Temporal asynchrony affects letter-speech sound integration in second language reading

The integration of visual letters with corresponding auditory speech sounds is crucial for literacy acquisition in alphabetic languages. Previous research exploring this process has shown that audiovisual (AV) congruency enhances auditory N1 amplitude, particularly in advanced English learners, during second language (L2) reading. In the current study, we investigated how the degree of temporal coincidence between letters and speech sounds influences AV congruency effects, using both behavioral and event-related measures. Native Japanese speakers with an intermediate level of English proficiency completed a priming task in English. During the task, visual letters or symbols preceded auditory sounds across different stimulus onset asynchrony (SOA) conditions. The behavioral experiment revealed shorter response times for congruent trials across all SOA conditions, indicating a facilitation effect of congruency. In the electroencephalography experiment, we found an increased early N1 response to congruent AV pairs in the bilateral frontotemporal scalp region during L2 reading, but only in the AV300 condition (with letters preceding speech sounds by 300 ms). Our findings highlight the importance of temporal relationship on N1 congruency effects. Specifically, a slightly longer SOA may help compensate for lower levels of automaticity in intermediate L2 learners, thus facilitating successful integration of letters and sounds during English reading. This study contributes not only to a better understanding of L2 reading mechanisms, but also provides a theoretical basis for examining the role of atypical letter-speech sound integration related to L2 reading difficulties, particularly those with limited proficiency.