The Neural Mechanisms of Private Speech in Second Language Learners' Oral Production: An fNIRS Study.

: According to Vygotsky's sociocultural theory, private speech functions both as a tool for thought regulation and as a transitional form between outer and inner speech. However, its role in adult second language (L2) learning-and the neural mechanisms supporting it-remains insufficiently understood. This study thus examined whether private speech facilitates L2 oral production and investigated its underlying neural mechanisms, including the extent to which private speech resembles inner speech in its regulatory function and the transitional nature of private speech. : In Experiment 1, to identify natural users of private speech, 64 Chinese-speaking L2 English learners with varying proficiency levels were invited to complete a picture-description task. In Experiment 2, functional near-infrared spectroscopy (fNIRS) was used to examine the neural mechanisms of private speech in 32 private speech users identified in Experiment 1. : Experiment 1 showed that private speech facilitates L2 oral production. Experiment 2 revealed that private and inner speech elicited highly similar patterns of functional connectivity. Among high-proficiency learners, private speech exhibited enhanced connectivity between the language network and the thought-regulation network, indicating involvement of higher-order cognitive processes. In contrast, among low-proficiency learners, connectivity was primarily restricted to language-related regions, suggesting that private speech supports basic linguistic processing at early stages. Furthermore, both private and outer speech showed stronger connectivity in speech-related brain regions. : This is the first study to examine the neural mechanisms of private speech in L2 learners by using fNIRS. The findings provide novel neural evidence that private speech serves as both a regulatory scaffold and a transitional form bridging outer and inner speech. Its cognitive function appears to evolve with increasing L2 proficiency.