Four functional magnetic resonance imaging (fMRI) experiments were conducted to investigate how the human brain processes and represents semantic information. Because previous neuroimaging research typically utilizes words having a precise and dominant meaning as stimuli and has not manipulated lexico-semantic ambiguity, a key feature of human language, as an experimental variable, we sought to examine the neural mechanisms for semantic ambiguity in monolinguals in Experiment 1. Subjects were instructed to generate a word semantically related to a visually presented character. Two types of characters were used, namely characters with high semantic ambiguity and characters without semantic ambiguity. Results indicate that precise-meaning characters provoked strong activity in left mid-superior temporal and bilateral inferior frontal gyri whereas peak brain activations occurred in left mid-superior frontal cortex for processing characters of high semantic ambiguity. This indicates that the neuroanatomical circuitry for semantic representation and analysis is modulated by lexical ambiguity and that the mid-superior frontal network works as a central executive system of cognition, guiding goal-related search and selection.

In Experiments 2 to 4, we investigate how semantic information is represented in the bilingual brain. Early Chinese-English bilinguals were asked to perform a lexical decision task on verbs and nouns (Experiment 2), to judge semantic plausibility of sentences in the two languages (Experiment 3). Past imaging work with monolinguals indicated that English verbs and nouns are represented in different brain regions. However, Chinese nouns and verbs are known to be associated with a wide range of overlapping areas in Chinese subjects without significant differences. Our results with bilinguals show that while Chinese nouns and verbs provoked common brain areas, the processing of English nouns and verbs recruited distinct neuroanatomical circuitries. In Experiment 3, significant differences were found between the processing of Chinese and English sentences. The left lingual and right inferior frontal gyri were activated more strongly in the Chinese condition whereas left inferior frontal gyrus and cerebellum showed more robust activity in the English condition. These results lend strong support to the proposal that early bilinguals deal with word categorical information of the two languages separately and that they are able to acquire language-specific information swiftly. Thus, the early bilingual’s brain is highly plastic in that it handles the two language systems in the light of their different design principles. In Experiment 4, late Chinese-English bilinguals were asked to make a semantic judgment on two presented stimuli. Compared to the fMRI results from native American English speakers performing the same semantic judgment task on English words, late bilinguals process the meaning of English words by recruiting brain areas responsible for the semantic processing of Chinese characters, suggesting that the semantic network in the brain interacts between native and second languages. In summary, these results provide compelling evidence for the proposal that the age of acquisition of L2 affects the cortical representation of semantic processes in bilinguals.