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Language experience and the brain: variability, neuroplasticity, and bilingualism
Judith F. Kroll
and Christine Chiarello
Department of Psychology, Center for Language Science, Pennsylvania State University, University Park, PA 16802, USA;
Department of
Psychology, University of California, Riverside, Riverside, CA, USA
ARTICLE HISTORY: Received 12 August 2015; Accepted 19 August 2015
In the last two decades, we have learned that there is far
greater neuroplasticity in adulthood than previously
understood. Experiences as varied as driving a taxi cab,
playing a musical instrument, physical exercise, or
playing video games change the brain both functionally
and structurally (e.g. Bavelier, Achtman, Mani, & Föcker,
2012; Erickson et al., 2011; Herholz & Zatorre, 2012;
Maguire et al., 2000). In this context, it should come as
no surprise that a life in two languages would do the
same. The specic consequences of bilingualism may
be unique, but the fact of using two languages actively
has implications for cognition and the brain networks
that support it (e.g. Abutalebi et al., 2012; Bialystok,
Craik, & Luk, 2012; Kroll, Dussias, Bice, & Perrotti, 2015).
The article by García-Pentón, García, Costello, Duñabeita,
and Carreiras (2015) asks what evidence exists to suggest
that bilingualism imposes structural changes in the brain
and what that evidence implies about whether the use of
two or more languages confers cognitive advantages to
bilingual speakers. Other recent reviews, notably Li,
Legault, and Litckofsky (2014), have also evaluated the
available structural data on the consequences of
The word hazyin the title of the García-Pentón et al.
(2015) paper suggests that there is a problem with the
existing research on this topic and the paper is indeed
oriented towards two problems. One is that the
authors claim that the quality of the data on the conse-
quences of bilingualism for neuroanatomical change is
variable: studies are underpowered, use a range of ana-
lytic methods that may not be comparable, and anatom-
ical results from similar studies lack convergence. The
other concerns the implications that this evidence
holds for whether bilinguals are advantaged relative to
In this commentary, we argue that neither of these pro-
blems creates haze. Rather, the approach taken to the
interpretation of structural imaging data requires
additional complexity that considers the relationship
between neural function and structure, particularly in
identifying the mappings between function to structure
and the role of individual differences. We suggest that
even if the mappings between neuroanatomical structure
and brain function can be understood in detail, the impli-
cations for potential cognitive advantages in behaviour
will require a theoretical model that maps behaviour to
brain structure and function, and that also takes into
account differences in the way that bilingualism is mani-
fest for different types of bilinguals across the lifespan
(e.g. Bialystok, Craik, Green, & Gollan, 2009; Luk & Bialys-
tok, 2013). Finding changes in neuroanatomical structure
as a function of bilingualism need not imply that there will
be cognitive advantages to bilinguals that are observed in
behavioural performance, nor vice versa. These behav-
ioural advantages need to be understood as well and
each of these investigations, at multiple levels of analysis,
requires a more sophisticated theoretical model.
Here, we consider what sort of framework is needed
to begin to address the apparent inconsistencies in the
evidence reviewed by García-Pentón et al. (2015). We
then consider the relationship of this work to the exten-
sive body of research on the consequences of bilingual-
ism for behaviour and ask what assumptions might need
to be made to begin to understand the causal mechan-
isms that underlie these consequences.
Understanding the neuroanatomical evidence
We take exception to the authorsguiding hypothesis (p.
9): if it is the case that bilingualism leads to enhanced
language related as well as domain-general executive
control processes, then structural differences may be
found in the neural regions that underlie these pro-
cesses. At the current state of our knowledge, the inves-
tigation of neurostructural correlates of bilingualism can
and perhaps should be explored independent of hypoth-
eses about putative advantages of bilingualism. The
guiding hypothesis is in fact conating two sets of
© 2015 Taylor & Francis
CONTACT Judith F. Kroll
Invited commentary on García-Pentón et al. (2015). The neuroanatomy of bilingualism: How to turn a hazy view into the full picture.
Downloaded by [Penn State University] at 05:28 12 September 2015
linking hypotheses that have yet to be fully explicated.
The rst link is needed to specify how structural differ-
ences should be related to differences in brain functional
activity. The second link is needed to specify how vari-
ations in the brain networks recruited for a given function
relate to individual differences in ability. Attempting to
directly associate brain structural variation with abilities
such as enhanced cognitive control by glossing over
these intermediate links will not lead to advances in our
understanding of bilingualism. Otherwise we are left
with rather naïve implicit theories of the sort: if an area
has a particular function, and someone is better at that
function, then that area should be structurally different.
We consider issues relating to each link below.
First, we need to examine the neurobiological under-
pinnings of the various in vivo structural measurements
currently available. For example, if cortical thickness in
a particular region differs between groups, is that due
to expanded neuropil in one group, group differences
in pruning, or differential rates of intracortical myelina-
tion? Similarly, group differences in white matter tracts
could be due to differences in axonal sprouting,
changes in myelination, or pruning of connections. And
group differences in local vs. long-range connectivity
will have differing consequences for a variety of neuroa-
natomical measures (Deng et al., 2014). Without at least
considering these issues, we cannot generate strong
hypotheses about how structural differences might
relate to neural function. It is important to avoid assump-
tions of the sort that more of something (e.g. thicker
cortex) is somehow better functionally. To be sure, the
neurobiological bases of current structural measures
are being actively investigated (Vandekar et al., 2015;
Wagstyl, Ronan, Goodyer, & Fletcher, 2015) and a
strong consensus is absent. Nevertheless, it behoves us
as (neuro)language researchers to think more deeply
about how bilingualism could affect brain structure or
brain activity (e.g. increased or decreased connectivity
between specic regions, sculpting or expansion of cir-
cuits within particular regions) and then seek out appro-
priate converging methods.
It is important to note, however, that this research
programme says little about differences in cognitive abil-
ities. Individuals could rely on differing structural and
functional networks, yet be equally adept at the cogni-
tive functions the networks subserve. Acknowledging
this possibility does not make the investigation of neuro-
structural correlates of bilingualism less important or
interesting. At a minimum, reliance on alternate neural
substrates opens up the possibility of differential out-
comes in the face of disease, injury, or senescence
issues with important public health implications for
various populations.
The second linking hypothesis concerns how brain
function might relate to cognitive ability. Function is
not ability, as was noted above. Exploring variations in
ability may benet from an individual differences per-
spective that acknowledges the wide variability in
human population in virtually every measurable trait
from cytoarchitecture (Zilles & Amunts, 2013) to math-
ematical ability (Lourenco, Bonny, Fernandez, & Rao,
2012). Often multivariate approaches are used that
require much larger sample sizes than those used in
the studies reviewed in the target article (e.g. Friedman
& Miyake, 2004). This would appear to be of particular rel-
evance to the study of bilingualism that involves con-
sideration of multiple interacting variables (e.g. age of
acquisition, prociency in various languages, and
degree of code switching) that cannot be submitted to
experimental control. The differing results across
studies that García-Pentón et al. document seem to us
not to indict the validity of investigating neurostructural
correlates of bilingualism, but rather to be the inevitable
result of a relatively new area of investigation in which
individual studies are probably underpowered to reliably
detect true differences. For example, there is substantial
between-subject variation in cortical surface area, thick-
ness, and local gyrication among healthy young
adults (N= 200) as indexed by the coefcient of variation,
and this measure can differ threefold across various cor-
tical regions (Chiarello, Vazquez, Felton, & McDowell, in
preparation). From this perspective, one would predict
differences across studies with small Ns and differing
measurement methods, even if the studies investigated
individuals with similar bilingual backgrounds. García-
Pentón et al. note that longitudinal studies obtain
more consistent ndings than cross-sectional investi-
gations. This is as would be expected if individual vari-
ation were a large contributor to the conicting cross-
sectional ndings. In addition, we suspect that age/sex
differences contribute to differences across studies.
Even relatively small differences in age (i.e. 20-year-olds
vs. 30-year-olds) are associated with signicant differ-
ences in measures such as cortical thickness (Zhou,
Lebel, Evans, & Beaulieu, 2013) and the authors do not
consider whether differences in the sex composition of
various studies may have inuenced the varying nd-
ings. Such variables may have little to do with bilingual-
ism per se, nor with particular measurement techniques.
García-Pentón et al. (2015) suggest that researchers
adopt similar measurement techniques in order to
promote more valid comparisons across studies.
Clearly, this would represent an improvement in many
respects, but would come with a cost as well. In a
science in which new methods are being developed at
a rapid pace, encouraging researchers to rely on older
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methods such as voxel based morphometry could have
the effect of ossifying the research area and increasing
the distance between basic neuroscience research and
the study of bilingualism. We suggest that even more
attention should be paid to the latest neuroscience nd-
ings and methods to better inform theoretical and
empirical study of the neural bases of bilingualism.
A framework for interpreting the
consequences of bilingualism
As the preceding comments make clear, interpreting
structural differences in the brain in response to individual
characteristics or experience is at an early stage of inves-
tigation that will require more sophisticated analytic
models to enable claims about how particular life experi-
ences create changes in the brain. Relating these ndings
to data from other levels of analysis, for example, from
behavioural or electrophysiological studies, is also
complex, but complexity itself does not mean that the
investigation is doomed. Rather, efforts towards develop-
ing theoretical frameworks will require a broader lens to
encompass the rapidly changing nature of the data that
are available, the relations across different levels of analy-
sis, and a theoretical commitment to how different forms
of bilingual experience across the lifespan are likely to
become apparent across the available methods.
Two trends in the recent literature have created
opposing tensions in achieving this goal. Recent critiques
of the bilingual advantage (e.g. Paap & Greenberg, 2013;
Valian, 2015) have focused on failures to replicate behav-
ioural studies that have reported executive function
advantages for bilingual young adults relative to age-
matched monolinguals. The failures to replicate have
been interpreted to suggest that the evidence for a bilin-
gual advantage has been greatly exaggerated. Notably,
these critiques, much like the García-Pentón et al.
(2015) analysis, are directed narrowly to a single
method, or to a single age group or type of bilingual,
and fail to consider converging evidence across different
measures, across the range of bilingual experience, or
between the consequences of bilingualism for cognition
and for language. Crucially, they fail to provide a prin-
cipled account of why, other than methodological short-
comings or variability, some studies report positive
effects of bilingualism and others do not. Some have
gone so far as to suggest that there is bias in the
report of these ndings that has created an illusion
that there is any evidence for bilingual benets (De
Bruin, Treccani, & Della Sala, 2015; but see Bialystok,
Kroll, Green, MacWhinney, & Craik, 2015).
Determining whether bilinguals and monolinguals
differ in brain structure or function or in behaviour is a
correlational exercise. It may provide clues but little in
the way of a causal analysis to understand how the
way that bilinguals use language may shape language
performance, cognition, and the neural systems that
support them. Another approach, and one that is more
complex but also far more promising, attempts to ident-
ify the factors that might be required to model the con-
sequences of bilingualism (e.g. Baum & Titone, 2014;
Green & Abutalebi, 2013; Kroll & Bialystok, 2013).
García-Pentón et al. (2015) appeal to the adaptive
control framework proposed by Green and Abutalebi.
Doing so provides a rst step, but one that is quite pre-
liminary, in speculating about why different structural
imaging studies on bilinguals have produced conicting
results. The adaptive control framework assumes that
bilinguals differ in the way that demands on cognitive
mechanisms, and their neural underpinnings, are
engaged by different types of language experience. Bilin-
guals differ in how the two languages are used, whether
others in the environment are also similarly bilingual,
whether they code switch between the two languages
or not, and if they do code switch, and whether it rst
requires a decision about they can code switch with a
particular interlocutor. This list is a partial illustration of
how different bilinguals may be even if they are similarly
procient in the two languages, share similar demo-
graphic characteristics, and come from the same age
and gender cohort. Considering developmental
changes across the lifespan creates additional complex-
ity (e.g. see Gold, Kim, Johnson, Kriscio, & Smith, 2013).
As we noted earlier, any and all of these factors may
potentially affect the way that brain networks are
adjusted in response to experience and may have both
functional and structural consequences. These are not
simple effects and how we understand the conse-
quences that are revealed in the brain and/or behaviour
will require adopting a theoretical framework that
acknowledges that it is unlikely that there will be a
single effect of bilingualism. If there are advantages,
they are likely to vary. In some instances, there could
very well be disadvantages or no effect of bilingualism
(see Bialystok et al., 2009, for a consideration of the
range of bilingual consequences).
As scientists, we seek explanations that are simple and
elegant. Phenomena that cannot be captured by a single
variable or that produce inconsistent ndings across
different tasks, measures, and people may be difcult
to investigate, but they are not impenetrable, noisy, or
hazy. They require that we develop deeper analyses
and new models. The variation in the data that results
from newly emerging methods should not be character-
ised as haze but as an opportunity to challenge the over-
simplication of earlier accounts. We need bold new
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hypotheses for relating these new methods to experi-
ence and some modesty about how deeply we under-
stand an experience as rich as life with two languages.
Disclosure statement
No potential conict of interest was reported by the authors.
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... Secondly, in a similar vein, to what degree can we claim structural variations in the brain to be associated with cognitive ability and not other dimensions of individual differences (Kroll and Chiarello 2016)? Currently, a more thorough analytical model to interpret the general consequences of bilingualism is still much needed (Kroll and Chiarello 2016). The release of this general model is perhaps the foundational step to secure before we can further consolidate scientific claims on effects of typological distance in the bilingual brain and whether they translate to any cognitive advantages at all. ...
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... Learning a second language has different benefits, such as improvements in cognitive, psychological, social and linguistic processes. 2,3 In this sense, English is one of the most widely spoken languages in the world, and is, therefore, considered a globalized language. 4 Early contact with this language, or any other language, enables the subject to develop a metalinguistic competence over the languages that transits. ...
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Introduction Learning a second language is an essential task in today's world, and is experienced by many children. The cognitive auditory-evoked potential (P300) is related to cognitive activity, attention and concentration, enabling the investigation of the effect of a second language on the central auditory pathway. Objective To analyze the effects of learning English on P300 latency and amplitude in children and to correlate them with age, time of exposure to English, and time in class. Method An observational, descriptive, cross-sectional and quantitative study, in which 33 children, aged between 5 and 9 years and 11 months, of both genders participated, 14 of them in the process of learning English (study group) and 19 without this experience (control group). All subjects had their P300 evaluated using the Intelligent Hearing Systems (IHS, Miami, FL, US) Smart EP equipment. A total of 300 binaural stimuli were used in 75 dBnHL, as well as 240 frequent and 60 rare stimuli, using the pairs /ba/ and /di/ respectively. Results There was a statistically significant difference regarding P300 latency between the groups, and children exposed to English classes had lower latency in this component. No statistical difference was found between P300 amplitudes. No correlation was observed regarding age, time of exposure to English, time in class, and electrophysiological responses. Conclusion The Children exposed to English classes had the most stimulating auditory pathway, because their P300 had lower latency, being a resource for the speech therapy clinic.
... The claim is that in a multi-causal world situation, the operation of complex, multivariate patterns is the norm, and factors of influence often push in opposite directions (Lieberson, 1991). In the present case, this entails that across different (i) conditions of testing, (ii) populations, and (iii) cognitive measures, the influence of a cluster of factors such as high level of education and/or high degree of language proficiency in two languages 2 can be outweighed by another cluster of factors such as type of bilingual trajectory, incidence, and context of language use (Luk & Bialystok, 2013;Kroll & Chiarello, 2016;Li et al., 2014;Bak, 2016a;Bialystok, 2016;Gullifer, Chai, Whitford, Pivneva, Baum, Klein & Titone, 2018;DeLuca et al., 2019;Beatty-Martínez, Navarro-Torres, Dussias, Bajo, Guzzardo Tamargo & Kroll, 2019). If some of these factors eventually cancel each other out or were never available in proportions sufficient to trigger neurocognitive adaptations, it would follow that different studies on bilingual cognition could reach contradictory results because of sampling issues, even when they employ the same tasks or recruit their subjects from the same linguistic community. ...
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Numerous studies have argued that bilingualism has effects on cognitive functions. Recently, in light of increasingly mixed empirical results, this claim has been challenged. One might ponder if there is enough evidence to justify a cessation to future research on the topic or, alternatively, how the field could proceed to better understand the phantom-like appearance of bilingual effects. Herein, we attempt to frame this appearance at the crossroads of several factors such as the heterogeneity of the term ‘bilingual’, sample size effects, task effects, and the complex dynamics between an early publication bias that favours positive results and the subsequent Proteus phenomenon. We conclude that any definitive claim on the topic is premature and that research must continue, albeit in a modified way. To this effect, we offer a path forward for future multi-lab work that should provide clearer answers to whether bilingualism has neurocognitive effects, and if so, under what conditions.
... For a comprehensive review regarding the overlap of EC and language control brain regions, see Wu et al. (2019). Hence, it has been proposed that this linguistic-specific practice exerted by bilinguals when managing their two languages transfers to non-verbal general-domain abilities through neuroplasticity Kroll & Chiarello, 2016;Mechelli et al., 2004). ...
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Aims and objectives/purpose/research questions We characterized the impact of several bilingualism-related factors on the executive control of Spanish-Catalan bilinguals. Design/methodology/approach Participants self-reported information regarding their age of acquisition, second language proficiency and frequency of natural language switching, and performed non-linguistic tasks tapping into specific executive control subcomponents, including inhibition, switching and updating. Data and analysis Data were analyzed by means of a structural equation model (SEM) approach. Findings/conclusions Results revealed that the frequency of natural language switching positively modulated the executive control performance of Spanish-Catalan bilinguals, while neither age of acquisition nor second language proficiency had an effect. Moreover, we found that the impact of natural language switching exerted general-processing influences, affecting all subcomponents of executive control. Findings are discussed in relation to context-specific effects on the cognitive system of a particular bilingual population. Originality The current study applied an SEM approach to provide new evidence on the previously ambiguous relation between bilingualism-related factors and executive control. Significance/implications Our findings suggest that the frequency of natural language switching does globally influence the executive control of Spanish-Catalan bilinguals.
... The life experiences of bilingual individuals are remarkably heterogeneous across individuals, given the many combinations of possible languages, social backgrounds, and migratory statuses, and within individuals across a lifespan. How to meaningfully capture and represent this variance in an experimental setting is not trivial [45,85,104,105] (see Outstanding Questions), and quite plausibly, inconsistency in the bilingual advantage literature may largely be due to insufficient representation of this diversity. Further, reliance on self-reports of bilingual proficiency, which have proven to be highly variable and unreliable [106], have additionally confounded which individuals are considered bilinguals, and thus, possibly crucially, influenced who enters the exploration of bilingual advantages. ...
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Traditional research in bilingualism has consistently found that switching languages is effortful, placing demands on neural systems of cognitive control. This finding runs counter to most bilinguals’ intuitive experience. We review a body of recent work showing that, in fact, when bilinguals switch languages voluntarily, both the behavioral cost of switching and the associated recruitment of cognitive control areas are greatly reduced or completely eliminated. This suggests that switching languages is not inherently effortful, but rather, particular communicative demands may make it costly. The new evidence also challenges the basic premise underlying the bilingual advantage hypothesis. We articulate a more nuanced version of it, in which the advantage is limited to bilinguals who frequently switch languages based on external constraints.
Technical Report
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The benefits of bilingualism and bilingual education
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The neuroanatomical bases of bilingualism have recently received intensive attention. However, it is still a matter of debate how the brain structure changes due to bilingual experience since current findings are highly variable. The aim of this review is to examine these structural studies from a methodological perspective and to discuss two major methodological problems that could give rise to this variability. The first problem is sample selection, an issue directly related to the heterogeneous nature of bilingualism. The second problem is the inconsistency in the methods used for the analysis of brain imaging data. This review reveals that although structural changes related to bilingualism have been reported in regions comprising language/cognitive control and language processing, these results are not yet sufficiently numerous or consistent to allow important generalizations to be reached. Consequently, current evidence offers ambiguous support for neural models of bilingualism. This shortcoming in the field is exacerbated by critical methodological differences between studies that only further complicate the matter. We conclude by identifying issues that should be taken into consideration so that studies are more comparable and results are easier to aggregate and interpret. We also point out future directions that would allow for progress in the field.
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MRI, enabling in vivo analysis of cortical morphology, offers a powerful tool in the assessment of brain development and pathology. One of the most ubiquitous measures used - the thickness of the cortex - shows abnormalities in a number of diseases and conditions, but the functional and biological correlates of such alterations are unclear. If the functional connotations of structural MRI measures are to be understood, we must strive to clarify the relationship between measures such as cortical thickness and their cytoarchitectural determinants. We therefore sought to determine whether patterns of cortical thickness mirror a key motif of the cortex, specifically its structural hierarchical organization. We delineated three sensory hierarchies (visual, somatosensory, auditory) in two species - macaque and human - and explored whether cortical thickness was correlated with specific cytoarchitectural characteristics. Importantly, we controlled for cortical folding which impacts upon thickness and may obscure regional differences. Our results suggest that an easily measurable macroscopic brain parameter, namely cortical thickness, is systematically related to cytoarchitecture and to the structural hierarchical organization of the cortex. We argue that measurement of cortical thickness gradients may become an important way to develop our understanding of brain structure-function relationships. The identification of alterations in such gradients may complement the observation of regionally localised cortical thickness changes in our understanding of normal development and neuropsychiatric illnesses. Copyright © 2015. Published by Elsevier Inc.
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The relation between bilingualism and cognition is informative about the connection between language and mind. From the perspective of language, the question is how bilingualism might help or hinder cognition – narrowly interpreted here as executive function. From the perspective of higher cognition, the question is what kinds of experiences improve executive function. Reported cognitive benefits from bilingualism range from none to substantial as a function of age, type of bilingualism (e.g., life-long balanced vs later-onset or infrequent use of the other language), syntactic relation between the two languages, socio-economic and immigrant status, task, and laboratory. To understand the variability and inconsistencies in results with bilingualism, I analyze concepts of executive function and cognitive reserve and examine the range of factors (such as active video game playing, education, musical training, and aerobic exercise) that are known to correlate with or to improve executive function. I suggest that a) “executive function” is a complex set of cognitive processes, the components of which are sometimes minimally correlated with each other, depending on the task; b) bilingualism is inconsistently correlated with superior executive function and delayed onset of dementia; c) all speakers (mono- or bilingual) have non-linguistic ways of improving executive function; and d) benefits from bilingualism – and all cognitively challenging activities – are inconsistent because individuals vary in the number and kinds of experiences they have that promote superior executive function.
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Over 90 years ago, anatomists noted the cortex is thinner in sulci than gyri, suggesting that development may occur on a fine scale driven by local topology. However, studies of brain development in youth have focused on describing how cortical thickness varies over large-scale functional and anatomic regions. How the relationship between thickness and local sulcal topology arises in development is still not well understood. Here, we investigated the spatial relationships between cortical thickness, folding, and underlying white matter organization to elucidate the influence of local topology on human brain development. Our approach included using both T1-weighted imaging and diffusion tensor imaging (DTI) in a cross-sectional sample of 932 youths ages 8-21 studied as part of the Philadelphia Neurodevelopmental Cohort. Principal components analysis revealed separable development-related processes of regionally specific nonlinear cortical thickening (from ages 8-14) and widespread linear cortical thinning that have dissociable relationships with cortical topology. Whereas cortical thinning was most prominent in the depths of the sulci, early cortical thickening was present on the gyri. Furthermore, decline in mean diffusivity calculated from DTI in underlying white matter was correlated with cortical thinning, suggesting that cortical thinning is spatially associated with white matter development. Spatial permutation tests were used to assess the significance of these relationships. Together, these data demonstrate that cortical remodeling during youth occurs on a local topological scale and is associated with changes in white matter beneath the cortical surface. Copyright © 2015 the authors 0270-6474/15/350599-11$15.00/0.
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The use of two or more languages is common in most of the world. Yet, until recently, bilingualism was considered to be a complicating factor for language processing, cognition, and the brain. The past 20 years have witnessed an upsurge of research on bilingualism to examine language acquisition and processing, their cognitive and neural bases, and the consequences that bilingualism holds for cognition and the brain over the life span. Contrary to the view that bilingualism complicates the language system, this new research demonstrates that all of the languages that are known and used become part of the same language system. The interactions that arise when two languages are in play have consequences for the mind and the brain and, indeed, for language processing itself, but those consequences are not additive. Thus, bilingualism helps reveal the fundamental architecture and mechanisms of language processing that are otherwise hidden in monolingual speakers.
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It is a widely held belief that bilinguals have an advantage over monolinguals in executive-control tasks, but is this what all studies actually demonstrate? The idea of a bilingual advantage may result from a publication bias favoring studies with positive results over studies with null or negative effects. To test this hypothesis, we looked at conference abstracts from 1999 to 2012 on the topic of bilingualism and executive control. We then determined which of the studies they reported were subsequently published. Studies with results fully supporting the bilingual-advantage theory were most likely to be published, followed by studies with mixed results. Studies challenging the bilingual advantage were published the least. This discrepancy was not due to differences in sample size, tests used, or statistical power. A test for funnel-plot asymmetry provided further evidence for the existence of a publication bias. © The Author(s) 2014.
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Normal aging is an inevitable race between increasing knowledge and decreasing cognitive capacity. Crucial to understanding and promoting successful aging is determining which of these factors dominates for particular neurocognitive functions. Here, we focus on the human capacity for language, for which healthy older adults are simultaneously advantaged and disadvantaged. In recent years, a more hopeful view of cognitive aging has emerged from work suggesting that age-related declines in executive control functions are buffered by life-long bilingualism. In this paper, we selectively review what is currently known and unknown about bilingualism, executive control, and aging. Our ultimate goal is to advance the views that these issues should be reframed as a specific instance of neuroplasticity more generally and, in particular, that researchers should embrace the individual variability among bilinguals by adopting experimental and statistical approaches that respect the complexity of the questions addressed. In what follows, we set out the theoretical assumptions and empirical support of the bilingual advantages perspective, review what we know about language, cognitive control, and aging generally, and then highlight several of the relatively few studies that have investigated bilingual language processing in older adults, either on their own or in comparison with monolingual older adults. We conclude with several recommendations for how the field ought to proceed to achieve a more multifactorial view of bilingualism that emphasizes the notion of neuroplasticity over that of simple bilingual versus monolingual group comparisons.
Structural asymmetry varies across individuals, brain regions, and metrics of cortical organization. The current study investigated regional differences in asymmetry of cortical surface area, thickness, and local gyrification, and the extent of between-subject variability in these metrics, in a sample of healthy young adults (N=200). Between-subject variability in cortical structure may provide a means to assess the extent of biological flexibility or constraint of brain regions, and we explored the potential influence of this variability on the phenotypic expression of structural asymmetry. The findings demonstrate that structural asymmetries are nearly ubiquitous across the cortex, with differing regional organization for the three cortical metrics. This implies that there are multiple, only partially overlapping, maps of structural asymmetry. The results further indicate that the degree of asymmetry of a brain region can be predicted by the extent of the region’s between-subject variability. These findings provide evidence that reduced biological constraint promotes the expression of strong structural asymmetry.