In the world, there are 55 countries with more than one official languages, and immeasurable regions with only one official language but also regional and local languages (www.uottawa.ca/clmc/). Understandably, countless people use more than one languages frequently for a long time. Long-term bilingual experience has an effect on multiple cognitive functions (see a review (Costa & Sebastian-Galles, 2014)). The effect of bilingualism on phonological processing received much attention. Bilinguals rely on one brain to process two distinct phonologies. Functional imaging studies showed that the brain activation patterns during phonological processing of the native and second language (respectively L1 and L2) for bilinguals were different (see reviews (Liu & Cao, 2016; Sulpizio, et al., 2020)), as well as the brain activation pattern during phonological processing of either language for bilinguals was different from that of the single language for monolingual (Cao, et al., 2013; Ma, et al., 2020; Parker Jones, et al., 2012). Functional changes are accompanied by anatomical changes (see reviews (Li, et al., 2014; Stein, et al., 2014)). White matter coordinates communications between different brain regions (Douglas, 2008) and interacts with the cortex function (Duffau, 2015). White matter has lifelong plasticity (Fields, 2015; Gibson, et al., 2014; McKenzie, et al., 2014) and is likely to be modulated by environment stimulus and behavioral experience, such as bilingualism (Li, et al., 2014). Therefore, it’s reasonable to hypothesize that long-term bilingual experience can shape different white matter structures of phonological-related tracts.
Diffusion tensor imaging (DTI) technique allowed a detailed observation of white matter structure in vivo without invasion (Basser & O¨zarslan, 2014). Characteristics of white matter can be described by the DTI indices, such as fractional anisotropy (FA) and diffusivity measurements including mean, axial and radial diffusivities (MD, AD and RD respectively) (Soares, et al., 2013). According to the dual-stream model of language processing, the dorsal cortical circuit including the perisylvian language areas is demonstrated to be involved in processing the phonological information in written and spoken languages (Hickok & Poeppel, 2007; Schlaggar & McCandliss, 2007). Fiber dissection studies claimed that the superior longitudinal fasciculus (SLF) is the core fiber connecting the dorsal language cortical regions (Sarubbo, et al., 2016). On the other hand, the ventral cortical circuit is generally considered to play an important role in semantic processing, while it also supports auditory comprehension during speech processing (Hickok & Poeppel, 2007). The core tracts connecting the ventral stream consist of the inferior longitudinal fasciculus (ILF) and the inferior fronto-occipital fasciculus (IFOF) (Sarubbo, et al., 2016). These two tracts were also claimed to correlates the mapping from orthography to phonology (Mei, et al., 2015; Tomasino, et al., 2020).
Accumulating DTI evidence revealed that alphabetic-alphabetic bilinguals had different white matter structures from their monolingual peers, especially in the SLF, ILF and IFOF (Anderson, et al., 2018; Gold, et al., 2013; Kuhl, et al., 2016; Luk, et al., 2011; Singh et al., 2017; Pliatsikas, et al., 2015; Schlegel, et al., 2012). However, the reproducibility of these studies is low. It’s considerable, because bilingualism is a multifaceted construct sensitive to age of L2 acquisition (AoA), L2 immersion time, language categories and etc. (Li, et al., 2014). Most of the current DTI studies on bilingualism recruited the first generation immigrants or local L2 learners as the bilingual samples, who generally learn L2 lately and use L2 for a short time (Cummine & Boliek, 2013; Kuhl, et al., 2016; Pliatsikas, et al., 2015; Rossi, et al., 2017; Schlegel, et al., 2012). In the SLF, short-term late bilinguals usually demonstrated higher FA and lower RD than monolinguals (Pliatsikas, et al., 2015; Rossi, et al., 2017; Schlegel, et al., 2012). This is similar to the white matter characteristics of the subjects after L2 training (Hosoda, et al., 2013; Mamiya, et al., 2018). However, for bilinguals born in bilingual societies, they learn L2 at an early age, and the stimulus of the two languages are long-term and continuous. Only a few DTI studies on bilingualism recruited long-term bilinguals who learned L2 at an early age (Anderson, et al., 2018; Luk, et al., 2011; Singh et al., 2017). Anderson et al. (Anderson, et al., 2018) and Singh et al. (Singh et al., 2017) respectively recruited lifelong bilingual samples from Canada and Hindi, and matched them well with the monolingual samples. They both reported bilinguals had higher AD in the SLF than monolinguals, and Singh et al. also reported bilinguals had higher MD in the SLF than monolinguals. This is inconsistent with the results of the studies focusing on late short-term bilinguals (Pliatsikas, et al., 2015; Rossi, et al., 2017; Schlegel, et al., 2012). It’s speculated that long-term immersion in a bilingual environment from an early age might induce different effects from relatively short-term L2 experience (Singh et al., 2017). In the world, there are countless regions with more than one language (www.uottawa.ca/clmc/), and nearly 66% of people are raised as bilingual speakers from childhood and use more than one language for a long time (Marian & Shook, 2012). The white matter structural characteristics of such long-term bilinguals are of our interest.
In addition, rare studies focused on logographic bilinguals. Literature (Li, et al., 2014) indicated that the language typology could affect the changes in bilinguals’ brain structure. Different from alphabetic languages, Chinese is a unique language best known for its logographic writing system (Deng, et al., 2013). Chinese characters have complex visual–spatial configurations (Cao, et al., 2013). The orthography-to-phonology mapping rule is extremely opaque in Chinese, while it’s relatively transparent in alphabetic languages (Cao, et al., 2017). In monolinguals, evidence from fMRI studies suggested that compared to alphabetic languages, mapping orthography to phonology in Chinese require extra involvement of cortex in the ventral stream (for details see the references (Bolger, et al., 2005; Sun, et al., 2011; Tan, et al., 2005)). For bilinguals, when processed as the L2 in the phonological tasks, compared to alphabetic language processing, Chinese processing also need greater activation in the ventral cortical regions similar to the regions required in Chinese processing for monolinguals (Cao, et al., 2017; Kim, et al., 2016). The ILF and IFOF are the two core tracts connecting the ventral cortex (Sarubbo, et al., 2016). Qi et al. (Qi, et al., 2015) found FA and RD of the right ILF could predict the Chinese achievement of native English speakers after short-term Chinese training. Besides, Cummine et al. (Cummine & Boliek, 2013) observed Chinese-English bilinguals had different DTI indices in the bilateral IFOF and right ILF from English monolinguals. These two studies suggested that using Chinese and another alphabetic language together might be related to the white matter tracts in the ventral pathway. However, there has been no study focusing on the whiter matter characteristics of bilinguals using two kinds of logographic languages. In addition, none of the above two studies focused on long-term bilinguals who learn L2 at an early age. Thus, we aim to investigate how the long-term bilingual experience of two logographic languages may shape the white matter structure. We hypothesized that the white matter structure in the SLF, ILF and IFOF might exhibit an effect.
Mandarin and Cantonese are two major Chinese and both logographic languages. In Guangdong Province, China, nearly half of the population speak both Cantonese and Mandarin (Huiming & Zhe, 2016). For Cantonese-Mandarin bilinguals, Cantonese is their native language (L1) mainly for daily communication and local medium, while Mandarin is the L2 for formal situations. As the official language, Mandarin is popularized nationwide throughout China. Mandarin is the teaching language in Guangdong, and children learn how to pronounce and write in Mandarin from primary school or even earlier. In terms of linguistic characteristics, Cantonese and Mandarin share the same set of characters and have similar grammatical structures (Tardif, et al., 2009). However, they shared the same pronunciation for only 21.5% of characters (Li, 1990). Thus, although Cantonese is defined as a dialect according to socioeconomic factors, it is still deemed as an individual language in the region of psycholinguistics (Chen, et al., 2004; Tardif, et al., 2009). Evidence from behavioral studies suggested that Cantonese-Mandarin bilinguals performed different phonological processing skills compared to their Mandarin monolingual peers over a wide age span (Chen, et al., 2004; Li, et al., 2011). As reviewed above, the SLF is the most recognized tract involved in phonological processing, while the ventral stream including the ILF and IFOF also play a role in speech comprehension and orthography-to-phonology mapping. It’s assumed that the white matter structure in SLF, ILF and IFOF might be different between Cantonese-Mandarin bilinguals and Mandarin monolinguals, and the different structure might have a relationship with phonological processing.
In the current study, we used the Tract-Based Spatial Statistics (TBSS) analysis to test our hypothesis. Tractography was conducted to create a binary mask of tracts of interests (TOI) for TBSS analysis to improve the anatomical accuracy and the power to detect significance (Hamalainen, et al., 2017). We measured the phonological processing skills and performed a post-hoc correlation between the phonological processing skills and the significant clusters revealed in the group-wise comparison. The post-hoc comparison could help us better understand the white matter plasticity induced by Cantonese-Mandarin phonological processing. The aims of the current study were: 1) to test whether the white matter structure in phonology-related pathways including SLF, ILF and IFOF is different between Cantonese-Mandarin bilinguals and Mandarin monolinguals. 2) to explore the relationship between the phonological processing skills and the significant clusters in the white matter structure.