Introduction


Second Language Acquisition Language acquisition has been intensely studied since it is a critical developmental stage. It is a very demanding cognitive task as it involves learning of memorization of tens of thousands of words in a language’s vocabulary, recognition based on visual and auditory cues, acquisition of literacy (for example, reading comprehension) and manipulation of prosody. From the neuroscience perspective, language acquisition therefore yields a great deal of questions of how different cognitive processes are accomplished by underlying neurological pathways.
Studying of such underlying mechanisms has been difficult due to the fact that spoken and written language is an exclusively human behavior and the lack of appropriate animal models limits researchers to non-invasive methods. To date, our knowledge regarding language acquisition involves specific brain regions that have been observed to show increased activity (based on brain imaging) during certain linguistic tasks. Moreover, second language acquisition is defined as learning of a non-native language. Many studies, using non-invasive brain imaging, tried to compare the neurological activities involved in manipulation of the two languages. It has been suggested that, on the neurobiological basis, the acquisition and manipulation of the second language is at least in part separate from the native language.1 Lastly, several studies showed levels of activity in certain brain region seem to predict the proficiency in cognitive verbal tasks.

1.Native Language Acquisition


1.1a. Cortical Organization

Numerous studies of functional imaging have mapped out major cortical regions activated during linguistic processes. Since language is a multifaceted skill, different aspects of linguistic knowledge involve different neural correlates needed for different computational demands and therefore elicit different patterns of activation of the cortex.
Lexicon (inventory of vocabulary): Studies investigating neurological processes involving lexical tasks elicits high levels of activation in the Broca’s Area (left Inferior Frontal Gyrus), Wernicke’s area (left Superior Temporal) and fusiform gyri,1all of which constitute the classical model of language faculty within the brain.
Syntax(grammatical rules of a language): Summarizing from functional imaging and lesion studies, Friederici’s model of syntax neural processes involves three stages: the first stage involves superior temporal gyrus, the second stage of lexical and syntactic integration involves Broca’s area (middle temporal gyrus, inferior frontal gyrus) and the third stage of syntax reanalysis involves the activation of posterior superior temporal gyrus and the basal ganglia. 2

1.1b. Functional Lateralization

The majority of brain activity involved in language is elicited in the left hemisphere. Many functional imaging and Event Related Potential (EPR) studies show asymmetrical activation of the two hemispheres during linguistic tasks (e.g. speech production.)3 Generally, neurological processes are much more frequently represented in the left hemisphere instead of the right hemisphere. Cerebral dominance, therefore, refers to the specialization of one hemisphere managing certain functions or computational tasks.4 Cerebral lateralization of language function begins at the age of 55 and the on-set of lateralization is unclear until recently, Badzakova suggested that a single gene may be responsible for lateralization of many functions in the human brain.4


2 . Second Language Acquisition

As mentioned earlier, Age of Acquisition (AOA) of second language (L2) is a significant influencing factor for acquisition efficiency, proficiency and the neural representation of L2. Different languages can exceptionally vary in terms of grammar and vocabulary (e.g. Spanish vs. Korean,) therefore learning an additional language possibly leads to additional recruitment of neural networks and/or modifications of existing ones. Through functional imaging studies, second language acquisition and utilization share most of the same neural correlates representing the native language. At the same time, however, scientists noted the second language of a late learner elicit higher activation in a few key regions and demonstrate a much less degree of functional lateralization.

2.1 a. Cortical Organization


Convergent neural networks:

Most brain imaging studies agree that people acquired a second language at later stages of life (late bilinguals) share the basic linguistic cortical representations with people who only know one language.6 Some studies such as conducted by Illes et al. also showed that the majority of neurological activations by L1 and L2 occur in the same regions in late bilinguals.7 These findings suggest that aspects of two languages are likely to be processed in similar ways, perhaps by same populations of neurons in very similar neural circuits.
In a study by Park et al., brain activation was compared during verbal tasks of monolingual speakers of English and bilingual speakers of Macedonian (L1) and English(L2).1 It was found that most of the activated sub-cortex for L2 in bilinguals closely resembles the sub-cortex activated by L1 tasks, which is also very similar to the monolingual control.

Divergent neural networks:

While majority of the sub-cortex activation pattern is the same for the native and the second language, many scientist point out that there are key differences between the bilingual and monolingual brain. In the same study done by Park et al., the investigators found that during verbal tasks involving L2, the inferior and middle frontal gyri, the superior parietal lobule and the angular gyri on the right hemisphere is significantly more activated in the right hemisphere compared to the monolingual brains.1 Bilingual brain lesion studies show that damage to the frontal cortex will result in inappropriate switching (of a few words) between languages8, suggesting that this region may be used for organizational demand such as separation of languages. Other additional neural networks are recruited for the additional demands involved in knowing and using one more language.

2.1. b. Degree of lateralization:

One of the key findings of the study by Park et al. was that the degree of left hemisphere lateralization is significantly reduced in late bilinguals.1 During verbal task involving the native language, subjects showed more activity in a number of right hemisphere cortical regions compared to the monolinguistic controls (such as right superior occipital gyrus, superior parietal lobule, insular lobe and middle temporal gyrus.) When utilizing L2, the bilingual subjects showed greater bilateral activation in superior temporal gyrus and other regions such as right inferior frontal occipital gyrus and the right cerebellum. 1
Such change in lateralization and recruitment of right cortical structures can be the response to new storage or processing demands of a new language. However, some of the activated right hemisphere structures are not previously known to be involved in linguistic functions and they could be recruited by higher computational demands of dealing with a new language (depending on the L2 proficiency.)


3. Proficiency

The level of proficiency of the second language is also an important moderating factor. Different bilingual individuals with varied levels of proficiency in L2 will different neurological representations.

3.1Cortical organization of L2 also depends on proficiency

One study by Abutalebi et al. found that the patterns of activation were different between individuals of high and low language proficiencies9. It was concluded, therefore that proficiency (rather than age) was the most important factor determining the linguistic cortical organization and neurological processing.
In terms of syntactic processing, studies have shown that as the language proficiency of a late bilingual improves, activation level increases in superior temporal gyrus at the expense of activity of inferior frontal gyrus. This trade off of activity is not observed in monolingustic individuals.

3.2A neurological marker for proficiency

A study by Tan et al. has demonstrated that during verbal tasks, the activity levels in left caudate nucleus and fusiform gyrus effectively predict individual’s proficiency level at L2 and their language learning ability (confirmed by a follow up experiment.) 10 Late bilinguals that had higher levels of activation in those regions also had performed better in L2 verbal tasks.
It is proposed that activation of the observed caudate-fusiform network is responsible for suppression of native language during the second language learning, and thus makes L2 acquisition more effective. This hypothesis is supported by the fact that involuntary and inappropriate language switch was observed in multilingual patients with caudate damage.


Reference:


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