Bilingual and monolingual brains compared: A fMRI study of semantic processing

Does early and sustained experience with two languages in childhood modify how language is processed in the adult bilingual brain? Does extensive exposure to two languages influence language processing in the bilingual brain as compared to monolinguals? The present study investigated how adult bilinguals process semantic information, both in speech and in print, across varying language contexts or modes (bilingual modes and monolingual modes). Using behavioral and neuroimaging (fMRI) measurements, two hypotheses were tested, the Neurological Signature Hypothesis and the Functional Switching Hypothesis. The Neurological Signature Hypothesis predicts that early highly proficient bilinguals will recruit neurological tissue to process language differently from monolinguals; hence, such differences may constitute a "neurological signature" of bilingualism and should be present across all language contexts. The Functional Switching Hypothesis predicts that bilinguals will recruit neurological tissue to process language similarly to monolinguals, but not across all contexts/modes. Early-exposed highly proficient Spanish-English bilinguals and English monolinguals (N=27) completed a Semantic Judgment (lexico-semantic level), a Passage Comprehension (sentences) and a non-linguistic Stimulus-Response Compatibility attention task in the fMRI scanner. During the Semantic Judgment task bilinguals were presented with words in Spanish and in English, one language at a time (monolingual mode), and in rapid alternation (bilingual mode). Monolinguals completed each language task in English only. Our results supported the Neurological Signature Hypothesis. When presented with one language at a time, bilinguals demonstrated greater recruitment of left dorsolateral prefrontal cortex (DLPFC) than monolinguals. When presented with two languages in rapid alternation, bilinguals demonstrated a more robust recruitment of right DLPFC and right inferior frontal cortex. During non-linguistic attentional tasks bilinguals showed higher behavioral accuracy during control and incongruous trials, but not during switch trials, as compared to monolinguals, and bilinguals also had an overall more robust recruitment of the left inferior parietal cortex. These findings elucidate how extensive early bilingual exposure modifies language organization in the brain, as well as how this change impacts other domains of cognition. This research further identifies the benefits of early and extensive bilingual education, including high dual language proficiency, skilled dual language literacy, and enhancement of general cognitive capabilities.