title: An investigation of speech reception thresholds elevation pure-tone thresholds can not explain
Creators: name: Dapper, Konrad
Publisher: name: UNSPECIFIED
Year: 2025

description: It is well established that the biomarker most predictive of speech reception thresholds (SRTs) consists of pure-tone thresholds (PTTs). In general, the assumption is that an elevation of PTTs causes an elevation of SRTs. However, PTTs can explain ~60% of the variance of speech reception thresholds, and it remains controversial whether the remaining variance can be explained by subcortical phenomena such as cochlear synaptopathy or which part is caused by central processing deficits such as limited working memory, or a combination of both. To investigate the origins of SRT elevation which cannot be explained by PTT elevation, we employed a combination of subcortical and cortical measurements.
In the first section I will describe detailed audiometric screening of our cohort, in which we introduce two new approaches to studying the aspect of speech comprehension: the pure-tone normalized OLSA threshold (PNOT) and a post hoc analysis of variance. Using them, we show that age does not affect SRT beyond PTT. Furthermore, we found that cochlear synaptopathy explains a small but measurable amount of additional variance of SRT. In contrast, we found that the function of outer hair cells measured via pulsed DPOAE could explain a larger amount of additional variance. 
In the second section, I will describe neuroelectric responses to phonemes recorded using a passive listening paradigm. Speech EEGs have been identified as an indicator of speech performance beyond PTT in hearing aid users. We confirm phoneme-evoked EEG response amplitude (PEA) as an indicator of speech comprehension. Specifically, PEA decreased with elevated SRT, PTT and increased age. We report here that temporal delays of phoneme-evoked EEG responses (PED) increase with age and PTT. The absolute duration of PED, its age-correlation, and the lack of PEA-lateralization suggest a predominantly thalamic generator of phoneme-evoked EEG responses. Most importantly, PED correlates with SRT and explains SRT-variance in quiet and in ipsilateral noise that PTT cannot. I will show that PED reflects both temporal fine structure (TFS) and temporal envelope (TENV) coding; thalamic activity may thus provide integrated information at the gate of the neocortex.
In the third section, I will expand the results of the previous two sections towards cortical speech processing. Cortical speech processing was investigated here using frequency-tagged MEG responses to speech in auditory scenes, including non-competitive and competing female and male voices. We found that attention alters cortical neural frequency-tagged responses. Furthermore, we have identified three factors that affect cortical speech processing. The first factor is the subject's age, which was correlated with increasing amplitude of cortical responses. The second factor is the delay of neural responses in the left hemisphere, which decreases with elevated SRT in noise. The neural response delay is particularly relevant when TFS coding is involved. Lastly, the third factor is the specificity of the neural response, which is linked to PNOT(quiet) and manifests as increased right-hemispheric activity and more stimulus-specific left-hemispheric responses to individual sentences. These findings can be explained as a combination of cortical and subcortical gating.
The combination of cochlear, subcortical and cortical factors measured in the same cohort provide a multi-dimensional picture of age-related hearing deficits. I sincerely hope that clinicians dealing with hearing deficits at the limits of pathology and the need to decide whether treatment is already necessary for a given patient, will benefit from the new perspectives opened here and possibly adopt new differentiated diagnostic tools. We have introduced new audiometric and neural measures which will hopefully advance further progress towards an objective measure of speech reception thresholds. Those more objective measures might in the future also enable more individualized therapeutic interventions.