Neural signatures of hierarchical linguistic structures in second language listening comprehension

Abstract

Native speakers excel at parsing continuous speech into smaller elements and entraining their neural activities to the linguistic hierarchy at different levels (e.g., syllables, phrases and sentences) to achieve speech comprehension. However, how a nonnative brain tracks hierarchical linguistic structures in second language (L2) speech comprehension and whether it relates to top-down attention and language proficiency remains elusive. Here, we applied a frequency-tagging paradigm in human adults and investigated the neural tracking responses to hierarchically organized linguistic structures (i.e., the syllabic rate of 4 Hz, the phrasal rate of 2 Hz and the sentential rate of 1 Hz) in both first language (L1) and L2 listeners when they attended to a speech stream or ignored it. We revealed disrupted neural responses to higher-order linguistic structures (i.e., phrases and sentences) for L2 listeners in which the phrasal-level tracking was functionally related to an L2 subject’s language proficiency. We also observed less efficient top-down modulation of attention in L2 speech comprehension than in L1 speech comprehension. Our results indicate that the reduced delta-band neuronal oscillations that subserve the internal construction of higher-order linguistic structures may compromise listening comprehension in a nonnative language.

Significance statement

Low-frequency neural oscillations are at the root of speech comprehension in a native brain. How a nonnative brain tracks hierarchical linguistic structures in L2 speech and whether it relates to attention and language proficiency has not been established. Our study recorded electrophysiological responses to the linguistic structures at the syllabic, the phrasal and the sentential rates for L2 listeners and found reduced tracking responses to the higher-order linguistic structures in L2 compared to L1, which relates to L2 proficiency at the behavioral level. Moreover, unlike native listeners, who automatically tracked speech structures without attention, nonnative listeners could not track higher-order linguistic structures in L2 speech during passive listening, indicating a different pattern of attentional modulation in a nonnative brain.