence for cochlear synaptopathy using non-invasive measures in humans remains limited. However, previous research has predominantly focused on young adults with normal hearing thresholds whose suspected synaptopathy has been based on self-reports of noise exposure history. It is quite likely that measurable synaptopathy only becomes evident later in life through the combined effects of ageing and noise exposure.

We are investigating the incidence of cochlear synaptopathy in humans by focusing on normal-hearing middle-aged adults (45-60 years old) with self-reported noise exposure history and comparing their data to a young control group (18-35 years old) without much noise exposure history. Since data collection is still in progress, we will focus on data from the young control group in this presentation.

To assess cochlear synaptopathy, we are measuring ABR wave I amplitudes in response to clicks presented at three different levels (95, 105, 115 dB peSPL), and envelope following responses (EFRs) in response to a 2.8 kHz transposed tone amplitude modulated at 176 Hz at three different modulation depths (0, -4, and -8 dB). To quantify the degree of cochlear synaptopathy, we will compute the slope of the click ABR wave I amplitude across levels and the spectral magnitude of the EFR at F0 across modulation depths.

To examine the potential effects of cochlear synaptopathy on speech perception in noise, we are measuring speech reception thresholds (SRTs) for both consonants and sentences at two different levels (40 and 80 dB SPL). SRTs are measured in two conditions, one where both the target and background noise are diotic (N0S0), and another where the target signal is inverted in polarity in one ear (N0Sπ) leading to a phase disparity across the ears. We compute the binaural intelligibility level difference (BILD) by taking the difference between the N0S0 and N0Sπ conditions. Of particular interest is the relationship between our electrophysiological measures and our speech-in-noise measures.

Given that cochlear synaptopathy is thought to selectively affect high-threshold low spontaneous rate fibres, we hypothesise that clicks presented at lower levels and transposed tones with shallower modulation depths will not be encoded as robustly in individuals with cochlear synaptopathy. We therefore predict that the middle-aged group will show shallower ABR and EFR slopes compared to the young controls. In addition, we expect that individuals with shallower ABR and EFR slopes will perform more poorly on the speech-in-noise task, particularly at higher stimulus levels.