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  • The experimental procedure was approved by the IRB of the Institute of Psychology, Chinese Academy of Sciences. All participants provided written, informed consent before taking part in the fMRI experiment. A total of sixteen healthy proficient early Chinese -Korean bilingual volunteers took part in the fMRI experiment. Data from two subjects were excluded from analysis for falling asleep during the scan. The mean age of the 14 remaining subjects was 22 years (range = 19–24; 13 females and 1 male). All subjects were Chinese whose first language was Korean. All subjects acquired the spoken language of their first language (Korean) and second language (Chinese) before 5 years of age and learned to read in these two scripts before 9 years of age when they were in elementary school. We examined the proficiency of these two languages in each subject. When tested, all subjects were able to show mastery on more than 3000 Chinese characters which are necessary for normal reading. They were able to speak in Chinese and read aloud an article from Chinese newspaper, proficiently. All participants were undergraduate students in Beijing. Chinese was the language used in most courses and examinations (seven of the fourteen participants majored in Korean and they had half of their courses taught and examined in Korean). Chinese was the official language and most students in their universities spoke Chinese. Both Chinese and Korean were used in their normal life. Therefore, all the participants in our study were proficient early bilinguals. All participants were fully right-handed and had normal or corrected-to-normal vision. None had neurological or psychiatric history. There were four categories of stimuli used in our study: single Chinese characters, single Korean characters, unfamiliar Chinese faces and line drawings of common objects. Written characters were compared with faces and line drawings to reveal category selectivity for visual words in the VWFA [6], [12], [33], [34]. Face and line drawings are thought to have a similar level of visual complexity with words. In particular, faces have the best category selectivity in the fusiform area known as the Fusiform face area (FFA) [35] and can serve as a good contrast stimuli. There were 60 images for each of the four categories of stimuli. Chinese characters used had high frequencies (>300 per million) and each had 6–8 strokes; each Korean character was chosen from primary Korean vocabulary and also had 6–8 strokes; line drawings of objects included buildings, tools and furniture; images of faces were black-and-white and half were male. The visual angle of each image was about 1.8 degrees in width and 2.4 degrees in height. The stimuli were presented in separate blocks, in a block-design paradigm with a simple content-irrelevant position judgment task. Each block lasted 20 s and consisted of 20 stimuli of the same category. Each stimulus was presented for 250 ms with an inter-stimulus interval (ITI) of 750 ms. A central fixation point in a rectangular area with a visual angle of 6.4 degrees in width and 4.6 degrees in height was present throughout each run. Participants were required to fix their eyes on the center point throughout each run. All stimuli pictures appeared pseudo-randomly in the rectangular area within each block. The center of each stimulus was slightly shifted from the center fixation point and participants were asked to judge whether the center of the picture was to the right or the left of the fixation point and respond with left or right key. Each run contained 12 blocks with 3 blocks for each category of stimuli. There was a 20 s interval between blocks and in the beginning and end of the run when participants only fixated on the fixation point and no task was required. Each run lasted 500 s. Each participant was scanned for 4 runs (Figure 1). Figure data removed from full text. Figure identifier and caption: 10.1371/journal.pone.0022765.g001 Schematic depiction of the experimental paradigm.In block-design runs, subjects viewed Chinese characters, Korean characters, unfamiliar faces and line drawings in separate blocks. In each block 20 stimuli were presented and each stimulus was presented for 250 ms with an inter-stimulus interval of 750 ms. Each run contained 12 blocks with 3 blocks for each category of stimuli. Brain images were obtained on a 3T Siemens Trio scanner with an eight-channel phase-array coil at Beijing MRI Center for Brain Research. The stimuli were generated on a PC and projected via a LCD projector onto a tangent screen located in the scanner. Subjects viewed the stimuli through a mirror located above their eyes. Functional images were acquired with an EPI sequence with parameters as follow: 13 slices approximately parallel to the base of the temporal lobe and covering the occipital lobe,occipito-temporal area and most of the temporal lobe; 4.0 mm slice thickness with no gap; field of view, FOV = 220×220 mm2; Matrix = 64×64; repetition time, TR = 2000 ms; echo time, TE = 30 ms; flip angle, FLA = 90°. For each subject, a high-resolution 3D structural data set (3D MPRAGE; 1×1×1.33 mm3 resolution, 144 slices and 1.33 mm slice thickness with no gap) was acquired for localization and visualization of the functional data in the same session after the functional runs. Parameters were as follows: FOV = 240×240 mm2, Matrix = 256×256, TR = 2530 ms, TE = 3.37 ms, IT = 1100 ms, FLA = 12°. Data were analyzed with BrainVoyager QX (Brain Innovation) software. Functional volumes in all runs for each participant were preprocessed, including slice scan timing correction, 3D motion correction and temporal filtering (High pass filter 0.006 Hz). All participants' head motion within any fMRI run was less than 2 mm. The functional data were then aligned with the anatomical data and transformed into the Talairach space. The voxel size of functional data was resampled to 1×1×1 mm3. The change of BOLD signals induced by Chinese characters and Korean characters were calculated against the fixation blocks for each participant. Activation maps induced by Chinese characters and Korean characters were generated using a GLM (general linear model) procedure at the individual level in a common Talairach space. Partly because of the mixed varieties of objects used in the line drawing stimuli, contrasting the characters with line drawing stimuli yielded less consistent activation map in the fusiform region. Thus we compared the activation of characters to that of the faces. The area that responded more strongly to the Chinese characters than faces in left occipito-temporal area was defined as VCFA (Visual Chinese character Form Area) and the area that responded more strongly to the Korean characters than faces was defined as VKFA (Visual Korean character Form Area). Further quantitative analysis was carried out in these two areas (VCFA and VKFA). We used individual analyses to conduct a within-subject comparison of these two scripts to avoid potential confounding factors in comparison of different subject groups. We compared the activation peak point distance, the overlap and the voxel numbers between VCFA and VKFA and response amplitude of Chinese and Korean characters in these two regions. Moreover, we also performed a multi-voxel pattern analysis (MVPA) of the data, to examine the detailed spatial pattern of activity across voxels [36]. We used the data of the first and third runs to define the ROI and data from the second and the fourth runs for correlation based MVPA. First, in each subject, with a GLM based procedure, we contrasted the Chinese characters and Korean characters together with fixation. We took the point with the highest statistical value (t value) as a center to draw a sphere with 6 mm radius. All the voxels within this sphere were included for MVPA analysis. Then, using the correlation based multi-voxel pattern analysis (MVPA) [37], we calculated the correlation coefficients between the pattern of response evoked by each category during the second run and the pattern of response evoked by each category during the fourth run. Our main interest is to investigate how similar the pattern of activation was between Chinese and Korean characters (C-K). In order to make this evaluation, we also obtained four types of within-category correlation coefficients: (Chinese-Chinese, Korean-Korean, Linedrawing-Linedrawing and Face-Face) and five between-category correlation coefficients (Chinese-Linedrawing; Korean-Linedrawing, Chinese-Face, Korean-Face, and Face-Linedrawing). Thus the correlation coefficient C-K could be evaluated against a set of within-category coefficients and a set of between-category coefficients.
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