In the last decade there has been a surge of interest in the use of neuroimaging tools such as event-related potentials (ERP) and functional magnetic resonance imaging (fMRI) to examine critical issues in the representation and processing of multiple languages in the brain. In 2001, David Green edited a special issue for Bilingualism: Language and Cognition on the cognitive neuroscience of bilingualism that involved studies of bilingual populations in English, German, Italian, and Japanese. According to a review by Vaid and Hull (2002), by 2001, there were at least 25 fMRI studies and 13 PET (positron emission tomography) studies of bilingual language processing in healthy individuals. This number has grown more rapidly since 2001. Many of these neuroscience studies of bilingualism have also appeared in top science journals and attracted widespread attention.

Neuroimaging studies of reading have identified unique patterns of activation for individuals reading in alphabetic and Asian languages, suggesting the involvement of different processes in each. The present study investigates the extent to which a cognitive prerequisite for reading, the understanding of the symbolic function of print, is common to children learning to read in these two different systems. Four-year-old children in Hong Kong learning to read in Cantonese and children in Canada learning to read in English are compared for their understanding of this concept by means of the moving word task. Children in both settings performed the same on the task, indicating similar levels of progress in spite of experience with very different writing systems. In addition, the children in Hong Kong benefited from the structural similarity between certain iconic characters and their referents, making these items easier than arbitrary characters. These results point to an important cognitive universal in the development of literacy for all children that is the foundation for skilled reading that later becomes diverse and specialized.

Bilingual reading can require more than knowing two languages. Learners must acquire also the writing conventions of their second language, which can differ in its deep mapping principles (writing system) and its visual configurations (script). We review ERP (event-related potential) and fMRI studies of both Chinese–English bilingualism and Chinese second language learning that bear on the system accommodation hypothesis: the neural networks acquired for one system must be modified to accommodate the demands of a new system. ERP bilingual studies demonstrate temporal indicators of the brain's experience with L1 and L2 and with the frequency of encounters of words in L2. ERP learning studies show that early visual processing differences between L1 and L2 diminish during a second term of study. fMRI studies of learning converge in finding that learners recruit bilateral occipital-temporal and also middle frontal areas when reading Chinese, similar to the pattern of native speakers and different from alphabetic reading. The evidence suggests an asymmetry: alphabetic readers have a neural network that accommodates the demands of Chinese by recruiting neural structures less needed for alphabetic reading. Chinese readers have a neural network that partly assimilates English into the Chinese system, especially in the visual stages of word identification.

Neural plasticity in speech acquisition and learning is concerned with the timeline trajectory and the mechanisms of experience-driven changes in the neural circuits that support or disrupt linguistic function. In this selective review, we discuss the role of phonetic learning in language acquisition, the “critical period” of learning, the agents of neural plasticity, and the distinctiveness of linguistic systems in the brain. In particular, we argue for the necessity to look at brain–behavior connections using modern brain imaging techniques, seek explanations based on measures of neural sensitivity, neural efficiency, neural specificity and neural connectivity at the cortical level, and point out some key factors that may facilitate or limit second language learning. We conclude by highlighting the theoretical and practical issues for future studies and suggest ways to optimize language learning and treatment.

In this study we examined ERP (event-related-potential) responses in the morphosyntactic processing of subject–verb agreements by L2 Chinese learners of English. Fifteen proficient L2 learners and fifteen native English speakers were presented with English sentences that varied in the grammaticality of the sentence with respect to subject–verb agreement. Our results indicate that late L2 learners show distinct ERP responses from native speakers in the processing of syntactic features that are absent in their L1, even when their behavioral patterns are similar to those of native speakers. The results are taken to support the proposal that language-specific experiences with L1 shape the neural structure of processing in L2.

The goal of this study was to examine the effect of the linguistic distance between a first language (L1) and a second language (L2) on neural activity during second language relative to first language processing. We compared different L1–L2 pairs in which different linguistic features characterize linguistic distance. Chinese and Korean native speakers were instructed to perform sentence comprehension tasks in two L2s (English and Japanese) and their respective L1s. Activation while understanding English sentences relative to understanding sentences in L1 was greater for the Korean group than the Chinese group in the left inferior frontal gyrus, bilateral posterior superior temporal gyri, and right cerebellum. Activation while understanding Japanese sentences relative to understanding sentences in L1 was greater for the Chinese group than the Korean group in the anterior portion of the left superior temporal gyrus. The results demonstrated that the location of the L2–L1 processing-induced cortical activation varies between different L1–L2 pairs.

Given that there are neural markers for the acquisition of a non-verbal skill, we review evidence of neural markers for the acquisition of vocabulary. Acquiring vocabulary is critical to learning one's native language and to learning other languages. Acquisition requires the ability to link an object concept (meaning) to sound. Is there a region sensitive to vocabulary knowledge? For monolingual English speakers, increased vocabulary knowledge correlates with increased grey matter density in a region of the parietal cortex that is well-located to mediate an association between meaning and sound (the posterior supramarginal gyrus). Further this region also shows sensitivity to acquiring a second language. Relative to monolingual English speakers, Italian–English bilinguals show increased grey matter density in the same region. Differences as well as commonalities might exist in the neural markers for vocabulary where lexical distinctions are also signalled by tone. Relative to monolingual English, Chinese multilingual speakers, like European multilinguals, show increased grey matter density in the parietal region observed previously. However, irrespective of ethnicity, Chinese speakers (both Asian and European) also show highly significant increased grey matter density in two right hemisphere regions (the superior temporal gyrus and the inferior frontal gyrus). They also show increased grey matter density in two left hemisphere regions (middle temporal and superior temporal gyrus). Such increases may reflect additional resources required to process tonal distinctions for lexical purposes or to store tonal differences in order to distinguish lexical items. We conclude with a discussion of future lines of enquiry.

Cognitive neuropsychological studies of bilingual patients with aphasia have contributed to our understanding of how the brain processes different languages. The question we asked is whether differences in script have any impact on language processing in bilingual aphasic patients who speak languages with different writing systems: Chinese and Mongolian. We observed a pattern of greater impairment to written word comprehension and oral reading in L2 (Chinese) than in L1 (Mongolian) for two patients. We argue that differences in script have only a minimal effect on written word processing in bilingual aphasia when the age of acquisition, word frequency and imageability of lexical items is controlled. Our conclusion is that reading of familiar words in Mongolian and Chinese might not require independent cognitive systems or brain regions.