What to do if we have nothing to rely on: Late bilinguals process L2 grammatical features like L1 natives

Journal of Neurolinguistics 40 (2016) 1e14

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Research paper

What to do if we have nothing to rely on: Late bilinguals process L2 grammatical features like L1 natives Hao Yan a, b, c, *, 1, Yu Mei Zhang d, 1, Min Xu a, Hong Yan Chen e, Yong Hui Wang f, ** a

Neuroimaging Laboratory, School of Biomedical Engineering, Shenzhen University Health Science Center, Shenzhen 518060, China Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China c Departments of Psychology and Linguistics, Xidian University, Xi'an 710071, China d Department of Neurology, Beijing Tiantan Hospital, Capital Medical University, China National Clinical Research Center for Neurological Diseases, Stroke Centre, Beijing Institute for Brain Disorders, and Beijing Key Laboratory of Translational Medicine for Cerebrovascular Disease, Beijing 100050, China e Department of Radiology, Beijing Tiantan Hospital, Capital Medical University, Beijing 100050, China f School of Psychology, Shaanxi Normal University, Xi'an 710062, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 December 2015 Received in revised form 13 April 2016 Accepted 13 April 2016

It is predicted that bilinguals rely on their first language (L1) to process the second language (L2). However, it remains largely unknown as to how the brain processes unique grammatical features of L2. To answer this question, we explored how Chinese-English bilinguals recognized English inflected verbs that are lacking in Chinese. By using a semantic consistency judgment task, we found that highly proficient late bilinguals processed dichotomic regular and irregular inflections the way English monolinguals did. Behaviorally, regular past tense verbs significantly primed recognition of verb stems, but irregulars did not enhance recognition of their simple forms. Brain imaging results showed that, in contrast to irregulars, late bilinguals additionally employed the procedural memory system of the inferior frontal gyrus (IFG), superior temporal gyrus (STG), middle temporal gyrus (MTG), supramarginal gyrus (SMG), cerebellum, and basal ganglia (BG) to process regulars. Such a differential brain activity pattern elicited by L2 syntax and semantics was distinctive from the way Chinese-English bilinguals processed their L1 syntax and semantics, which supported Ullman's declarative/procedural model. Native-like brain activity elicited by L2 grammatical features suggested that unique language features were processed through specialized neural substrates by late bilinguals either. Meanwhile, we also found that late bilingual learners with a high L2 proficiency still employed the cognitive control system (the BA47 and dorsal lateral prefrontal cortex, DLPFC) more heavily to process L2 syntax than L2 semantics. It supported the sensorimotor/emergentist (S/E) model which emphasized that cognitive control must be involved in L2 processing, and ran contrary to the fade-away prediction of the cognitive control process of the Convergent Hypothesis. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Language features English past tense Late Chinese-English bilinguals Event-related fMRI

* Corresponding author. Neuroimaging Laboratory, Shenzhen University Health Science Center, Shenzhen, 518060, China. ** Corresponding author. E-mail addresses: [email protected] (H. Yan), [email protected] (Y.H. Wang). 1 The authors contribute equally to this work. http://dx.doi.org/10.1016/j.jneuroling.2016.04.002 0911-6044/© 2016 Elsevier Ltd. All rights reserved.

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1. Introduction 1.1. Theoretical rationales of L2 processing It is becoming clear that the acquisition age of a second language (L2) affects neural activities during language processing (Dehaene et al., 1997; Frenck-Mestre, Anton, Roth, Vaid, & Viallet, 2005; Friederici, Steinhauer, & Pfeifer, 2002; Klein, Watkins, Zatorre, & Milner, 2006; Mahendra, Plante, Magloire, Milman, & Trouard, 2003; Perani et al., 2003; Weber-Fox & Neville, 2001; Wartenburger et al., 2003). In particular, there are three theoretical frameworks that have been offered to account for age of acquisition (AoA) effects in bilingual learners. The sensorimotor/emergentist (S/E) model (Hernandez, Li, & MacWhinney, 2005; MacWhinney, 2004) proposes that linguistic information which is learned earlier in life is dependent upon sensorimotor analysis and recruits phono-articulatory brain regions (Hernandez & Li, 2007), but later learned information preferentially enrolls brain areas underlying semantic and executive cognitive control (Hernandez, Hoffman, & Kotz, 2007). To be specific, Waldron and Hernandez (2013) claimed that early-learned information is preferentially processed in brain regions such as the putamen, anterior insula, inferior frontal gyrus, and motor cortices; later acquired linguistic information is mediated through regions involved in executive functioning, such as dorsolateral prefrontal cortex (DLPFC), and other frontal regions such as anterior inferior frontal gyrus (BA47). It postulates that late bilingual learners have already formed a consolidated and entrenched linguistic system in place, and they would employ direct lexical memory access strategy to a greater degree with special reliance on executive cognitive control and working memory circuits. Similarly, the Convergence Hypothesis (Abutalebi, 2008; Abutalebi & Green, 2007) predicts that late bilinguals utilize similar regions and networks of L1 to perform tasks in L2, but with additional reliance on cognitive control due to unmatched language proficiency. Generally, these two models can be categorized as the “single network hypothesis” (Abutalebi & Green, 2007), which proposes that adult individuals use their first language (L1) as a reference when processing L2 (Waldron & Hernandez, 2013). They share the opinion that late bilingual learners would display a greater extent of brain activity in regions implementing cognitive control and L1 processing. The disparities of the two theories can be summarized as the S/E model highlights different cognitive control mechanisms in early and late bilinguals, and the Convergence Hypothesis predicts a gradual fadeaway of cognitive control. However, the two theories both neglected how late bilingual learners processed L2 grammatical features. Different from a strong assumption that L2 must rely on L1, the declarative/procedural (D/P) model provided an alternative explanation for the AoA effects. The D/P model is based on the claim that language depends on a memorized “mental lexicon” and a computational “mental grammar” (Chomsky, 1965; de Saussure, 1959; Pinker, 1994). The mental lexicon is defined as a repository of stored information, including arbitrary sound-meaning pairings, word-specific information of grammatical properties, and words’ unpredictable morphosyntactic forms. As for the rules of grammar which are characterized by language regularities, Ullman (2001b) defines them as what underlie mental operations that manipulate words and abstract representations to construct phrase, sentences, and complex words, such as “walked”. According to the D/P model, the declarative memory system underlies the mental lexicon, whereas the procedural memory system subserves aspects of the mental grammar (Ullman, 2014). Under the D/P model of L2, the grammatical/procedural system is less available than lexical/declarative memory at later ages, especially after puberty (Ullman, 2001a, 2004). Fortunately, the availability of the lexical/declarative system allows it to compensate for the dysfunctional grammatical/procedural system, as some of the same or similar types of knowledge can be acquired by both systems. Therefore, adult L2 learners rely more heavily on declarative memory, not only for storing idiosyncratic lexical knowledge but also for memorizing complex forms and “rules” typically in a pedagogical context at the beginning. Since declarative memory provides a database from which grammatical rules can gradually and implicitly be abstracted by the procedural memory system, rule governed aspects of grammar should gradually rely on native like aspects of grammatical processing at higher levels (at least to some extent). Even though L2 bilinguals may not attain native-like language proficiency, native-like neural mechanism underlying L2 grammar can be finally achieved (Bowden, Steinhauer, Sanz, & Ullman, 2013). Although Ullman's theory provides a possible explanation about how late bilinguals process unique features of L2, its validity has not been demonstrated with neuroimaging evidence. In addition, the D/P model overlooked the role of cognitive control in L2 processing, which contradicted the observation that both early and late bilinguals utilize the cognitive control network differently or more efficiently than monolinguals (Abutalebi et al., 2012; Bradley, King, & Hernandez, 2013; Marian, Chabal, Bartolotti, Bradley, & Hernandez, 2014). To supplement the neurocognitive theory of L2 processing, it was quite worthy probing that (1) what to do when late bilinguals process unique L2 features which are lacking in L1, (2) and what is the role of cognitive control and semantics in processing unique L2 grammatical features. Given that syntax, especially morphosyntax, is more sensitive to AoA than semantics (Hernandez & Li, 2007), the current study planned to adopt tasks of English regular/irregular inflected verbs with late bilinguals. Since language similarity plays a role in the nature of neural activity (De Diego Balaguer, Costa, Sebastian-Galles, Juncadella, & Caramazza, 2004), the AoA effect should be more obvious between languages that are in stark contrast. The Chinese language lacks in grammatical morphology, as subject-verb agreement in Chinese sentences is not required (Chen, Shu, Liu, Zhao, & Li, 2007). It thus embodied profound implications if determining what brain areas Chinese learners would

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recruit to overcome the entrenchment from L1 for English morphosyntax. These results should help to elucidate whether cognitive control, or the procedural memory system, or both could fully explain L2 processing differences. 1.2. Dichotomic neural representatives of English regular and irregular inflected verbs In the English language system, regularly inflected verbs are produced with an affix “-ed” at the end of a stem, yielding concatenated forms such as ‘‘walk þ ed’’. Irregularly inflected words (no more than 200 yet more frequently used) such as ‘‘sang’’, cannot be decomposed into stems and affixes. As a certain dual mechanism model, the Words and Rules theory (Pinker, 1991, 1999; Pinker & Prince, 1988) postulates that a symbolic, rule-based system (procedural memory) dedicates to regular inflections for existing and novel forms, and a separate associative memory (declarative) works to store individual verbs (including the irregular past-tense forms). Even though connectionists alternatively differentiate regular and irregular verbs on a phonologicalesemantic continuum characterized by greater reliance on phonological processing for regular verbs and more involvement of semantic mechanisms for irregular verbs (MacWhinney & Leinbach, 1991; Marchman, 1997; Desai, Conant, Waldron, & Binder, 2006), the Words and Rules theory, which recognizes regulars and irregulars as representatives of grammatical/procedural and lexical/ declarative information (Ullman, 2001b), has been supported by a large number of behavioral (Pinker, 1991; Sereno & Jongman, 1997; Serratrice, Joseph, & Conti-Ramsden, 2003; Ullman, 1999), ERP (Luck, Hahne, & Clahsen, 2006; Newman, Ullman, Pancheva, Waligura, & Neville, 2007; Rodriguez-Fornells, Clahsen, Lleo, Zaake, & Munte, 2001), neuropsychological (Friederici et al., 2002; Tyler, deMornay-Davies et al., 2002; Longworth, Keenan, Barker, Marslen-Wilson, & Tyler, 2005; Tyler, Randall, & Marslen-Wilson, 2002; Ullman et al., 1997; Ullman, 2004; Ullman & Pierpont, 2005; Walenski, Mostofsky, & Ullman, 2007), and brain imaging studies (Bozic, Marslen-Wilson, Stamatakis, Davis, & Tyler, 2007; Tyler, Stamatakis, Post, Randall, & Marslen-Wilson, 2005). Previous research reported that accessing lexical representations from regular stems and idiosyncratic irregulars involves only neural representations of morphemic form and meaning (Marslen-Wilson & Tyler, 2007). In contrast, processing regularly inflected words may additionally activate the left IFG and posterior temporal regions (pSTG), which are connected by the arcuate fasciculus (AF) either directly or via posterior parietal cortex (IPL, SMG) to form a dorsal processing stream (Tyler, Stamatakis et al., 2005). These studies further specified that analysis of English regulars like “walked” involves access of lexical content associated with the stem “walk” (primarily mediated by the temporal lobe systems, the STG/MTG), phonological analysis of the {-d} morpheme (primarily mediated by the pSTG), and decoding of grammatical implication of the {-d} morpheme (primarily mediated by the IFG and basal ganglia, BG). Meanwhile, the anterior cingulate cortex (ACC) plays a monitoring role in morphosyntactic parsing and lexical access processes (Tyler, Stamatakis et al., 2005). In reference to a good history of neural mechanisms of syntax and semantics with monolinguals, if late Chinese-English bilinguals manipulated native-like neural substrates to process English regular inflected verbs, these areas would be significantly activated. It was the major concern in the current study. 1.3. Overlapped brain mechanisms of Chinese syntactic and semantic processing Void of activation difference between syntax and semantics served as a good reference for our study. Sentence comprehension in Chinese relies on contextual semantic processes but not syntactic analysis. Luke et al. adopted a semantic/ syntactic plausibility judgment task, and found that processing Chinese syntax and semantics relied on overlapping brain areas of declarative memory. This pattern is different from that of English, which are characterized by distinct brain regions subserving syntax and semantics (Luke, Liu, Wai, Wan, & Tan, 2002). Researchers interpreted unique Chinese syntax processing with a conjecture that Chinese syntax and semantics are closely inter-related with no transparent correspondence between lexical category and syntactic functions. Later on, this claim has been supported by another neuroimaging study at the lexicon level that neural representatives of Chinese nouns and verbs consist of overlapping brain areas in distributed networks, in contrast to a popular conviction that verbs in English and other Indo-European languages can elicit stronger activation in the frontal cortex, and nouns the temporal-occipital cortices (Li, Jin, & Tan, 2004). These results imply that processing Chinese sentences involves less grammatical computation, but more semantic inference in different contexts. Moreover, the finding of overlapping neural representatives of Chinese syntax and semantics has been further supported by a good many studies using event-related potential (ERP) with exquisite temporal resolution. Syntactic processes have been associated with a late centro-parietal positivity, referred to as P600 (Friederici & Weissenborn, 2007). Even though such a hallmark of syntax is always reported in Indo-European language studies, no significant P600 effect is observed during Chinese syntactic violation tasks (Ye, Luo, Friederici, & Zhou, 2006; Ye, Zhan, & Zhou, 2007; Yu & Zhang, 2008). Therefore, fMRI and ERP studies have been convergent to demonstrate that sentence comprehension in Chinese would naturally rely on semantic analysis. Hence, according to the D/P model, both Chinese syntax and semantics processing rely on the declarative memory system. 1.4. Research hypothesis and experiment design The null hypothesis of the current study is that direct contrast of regular and irregular English past tense forms yield no significant activation difference, let alone the contrast between inflections and simple words. It would suggest that Chinese

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learners relied on their L1 literacy and engaged an equal or no cognitive control to process L2 syntax and semantics. The alternative hypothesis was that late proficient bilinguals adopted additional brain areas to process L2 grammatical information compared to declarative information. In another words, we could observe that Chinese learners employed the STG, MTG, IFG, BG, and ACC to process English regulars (the procedural memory system which supported the D/P model), or the BA 47 and DLPFC (the cognitive control system which supported the S/E hypothesis), or both. In the current study, we adopted a priming paradigm (Tyler, Randall et al., 2002; Tyler, Stamatakis et al., 2005) and used four kinds of monosyllabic word pairs: the regular inflected verb, the irregular inflected verb, the decomposable word, and the base form of a verb (a simple word). We selected monomorphemic words only, with a rationale that recognition of monomorphemic forms involves only access of a stored full form, such that no phonological parsing is required. Therefore, the distinction of regulars and irregulars allows concentration on the core issue that access of regularly inflected verbs depends on the morphosyntactic and phonological parsing process (Pinker & Ullman, 2002). To exclude the possibility that recognition of morphologically complex words are simply based on phonological analysis of overlapped orthography, we included a decomposable condition (e.g., brothel, which can be segmented into its base form “broth” and a meaningless phoneme “el”) to contrast with regulars. This kind of comparison has been widely employed in previous studies, but with different names for decomposable words, such as the nonmorphological ending condition (Rastle, Davis, & New, 2004) and the additional phoneme condition (Tyler, Stamatakis et al., 2005). Reading English regulars involves not only processes of segmentation and stem access like decomposable words, but also engages grammatical property retrieval and phonological analysis of suffix. Differential neural responses between regulars and decomposable words could attribute L2 regular/irregular distinction to rule-based information vs. semantics rather than greater phonological similarity/form overlap of primes and probes in the regular condition. In addition, previous behavioral studies demonstrated inhibitory effects of orthographically related word primes on target recognition (e.g., brothel-BROTH) in longer SOA conditions (Grainger, Cole, & Segui, 1991; Rastle, Davis, Marslen-Wilson, & Tyler, 2000), This inhibitory effect would help to identify cortical regions implementing cognitive control in the late Chinese-English bilingual brain. The current research set a routine control condition of simple words, which are regular past tense verbs in their simple present forms, as the baseline. 2. Methods 2.1. Subjects Sixteen Chinese-English right-handed late bilinguals (all exposed to English after 12 years of age) participated in this study. There were four criteria for inclusion in this study: (a) very high English proficiency; (b) right-handedness as assessed with the Edinburgh Handedness Inventory (Oldfield, 1971); (c) normal or corrected-to-normal vision and (d) no known history of neurological impairment. All the experimental protocols were approved by the Medical Research Ethics Committee of Tiantan Hospital. The experiment was also conducted in accordance with the Declaration of Helsinki. Informed consents were obtained from all participants prior to scanning, and 15 USD would be paid after the experiment. One participant was excluded from data analyses due to excessive head movements during fMRI scanning. The remaining 15 participants were 9 females and 6 males (mean age 23 years, ranging from 21 to 28 years), and received a minimum of 10 years of formal training in English throughout high school and university life in China. 2.2. Language competence assessment The College English Test, better known as CET with a history of 24 years, is a nationally standardized, English as a Foreign Language test in the People's Republic of China. The CET is obligatory for university students in China who are not English majors. It is also a prerequisite for the bachelor's degree, and preferred by many employers in China. The purpose of CET is to examine English proficiency of undergraduate and postgraduate students in China and ensure that they reach the required English levels in listening comprehension, vocabulary, reading & writing skills, and translation competence. Only the students who can obtain a CET mark above 540 are treated as proficient language learners, and are qualified to take part in the oral examination of CET. There are two levels of certificates in the exam system, the basis level of CET 4 and the advanced level of CET 6. Participants who took part in the current study all have obtained a high mark in CET 6 test (average score of 553.2 ± 35.2, full mark 710), and would be included after an oral check by well-experienced English teachers. Based on self-reporting, all participants claimed that they have accepted formal pedagogical training of English in class for at least 10 years. 2.3. Materials and design We adopted a semantic consistency judgment task on visually presented word pairs. Participants were asked to judge if the first presented word (the prime, in lowercase letter form) was semantically consistent with the second presented word (the probe, in capital letter form). The study rested on a two-factorial design of repetitive measurements. The first factor was pair-word relations (semantically same vs. semantically different), and the second factor was the word type of primes (the regular inflected forms, the irregular inflected forms, the decomposable words, and the base forms of verbs). In all, there were

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eight test conditions, with 30 word-pairs in each condition. In the semantically same condition, word pairs of past tense forms were the regular inflected forms with their stems (e.g., walked-WALK), and the irregular inflected forms with its present tense forms (e.g., taught-TEACH). We had the decomposable word and the base form of a verb primed by itself in capital forms (e.g., settle-SETTLE, and solve-SOLVE). In the semantically different condition, the probe stimuli consisted of different words belonging to each condition (e.g., showed-FILL; fought-SAY; blame-DROP), except that prime and probe pairs in the decomposable condition were the nested words and their stems (e.g., thank-THAN). In all, there were 90 words selected for each condition. Since there lacks a word-frequency corpus specifically targeting at Chinese-English bilinguals’ language usage, the current research turned to choose simple and familiar words from vocabularies specified by the College Entrance Examination and College English Test band 4 (CET 4), which are both compulsory examinations for Chinese college students. To the corpus of contemporary American English (COCA), the frequency ranges of words in four conditions are not significantly different according to the heterogeneity analysis assessed with chi-square statistics (c2 ¼ 18.00, p ¼ 0.456). Particularly, it is difficult to find sufficient orthographic related verb pairs in a small word pool of no more than 4200 words. A recent fMRI study, concerning grammatical features at the lexical level, reported that late Chinese-English bilinguals recruited largely overlapping neural networks to process nouns and verbs in both Chinese and English (Yang, Tan, & Li, 2011). The current research was thus capable of employing both verbs and non-verbs (23/37 primes) to make up the stimuli battery in the orthographic condition, which are also routine practices in previous studies (Justus, Yang, Larsen, de Mornay Davies, & Swick, 2009; Rastle et al., 2004). 2.4. Procedure The task was implemented using E-Prime 2.0 (Psychology Software Tools, Inc), with experiment materials projected on a screen and displayed in random. Measures were taken to guarantee that all participants were able to see the stimuli presented in the middle of the vision field through a mirror. The whole experiment was composed of three sessions, with 80 trials each. Every trial began with a 500 ms “þ” fixation, followed by the prime stimuli. Both the prime and probe words appeared for 1000 ms, intercepted by a 400 ms blank. To make sure participants had sufficient time to respond, there was a 1600 ms blank after the probe. Participants were asked to make a semantic consistency judgment of the successive word pairs by means of two accessory key buttons (one for each hand). Response hand assignments were counterbalanced across subjects. Participants were instructed to respond as quickly and accurately as possible after the probe appeared. They may have a short rest of one minute between two sessions, motionless. 2.5. fMRI data acquisition The images were acquired on a Siemens Trio 3T MRI Scanner at Beijing Tiantan Hospital. A custom-built head holder was used to prevent head movements. Thirty-two axial slices (FOV ¼ 240 mm  240 mm, matrix ¼ 64  64, thickness ¼ 5 mm), parallel to the AC-PC plane and covering the whole brain, were obtained using a T2*-weighted single-shot, gradient-recalled echo planar imaging (EPI) sequence (TR ¼ 2000 ms, TE ¼ 30 ms, flip angle ¼ 75 ). Prior to the functional run, high-resolution structural images were also acquired using 3D MRI sequences with a voxel size of 1 mm3 for anatomical localization (TR ¼ 2700 ms, TE ¼ 2.19 ms, matrix ¼ 256  256, FOV ¼ 256 mm  256 mm, flip angle ¼ 7, slice thickness ¼ 1 mm). 2.6. fMRI data analysis Functional imaging analyses were carried out using statistical parametric mapping (SPM5, http://www.fil.ion.ucl.ac.uk/ spm/). The images were first slice-timed and then realigned to correct for head motions (subjects who had head movements exceeding 1.5 mm on any axis and head rotation greater than one degree would be excluded). The image data was further processed using spatial normalization based on the MNI space and re-sampled at 2 mm  2 mm  2 mm. Finally, the functional images were spatially smoothed with a 6 mm full-width-at-half maximum (FWHM) Gaussian kernel. Images in both semantic consistent and inconsistent conditions were combined. The statistics were color-coded and mapped in MNI space, while brain regions were estimated from Talairach and Tournoux after adjustments for differences between MNI and Talariach coordinates with a nonlinear transform. Parameter estimates for the fMRI signal induced by the four stimuli conditions in each voxel were obtained by a general linear model approach. Group analysis was carried out using a mixed-effects approach with threshold at p < 0.05 (FDR corrected). When there were no significant results at this conservative threshold, we would report activation results with a lower threshold in regions of interest (Rodd, Longe, Randall, & Tyler, 2010; Tyler, Marslen-Wilson, & Stamatakis, 2005). To confirm validity of the statistical differences observed in direct contrasts, a series of areas showing an increase in mean signal change were subjected to a subsequent region-of-interest (ROI) analysis. Mean activation from the peak voxel determined in the direct contrasts was calculated for each participant. These values were then used in a repeated-measures analysis of variance (ANOVA) of mean signal change. Spherical ROIs (10 mm) were defined centering on the peak of activation cluster using the appropriate contrasts. ROIs in this study were all language related areas (Friederici, 2002, 2011), including the left inferior frontal gyrus (BA44/45/47), left superior temporal gyrus (BA38/22/39), left inferior parietal lobule (the SMG, and the angular gyrus), left middle temporal gyrus (BA21/17), and basal ganglia (the caudate and putamen).

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3. Results 3.1. Behavioral study Response latency and accuracy data entered into a 2 (pair word relations: semantically same and semantically different)  4 (prime types: the regular inflected, irregular inflected, decomposable words, and simple words) analysis of variance (see Fig. 1). Response latency in the probe task was used as the primary dependent variable for the analysis of priming effects. Priming effects were calculated by comparing response times to targets preceded by a related prime against response times for targets after unrelated primes of the same condition. Trials with erroneous responses in the prime task (1.67%) were excluded from the analysis. A repeated measures analysis of variance (ANOVA) showed significant main effects of both word relations (F (1, 14) ¼ 50.93, p < 0.001), and the prime type (F (3, 42) ¼ 11.38, p < 0.001). Processing morphologically related words (regular past tense verbs) took less time (Mean ± SD ¼ 603.78 ms ± 96.99 ms) than the decomposable primes (633.81 ms ± 119.15 ms; t(29) ¼ 2.26, p ¼ 0.03). But the mean response latency of regular past tense verbs was equal to that of the base form condition (603.52 ms ± 100.49 ms; t(29) ¼ 0.028, p ¼ 0.98). In contrast, recognition of irregularly inflected word pairs took the longest time (640.12 ms ± 100.58 ms): it cost more time than both the regular infected verb (t(29) ¼ 3.79, p ¼ 0.001) and the control verb (t(29) ¼ 3.03, p ¼ 0.005). The two-way interaction was also significant, F (3, 42) ¼ 20.16, p < 0.001. Planned comparisons revealed a significant priming effect restricted to the condition of regular inflected verbs (t(14) ¼ 2.27, p < 0.05;

Fig. 1. Response time and error rate in recognizing four word pairs. (1) Probes of three conditions significantly primed the targets, except the irregularly inflected word pairs. The significant inhibition effect was found in the decomposable word condition. Subjects made significantly more errors in cases of irregulars and the decomposable words, which were caused greater competitions by unexpected words—irregular past tense verbs inhibited present tense forms, and the decomposable words the nested words. (2) “*” indicated significant threshold of p < 0.05, and “**” meant p < 0.001 after planned comparison between conditions.

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MD ¼ 34.97 ms, SD ¼ 59.70 ms), the control condition (simple verbs) (t(14) ¼ 5.62, p < 0.001; MD ¼ 81.25 ms, SD ¼ 56.04 ms), and the decomposable condition (t(14) ¼ 9.57, p < 0.001; MD ¼ 138.85 ms, SD ¼ 56.19 ms). As assumed, we also observed a significant inhibition effect of the orthographically related words against simple words (t(14) ¼ 5.15, p < 0.001, MD ¼ 59.08 ms). In contrast, there was no significant facilitation in the condition when irregular inflected verbs served as primes (t(14) ¼ 0.139, p ¼ 0.89; MD ¼ 2.01 ms, SD ¼ 55.76 ms). The same ANOVA was conducted with error rates as the dependent variable. We nevertheless observed a significant main effect of both word relations (F(1, 14) ¼ 11.65, p < 0.005) and the prime types (F(3, 42) ¼ 14.81, p < 0.001), with a significant interaction effect of the two factors (F(3, 42) ¼ 24.26, p < 0.001). Subjects made most errors in recognizing irregulars (Mean ¼ 4.01%, SD ¼ 5.16%) than the other three conditions (t1(29) ¼ 3.34, p ¼ 0.002; t2(29) ¼ 3.86, p ¼ 0.001; t3(29) ¼ 2.26, p ¼ 0.03). Planned comparisons of error rates revealed significant difference in the irregular inflected verb condition (t(14) ¼ 6.04, p < 0.001), of which the higher error rate fell on the semantically consistent condition (7.8% ± 4.68% vs. 0.22% ± 0.86%). However, participants made more errors in the semantically inconsistent condition when the nested word was preceded by an orthographically related prime word (0.44% ± 1.17% vs. 2.44% ± 3.2%) at the significant level of p ¼ 0.045 (t(14) ¼ 2.20). The two significantly higher error rates reflected conflicts of an ignored target. The findings that decomposable words inhibited semantic access to its nested word and incurred significant higher error rates in the semanticinconsistent condition confirmed that the orthographic complex word was also decomposed in recognition. 3.2. fMRI study Contrasting regular and irregular inflected verbs with the control condition and then with each other lead to a large scale of brain activation in traditional language related brain areas (Fig. 2). Regular inflected verbs differed from control verbs (simple verb) in the perisylvian and the subcotical areas. A large scale of activation clustered in the pars opercularis (BA44) in both left (coordinates [57 12 14]) and right (coordinates [55 3 20]) inferior frontal gyrus, the left posterior triangularis area (BA45, [57 11 18]).), and bilateral dorsal lateral prefrontal cortex (DLPFC, [48 2 39] [51 13 32]). In the temporal lobe, both the inferior temporal gyrus (BA20/19) and the middle temporal gyrus (BA21/37) were activated bilaterally. No significant activation in the superior temporal gyrus (STG) was detected, and few differences were found in the subcortical cortex except the thalamus. However, direct comparison between irregulars and the control verbs yielded no significant activation (see details in Table 1). The contrast between regulars and irregulars revealed several isolated clusters of activation, most of which lay in the left frontal and temporal cortex, the cerebellum, and the subcortical areas (see Table 1 and Fig. 2). Together with brain regions in

Fig. 2. Selected significant activation after planned contrasts between regular vs. irregular and control conditions. (1) Direct contrast of regulars and the control verb yielded a strong activation in Broca's area (marked by green circles in the purple frame), which is commonly interpreted as an important syntactic analysis area. Comparing regular inflections and irregular inflections yielded three activation clusters (marked by green circles in white frames). The first activation center consisted of the ventral part of the IFG (the pars orbitalis, BA47) and its neighboring areas in the anterior part of the STG (BA38), which was in charge of word sequence and working memory; the second activation center covered the posterior part of the STG (BA22/39), the angular gyrus (BA39), and the supramarginal gyrus (BA40) in the inferior parietal lobule, tackling visually presented phonological information; the third activation loci were the basal ganglia in the subcortical region. (2) Abbreviations: IFG for the inferior frontal gyrus, STG for the superior temporal gyrus, SMG for the supramarginal gyrus, and IPG for the inferior parietal gyrus. (3) The color bar indicated the range of T values for the activations shown.

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Table 1 Foci after planned comparisons between conditions (df ¼ 14; p < 0.05, FDR corrected). Regular verbs vs. Irregular verbs Talairach x Frontal Cortex Inferior Frontal Gyrus BA 44 IFG BA 45 IFG BA 47 DLPFC BA 6/9/46 Temporal Cortex Superior Temporal Gyrus BA 38 Superior Temporal Gyrus BA 22 Superior Temporal Gyrus BA 39 Middle Temporal Gyrus BA 21/37/39 Parietal Cortex SMG BA 40 Inferior Parietal Lobule BA 40 Angular Gyrus BA 39 Subcortical Cortex Putamen Caudate Cygulate Gyrus BA 31/32 Cerebellum Anterior Posterior

L R L L L R

y

z

Regular verbs vs. Simple verbs

t

V

Talairach

Value

Voxels

x

53 28 51 36

26 23 6 5

21 15 44 51

3.90 5.15 6.90 5.19

7 29 181 313

L R L R L R L R

48

19

11

3.98

13

44 50 46 51 63 46

56 58 54 61 35 64

14 14 12 18 5 3

3.41 5.42 3.69 5.04 4.79 6.59

11 20 11 30 106 67

L L R L R

40 38 36 36

45 47 38 76

34 39 57 30

7.57 5.36 3.84 3.94

26 108 17 7

L R L R L R

16 22 16 8 18 10

14 17 16 3 43 13

1 1 1 13 28 36

5.13 6.00 5.87 6.47 4.67 3.37

255 232 168 157 37 1

L R L R

2 32 36 34

64 55 62 57

5 19 34 19

4.51 7.80 5.67 7.20

217 416 582 994

y

z

Decomposable words vs. Irregular verbs

t

V

Talairach

Value

Voxels

x

y

z

32

17

14

6.73

5

40

1

55

10.57

43

57 55 57

12 3 11

14 20 18

4.88 5.10 3.97

36 10 7

48 51

2 13

39 32

5.34 4.61

70 52

44 38

66 72

7 29

4.87 5.46

61 53

44 34 38

41 50 35

37 45 42

7.00 7.00 4.69

11 322 65

38

74

30

5.49

12

10 16 16 24

66 43 75 79

3 13 32 35

4.13 6.15 6.01 6.89

12 149 295 830

t

V

Value

Voxels

Displayed are activation results after planned contrasts in cortical areas along the Perysilvian fissure. Abbreviations: IFG— inferior frontal gyrus; DLPFC— dorsal lateral prefrontal cortex; STG— superior temporal gyrus; MTG— middle temporal gyrus; SMG— supramarginal gyrus.

the DLPFC covering the left premotor cortex (BA6/9) and the middle frontal gyrus (BA46), such a contrast also showed a robust activation in the left inferior frontal gyrus, with the maxima at the pars orbitalis of BA47 (peaking at [28 23 15]), extending superiorly into the pars triangularis at BA45 (peaking at [53 26 21]). The regular/irregular comparison also produced a significant cluster of activation in left and right STG, including both the anterior and posterior parts (BA38, BA22/39). Activity was also found in the angular gyrus (BA39), ventral inferior parietal lobe (BA40), and supramarginal gyrus (SMG, BA40) in the inferior parietal lobe, the middle temporal gyrus (MTG) at BA21/37, cingulate gyrus and parahippocampal gyrus. Moreover, subcortical areas that were strongly activated included the bilateral basal ganglia, insula, and thalamus. Both the anterior and posterior lobes of the cerebellum were also strongly activated. No areas were found significantly more active for the irregulars compared to the regulars even at a lower threshold, which is consistent with previous findings (Tyler, Stamatakis et al., 2005). ROI analyses in the frontal, temporal, and basal ganglia areas (see Fig. 3) showed that signal magnitude change presented significant main effects in only three ROIs. In BA44, both regular (t1(14) ¼ 3.731, p ¼ 0.002) and irregular (t2(14) ¼ 3.829, p ¼ 0.002) inflected verbs showed significantly greater signal change than that of control verbs; in the anterior part of the superior temporal gyrus (BA38), regular inflections produced significantly higher changes than both irregulars (t1(14) ¼ 2.926, p ¼ 0.011) and the control verbs (t1(14) ¼ 4.265, p ¼ 0.001), but not the decomposable verbs (t1(14) ¼ 1.343, p ¼ 0.201); in the posterior part of the STG (BA39), signal change of the regular verbs were also significantly greater than that of the control verbs (t1(14) ¼ 3.305, p ¼ 0.005) and the irregulars (t1(14) ¼ 2.145, p ¼ 0.050), but not the decomposable verbs (t1(14) ¼ 1.397, p ¼ 0.184). The results indicated a special brain activity pattern in response to processes of syntactic parsing, morphological decomposition, and phonological analysis involved in regular inflected verb forms. To determine neural substrates of inhibition and eliminate the possibility that activation differences of regulars and irregulars in L2 were simply caused by form overlap, we contrasted decomposable words against simple verbs, regulars, and irregulars. As assumed, no significant activation was found by contrasting decomposable words against control verbs and regulars, and only a few clusters (the left BA47 and right DLPFC) survived the strict threshold of FDR corrected (p < 0.05) in

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Fig. 3. Signal change analysis in regions of interest. (1)Significant differences between regular inflections and other conditions were found in three important regions of BA44, BA38, BA39. (2) “*” indicating significant threshold of p < 0.05, and “**” meaning p < 0.001 after planned comparisons between conditions.

contrasting decomposable words against irregulars. To reject a possible type II error, the authors repeated all the above contrasts at a lower threshold of 0.01, uncorrected (Rodd et al., 2010; Tyler, Marslen-Wilson et al., 2005). Significant activation clusters of all these contrasts remained the same, but displayed significant activation in a larger scale. However, no additional neural clusters were found (see Fig. 4). 4. Discussion To probe how bilinguals process grammatical features of L2, the current research tested whether or not Chinese learners relied on declarative memory to process English morphosyntax. The key concern of the current study was whether ChineseEnglish bilinguals processed English regulars differently from English irregulars. In the present study, Chinese participants with high English proficiency showed different behaviors in response to English regulars and irregulars. As expected, a significant priming effect was observed in the regular inflected verb condition but not the irregular condition, indicating that a decomposition process might be involved in the recognition of regulars. The present study with bilingual subjects verified that regularly inflected word forms were processed via their constituent morphemes and thereby directly accessed the same lexical entry as target forms (Rodriguez-Fornells et al., 2001). Meanwhile, significantly high error rates occurred when irregular inflected verbs preceded their base forms. The greater difficulty in processing irregulars in the present study confirmed previous findings with English natives (Marchman, 1997; Ullman, 1999), which were characterized by longer RTs and higher error rates for irregular inflection (Desai et al., 2006; Tatsuno & Sakai, 2005). We reinforced that bilinguals do not reliably associate irregular past tense verbs with their simple forms (Basnight-Brown, Chen, Hua, Kostic, & Feldman, 2007), as the two forms of irregulars were exclusive and contradictory in usage. Brain imaging results also conformed to our alternative hypothesis. In reference to all the significant activated areas, we raised two chief proposals. We found that proficient Chinese-English bilinguals employed procedural memory to process L2 grammatical rules, since contrasting regulars against irregulars yielded stronger activation in the procedural memory system of the left IFG, a/pSTG, MTG, IPL, and BG, in the way English monolingual learners displayed (Kielar, Joanisse, & Hare, 2008; Kielar, Milman, Bonakdarpour, & Thompson, 2011; Tyler, Marslen-Wilson et al., 2005; Tyler, Stamatakis et al., 2005). It indicated that Chinese learners processed L2 specific grammatical rules like English natives, but did not rely on neural

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Fig. 4. Significant activation in regions of interest at different thresholds by contrasting regular verbs and decomposable words with irregular and control verbs respectively. To reject Type II error, clusters survived FDR correction at p < 0.05 were presented side by side with planned comparison results at the threshold of p < 0.01 (uncorrected). The comparison of decomposable words and irregular verbs is in contrast to that between regulars and irregulars. Contrasting decomposable and irregular verbs presented the activation of BA47 only. The results demonstrated the role of the pars obitalis in combination, and denied the possibility that decomposable words were processed the same way like morphologically complex words.

underpinnings of L1syntax. In addition, the left BA 47 and bilateral DLPFC which subserved executive control were also more strongly activated when late bilinguals processed L2 syntax than semantics. It supported the S/E model, and demonstrated that AoA effects on syntax were more profound than that on semantics. Detailed analyses were presented as follows.

4.1. Native-like neural mechanism underlying L2 specific morphosyntactic information According to previous research with English natives, processing English regulars may elicit stronger activation in cortical regions of the left IFG, a/pSTG, SMG/AG, cerebellum, BG and ACC. In the present study, stronger activation at BA44 and BA45 was found for processing regulars than simple words (Desai et al., 2006; Bozic et al., 2007). Contrasting regulars against irregulars lead to robust activation in areas including the pSTG which transiently stores phonemes (Wise et al., 2001) and implements phonological analysis of suffix (Tyler, Stamatakis, et al., 2005), the AG and SMG in the ventral inferior parietal lobule (vIPL) which are conventionally identified responsible for phonological processing of alphabetical languages (Buchsbaum, Hickok, & Humphries, 2001; Burton, Small, & Blumstein, 2000; Hickok, Buchsbaum, Humphries, & Muftuler, 2003), the aSTG (BA38) which are combinatorial in nature (Friederici, 2011; Rogalsky & Hickok, 2009) and underlie decomposition process (Bozic et al., 2007; Tyler, Marslen-Wilson et al., 2005; Tyler, Stamatakis et al., 2005; Ullman et al., 1997), the MTG (BA21/37) and ITG (BA19/20) which are believed to subserve semantic information (Friederici, 2011; Pinker & Ullman, 2002; Tyler et al., 2002; Tyler, Randall et al., 2002), and the BG and cingulate cortex which work in concert with the lateral prefrontal cortex and play a monitoring role in language processing (Chan, Ryan, & Bever, 2013; Tyler, Stamatakis et al., 2005). Altogether, the current study illustrated that late L2 bilinguals with high English proficiency employed the procedural memory like English natives to process language-specific syntax of L2, but did not rely on their L1 neural underpinnings. It revealed that late bilingual learners adopted language-specific neural substrates to process unique grammatical rules of L2. This finding was compatible with the claim that L2 is largely processed in the same brain areas as the first language, but also to

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some degree in different cortical regions from L1 (Bolger, Perfetti, & Schneider, 2005; Liu, Dunlap, Fiez, & Perfetti, 2007; Tan, Laird, Li, & Fox, 2005; Vingerhoets et al., 2003). Skeide, Brauer, and Friederici (2014) reported that it is only in the 9- to 10-year-old children that syntax-specific responses in the left IFG were observed. It demonstrates that syntax is segregated and gradually independent from semantics by neural segregation and local specification of frontal and temporal cortices. We postulated that such abstraction and segregation processes of linguistic rules in the monolingual brain were also applicable to bilinguals. As for late L2 learning, the increasing level of estrogen during childhood/adolescence improves declarative memory, but attenuates procedural learning abilities (Calabresi et al., 2000; Ullman, 2004). L2 learners are likely to adopt associative properties of lexical memory (rote memory) to maintain productivity in L2 at the beginning (Hartshorne & Ullman, 2006). However, practice is conducive to procedural learning, as is demonstrated by multiple studies, which report that adult acquisition of non-linguistic skills enhances procedural memory (Poldrack et al., 2001). Therefore, L2 syntax processing would gradually rely on the procedural system, segregating from its dependence on declarative memory. The primary finding that late bilingual learners processed L2 syntax like natives provided complementary fMRI evidence to a recent ERP study (Bowden et al., 2013). The authors reported that advanced L2 Spanish bilinguals and L1 Spanish speakers were statistically indistinguishable in all time-windows (typically LAN and P600) up to 900e1200 ms when processing Spanish sentences. It is contrasted with neural activity of L2 low group, who did not show any native-like processing of syntactic word order. The authors concluded that most or all aspects of grammar could potentially become native-like. In the current study, we further demonstrated that substantial native-like brain processing of grammar can be achieved by late bilinguals irrespective of language distance. It is worth noting that we did not rule out the possibility that late Chinese-English bilinguals utilized their non-linguistic procedural memory system to process L2 syntax. However, the connection between rules and language may be even deeper — rules were represented by language so that neural underpinnings of grammatical rules (primarily syntax) may be not qualitatively different from those necessary for executing non-linguistic rules (Buchweitza & Prat, 2013). In addition, no matter whether the linguistic and non-linguistic procedural memory system overlapped, late Chinese-English bilinguals adopted the declarative memory system to process their L1 syntax and the procedural memory system for the L2 syntax. This reflected that neural responses in the bilingual brain were determined by tasks per se rather than were constrained to the entrenched language system. Taken together, our result supported the D/P model, and declined the prediction that L2 processing will be accomplished via the same neural networks of L1. We held that even though physiological acquisition of literacy in L1 may have a significant effect on the acquisition of literacy in L2 (Nakada, Fujii, & Kwee, 2001; Yang et al., 2011), late bilinguals would manipulate neural areas functionally similar to that of natives to meet specific requirements of L2 language features. 4.2. More involvement of the cognitive control system in processing L2 syntax than semantics in the late bilingual brain Even though we held that neural responses were determined by tasks per se, we did not deny the function of a selection/ inhibition mechanism in the bilinguals. Our behavioral results indicated that processing decomposable words engaged an inhibition process of nested words. Therefore, the only activation of the left BA47 and right DLPFC in processing decomposable words suggested that these areas implemented the inhibition process in the late Chinese-English bilingual brain. It supported the S/E model which proposed that the cognitive control system in late bilinguals are composed of BA47 and DLPFC (Waldron & Hernandez, 2013). Since the neural substrates underlying cognitive control in the Chinese-English bilingual brain has been identified, the significant activation in areas of the left BA47 and bilateral DLPFC by the L2 syntax > semantics contrast can be interpreted as involvement of cognitive control. This proposal can thus explain why activation of DLPFC in processing English regulars has never been found in previous English past tense studies with monolinguals: the control system of monolinguals and bilinguals are different (Marian et al., 2014), and cognitive control was a compulsory process in late bilinguals. According to the single mechanism hypothesis, there should be a neural mechanism that was critical for selection of the correct word in the L2, and for inhibition or exclusion of other linguistic information such as the L1. Thus, processing L2 requires cognitive control in order to overcome processing in the first language that is dominant in practice. In the current study, L2 syntactic tasks elicited more neural activity in the control system than L2 semantics, which was represented by greater neural responses of English regulars in areas of the BA 47 and DLPFC. It reflected that late bilinguals required more involvement of cognitive control to inhibit inference from dominant L1 syntax. This conclusion was in accord with the previous studies which have found that tasks involving syntactic processing show larger AoA effects than tasks involving semantic processing (Wartenburger et al., 2003; Weber-Fox & Neville, 1996). It may be caused by conceptual overlap across languages, which avoided redundancy and was therefore less susceptible to AoA than syntactic and morphosyntactic processes (Hernandez & Li, 2007). What's more, our findings extended the S/E model by proposing that late bilingual learners with high L2 proficiency still adopted the cognitive control system to process L2 specific grammatical features which did not engage an explicit language competition. It maybe reflected the selection of a specific language pathway at the sight of linguistic visual inputs. Such a general language selection control system supported the S/E model, but ran contrary to the Convergent Hypothesis, which predicts that the cognitive control system may fade away with increased L2 proficiency. In sum, the contrast of decomposable words > irregulars determined the role of the BA47 and DLPFC as cognitive control in Chinese-English bilinguals. It helped to specify that stronger activation of the left IFG, a/pSTG, SMG/AG, cerebellum, and BG in

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processing regulars was not a subsidiary activation of the cognitive control system, but was aroused by language features. It lent support to our former claim that the native-like neural networks was aroused to specific cognitive requirement of regular past tense verbs: the decomposition process of suffix and word stems and the recognition of them. 4.3. Potential limitations and future directions The current study did not identify significantly robust activation in the irregular > regular contrast. We adopted the same experiment design after Tyler et al. (but different input modality), and also obtained null activation like their contrast of real and pseudo irregulars against regulars (Tyler, Stamatakis et al., 2005). However, our results were actually different from other studies, which have shown greater overall activation for irregulars than regulars (De Diego Balaguer et al., 2006; Desai et al., 2006; Oh, Tan, Ng, Berne, & Graham, 2011; Sahin, Pinker, & Halgren, 2006). We interpreted it as shared grammatical properties and unmatched phonological complexity between regulars and irregulars. De Diego Balaguer et al. (2006) reported greater left inferior (and middle) frontal activation for irregular vs. regular production, which was interpreted as shared grammatical processing of regulars and irregulars in the left opercular IFG. Similarly, Sahin et al. (2006) claimed that the regular-irregular distinction mapped imperfectly onto the computationmemory distinction, for regular verbs relied on rule computation and irregular forms involved a look-up process in semantic retrieval. Our study supported these proposals. We agreed that both regulars and irregulars embodied lexical information of grammatical properties. It was confirmed by the signal change analysis in the current study, as signal change analysis of the pars operaculais (BA44) showed no significant difference between the two kinds of inflections. Meanwhile, looking up for unpredictable morphosyntactic forms of irregulars increased working memory load in the left prefrontal cortex. It aroused equivalent activation in the left inferior prefrontal regions as regulars. Therefore, both regulars vs. irregulars contrast and irregulars vs. regulars contrast were not significant in this area. Another possible cause of no stronger activation for irregulars may be unmatched phonological complexity in the current experiment. Desai et al. (2006) found that robust activation of regulars against irregulars in the left STG disappeared in the matched phonological complexity condition. What's more, generation of irregular past tense forms may display more activation in regions of the right middle and superior frontal regions, bilateral middle temporal gyri (MTG) and right STG than regulars, if phonological complexity is well controlled (Oh et al., 2011). As we did not control phonological complexity difference between regular and irregular inflections (considering bilinguals' small vocabulary), no significantly activated areas were found by the irregular > regular contrast. A concomitant negative effect caused by unmatched phonological complexity was that neural responses for regulars were even stronger than irregulars in the middle temporal gyrus. Perhaps, this reversed contrast result may also be interpreted as late bilinguals exploited the declarative memory system more to retrieve rule knowledge. Future research can focus on influence of these factors with finer controlled materials. Hernandez et al. (2007) explored the role of regularity in modulating differences with early and late learners of Spanish through a gender decision task of regular and irregular items. They reported that late Spanish learners elicited increased activity in left IFG and the left anterior insula. Inconsistent results about the regularity effect between the current study and theirs may be caused by three possible reasons: (1) distinctive neural responses per se by different inflectional morphology tasks of English past tense verbs and Spanish nouns; (2) different experiment paradigms of our implicit semantic judgment task and their explicit gender decision task; (3) different language similarity of the native and nonnative languages in the two studies. 5. Conclusions The current study found that late proficient Chinese-English bilinguals, like English natives, manipulated procedural and declarative systems to process L2 grammatical and semantic information respectively. It demonstrated Ullman's D/P model both generally and specifically. We concluded that late bilinguals did not rely on L1 literacy to process unique L2 grammatical features. Actually, distinct language features were processed by specialized neural substrates, even though learned after the critical period. To our knowledge, the current study was the first fMRI study that demonstrated the D/P model concerning L2 processing. Meanwhile, we discussed an open question about the interface of language representation and cognitive control. We confirmed the compulsory role of cognitive control in processing L2, by demonstrating that proficient bilinguals of L2 still adopted cognitive control to process L2 grammatical features. It agreed with the S/E model, but was against the Convergent Hypothesis.

Acknowledgments This paper is supported by a 973 Grant from the National Strategic Basic Research Program of the Ministry of Science and Technology of China (No. 2012CB720701), the Fundamental Research Funds for the Central Universities (RW150401), China Postdoctoral Science Foundation (2015M582400), Key Basic and Clinical Medical Cooperation Project of Capital Medical University (16JL03), and the National Natural Science Foundation of China under Grant No. 31400962, 81371201, 31371026. There are no conflicts of interest.

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