Individual differences in acoustic acuity at supra-threshold level

Abstract

The ability to detect acoustic amplitude modulated envelopes differs even among listeners with normal hearing thresholds (NHTs), an effect that to some degree may correlate with noise exposure history. Moreover, at least one animal study suggests that auditory nerve fibers (ANFs) with lower spontaneous firing rates (high thresholds) are more vulnerable to damage from noise exposure compared to those with high spontaneous rate ANFs. Given that NHTs depend primarily on high-spontaneous rate fibers, even relatively severe loss of low-spontaneous ANFs could be present in listeners that have “normal hearing” according to traditional hearing screenings. Here, we evaluated the robustness of supra-threshold coding, which may relate to low-spontaneous ANFs health, in listeners with NHTs. Specifically, we measured the phase-locking-values (PLVs) of subcortical envelope following responses to SAM tones (carrier: 1500 Hz; modulation rate: 100 Hz) at two sensation levels (20/50 dB SL) and with two modulation depths (0/−6 dB). Results reveal huge individual differences in PLV strengths that are negatively correlated with behavioral amplitude modulation detection thresholds for noises at a moderate supra-threshold level (carrier: band-pass-filtered noise centered at 1500 Hz with 300 Hz bandwidth; modulation rate:10 Hz). Our data support the hypothesis that the number of operational low-spontaneous rate ANFs may differ human listeners with NHTs, and that deficit in low-spontaneous rate ANFs can lead to deficits in perceptual abilities.