John Fetterman, a Democratic candidate in a highly watched Pennsylvania Senate race against television personality Mehmet Oz, known as Dr. Oz, suffered an ischemic stroke—the obstruction of a vessel that supplies blood to a part of the brain—in May. The blockage causes brain cells to be starved of essential oxygen and nutrients. Within minutes, the cells start to die.

Five months later Fetterman sat down for an interview with NBC News where he used closed captioning technology to help manage the auditory processing issues caused by the stroke. “I sometimes will hear things in a way that’s not perfectly clear,” Fetterman told NBC News correspondent Dasha Burns. “So I use captioning so I’m able to see what you’re saying.” Oz and other Republicans have questioned Fetterman’s fitness for office and attacked him for needing accommodations.

Aphasia, or the inability to understand or express speech, is very common following a stroke, impacting an estimated third of people who have one. (Fetterman’s campaign has denied he has aphasia, but some of his symptoms are consistent with the condition.) Those who have a stroke in their left brain hemisphere, which serves as the center of language processing in most people, are particularly vulnerable. For the brain to recover, it must modify and adapt to this new injury, a process known as neural plasticity. But neuroscientists still have many questions about how the brain rewires, particularly with regard to language.

The fundamental mysteries include how the surviving brain regions after a stroke take over the functions of language and why this reorganization process is more successful in some people than others, says Stephen Wilson, head of the Language Neuroscience Laboratory at Vanderbilt University. “There’s a lot to learn,” he adds.

Part of answering these questions involves understanding how a healthy brain processes and creates language. When humans are young, both the right and left brain hemispheres are heavily involved in speech, according to a 2020 study published in the Proceedings of the National Academy of Sciences USA. The researchers found that, starting at around six years old, the neural network involved in language gradually becomes more confined to the left hemisphere—although the right hemisphere still plays a role in processing things such as the emotion in someone’s voice.

Young brains are very plastic and far more adept than older brains at responding to injuries. “In the womb and early after birth, neurons are just developing and connections are just forming, so with any stroke during that time, the brain has a massive ability to reorganize and rewire,” says Swathi Kiran, director of the Aphasia Research Laboratory at Boston University.

In fact, a study published this month in PNAS found that, following a stroke in an infant’s left hemisphere, the language network flips over to the right hemisphere. The investigation’s participants, who ranged from nine to 26 years old, have lived very normal and healthy lives despite their early-life strokes, says Elissa Newport, lead author of the study and director of the Center for Brain Plasticity and Recovery at Georgetown University.

How well the brain adapts, however, may be influenced by the size of the lesion caused by the stroke. I had a stroke while in the womb, resulting a hole in my left temporal lobe, a crucial language area. Interestingly, my language network remained in my left hemisphere, possibly because the lesion was small enough that there was sufficient healthy brain tissue left over for that network to operate, Newport suggests.

It’s unclear how long this ability to reorganize language to the opposite hemisphere—or successfully move it to the remaining healthy tissue—lasts. Scientists do know, however, that as people age, the brain’s neural networks, including the one for language, become more fixed. “When [this network] is damaged in adulthood, it’s not like you can just rebuild that circuit elsewhere,” Wilson says. “There are only certain parts of the brain that can do this.”

Consequently, the location of the stroke is a big determinant in whether someone will be able to recover from their aphasia or not. In a study published in April in Brain, Wilson’s lab recruited 334 adults with a left hemisphere stroke, including 218 who were experiencing aphasia, and found that those with a lesion near the front of the brain recovered well from their initial aphasia. This was the case even if the lesion extended into some parts of the left temporal and parietal lobes, respectively located behind the ear and at the back and top of the head.

But people who had significant lesions in the temporoparietal junction, an area in the back of the temporal lobe where it meets the parietal lobe, experienced notable language deficits that lasted far longer than they did in the other adults. This region of the brain has long been known as crucial for language—particularly a part of the back of the left temporal lobe called Wernicke’s area, which was discovered in 1874 by German neurologist Carl Wernicke.

The adult brain still has tremendous potential to heal itself within the first few months following a stroke, says Kiran, who has seen stroke patients recover from aphasia and go back to living a completely normal life. Experts disagree as to when this recovery process plateaus, but it seems to be around six to nine months after the injury, she says. Still, more and more research is showing that “the ability for the brain to continue to recover and reorganize continues all life long,” Kiran says.

This neural recovery process is likely occurring in Fetterman’s brain right now, although whether he will continue to suffer from residual auditory processing issues remains an unknown.

“I feel like I’m going to get better and better every day...,” Fetterman said during the NBC interview. “I believe I’m going to be able to serve effectively.”