Neural representation of linguistic feature hierarchy reflects second-language proficiency

Acquiring a new language requires a simultaneous and gradual learning of multiple levels of linguistic attributes. Here, we investigated how this process changes the neural encoding of natural speech by assessing the encoding of the linguistic feature hierarchy in second-language listeners. Electroencephalography (EEG) signals were recorded during English story listening from native Mandarin speakers with varied English proficiency and from native English speakers. We measured the temporal response functions (TRF) for acoustic, phonemic, phonotactic, and semantic features in individual participants and found a main effect of proficiency on the linguistic encoding. This effect of second-language proficiency was particularly prominent on the neural encoding of phonemes, showing stronger encoding of "new" phonemic contrasts (i.e. English contrasts that do not exist in Mandarin) with increasing proficiency. Overall, we found that linguistic feature representation in nonnative listeners progressively converged to that of native listeners with proficiency, which enabled accurate decoding of language proficiency. This detailed view advances our understanding of the cortical processing of linguistic information in second-language learners and provides an objective measure of language proficiency.