Second language experience modulates neural specialization for first language lexical tones

Recent neuroimaging studies have revealed distinct functional roles of left and right temporal lobe structures in the processing of lexical tones in Chinese. In the present study, we ask whether knowledge of a second language (English) modulates this pattern of activation in the perception of tonal contrasts. Twenty-four native Chinese speakers were recruited from undergraduate and graduate students at Beijing Normal University, China. Participants listened to blocks of computationally manipulated /ba/ syllables which were varied to form within- and across-category deviants at equal acoustic intervals from a standard tone while their cortical blood oxygenation was measured by functional near-infrared spectroscopy (fNIRS). Blocks were analyzed for peak blood oxygenation (HbO) levels, and several linear models were estimated for these data, including effects of deviant tone type (within- or across-category), behavioral differences in tone identification, age of earliest exposure to English (spoken), and proficiency in English. Functional changes in HbO indicated a significantly greater response to within-category contrasts in right STG, consistent with previous findings. However, the effect of deviant type in left MTG was significantly modulated by the age of participants' earliest English exposure: Average across-category activation exceeded within-category activation only for participants exposed to English after 13 years of age. While previous research has established the importance of left MTG in the categorical perception of lexical tones, our findings suggest that the functional specialization of this region is sensitive to second language experience, even in the processing of native language.