Can rollover be used to assess cochlear synaptopathy in listeners with normal audiograms?

While a level increase typically leads to better speech intelligibility (SI) at low presentation levels, it can result in decreased SI at high levels. SI declines with increasing level, so-called rollover (RO), have been observed in quiet and in noise for individuals with elevated hearing thresholds, and in noise for individuals with normal hearing thresholds. In the latter case, cochlear synaptopathy has recently been proposed to play a role. However, other factors, such as broadened auditory filters at high levels or a reduced ‘sharpness’ of the neural speech representation, could also contribute.
Thus, if RO occurs in individuals with normal audiograms, this finding on its own does not indicate cochlear synaptopathy. Instead, ‘normal’ and ‘abnormal’ contributions to RO would need to be disentangled. Since there are currently no clinically established tools for detecting cochlear synaptopathy, this cannot be done a priori (e.g., by testing groups with and without cochlear synaptopathy and then comparing RO for the two groups). Instead, it can be hypothesized that in case of ‘abnormal’ contributions, RO will be larger, and will correlate with other measures that presumably reflect cochlear synaptopathy.
The current study investigated RO and its potential relation to cochlear synaptopathy in listeners with normal audiograms. We measured SI in speech-like noise at multiple levels. We also conducted extended high-frequency audiometry and acoustic-reflex measurements. Furthermore, we assessed sensitivity to spectro-temporal modulations at the same levels as used for the SI measurements.
Here, we will present first results from this ongoing work. We expect to provide estimates of ‘normal’ RO magnitudes and, based on the relations with the other measures, to identify listeners with a high probability of abnormal RO and, thus, cochlear synaptopathy. Overall, we expect that this work will be a step towards improved hearing diagnostics and better rehabilitation of listeners with suprathreshold hearing disorders.