"Chapter 11 - Multilingualism and Brain Plasticity"

 I have read and analysed "Chapter 11 - Multilingualism and Brain Plasticity" of the Handbook of the Neuroscience by Christos Pliatsikas. I found this particularly challenging and there were lots of scientific terminologies which I had to research on the side. However I decided to read it twice and analyse and highlight the main parts I was interested in. 

A neuron has three main parts : cell body, dendrites and axon. The cell body analyse electrical signals from other neurons through dendrites and produce new electrical signals to communicate with other neurons which is carried out by the axon. The axon is covered by myelin which ensure effective communication between neurons. 

Grey Matter ( GM ) - high concentration of cell bodies

White Matter ( WM ) - high concentration of myelinated axons

A common method to measure GM density and volume in a specific area is the voxel-based morphometry ( VBM ).

GM in Cerebral Cortex 

Areas that seem to be often affected by being multilingual are in the parietal cortex, mostly in the left hemisphere but also sometimes bilaterally. This region is related to semantics and phonology of newly acquired words. Also the left ATL ( anterior temporal  lobe ) and hippocampus, which are linked to vocabulary acquisition and processing, are also found to be affected by being multilingual. The ventrolateral prefrontal cortex and dorsolateral prefrontal cortex also are affected and they are linked to executive control which allow the selection of the right language amongst several options. The anterior cingulate cortex (ACC) which is linked to select the right words when several language compete against one other ( language-switching events ) is also found to be affected. 

GM in Subcortical Structures

The subcortical brain structure make up 25% of the total human brain and it includes structures like thalamus, basal ganglia, hippocampus, putamen and caudate nucleus. Using VBM it was found that the GM volume of left caudate nucleus (LCN) is positively correlated to the phonemic fluency of L2. Also it was found an expansion of the globus pallidus which directly relates to the amount of time spent in the L2-speaking country which is linked to activation of L2 production. Another VBM study showed an increase in volume for the left putamen in female multilinguals ; but in this picture naming study, this region was only activated in their L3 which was acquired later in their lives compared to L1 and L2. This suggests that the left putamen might be linked to the acquisition and processing of languages acquired later in life. Also it was found that this region might be linked to language switching as in a study for simultaneous vs. sequential bilinguals it was found that simultaneous bilinguals had a bigger left putamen volume. 

GM in Cerebellum

The cerebellum has extensive connection with some of the subcortical structures mentioned in the previous paragraph and it has been suggested that it has a important role in  language control. Although there hasn't been much research about its contribution to the multilingual experience it has been found that it might have a crucial role in speech production and comprehension and grammatical acquisition and processing.

White Matter

WM is a network of axons in the brain which allow the exchange of information between different parts of the brain which are not necessarily next to each other. WM connect the cerebral cortex with the basal ganglia, the thalamus and the cerebellum. A common methos used to monitor WM  is measure how permeable the myelin of the axons is to water molecules. The study of WM showed how the earlier the acquisition of L2, the stronger the connections between brain regions. When comparing late and early bilinguals, the latter had a higher connectivity between a phonological network ( lots of WM tracts e.g. left ITG, IPL... ),semantic network and syntactic network.  Also another study showed how in sequential bilinguals the strength of the connectivity between the bilateral IFG and right IPL are positively correlated with the AoA of the L2.

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