Word count (excluding abstract, title page, references and methods): 6094.
Acknowledgements:The authors would like to thank Michael Broderick for his help with the semantic dissimilarity analysis. The authors also thank Adam Soussana and Ghislain de Labbey for their help with a pilot version of this experiment.
AbstractAcquiring 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. The neural encoding of acoustic, phonemic, phonotactic, and semantic features was measured in individual participants. 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 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. 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. be affected differently by proficiency, with some of them becoming more native-like than others for proficient L2 users. Part of the evidence comes from electro-and magneto-encephalography (EEG and MEG respectively) research, which showed the effect of proficiency at the levels of phonemes 13 , syntax 14,15 , and semantics 16 . These studies measured the changes in well-known event-related potentials, such as the MMN, N400, and P600. These approaches, however, use unnatural speech stimuli (e.g., isolated syllables or violative speech sentences) which do not fully and realistically activate the specialized speech cortex [17][18][19] . In addition, these approaches consider various levels of speech perception independently and in isolation. Language learning, on the other hand, involves simultaneous acquisition of novel phonetic contrasts 20,21 , new syllabic structures (phonotactics) 22 , and new words. A more complete view of the neural basis of language learning therefore requires a joint study of multiple levels of the linguistic hierarchy to advance our understanding of L2 perception by informing us on the precise effect of proficiency on the cortical processing strategies that underpin sound and language perception 23-25 .Previous effort in using naturalistic speech stimuli to study language proficiency showed a modulation of EEG phase-synchronization during naturalistic speech listening both at sub-cortical (FFR 26,27 ) and cortical levels (gamma EEG synchrony 28,29 ). Specifically, stronger synchron...