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Distinct activated cortical areas and volumes in Uygur-Chinese bilinguals

Objective The aim of this study is to evaluate variations in cortical activation in early and late Uygur-Chinese bilinguals from the Xinjiang Uygur Autonomous Region of China. Methodology: During a semantic judgment task with visual stimulation by a single Chinese or Uygur word, functional magnetic resonance imaging (fMRI) was performed. The fMRI data regarding activated cortical areas and volumes by both languages were analyzed. Results The first language (L1) and second language (L2) activated language-related hemispheric regions, including the left inferior frontal and parietal cortices, and L1 specifically activated the left middle temporal gyrus. For both L1 and L2, cortical activation was greater in the left hemisphere, and there was no significant difference in the lateralization index (LI) between the two languages (p > 0.05). Although the total activated cortical areas were larger in early than late bilinguals, the activation volumes were not significantly different. Conclusion Activated brains areas in early and late fluent bilinguals largely overlapped. However, these areas were more scattered upon presentation of L2 than L1, and L1 had a more specific pattern of activation than L2. For both languages, the left hemisphere was dominant. We found that L2 proficiency level rather than age of acquisition had a greater influence on which brain areas were activated with semantic processing.


introduction
The ability to acquire and use language is a unique and fundamental characteristic of humans. Bilinguals can be categorized based on the age of acquisition of their second language (L2). Early bilinguals learn both languages starting from infancy, whereas late bilinguals learn their L2 later than their first language (L1).
In contrast, some functional MRI (fMRI) studies showed that there is a shared neurological network for L1 and L2 in bilinguals [4][5][6]. language is an alphabetic language with eight vowels and 24 consonants and is written from right to left [7]. The Chinese language is a visually ideographic language written in a rectangular space, where the shape, sound, and meaning are all integrated. Thus, Chinese and Uygur languages are significantly different in the way they are written and read.

Study design and language stimulation scheme
All subjects were test-naïve prior to the start of the study. On day one, subjects were visually stimulated with monosyllabic notional Uygur words, and on day two, they were visually stimulated with Chinese characters delivered in the same manner. In the fMRI task presentation, a block was composed of alternating resting phases (with a non-word cognitive task) and stimulation phases (with a word cognitive task). Each block consisted of 16 randomly presented Chinese characters or Uygur words (24s) and the control condition (24 s). Each   partly activated (p < 0.001). Early bilinguals also exhibited activation in bilateral superior frontal gyri (BA6), right inferior occipital gyrus (BA18), and the insular cortex. The left hemisphere activation volume was significantly larger than the right (p < 0.05) (Fig. 3).
Comparison of brain activation with the Uygur or Chinese language In early and late bilinguals, there was extensive overlap in cortical areas activated by both L1 and L2 stimulation (Table 1)

Discussion
In this study, we investigated differences in Consistent with our data, a study comparing English-Spanish bilinguals during a semantic judgment task concluded that the same neurological structures were activated in bilinguals when semantically processing both L1 and L2 [5]. These results indicated that the left inferior frontal gyrus is a very important area for semantic implementation and semantic judgment processes.
In the current study, we found that the left middle temporal gyrus of both early and late bilinguals was activated during semantic processing of the Uygur language. Activation of the right temporal gyrus was different between bilinguals, with the right superior temporal gyrus activated in early bilinguals and the right middle temporal gyrus activated in late bilinguals. There was no meaningful activation of this area during semantic judgment with the Chinese language. By studying brain language function areas using a visual input vocabulary task and a listening vocabulary phonology task, Booth et al. [9] found that the superior temporal gyrus (BA22, BA42) was activated when processing text phonemes, whereas the left middle temporal gyrus was activated when performing semantic processing. In monolingual English language speakers, activation of the left middle temporal gyrus is linked to phoneme elements [10]. Modern Uygur uses the Arabic alphabet and it read left-to-right in the upper-to-lower direction and follows the rule of sound to shape transformation. When identifying alphabetic writings, pronunciation can be directly spelled out using the alphabetic composition before lexical processing. When performing semantic tasks, the phonetic system is automatically activated in order to activate, extract, and assist semantic judgment. The Uygur language is an alphabetic text and is therefore processed phonetically. In contrast, Chinese is a pictographic expression language, and its stimulation tends to automatically activate Comparison of activated cortical volume by L2 in early and late Uygur-Chinese bilinguals Perhaps the extent and strength of cortical activation by L2 is not related to the age of acquisition but to the degree of L2 proficiency [5,18,19]. However, other studies have found that some cortical areas are activated in a language-dependent manner, because the L2 age of acquisition is different [20,21]. All fluent Uygur-Chinese bilinguals in our study exhibited overlapping activated cortical areas when processing L2. This is consistent with the hypothesis that L2 proficiency level rather than age of acquisition affects activated brain areas during semantic processing. We did not detect any statistically Figure 5. Overlapped brain images of averaged activation areas with Uygur and Chinese language stimulation in late bilinguals. Note: Chinese language is red; Uygur language is orange; yellow indicates voxels that passed all statistical criteria for activation during both Uygur and Chinese language tasks.