Language experience and the brain: Variability, neuroplasticity, and bilingualism

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COMMENTARY
Language experience and the brain: variability, neuroplasticity, and bilingualism
Judith F. Kroll
a
and Christine Chiarello
b
a
Department of Psychology, Center for Language Science, Pennsylvania State University, University Park, PA 16802, USA;
b
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 specific 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
bilingualism.
The word “hazy”in 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
monolinguals.
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 authors’guiding 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 conflating two sets of
© 2015 Taylor & Francis
CONTACT Judith F. Kroll jfk7@psu.edu
Invited commentary on García-Pentón et al. (2015). The neuroanatomy of bilingualism: How to turn a hazy view into the full picture.
LANGUAGE, COGNITION AND NEUROSCIENCE, 2015
http://dx.doi.org/10.1080/23273798.2015.1086009
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linking hypotheses that have yet to be fully explicated.
The first 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 specific 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 benefit 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, proficiency 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 gyrification among healthy young
adults (N= 200) as indexed by the coefficient 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 findings than cross-sectional investi-
gations. This is as would be expected if individual vari-
ation were a large contributor to the conflicting cross-
sectional findings. 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 significant 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 influenced the varying find-
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
2J. F. KROLL AND C. CHIARELLO
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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 find-
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 findings
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 findings that has created an illusion
that there is any evidence for bilingual benefits (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 first step, but one that is quite pre-
liminary, in speculating about why different structural
imaging studies on bilinguals have produced conflicting
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 first
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
proficient 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 findings across
different tasks, measures, and people may be difficult
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-
simplification of earlier accounts. We need bold new
LANGUAGE, COGNITION AND NEUROSCIENCE 3
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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 conflict of interest was reported by the authors.
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