Acoustic Context Shapes Cortical Encoding of Spectral Discrimination in Normal and Impaired Hearing
The auditory change complex (ACC) provides a cortical measure of acoustic discrimination, yet the mechanisms shaping responses to spectrotemporally complex sounds remain poorly understood. We investigated how acoustic context and temporal-envelope variability influence cortical encoding of spectral contrast in normal-hearing (NH) and hearing-impaired (HI) listeners. Electroencephalography (EEG) was used to record ACCs and auditory steady-state responses (ASSRs) evoked by spectrotemporally modulated stimuli. We compared conventional spectro-temporal modulations (STMs) with a novel discrete spectro-temporal modulation (DSTM) paradigm, examined temporal roving, and evaluated cortical responses to spectral contrast in NH and HI listeners. ACCs elicited by DSTM stimuli were larger and more symmetrical across transition directions than those elicited by conventional STMs. Temporal roving markedly reduced responses to intensity changes while largely preserving responses to spectral changes, indicating a partial dissociation between intensity-driven and spectrally driven cortical discrimination. ACC responses decreased systematically with spectral contrast and were weaker and delayed in HI listeners. By contrast, ASSRs remained highly detectable and showed no systematic dependence on spectral contrast, supporting a distinction between contrast-sensitive cortical discrimination and ongoing envelope encoding. Temporal roving further enhanced separation between NH and HI listeners. ACC response strength was significantly associated with behavioural STM discrimination thresholds and explained variance beyond age and audiometric thresholds. These findings demonstrate that cortical representations of spectral discrimination are shaped by acoustic context and temporal-envelope statistics. Temporal roving attenuates intensity-driven responses while preserving spectrally driven discrimination, providing insight into cortical encoding of spectrotemporal contrast and its relationship to perceptual performance.