The Effects of Textual Enhancement and Individual Differences on L2 Incidental Vocabulary Learning via Bilingual Subtitles

Background. Bilingual subtitles can facilitate incidental vocabulary learning by providing simultaneous access to L2 forms and L1 meanings, but they also increase split-attention demands in multimodal processing. In such contexts, textual enhancement may either support noticing by increasing the perceptual salience of target words or add to cognitive load. Although previous research has examined bilingual subtitles, textual enhancement, and individual differences, these factors have rarely been investigated together within a single design. As a result, it remains unclear how textual enhancement operates in bilingual subtitle environments and to what extent its effects depend on learners’ working memory, language proficiency, and vocabulary size. Purpose. Given the lack of integrated evidence on how textual enhancement and individual differences jointly operate in bilingual subtitle contexts, this study aimed to clarify whether textual enhancement facilitates incidental vocabulary learning, and how this depends on learners' working memory, language proficiency, and vocabulary size. Results. Forty-two Chinese English majors watched a seven-minute documentary clip with bilingual subtitles in a within-subject, item-level design, in which half of the target words were visually enhanced, and half were not. Vocabulary gains were assessed through pre- and immediate post-tests measuring form recognition, meaning recall, and meaning recognition. The data were analysed using generalized linear mixed models with participants and items as random effects. Results. Textual enhancement significantly increased the likelihood of correct responses in form recognition (β = 1.608, p < 0.001, OR = 5.00) and meaning recall (β = 2.637, p = 0.004, OR = 13.98), but not in meaning recognition (β = 1.074, p = 0.161). Language proficiency significantly predicted form recognition (β = 0.506, p = 0.044), meaning recall (β = 0.818, p = 0.014), and meaning recognition (β = 0.646, p = 0.017). Working memory predicted form recognition (β = 0.486, p = 0.037) and meaning recognition (β = 0.622, p = 0.009), whereas vocabulary size predicted only meaning recognition (β = 0.529, p = 0.030). Conclusion. The findings suggest that, in bilingual subtitle contexts, textual enhancement primarily supports form-level learning rather than deeper semantic integration. Language proficiency emerged as the most consistent predictor of incidental vocabulary gains, while working memory and vocabulary size showed more selective effects depending on the type of vocabulary knowledge assessed. These results shed light on the boundary conditions under which visual enhancement is effective in cognitively demanding multimodal input.