Brain network differences in second language learning depend on individual competencies

Integrating new words into an existing semantic network is a core challenge of second language (L2) acquisition. We investigated how evidence-based learning strategies and individual performance relate to the neurocognitive dynamics of vocabulary learning. Eighty-three adults proficient in German or French learned 48 Finnish (L2) nouns over 14 days using a mobile app that systematically varied retrieval practice, corrective feedback, multisensory learning, and distributed learning. Before and after training, EEG was recorded during a translation recognition task designed to elicit the N400, an index of semantic integration. Vocabulary accuracy increased from 0.41% pre-learning to 75.5% post-learning (d_z = 3.96), and the N400 incongruity effect increased significantly, F(1, 75) = 99.52, p < .001, η²_g = .32, reflecting successful integration of new L2 words into the mental lexicon. High performers showed larger N400 responses, consistent with more differentiated semantic processing. In addition, performance group differences emerged in event-related potential (ERP) template-map preponderance (i.e., the proportion of epoch time points assigned to a given template map), indicating differences in processing dynamics and network configurations. Despite systematic manipulation of learning strategies, no single approach yielded consistent behavioral or neural advantages, suggesting that overall exposure and cumulative practice, rather than any specific strategy, may have been primary contributors to the observed learning gains. ERP template-map analyses further revealed that learning not only increased neural responses but also shifted map preponderance within the N400 window as a function of Session and Congruency, indicating a qualitative reorganization of semantic processing. These findings suggest that vocabulary learning success is more closely related to the degree of semantic integration than to any single instructional strategy.