Introduction

The neural background of language learning is still quite uncertain. There is no specific definition for language learning as it concerns too many areas. In simple terms it can be defined as acquiring the ability to communicate in your mother tongue or a second/foreign language.

Recently focus has been put on bilingualism as more and more people are exposed to multiple languages. This is due to a greater exposure with technology, immigration and ethnic diversity.

Neuroplasticity is the reconfiguration of the brain due to environmental needs for “specific motor behaviour or cognitive skills”. These changes later have an influence on developing further competences but it seems that there is an optimal age to obtain skills even though neuroplasticity occurs at all ages (even at senescence).

The method of language learning

The brain regions that are used for langauge development can be found in the inferior frontal and temporal cortices, within these location dominance is seen in the left hemisphere.

It is here in these areas that we can find one of the main components of language learning and development, the ventral and dorsal pathways. The inferior frontal and temporal cortices are connected via these two pathways. The ventral pathways main purpose is for auditory-to-meaning mapping, this is the process in which the brain goes through in connecting the auditory sound of a word to its meaning. The dorsal pathway is needed for supporting auditory-to motor mapping, this is the association between of the sound of the word and the brains skills used for articulatory purposes.

Recently studies have shown that syntactic processing may also heavily rely on the dorsal pathway, in particular when sentences are complex. The temporal cortex is connected to the premotor cortex by the dorsal pathway, which tracks through the inferior parietal cortex (IFC) and parts of the superior longitudinal fasciculus (SLF). The ventral pathway connects the temporal cortex to Brodmann Area (BA) 44, which is located in the frontal cortex as is specific to humans and other primates, as part of Broca’s area via the arcuate fasciculus (AF). The ventral pathway also seems to be responsible for more than one function: it is assumed to support sound-to-meaning mapping , as well as local syntactic structure building or syntactic processes in general.

The ventral pathway consists of two fiber tracts that run closely together the uncinate fascicles and the extreme capsule fibber system. The uncinate fasciculus , which connects the anterior ventral inferior frontal cortex to the temporal pole, and the extreme capsule fiber system, which mediates the inferior fronto-occipital fasciculus (IFOF), which connects the inferior frontal cortex along the temporal cortex to the occipital cortex.

Currently there are thought to be two functionally and structurally different dorsal pathways. There is also considered to be two ventral pathways in their possible relevance for semantic and syntactic processing during language comprehension.

Auditory perception is the initial stage in the auditory language comprehension process with this taking place in the temporal cortex. It is in the anterior region of Heschl's gurus that auditory processing takes place, this is in the left superior temporal gyrus. Here, we can see where a words phonological form or processing of the auditory word takes place.

With early age development data from diffusion weighted MRI suggest that newborns do not have a fully developed dorsal connection between the superior temporal gyrus to the inferior frontal gyrus. Researchers believe that the dorsal connection is terminated in the premotor cortex.

Learning language

Age of acquisition and proficiency are believed to be the two main variables influencing experience-dependent skill learning and brain development.

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Evidence shows that both endogenous and experiential factors influence language acquisition.

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However there may be varying optimal periods for different aspects of languages such as phonology, syntax, morphology and semantics.

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Phonology begins very early (exposure in utero) and develops extensively in the first year of life. A problem in phonological development in a child may result in reading and spelling difficulties in later child hood.

Infants exposed to two languages from birth are able to differentiate them from very early on, even before they speak their first words.

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• by 6 months old they start to lose discrimination of contrasting non-native vowels and consonants. By 9 months old their babbling resembles their native language’s phonetics.

At 12 months they are turned to the language they have been exposed to and will gain an accent undistinguishable from a native speaker.

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Bilingualism

There are two types of bilingualism. Simultaneous bilinguals learn two languages at the same time from birth. Sequential bilinguals meanwhile learn their second language later on in life.

People acquiring a second language at a later age are rarely able to obtain a native-like accent despite years of practice and high proficiency in other aspects of the language. Simultaneous bilinguals are usually able to speak both languages with a native-like accent although some may occur.

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è Study Korean immigrants in America. Children had less of an accent that the adults when speaking English however if they were over 6 years old they all still had some detectable accent after 4 years of immersion. 6-14 years old considered sequential bilinguals.

Phonology is the aspect of language learning that is most affected by age.

If a language is learned as a child but then disused or discontinued then memories of language exposure persist.

Early bilingualism may lead to the development of new synapses, myelination and pruning of connections of neural circuits. The brain matures with a further development of neural networks. Neural maturation is especially extensive in the first few years of life so this is the period where it would be most sensitive to sensory experiences such as exposure to further languages and so would undergo the most neuroplasticity.

Parvalbumin cells seem to be activated in the cortex by experiential inputs. These can turn the structural and functional change capacities of different brain regions on and off. In language learning these seem to be constrained by the degree of neurobiological brain maturation.

When exposed to multiple languages from birth an infant has an increase in their complexity of sociolinguistic and sensorimotor processing so as to better interact with their environment. This leads to the optimal neuroplasticity period lasting longer. Simultaneous and sequential bilingual both undergo brain changes but the substrate it manifests on is different. Macroscopic differences in adults are subtle as they are more apparent on the microscopic level. Overall simultaneous bilinguals have enhanced cognitive and language processes which facilitate overall brain development.

It is still unsure whether accommodating two languages affects the same brain regions or if more needs to be recruited. The impact of age on language acquisition is also not entirely understood.

Most studies suggest optimal intervals to acquire languages, this being especially important for phonology. However others also argue that it is a progressive and linear decline of L2 proficiency potential with age.

Position Emission Tomography and fMRI are used to try and see the correlation between age of acquisition and different functional signatures in the bilingual brain.

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