Hidden hearing loss is hitting people of all ages. Neuroscientists are still debating why.

Those affected can hear all right, but when it’s noisy, they can’t understand.
Hearing loss
Recent work seems to underscore the importance of protecting your ears—even when you’re young. Some of the hearing loss we’ve blamed on age might be due to how many fire trucks we’ve stood next to. Anthony Gerace

Tucked inside the air traffic control tower in Portland, Maine, Samantha Bassett was busy making sure planes didn’t crash into each other. All systems seemed normal that day in May 2014. Aircraft pinged their positions on radar while a constant and chaotic barrage of updates and requests flowed through her headset.

Then, out of nowhere, a burst of interference screamed into Bassett’s right ear. Those in the business call this phenomenon “getting sidetoned.” It can happen when lightning strikes, equipment malfunctions, or radio signals cause feedback like you might hear at a rock concert.

Bassett simply took off the headset, switched on speaker mode, and kept working. The planes must go on, after all.

Within the hour, though, Bassett got a headache and grew nauseous. She ended up going to see her doctor.

She knew something was wrong, but the results from an audiogram—a test of how well the ear picks up sound across different frequencies—​looked normal. She could detect soft high and low tones, and the ones in between. “You don’t have any hearing loss,” her doc told her. “You seem fine.”

But Bassett wasn’t fine. The nausea went away quickly, but loud environments continued to induce headaches. Her interactions started to change. In bars and restaurants, she couldn’t track the chatter. “I could see people talking, I saw their lips moving, and I knew sound was coming out,” she says. But she couldn’t decipher what that sound meant. She began to smile and nod a lot. At work, when plane traffic got heavy, she had to concentrate to interpret what she heard. “Before this happened, I could follow three to four conversations at once—because that’s what air traffic controllers are trained to do,” she says. Now, everything seemed to become harder.

For the next few years, Bassett continued to see specialists and search for answers, until she learned about a recently discovered phenomenon called hidden hearing loss. Usually, we expect that people’s sonic perception degrades because the receptors that detect sound get damaged and can’t pass the signals onward toward the brain. In a groundbreaking 2009 mouse study, though, Harvard auditory neuroscientists Charles Liberman and Sharon Kujawa found that sometimes the problem resides in another part of the ear: The receptors are fine, but some of the synapses that should transmit the messages have withered. As Liberman sees it, the microphone is good, but the stereo jack is damaged.

A person with this sort of damage can detect quiet sounds just fine, so audiograms don’t register any anomalies. But when surrounded by noise—where chitchat bounces off minimalist walls, machinery rumbles against a colleague’s instructions, or music blares from speakers—they can’t pick out the sounds they care about. Some individuals with these symptoms experienced a single blast, as Bassett did. Others were exposed to lower decibel levels over time, like listening to their orchestra practice, working in an engineering lab, or even mowing the lawn every Saturday. Some take prescription drugs that harm the delicate ear. Some have autoimmune disorders. Some are in their 20s. Some are in their 80s. The triggers vary, but the results appear the same: People can hear, but when it’s noisy, they can’t understand.

There are no statistics on how many might be affected, or exactly how much exposure would make you susceptible. Doctors can’t point to a single living person and definitively say they have hidden hearing loss. That’s because they can’t dissect your head, remove your inner ear, and see that your synapses are screwed up—which is currently the definitive biological test for the disorder. So the victims of this aural anomaly are, like Bassett, told repeatedly there’s nothing wrong. That’s why Kujawa, Liberman, and international groups are racing to understand the condition. Their research is leading the biotechnology industry toward treatments that could reverse the damage by coaxing synapses to regrow and give people back their normal, clamorous lives.

It’s a scientific path that matters to everyone in the modern world, unless they live under very quiet rocks. We’re exposed to more noise than ever. And it might be hurting us more than we realize.

Hearing loss
Hair cell balding can cause profound hearing loss. That’s why audiologists, the specialists who treat such problems, have stuck with the traditional audiogram to diagnose aural issues. Anthony Gerace

On a too-hot summer day in July, Kujawa sits in her office at Massachusetts Eye and Ear, a Harvard teaching hospital housed in, as she describes it, “a building that’s kind of been pasted together,” with a third floor that connects to the others only in some places. A bookshelf displays, perhaps lower down than you’d imagine, the etched-glass cylinder of her 2017 Callier Prize in Communication Disorders. Out the window, the Charles River silently flows. “Patients tell us all the time that they don’t hear as well as they used to,” Kujawa says, “and then they go into the clinic, and audiologists do the usual things and say, ‘You’re good.’” The patients, she says, “know they’re not good.”

No one really knew why until Kujawa and Liberman discovered hidden hearing loss, causing what many in the field call a paradigm shift—changing how researchers think about the ear’s inner workings and the definition of hearing loss. In the traditional view, the organ simply grows less adept at detecting sound. When noise enters, it hits the eardrum and vibrates those tiny bones whose names you had to memorize in seventh grade. The action sends pressure waves through the liquid in the cochlea, the snail shell of the inner ear. Hair cells live there, and their tips bend in response, producing electricity that releases neurotransmitters at the other end. These drift across the synapses to nerve fibers, sparking more current. The brain speaks this electrical language and turns the juice into conversation, cuckoos, car horns.

Hair cell balding can cause profound hearing loss. That’s why audiologists, the specialists who treat such problems, have stuck with the traditional audiogram to diagnose aural issues. They play a series of sounds at a range of frequencies and volumes. If you can hear across the octaves, even when the tone is quiet, doctors say you’re normal. But that, Liberman says, is not a nuanced test. He draws an analogy: “It’s like going to an eye doctor and asking, ‘Is the chart on the wall?’ instead of ‘Can you read the bottom line?’” It tells the examiner that your eyes can pick up light, sure—but it doesn’t tell them whether your brain can transpose those photons into letters.

Liberman, who has the air of a concerned father, first worked with Kujawa when she was a postdoc. Today, his office—complete with commissioned illustrations of the inner ear and a joke jar labeled “the ashes of old bosses”—is a few doors down from hers.

Kujawa first detected clues of hidden hearing loss after she left her postdoc position for a faculty job at the University of Washington in the late ’90s. There, she was looking into data from an ongoing long-term research project called the Framingham Heart Study, which launched in 1948. As its name suggests, it deals primarily with cardiovascular data, but participating doctors also administered hearing tests, surveying more than 5,000 people from the Massachusetts factory town of the same name and continuing to do so over decades. Kujawa found something surprising: The ears of people who had been exposed to noise kept getting worse over time, faster than in those without noise damage. Scientists had thought that after, say, a truck backfired near your head, you would either immediately suffer the ill effects or quickly recover. You’d maybe feel like you had cotton in your ears for a day or two, and then bounce back. The data, though, seemed to show that problems could be delayed or ongoing.

Kujawa didn’t know why this happened, but she thought she could test it. In 2001, she joined the faculty at Mass Eye and Ear and continued collaborating with Liberman. It was there, in 2009, that the two conducted the definitive study that established hidden hearing loss as A Thing. The experiment was, at base, simple: They played 100-decibel noise—about the same level as using a lawn mower—at mice for two hours. They waited a few days or weeks, then they autopsied the subject’s wee ears. The pair saw something they didn’t expect. The rodents’ hair cells were intact, but 50 percent of the synapses were gone. “Literally half the connections … That was terrifying,” Liberman says.

The takeaway was this: You could be exposed to sound that wasn’t loud or sustained enough to fry hair cells but could still cut wires to the brain. The neural connections were more delicate, and they degraded earlier and easier than the hair cells. Two years later, other researchers named this neurological phenomenon hidden hearing loss. “Hidden” because in humans, there’s no simple way to see if those synapses snap, and the deficiency doesn’t directly reveal itself in any standard clinical tests. You can lose nearly 90 percent of the electrical connections before a doctor could tell something was wrong. “If the hair cells are still functioning normally,” Liberman says, “the audiogram can still be completely normal.”

If you’ve lost that high a percentage of your ear-brain hookups, you don’t have enough processing power to decipher all the sounds that the hair cells detect. Researchers have now seen evidence of hidden hearing loss in dead mice, guinea pigs, rats, chinchillas, and nonhuman primates. But people, though their ears curl and conduct current like those animals, are more difficult to study than their mammalian counterparts because you can’t simply dissect a live ear.

There’s a lot researchers don’t know about what hidden hearing loss means: how big a deal it is, how commonly it happens, how to identify the underlying biology without an autopsy. But Kujawa and Liberman are working on studies that aim to tease results from both animals and humans. They’ll work out the anatomy and physiology from dead body parts and living animals, and compare it to data from the real-life corporeality of folks like Bassett, who’s participating in one of Mass Eye and Ear’s projects.

It took Bassett, now in her early 40s, a long time to find these researchers and learn about the condition. About a year after her injury, perplexed doctors sent her to Mass Eye and Ear. At first, even her doctor there—who was outside Kujawa and Liberman’s group—agreed with the others. But when Bassett wouldn’t back down, they gave her a deeper kind of test. With electrodes stuck to Bassett’s head, they looked at her brain activity when she listened to sounds while sleeping. The assessment—called an auditory brainstem response exam—measures the spikes and dips from all the nerve fibers transmitting audio to gray matter; specialists routinely use it for infants or young children, who lack the verbal skills for a normal audiogram. That’s when the doctors finally saw something wrong. Bassett’s injured ear could hear, sure, but she wasn’t getting the message. Scientists still can’t match that result with evidence of wasted-away synapses in breathing patients, but it’s progress in the right direction. Bassett felt like she wasn’t crazy, though she still didn’t have a name for the condition or know that others shared it.

Things started to change only in 2019, when her doctor helped connect her to audiologist Stéphane Maison, who works with Kujawa and Liberman. As Bassett ticked off her symptoms, Maison responded: “Yep. Yep. Yep. Yep.” The problem had a name, and lots of other people felt cut off from the world in the same way; they ranged from middle-aged office workers to musicians with a lot of concert experience. “He’s the first one who said: ‘This is real. I believe you,’” Bassett recalls.

The electrode test Bassett underwent could potentially contribute to future diagnoses. But right now, the response it measures simply correlates with the condition’s symptoms, and noise in the data and other variables can influence the results.

To prove what underlies hidden hearing loss in humans, you have to study autopsied ears, which Liberman says “tell the truth.” A hard-bound folder, sitting on a counter in his lab, holds microscope slides of see-through slices of the organ, part of the clinic’s 2,500-ear archive, donated by former clinic patients and other individuals. Many samples come with an audiogram so scientists can see what sort of physical damage matches up to what types of aural decline happened when the subjects were alive.

Toward the back of the room, several shelves hold the kinds of amber-liquid jars you see in mad-scientist movies, each containing a temporal bone, where the cochlea resides. They dangle in plastic blocks, as if listening to the liquid.

With samples like these—impossible to get while their owners were alive—Liberman can stain specific types of cells with different proteins, hit them each with a frequency of light, and watch them gleam in a rainbow of colors. With the resulting images, he can count the person’s neural connections and hair cells. The latter line up like little violet teeth—or dark, blank spots where they’re missing. The ends of the auditory nerve look like green jellyfish; the sheaths around the nerve fibers licorice red. It’s paint-by-number science. If only it were so uncomplicated in living humans.

Liberman and Kujawa hope they can combine the anatomy lessons from deceased humans and animals with the aural and brain tests in study participants to decide how to diagnose hidden hearing loss, understand enough about how it works to fix it, and lock down its causes precisely enough that, maybe someday, we can better prevent it.

Hearing loss
Scientists are just beginning to delve into the ear-brain hookups that are at the root of hidden hearing loss. Anthony Gerace

Before her accident, Bassett had been ultraprotective of her ears. She began working at airports when she was just 17. Her first job was answering the phone, but before long she was out on the runway, chasing away animals, driving fuel trucks, parking planes. “I had a boss who kind of treated me like his own kid, so he was constantly like, ‘Wear your earplugs; wear your headphones,’” she says. She even took the plugs to concerts.

Research seems to underscore the importance of protecting your ears from quotidian sounds that you wouldn’t have given a second thought—even when you’re young and feel untouchable. Liberman and Maison recently did a study of college kids: About 35 percent of their subjects, mostly audiology students, had used safeguards, while the other 65 percent—mostly pop music students at several Boston schools—had been less careful. “A lot of them are really abusing their ears,” Liberman says.

Both groups had normal standard audiograms. But when the scientists looked at the kids’ brains, using a test similar to Bassett’s, the music students showed more signal from the hair cells compared with their cochlear neurons. In other words, some of the message was getting lost. These subjects also couldn’t recognize words as well when there was background noise or an echo, or when the sound was sped up.

This was a small pilot study, but Liberman and Maison plan to gather a larger population of people and track them to see how their hearing changes over time. Aging, Liberman suspects, isn’t the only thing causing the decline we experience long-term. Some of it is the result of exposure. “If we lived on a deserted island and were not constantly barraged by environmental noises created by human machines that our bodies did not evolve to protect ourselves against,” he muses, “would our hearing deteriorate as much as it does?” Old studies of tribespeople in Sudan in the early 1960s—their ability pristine compared with city dwellers of the same era—suggest it would not. Unlike the eyes’ tendency to grow farsighted no matter what, some of the decline we’ve blamed on age might be due to how many fire trucks we’ve stood next to.

Other researchers come at hidden hearing loss from different perspectives. Gabriel Corfas—a neuroscientist at the University of Michigan’s Kresge Hearing Research Institute who has collaborated with Liberman—thinks there’s more to the condition than shrinking synaptic power: In his view, it’s a symptom that can be caused by more than one problem. His research shows that when a mouse’s ears lose the myelin that insulates the neurons, the critter experiences the symptoms of hidden hearing loss, even though its synapses are fine. He theorizes that autoimmune disorders like Guillain-Barré syndrome—which is associated with food poisoning, the flu, hepatitis, and the Zika virus—strip the body of this myelin, and so might produce that result.

Colleen Le Prell of the Callier Center for Communication Disorders in Dallas (the place that gave Kujawa her award) harbors a deeper doubt about this new condition. Le Prell, whose work focuses on preventing human hearing loss, has found no evidence that recreational noise affects the ear. She asked adults in their 20s to keep track of the time and volume when they went to a loud place, and she measured their hearing and speech-recognition abilities before and after. In participants who opted for a lot of high-decibel fun, Le Prell found no indication that they experienced any permanent changes. She considered the sounds produced by wiggling hair cells, the subjects’ ability to comprehend words in both quiet and boisterous environments, and the electrical impulses within the ear. The kids seemed all right, at least if they quantified their activities accurately—not a guarantee.

Meanwhile, another group, at University College London, is trying alongside Mass Eye and Ear to develop a diagnostic test—and to see if those are worth doing in the first place. According to speech and hearing scientist Tim Schoof, the group is using the electrode-based exam, as well as tests of how well participants can decipher specific sounds against background noise, to compare clamor-averse young people with adults over 45 who have been exposed to loud environments. They’ve recruited through musicians’ groups, as well as shooting- and motor-sports clubs.

Back at Harvard, Kujawa continues to find motivation in the many emails she gets from distressed people who come across her research. “They recognize their problems in it,” she says. “They are looking for answers because the answers we’ve given them haven’t been very satisfactory.”

Things are starting to look up. Already, a few companies—some staffed by people who used to work with Kujawa and Liberman—are working on therapeutics, like chemicals called neurotrophins that could help neurons regrow their synapses. If the connections to the brain could blossom again, and the hair cells were fine, hearing could return to baseline.

Even without treatments, awareness has made life better for Bassett, in that she now understands what might be happening inside her head. “It was just such a huge relief to hear that there’s something wrong,” she says.

“We would be hard-pressed in our society to find somebody who has never had a noise exposure that was maybe too loud,” Kujawa says. The Occupational Safety and Health Administration sets limits—​based on length, loudness, and frequency—​on what sound levels are allowed and the situations in which protection is required. But those guidelines are based on what causes hair-cell loss. Kujawa and Liberman say we don’t know enough yet to determine exactly which levels are safe for synapses. There’s no public-awareness campaign urging people to wear earplugs when they cut the grass, which Liberman does, or stick their fingers in their ears when an ambulance drives by, as Schoof does. But maybe there should be, even though there aren’t yet hard-and-fast delineations for what “too loud” means. As researchers learn more about the ear’s hidden frailties, everyone else should too, so we stop thinking that hearing loss is a thing that just happens when you’re old, and no matter what.

Sidetone-type accidents are going to occur, sure, but much of hidden hearing loss might be within our control. We can decide to wear earplugs to the runway, the factory floor, or jam-band practice, and then maybe, at the dinner parties of the future, we’ll do more than just nod and smile.

This story originally published in the Noise, Winter 2019 issue of Popular Science.