Animal research has shown that noise exposure can lead to cochlear synaptopathy (CS), primarily affecting low-spontaneous-rate, high-threshold fibers in the auditory nerve. Consequently, it can be assumed that the effects of CS will be more pronounced for high-intensity stimuli compared to low-intensity ones. In humans, CS and its correlates proved challenging to find, but recently Shehorn et al. (2020; doi:10.1016/j.heares.2020.107982) reported that in listeners with normal audiograms lower speech intelligibility (SI) at very high presentation levels (104 dBA) was associated with reduced middle-ear muscle reflex (MEMR) magnitude. Further, the MEMR magnitude, measured with a research protocol, was associated with higher estimated lifetime noise exposure.

SI declines with increasing level, so-called rollover (RO), have also been observed in individuals with normal audiograms. While RO could be CS-related, broadened auditory filters or a reduced ‘sharpness’ of the neural speech representation at higher presentation levels could also cause RO. In other words, RO occurrence in individuals with normal audiograms does not necessarily indicate CS. Instead, ‘normal’ and ‘abnormal’ contributions to RO need to be disentangled.

Here, we hypothesized that abnormal contributions will lead to larger RO and correlations with experimental measures presumably reflecting CS. To test these hypotheses, we measured SI in speech-shaped noise for 25 young adults with normal audiograms. Two ecologically valid presentation levels (65 and 80 dB SPL) were used. Three additional measures, presumably sensitive to the effects of CS, were included: extended high-frequency audiometry, MEMR magnitude, and noise exposure structured interviews. All measurements were conducted using clinically feasible protocols.

The results showed some degree of RO in most listeners (median: 9%, range: 0 to 21%). The extended high-frequency audiometry showed a broad range of thresholds (range: -10 to 55 dB HL). Further, a negative correlation was found between noise exposure history and MEMR magnitude, in good agreement with previous research. However, no simple relations were found between RO and other measures potentially reflecting CS in the tested group.

Taken at face value, the last finding suggests that CS may not be assessable with RO at 65-80 dB SPL. However, the majority of listeners tested in the current study can be considered normal hearing, and were unlikely affected by CS. Therefore, the median RO of 9% can be considered an estimate of normal RO, establishing a baseline for future research involving more heterogeneous listener groups.