Cortical processing of speech sounds in individuals with cochlear hearing loss and auditory neuropathy

Cortical auditory evoked potentials (CAEPs) provide valuable insights into how the auditory cortex processes sound. The cochlear hearing loss (CHL) and auditory neuropathy (AN) both disrupt the transmission of auditory information from the peripheral to the central auditory system, although the nature of the disruption differs. We hypothesised that these two forms of sensori-neural hearing loss may have differential effects on cortical processing of the sound. To test this hypothesis, we recorded CAEPs from 25 normal-hearing (NH), 25 CHL, and 25 AN participants in response to the /da/ stimulus using a 64-channel electroencephalograh . Three analyses were performed: conventional peak latency and amplitude analyses, whole-scalp amplitude analyses across all time points, and source-level analyses at selected time windows. Generalised linear mixed-model analyses revealed that the CHL group showed significantly larger N1 amplitudes compared to the NH and AN groups. The AN group exhibited significantly prolonged P1, N1, and P2 latencies compared to both NH and CHL groups. Additionally, the CHL group demonstrated a delayed P2 latency relative to the NH group. The results of whole-scalp amplitude analyses were consistent with and complemented the peak-based findings. Source localization revealed increased activation in the right superior temporal gyrus and insula in the CHL group compared to the NH group during the N1 time window. The AN group showed enhanced activation in the left parietal region (precuneus, BA7) compared to the NH group during the N1 time window. No significant group differences were observed for P2 sources. These findings suggest distinct cortical reorganization patterns in CHL and AN, indicating that early sound detection (N1) mechanisms differ between the groups, whereas later stimulus evaluation (P2) appears to be similar.