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After a concussion, the brain may no longer make sense of sounds

Researchers are studying athletes and military personnel to learn more about how a concussion can affect the brain's ability to understand sound.
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Researchers are studying athletes and military personnel to learn more about how a concussion can affect the brain's ability to understand sound.

Headaches, nausea, dizziness, and confusion are among the most common symptoms of a concussion. But researchers say a blow to the head can also make it hard to understand speech in a noisy room.

"Making sense of sound is one of the hardest jobs that we ask our brains to do," says Nina Kraus, a professor of neurobiology at Northwestern University. "So you can imagine that a concussion, getting hit in the head, really does disrupt sound processing."

About 15% to 20% of concussions cause persistent sound-processing difficulties, Kraus says, which suggests that hundreds of thousands of people are affected each year in the U.S. The problem is even more common in the military, where many of the troops who saw combat in Iraq and Afghanistan sustained concussions from roadside bombs.

From ear to brain

Our perception of sound starts with nerve cells in the inner ear that transform pressure waves into electrical signals, Kraus says. But it takes a lot of brain power to transform those signals into the auditory world we perceive.

The brain needs to compare the signals from two ears to determine the source of a sound. Then it needs to keep track of changes in volume, pitch, timing and other characteristics.

Kraus's lab, called Brainvolts, is conducting a five-year study of 500 elite college athletes to learn how a concussion can affect the brain's ability to process the huge amount of auditory information it receives. And she devotes an entire chapter to concussion in her 2021 book, Of Sound Mind: How Our Brain Constructs a Meaningful Sonic World.

College athletes who sustain a concussion usually have normal hearing — at least when it comes to detecting faint sounds, Kraus says. Yet they often fail something called "the speech-in-noise test."

"You have the athlete listen to a sentence that is embedded in increasingly loud noise," she says. Those with normal brain function will be able to understand the sentence even when there is a lot of background noise. But many of those recovering from a concussion will fail the test.

A concussion may also leave athletes hypersensitive to sounds.

To learn more, Kraus's lab has been analyzing the electrical signals in areas of the brain that process auditory information.

"You just need to put on a couple of scalp electrodes and stick some earbuds in a person's ear and play some sounds," she says, adding that you also need some fancy equipment to capture what the electrodes detect, and a lot of expertise to interpret the result.

The approach can reveal which sound-processing areas in the brain have been affected by a head injury, even if the person being tested is asleep. "This gives you an objective way of assessing brain health," Kraus says.

Researchers are only beginning to understand how a concussion disrupts brain circuits, she says. But early evidence suggests that the brain injury causes "bottlenecks" — preventing information from flowing quickly from one brain area to another.

Most athletes recover from a concussion in a week or two, Kraus says. For those with lingering symptoms, she's experimenting with something called rhythm therapy, which has its roots in dancing.

"The athlete needs to listen to sounds and kind of move their whole body so they can align their movement with what they're hearing," she says. The idea is to strengthen the pathways needed to process sound.

Concussion from bomb blasts

The military has been studying the link between head injury and sound processing since about 2005, says Melissa Papesh, a research investigator at the Veterans Affairs National Center for Rehabilitative auditory research in Portland, Ore.

During the wars in Iraq and Afghanistan, she says, the VA began to see something odd in military personnel.

"We have all of a sudden this large influx of relatively young and middle aged people," she says. "They're coming into our audiology clinics and saying, 'hey, I'm having problems hearing.'"

Except when Papesh runs tests on their hearing, it looks "essentially normal." Surprisingly, their ears are fine. But their brains can't process what they're hearing.

Like athletes with concussions, these patients have trouble separating speech from background noise. Many also have problems processing rapidly spoken speech, she says.

VA scientists knew that the blast wave from a roadside bomb could cause a concussion without leaving any visible sign of injury. So they took a closer look at the patients who had trouble processing sounds, and found that many had been exposed to one or more blasts.

Some veterans still have auditory symptoms more than a decade after being exposed to a bomb blast, Papesh says.

Now that most military personnel have left the battlefield, the VA is looking at another potential source of brain injury. It involves exposure to lots of smaller blast waves that pass through the brains of people who fire high-powered weapons or use explosives in training exercises.

"That will be a big area of research in the future," Papesh says. The military "wants to prevent this kind of stuff before it becomes a chronic problem for veterans."

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Jon Hamilton is a correspondent for NPR's Science Desk. Currently he focuses on neuroscience and health risks.