Perception of non-native sounds in a second language: Electrophysiological evidence of neuroplasticity in the phonological system

Second language learners frequently encounter difficulty in perceiving specific non-native sound contrasts. This phenomenon called phonological deafness rather occurs if the second language (L2) is learned after early childhood and is quite persistent even when high L2 proficiency is attained (Dupoux et al., 2008). However, if the neuronal underpinnings of phonological processing are plastic to a certain degree, late L2 learners should be able to reach the capacity to distinguish non-native phonemic contrasts (Best & Strange, 1992; Flege et al., 1997; Iverson et al., 2012). In the present study, our goal was to examine the extent to which the phonological system in late L2 learners is adaptable. We designed an ERP experiment in which the capacity to discriminate second language phonemic contrasts mediated lexical access. We used a semantic violation paradigm in which the difference between semantically congruent and incongruent items was implemented by a phonemic contrast that was unique to the second language, English, but absent in the first language, French (e.g., /ɪ/ - /i/: ship – sheep). Twelve young adult native speakers of French with intermediate proficiency in English participated in the ERP experiment. Participants heard sentences that contained either a semantically congruent item (e.g., The anchor of the ship was let down) or an incongruent one (e.g., *The anchor of the sheep was let down) and were asked to perform a grammaticality judgement. Preliminary results reveal that second language learners of English showed a larger centro-parietal negativity between 300-500 ms after the onset of semantically incongruent words as compared to congruent target words, i.e. an N400 effect. This finding indicates that L2 learners were sensitive to the semantic incongruency mediated by a phonemic contrast. Critically, the N400 effect size varied as a function of L2 proficiency, i.e. the more proficient the participants, the larger the N400 effect size. Thus, the sensitivity to phonemic contrasts of a second language seems to play a significant role in lexical access. With an increasing capacity to discriminate second language phonemic contrasts, the access to lexical information is facilitated. These findings show that even late learners of a second language can develop a perceptual sensitivity to discriminate non-native sound contrasts, at least at the segmental phonological level, which also indicates that neuroplasticity in the phonological system allows for a certain adaptation to linguistic environmental constraints. Further investigations should explore how targeted training can improve the sensitivity to second language phonemic contrasts.