Here we review the neural correlates of cognitive control associated with bilingualism. We demonstrate that lifelong practice managing two languages orchestrates global changes to both the structure and function of the brain. Compared with monolinguals, bilinguals generally show greater gray matter volume, especially in perceptual/motor regions, greater white matter integrity, and greater functional connectivity between gray matter regions. These changes complement electroencephalography findings showing that bilinguals devote neural resources earlier than monolinguals. Parallel functional findings emerge from the functional magnetic resonance imaging literature: bilinguals show reduced frontal activity, suggesting that they do not need to rely on top-down mechanisms to the same extent as monolinguals. This shift for bilinguals to rely more on subcortical/posterior regions, which we term the bilingual anterior-to-posterior and subcortical shift (BAPSS), fits with results from cognitive aging studies and helps to explain why bilinguals experience cognitive decline at later stages of development than monolinguals.

Bilinguals often outperform monolinguals on executive function tasks, including tasks that tap cognitive flexibility, conflict monitoring, and task-switching abilities. Some have suggested that bilinguals also have greater working memory capacity than comparable monolinguals, but evidence for this suggestion is mixed. We therefore conducted a comprehensive meta-analysis on the effects of bilingualism on working memory capacity. Results from 88 effect sizes, 27 independent studies, and 2,901 participants revealed a significant small to medium population effect size of 0.20 in favor of greater working memory capacity for bilinguals than monolinguals. This suggests that experience managing two languages that compete for selection results in greater working memory capacity over time. Moderator analyses revealed that largest effects were observed in children than other age groups. Furthermore, whether the task was performed in the first (L1) or second (L2) language for bilinguals moderated the effect size of the bilingual advantage; this factor is often overlooked and our results point to the importance of defining language variables that influence critical cognitive outcomes.

Three studies examined the hypothesis that bilinguals can more rapidly disengage attention from irrelevant information than monolinguals by investigating the impact of previous trial congruency on performance in a simple flanker task. In Study 1, monolingual and bilingual young adults completed two versions of a flanker task. There were no differences between language groups on mean reaction time using standard analyses for congruent or incongruent trials or the size of the flanker effect. Sequential congruency effects (SCEs) however, which account for previous trial congruency, were smaller for bilinguals than for monolinguals. This finding was strongest at the shortest response-to-stimulus interval (RSI). Study 2 replicated this effect using a slightly different flanker task and a shorter RSI than study 1. Study 3 showed that at long RSIs, where behavioral SCE differences between groups disappear because of sufficient time to recover from the previous trial, event-related potentials demonstrated a continued influence of previous trial congruency for monolinguals but not bilinguals at both the N2 and the P3, replicating the reaction time effects in Studies 1 and 2. Together, these studies demonstrate that bilinguals experience less influence from previous trial congruency and have greater ability to disengage attention from the previous trial in order to focus attention on the current trial than is found for monolinguals.

Several researchers have suggested that learning and using a second language requires domain-general executive functions, and many have shown that bilinguals outperform monolinguals on tasks that tap into these processes. However, recent behavioral studies and meta-analyses reporting failed replications have called into question whether or not bilingualism leads to changes in domain-general executive functions. The present paper argues that there are several reasons, often overlooked, that lead to failed replications, and that when group differences do appear on EF tasks, despite these issues, performance favors bilinguals far more often than monolinguals. The present paper reports a Bayesian analysis of 167 independent studies to support this claim with a Bayes Factor classified as ''decisive'' evidence for the alternative (BF 10 = 2.91 9 10 8 ), ruling out chance outcomes that would predict an equal number of studies favoring monolinguals to studies favoring bilinguals. These findings could not be explained by publication bias, year of publication, or sample size. Critically, these findings are not at odds with recent meta-analyses examining overall effect sizes, but rather, highlight the need to determine when, rather than if, bilinguals outperform monolinguals on EF tasks.

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Background:Fluoride exposures have not been established for pregnant women who live in regions with and without community water fluoridation.Objective:Our aim was to measure urinary fluoride levels during pregnancy. We also assessed the contribution of drinking-water and tea consumption habits to maternal urinary fluoride (MUF) concentrations and evaluated the impact of various dilution correction standards, including adjustment for urinary creatinine and specific gravity (SG).Methods:We measured MUF concentrations in spot samples collected in each trimester of pregnancy from 1,566 pregnant women in the Maternal–Infant Research on Environmental Chemicals cohort. We calculated intraclass correlation coefficients (ICCs) to assess variability in MUF concentrations across pregnancy. We used regression analyses to estimate associations between MUF levels, tea consumption, and water fluoride concentrations as measured by water treatment plants.Results:Creatinine-adjusted MUF values (mean±SD; milligrams per liter) were almost two times higher for pregnant women living in fluoridated regions (0.87±0.50) compared with nonfluoridated regions (0.46±0.34; p<0.001). MUF values tended to increase over the course of pregnancy using both unadjusted values and adjusted values. Reproducibility of the unadjusted and adjusted MUF values was modest (ICC range=0.37–0.40). The municipal water fluoride level was positively associated with creatinine-adjusted MUF (B=0.52, 95% CI: 0.46, 0.57), accounting for 24% of the variance after controlling for covariates. Higher MUF concentrations correlated with numbers of cups of black (r=0.31–0.32 but not green tea (r=0.04–0.06). Urinary creatinine and SG correction methods were highly correlated (r=0.91) and were interchangeable in models examining predictors of MUF.Conclusion:Community water fluoridation is a major source of fluoride exposure for pregnant women living in Canada. Urinary dilution correction with creatinine and SG were shown to be interchangeable for our sample of pregnant women. https://doi.org/10.1289/EHP3546

Bilingualism can delay the onset of dementia symptoms and has thus been characterized as a mechanism for cognitive or brain reserve, although the origin of this reserve is unknown. Studies with young adults generally show that bilingualism is associated with a strengthening of white matter, but there is conflicting evidence for how bilingualism affects white matter in older age. Given that bilingualism has been shown to help stave off the symptoms of dementia by up to four years, it is crucial that we clarify the mechanism underlying this reserve. The current study uses diffusion tensor imaging (DTI) to compare monolinguals and bilinguals while carefully controlling for potential confounds (e.g., I.Q., MMSE, and demographic variables). We show that group differences in Fractional Anisotropy (FA) and Radial Diffusivity (RD) arise from multivariable interactions not adequately controlled for by sequential bivariate testing. After matching and statistically controlling for confounds, bilinguals still had greater axial diffusivity (AD) in the left superior longitudinal fasciculus than monolingual peers, supporting a neural reserve account for healthy older bilinguals.

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