Causal evidence for the involvement of Broca’s area in second language acquisition: A longitudinal HD-tDCS study

Abstract

A wealth of correlational evidence suggests that Broca's area (the left inferior frontal gyrus) plays a role in second language acquisition. With the use of highly focal non-invasive brain stimulation, evidence for a targeted brain region's causal role in some cognitive behavior can be assessed. Over three sessions, each one week apart, we used online anodal high-definition transcranial direct current stimulation (HD-tDCS) over Broca's area during a novel foreign grammar training session. During training, participants who were naive to Spanish were tasked with acquiring present tense conjugation rules for the three Spanish verb endings. In Session 1, we observed significant improvement in performance in two linguistic (reception and production) tasks but not in a non-linguistic (working memory) task. Sessions 2 and 3 were subject to a ceiling effect, which obscured any potentially long-lasting effects of the stimulation. We interpret these results as support for the idea that Broca's area has languagelike specificity that is not limited to receptive or productive processes and as evidence that Broca's area plays a causal role in foreign grammar acquisition.