Imagine you're standing on a busy city street corner with cars rushing past, there's traffic exhaust in the air, the cement is hot beneath your feet, there's a dog barking in the distance and people are shuffling by.
All those sights, sounds and smells are all around you, the brain never stops working to make sense of all that sensory information. Our eyes take in a lot but scientists say we process what we hear faster than what we see, and sound has a powerful effect on the nervous system.
Fourteen-year-old Addison Zehren says she began to notice that power in the days after she hit her head while swimming. She didn't know it at the time, but when Addison miscalculated her distance from the side of the pool and swam head first into the wall, she suffered a concussion. That trauma changed the way her brain processed sensory input.
Addison didn't think much of that bump to her head until she got out of the water—and did what lots of teen girls do—she checked her phone.
"I noticed that my eyes were darting off different places," she said.
In the days following the accident, Addison was hypersensitive to light and had to wear sunglasses indoors. Loud noises felt oppressive.
"The first time I went back to the lunchroom someone had screamed behind me and I immediately got a headache," Addison said. "I'm like, I have to leave."
Researchers are beginning to study the way the brain processes sound to better understand--possibly even diagnose--trauma, like a concussion.
"You can imagine that getting hit in the head would disrupt, especially, this very delicate machinery," said Nina Kraus, a biologist and neuroscientist at Northwestern University, just outside of Chicago.
"And we are interested in trying to understand the biological basis for this."
The team at BrainVolts, Kraus's auditory neuroscience lab, studies how the brain understands sound, and how that understanding changes based on life experience, disease, or injury.
Of all the things we ask our nervous system to do—retrieve information from our skin, our eyes, and our noses--Kraus says making sense of sound is one of the most complex tasks. To tackle this big job, the brain draws on a lifetime of sound experience to create a kind of map or guide to processing what we hear.
To explain what she studies, Kraus tells people to think about a mixing board--like the one a DJ or audio engineer uses when she's mixing a song.
Imagine your brain is the audio engineer moving various sliders on the mixing board up and down as sound enters your ears. Inch the slider up a bit and one sound fades into the background, making other sounds more audible and clear.
Each slider corresponds to a different sound ingredient. The brain might have one slider represent timing--how fast or slow a sound comes in. Another might be for pitch--how high or low you interpret the sound. One might be for timbre--how robust or thin a sound is.
But a blow to the head can jostle those imaginary sliders, sending them out of whack, causing confusion, disorientation and general disorder.
Scientists can measure those settings. The first step is to track electrical currents in the brain. Any time you hear a sound, like a voice, the neurons in your brain produce electricity. In Kraus' lab, researchers place electrodes on each study participant's head to capture the electrical impulses as the participant listens to different sounds, such as an audio clip from a movie.
For most listeners, the movie clip sounds normal--audible, clear. But someone with brain trauma may hear something very different because the auditory parts of the brain are altered.
With those imaginary mixing board sliders out of whack, the voice may sound garbled, broken, nearly swallowed up by noise and static. A listener with a concussion may even strain to hear what's being said.
Researchers can measure the electricity produced while listening to a sound and create a visual representation of the brainwave, which looks a lot like a sound wave--a solid but wavy line with peaks and valleys smushed in close together.
Kraus says the idea is to use these pictures to create a measurable baseline or range called a biomarker.
Once the experts know what it looks like when a brain is processing sound "normally," then that biomarker might be used to diagnose trauma in the brain. In a December study of pediatric patients, Kraus' team put this idea to the test.
The team correctly diagnosed concussions in 90 percent of pediatric patients including 14-year-old swimmer Addison. Although the study only included 40 participants, Kraus says she suspects the research could lead to a reliable, objective diagnostic tool for concussions, which is something physicians don't really have right now.
"There isn't really a gold standard you could say for diagnosing a concussion," says Kristen Dams-O'Connor, a neurologist at Mt. Sinai Hospital in New York, and co-director of the Brain Injury Research Center.
Dams-O'Connor says concussions are most often diagnosed through subjective assessments. Physicians, trainers, or other experts use their best judgment to look for symptoms like headache, amnesia, confusion, dizziness, imbalance, or blurred vision. And they might monitor someone who hit his or her head for a few days and watch for things like vomiting or seizure.
Although certain tests may be able to pinpoint slight disruptions in the brain that could indicate a concussion, Dams-O'Connor says the most cutting-edge solutions for objectively diagnosing concussions are not accessible for most people.
"There's a lot of research looking into biomarkers, objective biomarkers, many of those options are very expensive, they're invasive," she says.
Dams-O'Connor says the Northwestern study findings are preliminary, but exciting, and may be an early step toward an inexpensive and less invasive test.
"The hope is that this information could really be used in an individual to determine when they are healthy enough to return to sports," Kraus says. "Because you really don't want to return before the brain is healed."
Kraus says beyond improving sports medicine, being able to access this information could lead to a new world of understanding the nature of the brain. She uses a recording of electrical impulses in the brain to help illustrate what she means.
The recording begins with a clip from Mozart's Eine kleine Nachtmusik serenade, as one would normally hear it. Then partway through, the sound changes, shifting into a recording of the brain playing its own version of the tune using electricity.
Using this same approach of recording and capturing what the brain actually hears and how it processes that sound, may lead to a deeper understanding of how trauma impacts our nervous systems.
"I am thrilled that we have a way of accessing this information and trying to make some sense out of it," Kraus says. "This robustness in the signal processing that enables you to look at what is a kind of miracle of all the different sound ingredients operating in the brain."