Testing for evidence of cochlear synaptopathy in normal-hearing young adults with varying noise exposure history

Loss of high-threshold cochlear synapses following mild noise exposure is suggested to degrade amplitude modulation (AM) encoding in cases where hearing thresholds are normal. However, the relationship of AM encoding to noise exposure history has not been consistent in previous studies. We investigated this relationship in young adults with normal audiograms in two studies using different methods. Study 1 (N = 25) measured the ~80 Hz electrophysiological envelope following response (EFR) and behavioural AM detection thresholds. Both measures were taken in quiet and in a narrowband background noise designed to attenuate low-threshold synapse contributions. When subjects were divided into two groups based on their noise exposure history, subjects with more noise exposure had smaller EFRs (p = 0.0198). AM detection was also poorer in these subjects, but this difference fell short of significance (p =0.067). Study 2 (ongoing) also measures the EFR but employs an additional wider band of background noise to attenuate possible off-frequency contributions of low threshold fibers to AM coding. In addition, AM discrimination is tested instead of AM detection. The question is whether these modifications will reveal a more robust effect of noise exposure history on AM coding than did Study 1. [Work supported by NSERC of Canada.]