ABSTRACT a direct correlation between age of acquisition and




            Over the past decade, researchers
have been investigating how language affects the brain centers, and they have
been utilizing several different neuroimaging techniques to aid in
understanding this process. The primary goal of this literature review was to
explain the level at which bilinguals process language differently from
monolinguals, and how it reconfigures the structural network of the brain.
Further, how the age at which an individual learns their first and second
language contributes to the aforementioned was explored. Several articles were
analyzed to provide an overview of the different studies conducted to explain
the aims of this review.  Several studies
were discussed to exemplify how the left hemisphere of the brain has the
strongest neuronal activations during the use of language, and the concept of
executive functioning in regards to bilinguals. In current literatures, the
areas of the brain that serve great functional importance to the bilingual
brain are as follows: inferior parietal cortex, inferior parietal lobe,
inferior frontal gyrus, superior temporal gyrus, dorsolateral prefrontal cortex
and lastly the rostrolateral prefrontal cortex. Additionally, there is a direct
correlation between age of acquisition and language proficiency. These studies
are very detailed in both their findings and explanations, but continued
research in these areas can possibly lead to new discoveries that indicate
areas in either the left or right hemisphere of the brain being activated when
monolinguals learn a second language.  

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bilingualism, imaging, structure, welsh, Chinese, function



Language contributes to the uniqueness of human
abilities and encompasses the usage of different parts of our brain. Most
humans reside in different developed environments and are surrounded by
individuals from various cultural backgrounds. Therefore, we have all
encountered someone who knows more than one language, including ourselves. The
Center for Immigration Studies reported that approximately 66.8 million
Americans, five years of age and older , speak more than one language including
English, which has risen by about 2.2 million since the year 2010 (Camarota et al, 2014).  This aforementioned ability is called
bilingualism, which researchers define as a multidimensional phenomenon
attributed to humans who use one or more spoken languages regularly throughout
their daily lives, and this capability can be measured by the frequency at
which an individual uses his or her language and their capacity to develop this
skill (Buchweitz & Prat, 2013). According
to Bialystok et al (2012), bilingual
individuals have a stronger advantage than monolinguals during old age because it
lessens the risk of cognitive decline, being that they are unknowingly forced
to use their brains frequently when switching between their languages. Many
studies have shown how the age of acquisition of a second language can
significantly contribute to the structural and functional connectivity in the
brain, whether it was learned “sequentially” or “simultaneously” (Berken et al¸2016).  Researchers and scientists have utilized
several different types of neuroimaging techniques to provide quantitative data
and visual evidence that revealed how processing more than one language can
affect the way in which the brain functions and the structural changes that can
result.  The primary issues regarding the
neural basis in bilingualism that will be investigated are the following: different
aspects of processing language in association with the different areas of the
brain that are affected as a result and the connection between age of
acquisition and its effect on language proficiency.


Readings assignment)

            The articles in the areas of
research pertaining to the neural basis in bilingualism were found through
several online databases. A majority of the articles were found through United
States (US) National Library of Medicine for the National Institutes of Health,
specifically called “PubMed”. This database is tailored towards scientific
articles, and the specific format used to search for these articles is
“structure AND bilingual brain”. The articles that came up were then
additionally refined to a specific type of article (journal), text availability
(free full text), and publication dates (6 years). The articles I chose were
the ones with key words or phrases such as “brain structure”, “bilingualism”,
“function”, etc. The journal of neuroscience was another database that was
largely utilized. In the search box, I typed “bilingualism AND brain” A list of
articles would come up and I would click on the ones that were relevant to my
studies. For both of these databases, I would first read the abstract of each
article and then move on to the introduction and discussion to ensure that the
information was best suited for my discussion and if it was, then I would
download the digital copy to begin analyzing. The only issue that I faced when
beginning to construct this scientific review was choosing the best and most
relevant articles when evaluating them because some of them discussed the same
information, while others contained necessary information. Lastly, I utilized a
neuroscience textbook n aim of defining and
giving the functions of specific brain areas.



are very distinct, complex structural components of the bilingual brain that
are activated during language processing. Both the left and right hemispheres
of the brain utilize cognitive abilities when pertaining to language, however
the left hemisphere plays a primary role in language itself because it plays a
role in writing, speaking, hearing, and interpreting (Purves et al, 2018). Firstly, the inferior
parietal lobe and cortex are responsible for processing sensory, visual,
auditory and vestibular signals, which allows us to integrate our senses when
tasting or touching something. It also gives us a sense of the “neutral
construct” of our bodies, where the different parts are located and how
movement of these parts are initiated (Purves et al, 2018). Secondly, the caudate nucleus controls how the brain
learns in the act of storing and processing memories, which is important
throughout development and language processing. However, the left caudate
nucleus specifically contributes to a person’s communication skills. The
dorsolateral prefrontal cortex plays a role in the planning and organization of
behavior, whereby a reflex response occurs when sensory information is inputted
into the brain followed by a motor output. For example, if you are stuck in
traffic causing you to be potentially late for work, then you would think of an
alternative route to arrive on time (Purves et
al, 2018). According to Wong et al (2015),
there was a large difference in grey matter density for bilinguals as compared
to monolinguals in both the inferior parietal cortex and inferior parietal
lobe, which are all effects of how well the individuals knew and used the
language as well as their heightened control over cognition. Grey matter is the
neuronal cell bodies that constitute the brain tissue, and its density in the
specific region of the brain that positively correlates with certain skills and
cognitive abilities (Purves et al, 2018).  Most of the results from the aforementioned
were obtained using voxel-based morphometry (VBM), which is a technique that
uses statistics to reveal anatomical differences in the brain between groups
(Whitwell, 2009). Nonetheless, there were additional findings that showed
higher “grey matter volume in the left caudate nucleus in bilinguals than
monolinguals because the caudate nucleus works synonymously with the
dorsolateral prefrontal cortex,  and
since these areas are responsible for executive functions such as selective
inhibition and working memory; the bilingual brain shows a more significant
activation because bilinguals induce these functional changes when they switch
from one language to another due to their enhanced cognitive control (Lehner et al, 2016).  Lastly, the inferior frontal gyrus (responsible
for comprehending language), superior temporal gyrus (responsible for encoding
language content), dorsolateral and rostrolateral prefrontal cortexes were
specific areas in the brain that showed higher activation in bilinguals than
monolinguals during a reading task. The rostrolateral prefrontal cortex mainly
processes abstract thoughts, analogical reasoning and “episodic memory
retrieval”, which all require high cognitive control (Westphal et al, 2015).

The differences in the monolingual brain
performing a language task similar to the aforementioned can be seen through a
study performed by Bingjiang et al (2016),
which explains why the human brain responds to spoken language not only by
understanding and identifying words, but also deriving their meanings through
“contextual information”. These researchers conducted a study to test how the
brain maps the correlation between sound and meaning through the processing of
speech by using Functional Magnetic Resonance Imaging (fMRI),  which demonstrates regional and time-varying
changes in brain metabolism (Glover, 2011). The participants were thirty
right-handed Mandarin Chinese speakers all with normal hearing or
corrected-to-normal vision. There were no reports of the subject’s having any
language or mental impairment, but one female displayed large head motions
during the fMRI scanning, so she was excluded from the results (Bingjiang et al, 2016). There were three types of
auditory stimuli (total of 84) presented to the participants: time-versed
phrases (TPs), expected phrases (EPs), and unexpected phrases (UPs). The Eps
were Chinese idioms with three to five characters and UPs were the idioms where
the first two characters were kept and the last was replaced with illegitimate
characters. TPs were made from both UPs and Eps to make a “low-level acoustical
match”, so that the spectrums of the sounds and “voice identity” were kept and
allowed the idioms to seem less logical. The fMRI results revealed that the
left anterior superior temporal gyrus (aSTG) had stronger activation analogous
to UPs and as well as the ventral inferior frontal gyrus (IFG) relative to the
“phonological-semantic prediction of spoken words” (Bingjiang et al, 2016). Additionally, the study
found that the superior temporal lobe is responsible for processing and
organizing words and phrases of different lengths and regions of the brain from
the left aSTG to the anterior superior temporal sulcus (STS) become activated
in this process. The superior temporal sulcus can be differentiated into two
functional aspects: language processing and social cognition (Redcay, 2008). For
example, a person is socially cognizant when they are able to be empathetic towards
other individuals, and also to make eye contact when have a respectful conversation
with another person. Lastly, the posterior middle temporal gyrus (primarily
responsible for encoding language context and motion discrimination) displayed
relatively strong activation for both UPs and Eps.



            According to Wong et al (2015), the structural components
of the brain serve as a visual aid in understanding these “architectural
language networks”, and the functions explain how these networks are aligned
through either personal experiences or different contexts (Wong, Ying &
O’Brien, 2015). Studies indicate that bilingual individuals are constantly
selecting and omitting specific words from their vocabulary when switching
between their first and other spoken languages, which enables them to have a
select advantage over monolinguals in the level of their cognitive abilities.
There is the concept called executive functioning, whereby it is a set of
nonverbal, neurological functions that assist individuals in managing simple or
complex daily tasks. Inhibition, shift and update are the main executive
functions that are relevant to language. Inhibition encompasses the ability to
stop an automatic, behavioral response as needed and shift is the ability to
freely transition between situations or tasks. Finally, update refers to the brain’s
capacity to retain information and replace it with newly acquired information
(Wong et al, 2015).

            A study was conducted to test the
frequency of executive functioning in Welsh-English bilinguals. The principal
investigators, Wu and Thierry (2013), formulated two hypotheses stating: 1) if
bilinguals have a universal, fixed advantage for executive functioning, then
cognitive control is independent of linguistic context or 2) if the
aforementioned advantage is dependent on context, then cognitive control is
possibly prominent when bilinguals are exposed to each of their spoken
languages. There were eighteen Welsh-English bilinguals who participated in the
study and they were right handed with correct-to-normal vision (nine males and
nine females; 20.4 +/- 2.1 years). These individuals learned both languages at
the toddler stage and spoke only Welsh and English. Therefore, the subjects
rated their proficiency in both languages on a scale from 1 to 10 with 1 being
“very poor” and 10 being “perfectly fluent” in regard to their level of
speaking, reading, listening and writing (Wu et al., 2013).  The results
of the ratings were English (8.9 +/- 1.1) and Welsh (9.8 +/- 0.5).

            The stimuli were arrow displays with
five horizontal arrows and the third arrow either did or did not match the
direction of the “flanking” arrows. The different arrangements of the arrows
included: all five pointing to the right, all pointing to the left, each
pointing to the left with the third arrow pointing to the right, and lastly each
arrow pointing to the right with the third arrow towards the left. A total of
270 “high-frequency nouns” in both English and Welsh were displayed across 3
blocks of 90 arrow arrangements. The monolingual blocks had words in either
English or Welsh only and the bilingual blocks had half in Welsh and the
remaining half in English. There were 18 different stimuli created so that the
presentations were random among each participant (Wu et al., 2013). 

series of stimuli were presented to the participants, who were approximately 1
meter away. They were directed to press the right button if the stimuli, which
is an arrow display, pointed right and the same for the left. In addition to
the arrow displays, there were a set of words intermixed with them so as to act
as both a distractor and an indicator of the subjects’ cognitive abilities. The
participants stared at a blank screen for 200 milliseconds (ms) and then
another display for 500 ms, where a cross appeared at the center of the screen.
The different stimulus, then appeared on the screen for about 1500 ms and then
at different intervals for 2,000
ms. At the end of the trials, there was a questionnaire that contained the same
words from the displays with approximately 270 added words and the participants
had to pick the ones they recalled seeing altogether.  (Wu et
al., 2013). 

The electrophysiological data (ERP) was
recorded throughout the study to measure the mean amplitude of
non-corresponding and corresponding conditions through the 63 electrodes arranged
across each person’s scalp. The data indicated a slightly elevated amplitude
when a combination of the languages was presented at a duration of 400 to
800ms, and an increasingly high amplitude for non-correspondence in either
Welsh or English. On the contrary, the behavioral study indicated that the
reaction times for the correspondence factor in Welsh and English contexts were
about the same at 610ms and 630ms in mixed contexts. In the non-corresponding
trail, English had the highest mean reaction time (690ms) as opposed to Welsh
and mixed contexts both slightly decreasing at 660ms. According to the results,
the subjects facilitated the non-corresponding trails better, as indicated by
the decreased reaction times for the mixed blocks, because the different blocks
for the corresponding trails did not display any significant differences.
Nonetheless, the results support the second hypothesis that states executive
function depends upon the language context in bilinguals. In other words,
bilinguals can incorporate and utilize different inhibitory mechanics when
presented with a mixture of words from their spoken languages (Wu et al., 2013). 



Age of Acquisition (AoA) involves comprehending
a piece of information at an earlier stage of development, thus fostering a
quicker response time in adulthood. Some studies used different imaging
techniques to show how this phenomenon can affect the white or grey matter of
the brain (Lake & Cottrell, n.d). Grogan et al (2012) studied multilinguals with differences in their
lexical (lingual) efficiency in comparison to their various number of spoken
languages. A total of sixty-one right handed individuals from the United
Kingdom participated in the study. They were all between the ages of 18 and 29
years and learned English after their native language. Of the sixty-one, 31
were multilinguals who spoke English and several other languages (a range of 3
to 6 languages with 1 of them being native), and 30 were bilinguals who spoke
English and their native language (Grogan et
al., 2012).

The criteria for the AoA of the English
speakers in this study were: when did the subjects first learn English, how
much English was used, and how they rated their language proficiency with 1
being low proficiency and 9 being high proficiency. Additionally, the
researchers measured the subject’s efficiency in producing English words
through a “letter fluency” assignment, where the subjects wrote down as many
words as they can starting with the letter “s” in one minute. A letter or two
were changed from actual English words to measure how well they could recognize
English words, which was measured through a computerized lingual decision assessment
The trials consisted of 60 illegitimate words and 60 legitimate words divided
into a total of six blocks, whereby the subjects press a button for the words
they thought to be legitimate (Grogan et
al., 2012).

The data were collected using magnetic
resonance imaging (MRI) to measure the speed and accuracy of the indicated
response by taking high quality images of the brain. Also, voxel-based morphometry
(VBM) images were taken to identify the structures in the brain (Wong et al., 2015).  These images were either modulated, which
means they are fixed to quantify the volume of a given area and the unmodulated
images calculate tissue density. The researchers wanted to compare the brain
images for bilingual and multilingual groups without AoA, and each group
separately. Essentially, both groups were tested for the findings of a
correlation between grey matter and lexical efficiency (excluding AoA), grey
matter with AoA (excluding lexical efficiency) and both together (Grogan et al., 2012). The unmodulated images
indicated an increased density of grey matter in multilinguals in the posterior
supramarginal gyrus (pSMG) of both the left and right hemispheres of the brain.
The supramarginal gyrus makes up part of the parietal lobe of the brain,
previously discussed, and it plays a major role in emotional responses
(empathy) and phonological processing (Purves et al, 2018). The lexical efficiency trial showed a direct
correlation between the aforementioned criteria and grey matter density. An
increase in high lexical efficiency directly correlated to an increase in grey
matter and vice versa, but causes a low AoA in the left inferior frontal cortex
(Grogan et al., 2012).



The phenomenon known as bilingualism and its
effect on the human brain allows a simple concept like language to become
vastly interesting. Often times, we understand that a second language is
developed through our parents or being learned. However, one rarely considers
how the neural network becomes altered as a result of processing more than one
language at once, and the different types or levels of this. The studies
discussed thoroughly explain how the capabilities of a bilingual individual can
alter the brain structurally and functionally in regards to the different types
of executive functioning, such as inhibition, shift and update, as well as the
correlation between language proficiency and age of acquisition.  The study performed by Wong et al (2015) measured the differences in
grey matter density in bilinguals and monolinguals using voxel-based
morphometry. Mostly the left hemisphere of the brain was affected since it
plays a role in language, reasoning and working memory (Lehner et al, 2016).  The specific areas that exhibited increased
grey volume density in bilinguals when compared to monolinguals were both the
inferior parietal cortex and lobe (sensory, visual and auditory processing that
serves great importance when learning a new language), the inferior frontal
gyrus (understanding language), superior temporal gyrus, dorsolateral and rostrolateral
prefrontal cortex (major functions of planning and organizing behavior as well
as processing utilizing the highest level of cognitive skills and
decision-making).  Consequently, a
similar study to the aforementioned was discussed briefly to establish the
areas activated in a monolingual brain during language processing and they were
slightly similar to the ones in the bilingual brain. The activated brain areas
were the left anterior superior temporal gyrus, ventral inferior frontal gyrus,
anterior superior temporal sulcus (executing social cognition by understanding
different social cues such as empathy and kindness), and posterior middle
temporal gyrus (language content and deciphering motion). Lastly, studies were
discussed to show how the age at which a person acquired their first and second
languages can affect different cognitive abilities and proficiency in language.
For future applications, additional studies should be performed to investigate
and discover if the same or newer areas in the left or right hemispheres of the
brain are being activated when learning a second language (both writing and