Understanding Cortical Activation Associated with Second Language Acquisition

Abstract

Repetitive transcranial magnetic stimulation (rTMS) performed at a low frequency has been found to have an inhibitory effect on cortical function, which may result in behavioural consequences. Accordingly, rTMS may provide a valid model of focal cortical lesions, such as stroke, in healthy individuals. rTMS has been further proposed as a tool to inhibit high levels of cortical activity in individuals with aphasia in right hemisphere (RH) language homologues following a stroke that damages primary language regions. It is thought that while RH activity is compensatory during the early stages of recovery from stroke, it is maladaptive in terms of long-term recovery. RH activity has also been reported in healthy, low-proficiency bilinguals during second language (L2) production; thus, it has been proposed that RH activity may be better classified as reflecting effortful language production. Furthermore, given similarities in RH activity during language production in low-proficiency bilinguals and aphasics, non-disabled controls learning an L2 may provide a useful analogue to study aphasia. The research presented in this dissertation examines learning of Spanish as an L2 in unilingual, English speaking adults in an attempt to understand which brain regions are associated with L2 acquisition, and the role that the RH plays in language functioning. Results of this project indicate that adults were able to learn novel Spanish vocabulary through a computer-based language-training program, but that learning was partially dependent on age and information processing capabilities. Unlike previous research that has demonstrated a more extensive network for L2 production, including regions of the RH, we found a less extensive network for Spanish than for English, although English language production was associated with RH activity. Furthermore, inhibitory rTMS produced no effects on lexical retrieval in either English or Spanish, indicating that the virtual lesion induced by TMS is not sufficient to mimic an aphasic stroke. Overall, our results indicate that while the RH does play an important role in language production, that role may differ between healthy and aphasic individuals. Also, neither L2 acquisition nor virtually induced lesions via TMS appear to provide an ideal model of post-stroke aphasia.