Age of acquisition impacts the brain differently depending on neuroanatomical metric.

Although researchers generally agree that a certain set of brain areas underlie bilingual language processing, there is discrepancy regarding what effect timing of language acquisition has on these regions. We aimed to investigate the neuroanatomical correlates of age of acquisition (AoA), which has been examined previously, but with inconsistent results, likely influenced by methodological differences across studies. We analyzed gray matter density, volume, and thickness using whole-brain linear models in 334 bilinguals and monolinguals. Neuroanatomical correlates of AoA differed depending on gray matter metric. Relative to early bilinguals, late bilinguals had thicker cortex in language processing and cognitive control regions, and greater density in multiple frontal areas and the right middle temporal and supramarginal gyri. Early bilinguals had greater volume than late bilinguals in the left middle temporal gyrus. Overall, volume was the least sensitive to AoA-related differences. Multiple regions not classically implicated in dual-language processing were also found, which highlights the important role of whole-brain analyses in neuroscience. This is the first study to investigate AoA and gray matter thickness, volume, and density all in the same sample. We conclude that cognitive models of bilingualism should consider the roles of development and neuroanatomical metric in driving our understanding of bilingual and monolingual language organization.

Parietal lobe volume distinguishes attentional control in bilinguals and monolinguals: A structural MRI study.

Research suggests that bilingualism is associated with increases in parietal gray matter volume (GMV). These parietal GMV increases are a source of variability that may help explain the reported bilingual/monolingual differences in attentional control. The current study examined how parietal GMV variability and a participant's language background predicted Simon task performance. GMV measures were extracted from the bilateral angular and supramarginal gyri from participants' MRI scans using Freesurfer image analysis suite. Contrary to expectations, bilinguals did not outperform monolinguals on the Simon task. In fact, bilinguals had slower response times across all conditions of the task (incongruent, congruent, and neutral) than monolinguals. In addition, GMV in the right supramarginal gyrus was negatively associated with response times for congruent trials for bilinguals, and positively associated with these response times for monolinguals. The difference in the relationships between parietal GMV and task performance suggests that bilinguals rely on spatial attention to complete the Simon task, while monolinguals may rely on verbal attention. These results help to connect bilingual advantages in tasks requiring spatial attention (e.g., attentional control) with bilingual disadvantages in tasks requiring verbal attention (e.g., verbal fluency).

Anterior insular thickness predicts speech sound learning ability in bilinguals.

A previous fMRI study of novel speech sound learning, tied to the methods and results presented here, identified groups of advanced and novice learners and related their classification to neural activity. To complement those results and better elucidate the role of the entire neural system in speech learning, the current study analyzed the neuroanatomical data with the goals of 1) uncovering the regions of interest (ROIs) that predicted speech learning performance in a sample of monolingual and bilingual adults, and 2) examining if the relationship between cortical thickness from selected ROIs and individual learning ability depends on language group. The ROIs selected were brain regions well-established in the literature as areas associated with language and speech processing (i.e., Transverse Superior Temporal Gyrus, anterior insula and posterior insula, all bilaterally). High-resolution brain scans (T1-weighted) were acquired from 23 Spanish-English bilinguals and 20 English monolingual adults. The thickness of the left anterior insula significantly predicted speech sound learning ability in bilinguals but not monolinguals. These results suggest that aptitude for learning a new language is associated with variations in the cortical thickness of the left anterior insula in bilinguals. These findings may provide insight into the higher order mechanisms involved in speech perception and advance our understanding of the unique strategies employed by the bilingual brain during language learning.

Neuroanatomical profiles of bilingual children.

The goal of the present study was to examine differences in cortical thickness, cortical surface area, and subcortical volume between bilingual children who are highly proficient in two languages (i.e., English and Spanish) and bilingual children who are mainly proficient in one of the languages (i.e., Spanish). All children (N = 49) learned Spanish as a native language (L1) at home and English as a second language (L2) at school. Proficiency of both languages was assessed using the standardized Woodcock Language Proficiency Battery. Five-minute high-resolution anatomical scans were acquired with a 3-Tesla scanner. The degree of discrepancy between L1 and L2 proficiency was used to classify the children into two groups: children with balanced proficiency and children with unbalanced proficiency. The groups were comparable on language history, parental education, and other variables except English proficiency. Values of cortical thickness and surface area of the transverse STG, IFG-pars opercularis, and MFG, as well as subcortical volume of the caudate and putamen, were extracted from FreeSurfer. Results showed that children with balanced bilingualism had thinner cortices of the left STG, left IFG, left MFG and a larger bilateral putamen, whereas unbalanced bilinguals showed thicker cortices of the same regions and a smaller putamen. Additionally, unbalanced bilinguals with stronger foreign accents in the L2 showed reduced surface areas of the MFG and STS bilaterally. The results suggest that balanced/unbalanced bilingualism is reflected in different neuroanatomical characteristics that arise from biological and/or environmental factors.

Subcortical volume and language proficiency in bilinguals and monolinguals: A structural MRI study.

The current study focused on an understudied but most prominent bilingual population in the U.S. - heritage bilinguals. The current study combined data from eight MRI studies to examine the relationship between language experience and subcortical gray matter volume in 215 heritage Spanish-English bilinguals and 145 English monolinguals, within and between groups. For bilinguals, higher Spanish (L1) proficiency was related to less volume in the bilateral globus pallidus, and higher English (L2) proficiency and earlier English AoA were related to greater volume in the right thalamus, left accumbens, and bilateral globus pallidus. For monolinguals, higher English proficiency was associated with greater volume only in the right pallidum. These results suggest that subcortical gray matter structures are related to the learning of a second language. Future research is encouraged to understand subcortical adaptation in relation to L1 and L2 acquisition from a developmental perspective.

The neural basis of non-native speech perception in bilingual children.

The goal of the present study is to reveal how the neural mechanisms underlying non-native speech perception change throughout childhood. In a pre-attentive listening fMRI task, English monolingual and Spanish-English bilingual children - divided into groups of younger (6-8yrs) and older children (9-10yrs) - were asked to watch a silent movie while several English syllable combinations played through a pair of headphones. Two additional groups of monolingual and bilingual adults were included in the analyses. Our results show that the neural mechanisms supporting speech perception throughout development differ in monolinguals and bilinguals. While monolinguals recruit perceptual areas (i.e., superior temporal gyrus) in early and late childhood to process native speech, bilinguals recruit perceptual areas (i.e., superior temporal gyrus) in early childhood and higher-order executive areas in late childhood (i.e., bilateral middle frontal gyrus and bilateral inferior parietal lobule, among others) to process non-native speech. The findings support the Perceptual Assimilation Model and the Speech Learning Model and suggest that the neural system processes phonological information differently depending on the stage of L2 speech learning.

Cortical Thickness Is Related to Variability in Heritage Bilingual Language Proficiency.

Research suggests that bilingual experience is associated with gray matter changes, such that initial language gains are associated with expansion and language expertise is associated with renormalization. Previous studies on language proficiency development primarily focused on between-subjects, quasiexperimental comparisons of monolinguals and bilinguals. This study proposes a new paradigm to examine language expertise and cortical thickness within heritage bilinguals (n = 215), as well as between bilinguals and monolinguals (n = 145), using data combined from eight previous magnetic resonance imaging studies. In general, results highlight variability within bilinguals, finding relationships between cortical thickness and English proficiency that are relatively consistent within monolinguals, but inconsistent within bilinguals. In all participants, higher levels of proficiency in English-monolinguals' only language and bilinguals' second but stronger language-were negatively related to cortical thickness. In bilinguals, higher proficiency in the weaker, albeit first learned, language was positively related to cortical thickness. Moreover, there was an interaction between language group and English proficiency in predicting cortical thickness, such that the relationship between proficiency and thickness was stronger in monolinguals than in bilinguals. Findings also demonstrate that the regions associated with language expertise differ between bilinguals and monolinguals. Future directions for cognitive-developmental neuroscience research in bilinguals are suggested, particularly the longitudinal examination of cortical changes in relation to bilingual experiences.

Highly proficient, balanced bilingualism is related to thinner cortex in two cognitive control regions.

Whereas some bilinguals have one language that is dominant, others attain high proficiency in both languages. This variation is likely explained by a combination of environmental and genetic factors; however, there is a lapse in research on the neural underpinnings of bilingual proficiency. No study to date has examined how highly proficient bilingualism that is balanced relates to brain morphology in adults. Our present study analyzed the brains of 200 Spanish-English bilingual adults. Bilingual proficiency was measured and weighted by the degree of balance across the two languages. It was found that having higher dual language proficiency was related to thinner cortex in two regions: the left anterior prefrontal cortex (PFC) and the right anterior cingulate cortex (ACC). Neither English nor Spanish proficiency alone could account for neuroanatomical differences. Our findings suggest that thinner cortex of the left anterior PFC and the right ACC in adults with highly proficient, balanced bilingualism is how the adult brain reflects a lifetime of learning to flexibly adapt and utilize both languages and suggests the involvement of these structures in maintaining and increasing dual language proficiency.

Speech sound learning depends on individuals' ability, not just experience.

AIMS: The goal of this study was to investigate if phonetic experience with two languages facilitated the learning of novel speech sounds or if general perceptual abilities independent of bilingualism played a role in this learning. METHOD: The underlying neural mechanisms involved in novel speech sound learning were observed in groups of English monolinguals (n = 20), early Spanish-English bilinguals (n = 24), and experimentally derived subgroups of individuals with advanced ability to learn novel speech sound contrasts (ALs, n = 28) and individuals with non-advanced ability to learn novel speech sound contrasts (non-ALs, n = 16). Subjects participated in four consecutive sessions of phonetic training in which they listened to novel speech sounds embedded in Hungarian pseudowords. Participants completed two fMRI sessions, one before training and another one after training. While in the scanner, participants passively listened to the speech stimuli presented during training. A repeated measures behavioral analysis and ANOVA for fMRI data were conducted to investigate learning after training. RESULTS AND CONCLUSIONS: The results showed that bilinguals did not significantly differ from monolinguals in the learning of novel sounds behaviorally. Instead, the behavioral results revealed that regardless of language group (monolingual or bilingual), ALs were better at discriminating pseudowords throughout the training than non-ALs. Neurally, region of interest (ROI) analysis showed increased activity in the superior temporal gyrus (STG) bilaterally in ALs relative to non-ALs after training. Bilinguals also showed greater STG activity than monolinguals. Extracted values from ROIs entered into a 2×2 MANOVA showed a main effect of performance, demonstrating that individual ability exerts a significant effect on learning novel speech sounds. In fact, advanced ability to learn novel speech sound contrasts appears to play a more significant role in speech sound learning than experience with two phonological systems.

The effect of age of acquisition, socioeducational status, and proficiency on the neural processing of second language speech sounds.

This study investigates the role of age of acquisition (AoA), socioeducational status (SES), and second language (L2) proficiency on the neural processing of L2 speech sounds. In a task of pre-attentive listening and passive viewing, Spanish-English bilinguals and a control group of English monolinguals listened to English syllables while watching a film of natural scenery. Eight regions of interest were selected from brain areas involved in speech perception and executive processes. The regions of interest were examined in 2 separate two-way ANOVA (AoA×SES; AoA×L2 proficiency). The results showed that AoA was the main variable affecting the neural response in L2 speech processing. Direct comparisons between AoA groups of equivalent SES and proficiency level enhanced the intensity and magnitude of the results. These results suggest that AoA, more than SES and proficiency level, determines which brain regions are recruited for the processing of second language speech sounds.

Age of acquisition and proficiency in a second language independently influence the perception of non-native speech.

Sensorimotor processing in children and higher-cognitive processing in adults could determine how non-native phonemes are acquired. This study investigates how age-of-acquisition (AOA) and proficiency-level (PL) predict native-like perception of statistically dissociated L2 categories, i.e., within-category and between-category. In a similarity task, participants rated the level of similarity between pairs of English syllables from 1 (similar) to 4 (dissimilar). Early L2 acquisition predicts accurate within-categorization and high proficiency in late L2 acquisition predicts improved between-categorization. Our results suggest that the manner in which bilinguals learn to categorize non-native sounds depends on the cognitive processes available at the age of L2 exposure.