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You might not put a lot of thought into what it takes to speak, but speaking keeps your brain busy. In every conversation, multiple regions activate to process sounds, give words meaning and control the muscles that move your mouth.

If someone knows more than one language, the process has even more layers – as do the challenges when such a person has a stroke that limits their ability to speak.

Difficulty speaking clearly or understanding language is called aphasia. Its most common cause is stroke, but aphasia also can result from brain injuries or other diseases. And it can be "devastating," said Dr. Mira Goral, a professor of speech-language-hearing sciences at Lehman College and the Graduate Center of the City University of New York.

An estimated 2 million people in the U.S. live with aphasia, commonly as a result of stroke, according to the . It's not clear how many people have what is often called bilingual or multilingual aphasia. But in the U.S. alone, about 20% of the population speaks a language besides English at home, according to the .

And the numbers affected are likely to grow, said Dr. Swathi Kiran, founding director of the Center for Brain Recovery at Boston University. She noted that the U.S. Hispanic population has been projected to reach 71 million by 2030, and the largest increase in stroke prevalence is expected to be among Hispanic males.

The condition, however, doesn't know borders. "Most of the world is either bilingual or multilingual," Kiran said.

Language is just one form of communication, Goral said. It involves vocabulary, grammar, the sound of a spoken word or the form of the written one. It requires understanding of tone and context. And there is no single spot in the brain that handles all those functions. "Each of these linguistic aspects are associated with particular networks in the brain," Goral said, and "many, many layers" combine to form "this amazing thing" that we use to communicate.

In a multilingual person, things get even more interesting.

Brain regions at work

If a brain is like a computer, Kiran said, a language is like software. And multiple languages are like different programs that run on the same equipment. "It's not that when you're learning one language, a certain part of your brain is involved, and when you speak another language, a different part of your brain is involved. It's actually the same structure."

A stroke affects everyone differently, Goral said, but patterns emerge.

The most common type of stroke is an ischemic stroke, when a vessel supplying blood to the brain is obstructed. And the most common place for that kind of stroke is in the left middle cerebral artery, which supplies blood to parts of the brain involved in speech and language. (In most people, most language is processed in the left half of the brain.)

The exact effect of a stroke depends on which part of the brain is damaged.

"If there's a reduction in the blood flow in the main artery, then a person has something called global aphasia," Kiran said. Many regions of the brain will be affected, and the person will have trouble speaking, understanding, reading and communicating.

Depending on which branch of the artery is blocked, the damage may not be that extensive. If the branch affecting the frontal lobe is affected, the person will have trouble speaking fluently but will understand what's being said to them, Kiran said. But if another artery is affected, people may be able to speak fluently but have trouble understanding what's being said.

Recovering language

In multilingual people who develop aphasia, Goral said, the most common pattern is to experience the aphasia in all of their languages as opposed to just one. This might be because the same structures and networks of the brain are involved in all languages, and stroke can damage the network that allows a bilingual person to toggle back and forth between languages. This could play into the way people recover their language abilities after a stroke, Kiran said.

After a stroke, people may remember their first language, Kiran said. "Sometimes people remember the second language."

How much someone used a language before their stroke is emerging as a key factor. Recent research, she said, suggests that "if you're using both languages a lot before your stroke, they're both probably going to be equally affected." But if you primarily used a newer language more than the one you originally learned, that one might actually come back more quickly.

Kiran said there has been "a lot of buzz" around research suggesting that being bilingual might help protect against dementia or aid recovery after a stroke. It's a "hotly debated" area, she said, but "the theory is that switching back and forth between these languages requires a certain kind of cognitive control" that strengthens thinking ability.

Many people with post-stroke aphasia can improve their language ability over time, Goral said, especially in the early weeks following a stroke. Coupled with speech language therapy, "people can regain a lot of their linguistic and communication abilities."

But, people likely receive therapy in just one language, Goral said, especially in the U.S., where most speech language therapists speak only English.

Many studies have been conducted to try to find out whether the best results for treating aphasia come from treating someone in their first language or one they learned later, she said. "The evidence suggests that treatment can be beneficial in any language."

Whether treatment in one language also helps recover other languages is less clear, Goral said. "The results are mixed. Sometimes we get good carryover to the non-treated language, but sometimes it doesn't happen." Many factors come into play, she said, such as how well someone spoke the language before the stroke or what the people around them are speaking.

Helping people recover all their languages is important, Goral said.

"Multilingual people value all of their languages and the cultures that are associated with the identity that is associated with each of these languages," she said. "So it's important to assess and treat all of the languages that are relevant to the person who is multilingual. And so to the extent we can, we should offer services in all of the languages that are relevant to that person."

That includes paying attention to people who mix their languages, which she called "a very common phenomenon of bilingual people." Many of the Spanish-English speakers she works with in New York tend to use both languages in the same conversation, sometimes in the same sentence, and it could be worthwhile to assess and help them in "the same linguistic context that they are used to and are comfortable with."

Unfortunately, Kiran said, not enough non-English-speaking therapists or interpreters are available to provide diagnosis, counseling and treatment in the language patients feel more comfortable with.

Speech therapy challenges and future directions

The need for experts who can provide speech language therapy to multilingual people is expected to grow as the population ages, researchers say. And providing care in multiple languages is important at even the most basic levels, Kiran said.

"This is a problem we see all the time in Boston," she said, where she sees many speakers of Haitian Creole, Vietnamese and Spanish, "and it's hard to even get across the most basic 'How do you access care?' kinds of questions."

Kiran's ongoing research uses machine learning to create a "digital twin" of the brain's language networks. Her team's work, which includes published in the journal Stroke in January, is "telling us a lot about how people learn languages and lose languages and recover languages" and could guide health care practitioners as they make decisions about therapy.

Such research, Kiran said, is "the new frontier for bilingual aphasia."