How to analyse age effects in electrophysiological signatures of second language grammar processing

There is a debate on the nature of the relationship between age of acquisition (AoA) and grammatical processing in second language (L2) learners (Birdsong, 2005). On the one hand, it has been argued that the ability to learn the grammar of an L2 decreases continuously with age due to general cognitive declines (Bialystok & Hakuta, 1999). On the other hand, in line with the Critical Period Hypothesis, this relationship has been claimed to be discontinuous (Johnson & Newport, 1989). Typically, electrophysiological studies on this topic have used statistical techniques that assume a linear relationship between event-related potential (ERP) amplitudes/latencies and a predictor variable (e.g., linear mixed effect regression modelling, LMER). However, these methods are at odds with the proclaimed non-linear relationship between AoA and the outcome variable. Here, we used generalized additive mixed modeling (GAMM; Wood, 2006) on our data and compared those outcomes with the LMER technique. Specifically, we investigated the effect of AoA on the P600 effect, which was taken as an indicator of native-like processing, in L2 processing of tense marking. Late L2 learners listened to Dutch sentences that were either well-formed or included a tense marking violation while ERPs were recorded. Results demonstrated that all L2 learners show a P600 effect in response to the tense marking violations. Additionally, there appeared to be a linear relationship between AoA and the size of the P600 effect. Learners with higher AoA showed a smaller P600 effect. Even though our analysis techniques allow for non-linear relationships, no such effects were found. Our findings emphasize the linear relationship between AoA and L2 grammatical processing and stress the importance of extending statistical analyses to non-linear relationships between AoA and L2 grammar processing. Bialystok, E., & Hakuta, K. (1999). Confounded age: Linguistic and cognitive factors in age differences for second language acquisition. Second Language Acquisition and the Critical Period Hypothesis, 161–181. Birdsong, D. (2005). Interpreting age effects in second language acquisition. In J. F. Kroll & A. M. De Groot (Eds.), Handbook of bilingualism: Psycholinguistic approaches (pp. 109–127). Johnson, J. S., & Newport, E. L. (1989). Critical period effects in second language learning: The influence of maturational state on the acquisition of English as a second language. Cognitive Psychology, 21(1), 60–99. http://doi.org/10.1016/0010-0285(89)90003-0 Wood, S. N. (2006). Generalized Additive Models: An Introduction with R. Boca Raton: Chapman & Hall/CRC. http://doi.org/10.1111/j.1541-420.2007.00905_3.x