What Is Different in the Bilingual Brain?

Research on the bilingual brain has gone through several stages over the years: the study of aphasic polyglots, experimental work on language lateralization in bilinguals, and now brain imaging studies that examine language processing and neural structures and connections between them.

One of the leading researchers in this field is Dr. Ping Li, Professor of Psychology and Linguistics at Pennsylvania State University. He works on the neural and computational bases of language representation and learning and has kindly accepted to answer a few of our questions. We wish to thank him wholeheartedly.

Before addressing the issue of what is different in the bilingual brain, as compared to the monolingual brain, can you quickly go through what is clearly similar?

It may be helpful to say at the outset that we are talking about the human brain, bilingual or not, which is the only brain that can learn and use complex natural languages for communication. No brain of any other species on our planet has language like ours, despite claims that other animals may also have sophisticated communication systems.

Against this backdrop, then, the similarities between the bilingual and monolingual brains will be more important than the differences. For example, given the physical constraints of the human brain–its neuroanatomical substances–we must be using more or less the same neural structures to learn and use different languages, whether these are English, Chinese, French, or Spanish. In other words, we cannot imagine that each of the world’s 7000+ languages occupies a different part of the brain. Now, this is not to deny that different languages will engage the neural structures in different ways, a position I myself dearly embrace and which you described elsewhere (see here).

Does evolution also play a role?

From an evolutionary perspective, human language has had a long history–at least one hundred thousand years–and has evolved into a very complex communicative system. Evolution has determined that something as complex as human language simply cannot be supported by a single area in the brain. Rather, a great deal of brain resources needs to be dedicated to language. Recent neuroimaging evidence shows that language processing involves not only the classical Broca’s and Wernicke’s areas but the entire brain, from frontal to temporal to parietal lobes.

Concerning this, note that there is an area in the brain that we can call "the visual cortex" (for visual processing), but there is no such area that we can call "the language cortex," to the disappointment of some who look for a "language gene" or a "language area." Because of this, there is also no "monolingual cortex" or "bilingual cortext." The most plausible scenario, as has been argued by David Green and his colleagues, is that the brain uses the same neural structures and resources to handle different languages, but in different ways, even in the same individual.

For many years, we were led to believe that bilinguals were language lateralized differently than monolinguals? Is there any truth to that?

Although the idea of different brain lateralization patterns for bilinguals versus monolinguals made sense initially, like many intuitively appealing ideas, the more we know about the linguistic brain, the more unlikely this view has now become.

I want to illustrate my point with one simple example. Take monolingual English speakers who learn Chinese and acquire lexical tone, an essential aspect of the language for listening and speaking Chinese. Now, we know that native Chinese speakers typically use the left hemisphere to process lexical tones (although the right hemisphere is also engaged to some degree), given that tones are phonological units marking different word meanings (for example, /pa/ means squat if pronounced in Tone 1 and crawl in Tone 2).

Native English speakers learning Chinese initially treat such tonal differences simply as acoustic variations (high pitch in Tone 1 and low-then-high pitch in Tone 2), and use the right hemisphere to process them. But once they are fluent bilinguals, they start to treat these tonal differences as phonological, and not just acoustic, variations. The difference between Tone 1 vs. Tone 2 is now just as important as that between /ba/ and /pa/. Hence, there is a stage where bilinguals shift from relying on the right hemisphere to the left hemisphere, as lexical tones become linguistically meaningful to them.

Tell us more about the use of the right hemisphere.

I mentioned above that even native Chinese speakers use the right hemisphere to some extent, so the story is even more complicated. In one of our recent studies, we found that comprehension of Chinese idioms by native speakers engages the right hemisphere much more strongly than we previously thought based on data from figurative language processing. This is because the understanding of Chinese idioms requires a lot of historical, social, and cultural background knowledge and the right hemisphere plays a huge role here in integrating such knowledge.

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So, the dichotomy between monolinguals vs. bilinguals with regard to left-vs.-right brain lateralization, without regard to specific linguistic features or components, does not seem to be a fruitful direction for future work.

Over the years, what differences in neural structures and connections have clearly emerged between monolinguals and bilinguals?

From what has been said so far, we should be careful when we speak of "the monolingual brain" versus "the bilingual brain," because language is complex and the brain is complex. This said, being a bilingual does have significant implications for the brain, both for how the brain represents and processes the two or more languages (function), and for how the brain changes as a result of learning languages (structure).

Bilingual experience, as compared with other types of experience, is somewhat unique in terms of neuroplasticity, i.e. functional and physical changes in the brain induced by activities performed regularly. For example, given how complex language is, you cannot learn a new language in a day despite what various commercial products claim for speedy language learning. If you are serious about learning, you have to, at the very least, remember thousands of words (lexicon), learn the sound system (phonology), acquire the writing system (orthography), learn the complex grammar (syntax), and learn the subtle ways to express yourself (pragmatics).

To learn all of these components of language, you need to consistently use many parts of the brain. For example, the lexicon engages the frontal and parietal cortical regions, phonology uses your frontal and temporal regions, orthography uses your occipital and temporal-parietal regions, syntax engages your frontal and subcortical regions, and pragmatics relies on both the left and the right hemispheres.

What impact does this have on the bilingual brain?

When you spend a couple of weeks, months, or even years learning a second language, the net effect of this effort is that your entire brain is exposed to, and trained by, the auditory and visual features of the language (see the Chinese tone example above).

Neuroscientists have discovered that when bilingual brains are compared with monolingual brains as a whole, specific brain regions are more active when doing specific linguistic tasks (phonology, orthography, syntax). These regions also become strengthened in terms of the amount/volume of neural substances, i.e. gray matter and white matter.

Thus, a recent trend in the neurosciences of bilingualism is to study how these areas become better connected in the bilingual’s brain. For example, in one of our studies we showed that students learning Chinese for only six weeks display a more integrated neural network that connects the superior temporal gyrus with the frontal and parietal cortex, and this contrasted with students who have not learned Chinese within the same time period.

The second part of this interview can be found here.

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