More than half of the world’s population are actively learning or speaking a second language. Research in this field thus far has focused primarily on the second language processing alone, but neglect the interaction of the two languages. In addition, majority of the previous studies investigated this issue using bilingual adults. Do bilingual children recruit both native and second language neural networks in second language reading as adults do? Does second language and native tongue affect one another in bilingual children’s brain?
To answer these questions, we tested 28 early Chinese-English bilingual children, assigning participants phonological and orthographical processing tasks in both languages while performing brain scans using fMRI. Phonological tasks required participants to determine whether or not displayed Chinese characters were homonyms, while orthographical tasks required these children to judge whether or not the given stimuli were visually similar. In addition to these tasks, participants also undertook four behavioral tests to assess their proficiency in both languages. These tests comprised of a Chinese character recognition test, a Chinese reading fluency test, and an English dictation test.
From whole-brain analysis, we found that the two tasks recruited largely similar brain networks across both languages despite of some language differences. Based on an meta-analysis on cross-language comparison, we defined our regions of interest (ROI). ROI analysis revealed that some Chinese-specific regions (bilateral inferior occipital gyrus, cingulate gyrus, and right fusiform gyrus) were significantly activated in English tasks. Similarly, some English-specific regions (left fusiform gyrus, inferior frontal gyrus, and medial frontal gyrus) were significantly activated in Chinese tasks. Among these regions, Increased activation of the Chinese-specific cingulate gyrus was negatively correlated with English dictation test scores, suggesting that the higher the second language proficiency was, the less the native language network was involved. However, while increased activation in the English-specific left superior temporal gyrus was negatively correlated with Chinese reading fluency and character recognition test scores, activation in the English-specific left inferior frontal gyrus was positively associated with greater Chinese reading fluency scores - suggesting that second language processing may be affected by native language proficiency.
Summarily, these results suggest that while bilingual children do develop a distinct neural network to process their second language, it is also partly supported by the brain’s native language network. Additionally, this study further indicates that the neural networks supporting native and second language in bilinguals' brain interact with one another, and this interaction is affected by language proficiency.