Neurological studies of multilingualism are carried out with functional neuroimaging,[1] electrophysiology, and through observation of people who have suffered brain damage.
These patterns are explained by the dynamic view of bilingual aphasia, which holds that the language system of representation and control is compromised as a result of brain damage.
Studies with bimodal bilinguals have also provided insight into the tip of the tongue phenomenon, working memory, and patterns of neural activity when recognizing facial expressions, signing, and speaking.
Participants in the studies who had transient language exposure as an infant or were multilingual showed greater brain activation in non-verbal working memory patterns, compared to monolingual speakers.
[4][18][19] The dynamic approach offers a satisfactory explanation for the various recovery times of each of the languages the aphasic has had impaired or lost because of the brain damage.
[19] Research affirms with the two approaches combined into the amalgamated hypothesis, it states that while languages do share some parts of the brain, they can also be allotted to some separate areas that are neutral.
PET scans from these studies show that there is a separate region in the brain for working memory related to sign language production and use.
[21] Studies with bimodal bilinguals have also provided insight into the tip of the tongue phenomenon and into patterns of neural activity when recognizing facial expressions.
[citation needed] Researcher Ellen Bialystok examined the effect of multilingualism on Alzheimer's disease and found that it delays its onset by about 4 years.
For example, increased familiarity with a language has been found to lead to decreases in brain activation in left dorsolateral frontal cortex (Brodmann areas, 9, 10, 46).
The putamen, therefore, plays a critical role because the articulation process places greater demand on brain resources, when one is producing a second language learned late in life.
Even when the second language is acquired later in life (up to age five), L2 production in highly proficient bilinguals reveals activation of similar brain regions as that in L1.
[31] Word generation (phonemic verbal fluency) has also led to larger foci of brain activation for the least fluent language(s) within multilinguals (observed using fMRI).
Using functional magnetic resonance imaging (fMRI), representations of L1 and L2 have been found in spatially isolated parts of the left inferior frontal cortex of late learners (Broca's area).
[28] With the use of positron emission tomography (PET), research has shown that brain regions active during translation are outside classical language areas.
[35] Translating from L1 to L2 and vice versa activates the anterior cingulate and bilateral subcortical structures (i.e. putamen and head of caudate nucleus).
[28] Functional neuroimaging research has shown that very early bilinguals display no difference in brain activation for L1 and L2—which is assumed to be due to high proficiency in both languages.
Also, brain activation of these two orthographically and phonologically outlying languages showed striking overlap (i.e. the direct contrast did not indicate significant differences).
Based on the evidence we can conclude that the bilingual brain is not the addition of two monolingual language systems, but operates as a complex neural network that can differ across individuals.
Evidence, mentioned previously, has shown that differential cerebral activation in anterior brain structures (e.g. Ba and the basal ganglia) is related to poor performance on word generation and production.
The human ability to learn multiple languages is a skill thought to be mediated by functional (rather than structural) plastic changes in the brain.
Learning a second language is said to increase grey matter density in the left inferior parietal cortex, and the amount of structural reorganization in this region is modulated by the proficiency attained and the age at acquisition.
[46] It is thought that these effects are due to the cognitively demanding skill of handling more than one languages, which requires more efficient connectivity between areas in the grey matter of the brain.
[19][4][18] This theory is supported by the functional imaging data of normal bilinguals and holds that fluency in a language is lost because of an increase in the activation threshold.
[17] An example follows where, in a Friulian and English pair, the English stimuli included “mat, cat, bat, hat” and the Friulian counterpart (which included 4 words that differed solely by one initial phoneme) was represented as “‘cjoc, c¸oc, poc, toc’ (drunk, log, chicory, piece).”[20] The response of the patients are recorded and processed with computer programs that indicate the percentage of correct answers for each linguistic skill.
Stroke patients (bilinguals) with aphasia also perform better in other cognitive tasks that measure attention and ability to organize and retrieve information.
This is relevant since in some patients the automatization of language is impaired, highly correlated to basal ganglia lesions and anterior parietal cortex.
[64] Nonetheless, age of acquisition also shows to be a factor in the degree of recovery of stroke patients due to differences in language mapping and the amount of grey matter developed.
Studies have shown stroke patients are able to benefit more from rehabilitation and recover faster if they have acquired a new skill that requires high cognitive ability due to more extensive brain training.
An fMRI study conducted by Deen B, Koldewyn K, Kanwisher N, Sax R concluded that the first cognitive function attributed to the superior temporal sulcus was language comprehension.