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Active HD-tDCS over the LIFG during L2 grammar acquisition reverses correlation between accuracy and ERP amplitude over the left frontotemporal region
Poster C93 in Poster Session C, Wednesday, October 25, 10:15 am - 12:00 pm CEST, Espace Vieux-Port
Daniel Gallagher1, Shinri Ohta1; 1Kyushu University, Fukuoka, Japan
Active (high-definition) transcranial direct current stimulation ((HD-)tDCS) over the left inferior frontal gyrus (LIFG) has been shown to cause an improvement in language processing during various linguistic tasks, including vocabulary acquisition, sentence comprehension, and artificial grammar acquisition. Recently, we showed that HD-tDCS over the LIFG improves L2 grammar acquisition, however, neurophysiological correlates induced by this stimulation have not yet been elucidated. In this study, 50 native Japanese speakers naïve to Spanish completed a 20-minute training of present-tense verb conjugations for the three Spanish verb types (-ar, -er, -ir). During training, half of the participants received active HD-tDCS over the LIFG (the active group), while the other half received sham stimulation in the same location (the sham group). After training, participants completed a mixed-task phase consisting of two linguistic tasks (reception, production) and two working memory tasks (word recall, animal call recall). The reception task required participants to identify whether a given verb was conjugated correctly according to the given subject pronoun. The production task required the participant to verbalize the correct conjugation when given a subject pronoun and an unconjugated verb. The two working memory tasks were used as non-linguistic control tasks. ERPs were recorded during the reception task and aligned to the critical syllable onset (i.e., the syllable of conjugation disambiguation). (Generalized) linear mixed effects ((G)LME) models were used to test the behavioral data for significant effects of stimulation. GLME models revealed that linguistic (but not non-linguistic) task accuracy was significantly improved with active stimulation, while LME models revealed no significant effects of stimulation on response time. Due to the exploratory nature of this study, to analyze ERPs, we first visually inspected whole-epoch (0¬–2000 ms) topographical maps of both groups to select regions and time-windows of interest, primarily in the difference condition (morphosyntactic violation – control). Subsequent statistical analyses revealed several significant effects of stimulation demonstrating that while the two groups had similar ERPs, the sham group exhibited overall stronger and longer-lasting potentials. Additional tests for correlations between accuracy and ERP amplitude revealed that the active group exhibited a reversal of the sham group’s positive correlation between accuracy and ERP amplitude over left frontotemporal electrodes. That is, in the sham group, as accuracy improved, left frontotemporal ERPs became stronger and more positive, while in the active group, improvements in accuracy were characterized by weaker and more negative deflections. In sum, behavioral data corroborated the result from our previous study that HD-tDCS over the LIFG differentially improves L2 grammar acquisition. Meanwhile, ERP results indicated two primary effects of active stimulation: generally improved cognitive efficiency and a reversal of the correlation between accuracy and left frontotemporal ERP amplitude. Taken together, we interpret these results to indicate that HD-tDCS over the LIFG during L2 grammar acquisition caused the active group to more quickly reach a later stage of language learning (e.g., the consolidation phase), while the sham group remained at an earlier stage (e.g., shallow learning phase).
Topic Areas: Language Development/Acquisition, Multilingualism