A recent neuroimaging studies to investigate the neural correlates

A Review of the Cognitive
Neuroscience of Executive Function in Bilinguals

Introduction

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Executive function refers
to three main functions including; updating information in
the working memory, inhibiting spontaneous responses and shifting between tasks
and mental sets (Miyake et al., 2000). These executive functions involve the frontal lobes and premotor cortex of the brain and although they are moderately correlated,
they are clearly separable functions. (Miyake et al., 2000)

In recent years there has been a contentious debate about whether
bilinguals have enhanced cognitive
control compared
to monolinguals which may give them an advantage in cognitive tasks and
executive functions. Hernandez et
al., 2015). This notion is known as the bilingual advantage (Kroll & Bialystok, 2013) and has been supported through results from non-linguistic
cognitive control tasks such as the Simon task and Stroop test, (Bialystok, 2009) (Soveri, Rodriguez-Fornells & Laine,
2011). Bialystok and colleagues (2004) ). These findings supporting the bilingual advantage have not
been consistent (examples) and the differences observed between
monolingual and bilinguals are more likely due to uncontrolled factors such as
socioeconomic status (SES), socio-cultural demographics, language-switching and
individual differences which can be difficult to separate from bilingualism (YANG, YANG & LUST, 2011) (Anton
et al 2014) (Hernandez, Greene, Vaughn, Francis
& Grigorenko, 2015).

Presently, no consensus has been reached about which facet
is specifically improved in bilinguals executive function however, research by Bialystok et al. (2004), (Bialystok, Craik, Green & Gollan, 2009)and Peal and Lambert (1962) suggest the inhibition and shifting
components of executive function may be advanced since bilinguals have demands
for language switching and must deal with interfering responses from their
second language(Miyake et al., 2000). (Stocco, Yamasaki, Natalenko & Prat,
2012).Bialystok, 2001; Bialystok & Martin, 2004; Carlson & Meltzoff, 2008; Festman, Rodriguez-Fornells, & Münte, 2010). This essay will focus on shifting in bilinguals
with an aim of using recent
neuroimaging studies to investigate the neural correlates of shifting and will explore
the brain processing differences while shifting between bilinguals and
monolinguals, taking inot account the research limitations. Finally, this essay
will discuss how differences in the brain while shifting may be related to the behavioural findings in bilinguals and monolinguals (Luk
et al., 2012). Until now, bilingual
language processing has been studied primarily at the behavioural level with researchers
focused on which group is better. This essay joins with (Hernandez et al., 2015) to advocate
that researchers strive to understand more about cognitive development, executive
function and language processing in both bilinguals and monolinguals instead of
proving who has the advantage. (Hernandez
et al., 2015)

Shifting in the bilingual brain

Shifting is one of the key executive
functions, it refers to a person’s ability to shift back and forth between
several tasks or mental sets. Shifting is also commonly referred to as task-switching.
Hofmann,
Schmeichel & Baddeley, 2012). Recent neuroimaging studies suggest a neural overlap between language
control and non-verbal cognitive control in bilinguals especially involving the
basal ganglia (De Baene et al., 2015) Abutalebi and Green (2008) Garbin et
al., 2010, 2011; Guo, Liu, Misra, & Kroll, 2011; Abutalebi & Green, 2008; Wang, Xue, Chen, Xue, & Dong, 2007; Crinion et al., 2006; Hernandez, Dapretto, Mazziotta, & Bookheimer, 2001; Hernandez et al., 2000). Additionally De Baene
et al., 2015) examined the overlap in brain activation in between
linguistic switching activities and a closely matched non-linguistic switching
task in highly proficient Spanish–Basque bilinguals, results from the Fmri strongly
suggests that bilinguals share similar brain circuits for language control and
general cognitive control. Using the activation likelihood
estimation method (Luk et
al., 2012) examined the
neural regions involved in bilingual language switching, their results
highlighted frontal left lateralized clusters including; middle frontal gyrus
(BA 9, 46), midline pre-supplementary motor area (BA 6), left inferior frontal
gyrus (BA 44 and 47), left middle temporal gyrus (BA 37), right superior
temporal gyrus (BA 22), right precentral gyrus (BA 6) and bilateral caudate nuclei
(Figure 1).(Luk et al., 2012), five of these eight regions were also identified
by (Green & Abutalebi, 2008). Although (Green & Abutalebi, 2008), and luk highlight the activation of
ACC in bilingual language switching, (Velanova, Wheeler & Luna, 2008 suggests that ACC is more
involved with error monitoring and detection as less activity is detected during
correct trials and more activity during error trials). (Luk et al., 2012. Overall, the
prefrontal cortex and basal ganglia are involved in both linguistic switching and non-linguistic
switching, this neural
computation overlap suggests that language-switching
is key in understanding executive function in the bilingual brain and helps us to understand how the shifting occurs in the
brain. (e.g., Hernandez,
Martinez, & Kohnert, 2000; Rodriguez-Fornells
et al., 2005). (Stocco,
Yamasaki, Natalenko & Prat, 2012 (basal ganglia involved in shifting (Yehene,
Meiran, & Soroker, 2008)(e.g., Lehtonen
et al., 2005; Price,
Green, & von Studnitz, 1999))

The Conditional Routing
Model

In 2010, Stocco and colleagues proposed the conditional routing model
which provides a neural framework explaining the enhanced executive function in
bilinguals. The conditional
routing model is based on evidence indicating the neural mechanisms which
control language are connected to the neural mechanisms of the shifting
component in executive function(Stocco,
Yamasaki, Natalenko & Prat, 2012), particularly the basal ganglia circuitry(REF)
and evidence that this circuit is involved in tasks where bilinguals outperform
monolinguals (REF).(Stocco, Lebiere
& Anderson, 2010) (Stocco, Yamasaki, Natalenko & Prat, 2012)

Bilinguals’ constantly need to apply and switch between languages
and grammatical rules and according to the conditional routing model the basal
ganglia circuit is responsible for this selection and switching, in particular
the striatum (Stocco, Yamasaki, Natalenko & Prat, 2012). The basal ganglia are a set of
distinct grey matter nuclei located in the middle of the brain which form a
complex circuit that send signals to the frontal lobe(Albin,
Young, & Penney, 1989; DeLong,
1990) (Stocco,
Yamasaki, Natalenko & Prat, 2012). The largest structure within the basal ganglia is the
corpus striatum (referred to as striatum), which includes the caudate
nucleus(CN) and putamen (Purves, Augustine &Fitzpatrick, 2001). The
striatum is the input station of the circuit and connects
to the cortical regions and controls signals to the prefrontal cortex, which is
associated with higher order functions such as shifting(Garbin et al., 2010).The caudate
nucleus receives and transmits information to the dorsolateral prefrontal
cortex and plays a strong role in cognitive control. Abutalebi,
2008). (Hernandez et
al., 2015). When bilinguals engage in shifting activities or language
switching the striatum routes information to the frontal cortex and (Stocco, Lebiere & Anderson, 2010) propose
that extensive bilingual experience
reinforces the basal ganglia’s ability to manage signals across cortical
regions, resulting in bilinguals enhanced performance in executive
function tasks such as x and y. (Hernandez
et al., 2015) (Garbin et al., 2010)

The conditional routing model is supported by observations
of patients with selective damage in the basal ganglia caused by diseases such
as Parkinson’s and Huntington’s. This damage leads to a decline in the shifting
component of executive function, finally resulting in poor abilities to switch
efficiently between tasks and mental states (Stocco, Yamasaki, Natalenko &
Prat, 2012)(REF and images of damaged basal ganglia). Additionally, bilingual
patients with injuries impacting the basal ganglia circuit tend to have uncontrolled
language-switching (Fabbro,
2001)(Stocco,
Yamasaki, Natalenko & Prat, 2012)

Differences between the monolingual
and bilingual brain

According to the conditional routing model, bilinguals rely
on the basil ganglia for shifting and language switching (Stocco, Yamasaki,
Natalenko & Prat, 2012) however, research by Garbin et al. (2010) highlights significant brain network differences between
monolingual and bilinguals when task switching. Garbin et al. (2010) found larger oxygen consumption in the
right inferior frontal gyrus (IFG) and the anterior cingulate cortex of
monolingual participants, while bilinguals show a reduced switching cost and
activated the left IFG and striatum while shifting. Furthermore, the size of the switch costs was negatively
associated with the activity of the left caudate nucleus in bilinguals,
suggesting that the recruitment of the CN results in faster switches (Stocco,
Yamasaki, Natalenko & Prat, 2012). Research
by (Vaughn et al., 2015) focused
on differences in brain activity during the Simon task based across a range of
bilinguals, considering age of acquisition and proficiency, results suggest later
age of acquisition predicts greater activity in the left inferior parietal
lobule, while better proficiency predicts less recruitment of the dorsolateral
prefrontal cortex and anterior cingulate cortex. These findings suggest that
the age of second language acquisition may predict how the brain handles cognitive
tasks, this is important as it suggests variations in language experience may
influence brain networks, executive
function and behaviour(Vaughn et al., 2015).

Another significant difference between the bilingual and the
monolingual brain is that the bilingual brain relies on the left caudate
nucleus to switch between tasks, while the monolingual brain does not (Garbin et
al., 2010)(Stocco, Yamasaki, Natalenko & Prat, 2012). Regardless of the bilingual advantage, bilingual’s
neural activity during these shifting tasks are different to monolinguals,
which indicates that the bilingual brain processes these executive function
tasks differently. These researchers provide
information about the potential neural network differences between bilinguals
and monolinguals while task switching however, further research is needed to
investigate the neural differences between monolingual and bilinguals across
the other executive functions.