Incremental or delayed processing? L2 learners’ active gap-fillingin sentence comprehension

The contrast between first language (L1) and second language (L2) sentence processing provides valuable insights into how linguistic knowledge and cognitive mechanisms shape real-time comprehension. While L1 speakers are known to engage in incremental parsing–integrating linguistic input as it becomes available–it remains debated whether adult L2 learners adopt similar strategies or rely on delayed, non-structural parsing as proposed by the Shallow Structure Hypothesis (SSH). To investigate this, the present study employed the visual world eye-tracking paradigm to examine how Japanese L2 learners of English process structurally ambiguous wh-questions. Results revealed a strong preference for main clause (MC) interpretations, mirroring native speaker behavior. Crucially, eye-tracking data showed early fixation patterns consistent with active gap-filling, providing evidence against SSH. Furthermore, L2 learners demonstrated an even stronger commitment to the MC analysis than native speakers, suggesting that while their syntactic processing is guided by incremental structural parsing, it may also reflect a greater reliance on initial interpretations.