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 relevant for language are located in the inferior frontal and temporal cortices with dominance in the left hemisphere.
These cortices are connected via ventral and dorsal pathways with the ventral pathway mainly for auditory-to-meaning mapping, this is the process the brain goes through in connecting the auditory sound of a word to its meaning. The dorsal pathway supporting auditory-to motor mapping, this is the association between of the sound of the word and the brains skills used for articulatory purposes.
Recent evidence suggests that the dorsal pathway is also involved in syntactic processing, in particular when sentences are complex. The dorsal pathway connects the temporal cortex to the premotor cortex (PMC) via 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 fasciculus (UF), which connects the anterior ventral inferior frontal cortex to the temporal pole, and the extreme capsule fiber system (ECFS), which mediates the inferior fronto-occipital fasciculus (IFOF), connecting the inferior frontal cortex along the temporal cortex to the occipital cortex.
The present model of the brain assumes two functionally and (partly) structurally different dorsal pathways, and considers two ventral pathways in their possible relevance for semantic and syntactic processing during language comprehension.
Learning language
Age of acquisition and proficiency are believed to be the two main variables influencing experience-dependent skill learning and brain development.
Evidence shows that both endogenous and experiential factors influence language acquisition.
However there may be varying optimal periods for different aspects of languages such as phonology, syntax, morphology and semantics.
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.
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.
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.