Neural tracking of auditory and lexical processing in bilingual children and adult L2 learners under simulated cochlear implant listening conditions

This study explores auditory and lexical processing in Spanish-English bilingual children and adult second language (L2) learners under simulated cochlear implant (CI) listening conditions. EEG recordings were analysed using multivariate temporal response functions (mTRFs) while participants listened to narrative speech in English and Spanish, presented either in its original form or vocoded to simulate CI processing. Lexical features, including frequency, predictability, concreteness, arousal, and neighbourhood density, were estimated using a large language model and used to model neural responses. Both groups showed sensitivity to acoustic and lexical features; however, children exhibited greater difficulty with lexical prediction when speech was vocoded. They also showed enhanced theta-band tracking under degraded conditions, suggesting increased reliance on acoustic cues. In contrast, their lower theta tracking in clear speech may reflect an ongoing developmental process in auditory linguistic networks. Adults exhibited stronger and more consistent tracking of lexical frequency and neighbourhood density, along with a language effect favouring Spanish. These results highlight developmental differences in the robustness and flexibility of lexical prediction under spectral degradation. They demonstrate that auditory and lexical neural tracking can be jointly measured in naturalistic listening tasks, providing insight into how age affects lexical processing under perceptually challenging conditions. [Work supported by ESRC, UK.]