Distinct Neural Signatures of Novel Word-Referent Mapping in Monolingual and Bilingual Toddlers

Evidence suggests that bilingualism impacts children's lexical and brain development. Yet, the development of the brain's language network in early childhood-a period marked by rapid vocabulary growth-remains poorly understood, particularly with respect to differences between bilingual and monolingual children. This study used eye tracking and functional near-infrared spectroscopy (fNIRS) to examine the cortical representations of lexical acquisition in frontal and temporal lobes. We relied on a fast mapping task that included three trial types: Familiar (familiar label-referent mapping; e.g., ball), Disambiguation (novel label-referent mapping; e.g., dax), and Retention (long term retention of two novel referents). Sixty English-learning monolingual and bilingual children (27-32 months), from a majority English-speaking society, participated in the study. Results revealed that monolinguals showed increased looking to the novel referent during disambiguation trials, with a similar trend during retention. In contrast, bilinguals did not show reliable novel label-referent mapping behavior. Brain activity differences between the language groups emerged in right temporal regions during disambiguation trials, with bilinguals showing increasing HbO activity in this region, than monolinguals. Bilinguals' right hemisphere greater activity may reflect reduced automaticity in the use of disambiguation. These results offer foundational insight into neural markers supporting lexical acquisition, especially for those who may struggle in language acquisition such as late talkers.