L1 vowel perceptual boundary shift as a result of L2 vowel learning

The current study investigated second language (L2) vowel learning influence on first language (L1) vowel perception. We examined how late L2-English learners’ perception of L1-Japanese vowels is influenced by learning to perceive a new L2-English vowel. The study compared L1/L2 perception task results from 60 late L1-Japanese learners of L2-English with those of monolingual Japanese (N = 21) and English speakers (N = 16). To further test hypotheses put forward in the revised Speech Learning Model (SLM-r: Flege & Bohn, 2021), that L2 input distribution is associated with L1/L2 phonetic learning, information on L2-learner participants’ L2 dominance was gathered. The results showed clear L1 perceptual drift in a subgroup of L2-learner participants who were NOT nativelike in L2 English /i-ɪ/ categorization but were L2 dominant. The results support the claim that L2 input plays an important role in reorganizing the L1 phonetic system. However, they also highlight the importance of separating L2 dominance related factors (e.g., L2 input/use) and L2 perceptual ability in investigating L1-L2 phonetic interaction.