The Bilingual Brain

There is substantial evidence that bilingualism, from an early age, alters the brain. The bilingual brain is not just a sum of two monolingual brains: it differs qualitatively from the brains of monolinguals (Abutalebi, Cappa & Perani 2001). Both structural and functional differences have been found. These brain differences are believed to be the root of the enhanced cognitive performance and decreased language proficiency consistently found in bilingual people. All of the differences between monolingual and bilingual individuals can be traced to a neural basis.

Brain Structure

graymatter.jpg The brain is made up of two types of tissue, grey and white matter. Grey matter contains most of the nerve cells in the brain. These nerve cells, called neurons, carry information to and from different parts of the brain, including linguistic information. Studies have found that bilingual adults have denser gray matter than monolingual adults, especially in the left hemisphere, where language information is processed (Mechelli 2004). This effect is moderated by the age at which the second language was learned and the proficiency in the second language. Individuals who learned two language before the age of five had significantly denser gray matter, as did individuals who were more proficient in their second language, according to Mechelli (2004).

Brain Function

Researchers using neuroimaging techniques such as functional MRI (fMRI) and positron emission tomography (PET) can discover differences in brain activity between monolinguals and bilinguals.

The Left Hemisphere

Language Production

There is evidence that bilingual people have differences in cerebral activation in anterior brain structures, such as Broca's area and the basal ganglia during work production tasks (Abutalebi, Cappa & Perani 2001). More extensive activations in these areas are associated with production in the less proficient language, possibly because of the extra effort required to produce words.

brain_pieces2.jpg

Language Comprehension

Language comprehension studies appear to provide more reliable results compared to language production studies, according to Abutalebi, Cappa & Perani (2001). In individuals who learned both language from an early age, comprehension of each language seems to be processed in a single neural machinery in the temporal and frontal lobes of the left hemisphere (Abutalebi, Cappa & Perani 2001). Less proficient bilinguals, however, show a different pattern of activation for each language. This supports a theory that increasing language proficiency engages a common network in the language area of the brain, according to a review of multiple studies by Abutalebi, Cappa & Perani (2001).

Non-Linguistic Tasks

A study of Spanish-Catalan bilinguals found that bilinguals activated the left anterior frontal lobe (Broca's area) to respond to stimuli where executive function is used, whereas monolinguals activated the right inferior frontal cortex to respond to the same stimuli (Garbin et al. 2010). This provides evidence that bilingual individuals process information differently than monolinguals. Garbin et al hypothesized that this was because the switching back and forth between languages led to the use of brain areas used for language when performing non-linguistic cognitive tasks.

The Right Hemisphere

Studies have also found differences in right hemisphere activation for bilingual and monolingual individuals. For example, in a study by Kovelman, Baker and Petitto (2008), bilingual and monolingual individuals had similar speed and accuracy in during a cognitive task, but differed in the areas of the brain that were activated. When bilingual individuals were switching between languages, they showed more activation in the right hemisphere than monolinguals, specifically in the dorso-lateral prefrontal cortex. This area is known to be a source of attention and control. This allowed the researchers to conclude that there may be a “neural signature” of bilingualism- increased activation in this particular area of the brain.