Phase coding in phoneme processing slows with age

The comprehension of phonemes is a fundamental component of speech processing which relies on both, temporal fine structure (TFS) and temporal envelope (TE) coding. EEG amplitude in response to phonemes has been identified as indicator of speech performance in hearing aid users. Presbyacusis may also alter neuro-electric responses to phonemes, even with minimally or unaffected hearing thresholds. Elevated speech reception thresholds (SRT) in absence of pure-tone threshold (PTT) elevation suggest central processing deficits. We therefore collected audiometric data (PTT, SRT) and EEG during passive listening in 80 subjects, ranging in age from 18 to 76 years. We confirm phoneme-evoked EEG response amplitude (PEA) as indicator of speech comprehension. Specifically, PEA decreased with elevated SRT, PTT and increased age. As novel observation, we report the temporal delay of phoneme-evoked EEG responses (PED) to increase with age and PTT. The absolute duration of PED, its age-correlation, and the lack of PEA-lateralization combined with the frequency of phoneme stimuli used here suggest a predominantly thalamic generator of phoneme-evoked EEG responses. Hearing loss in extended high-frequencies affects PED more than PEA. In our sample, neural compensation for increased PTT came at the cost of decreased temporal processing speed. Most importantly, PED correlates with SRT and explains SRT-variance in quiet and in ipsilateral noise that PTT cannot. PED was a better predictor of TFS coding in quiet and of TE coding in ipsilateral noise. As PED reflects both TFS and TE coding, thalamic activity may provide integrated information at the gate of neocortex. Significance StatementIntact speech comprehension is essential for social participation which protects against depression and dementia. Age-related hearing loss is a growing problem in aging societies, as hearing deficits constitute the third most important modifiable risk factor for cognitive decline. This work uses electrical brain responses to phonemes in a cohort covering age 18 to 76 years. As the temporal delay of phoneme responses showed the most significant correlations with age and high-frequency thresholds, we demonstrated that speed of neural processing seems essential for speech comprehension. The observed neural signals likely originate from thalamus which receives feedback from neocortex and is embedded in cognitive processing. Developing objective markers for speech processing is key for ensuring cognitive fitness in aging.