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Matt Garlock has trouble making out what his friends say in loud bars,
but when he got a hearing test, the result was normal. Recent research may have
found an explanation for problems like his, something called "hidden
hearing loss." Scientists have been finding evidence that loud noise -
from rock concerts, leaf blowers, power tools and the like - damages our
hearing in a previously unsuspected way. It may not be immediately noticeable,
and it does not show up in standard hearing tests.
But over time, Harvard researcher M. Charles Liberman says, it can rob
our ability to understand conversation in a noisy setting. It may also help
explain why people have more trouble doing that as they age. And it may lead to
persistent ringing in the ears.
The bottom line: "Noise is more dangerous than we thought."
His work has been done almost exclusively in animals. Nobody knows how
much it explains hearing loss in people or how widespread it may be in the
population. But he and others are already working on potential treatments.
To understand Liberman's research, it helps to know just how we hear.
When sound enters our ears, it's picked up by so-called hair cells. They
convert sound waves to signals that are carried by nerves to the brain. People
can lose hair cells for a number of reasons - from loud noise or some drugs, or
simple aging - and our hearing degrades as those sensors are lost. That loss is
what is picked up by a standard test called an audiogram that measures how soft
a noise we can hear in a quiet environment.
Liberman's work suggests that there's another kind of damage that
doesn't kill off hair cells, but which leads to experiences like Garlock's.
A 29-year-old systems engineer who lives near Boston, Garlock is a
veteran of rock concerts. "You come home and you get that ringing in your
ears that lasts for a few days and then it goes away," he said.
But after he went to Las Vegas for a friend's birthday, and visited a
couple of dance clubs, it didn't go away. So he had the audiogram done, in
2015, and his score was normal.
Last fall, he came across a news story about a study co-authored by
Liberman. It was a follow-up to Libermans' earlier work that suggests loud
noise damages the delicate connections between hair cells and the nerves that
carry the hearing signal to the brain.
The news story said this can cause not only persistent ringing in the
ears, but also a lingering difficulty in understanding conversations in
background noise. After the Vegas trip, Garlock sensed he had that problem
himself.
"I notice myself leaning in and asking people to repeat themselves,
but I don't notice anybody else doing that," he said. Garlock emailed one
of Liberman's colleagues and volunteered for any follow-up studies.
It's hard to be sure that Garlock's situation can be explained by the
research. But the seeming contradiction of hearing problems in people with
perfect hearing tests has puzzled experts for years, says Robert Fifer of the
University of Miami's Mailman Center for Child Development.
He's seen it in Air Force personnel who worked around airplanes and in a
few music-blasting adolescents. "We didn't have a really good explanation
for it," said Fifer, who's an official of the American
Speech-Language-Hearing Association.
But the work by Liberman and others helps solve the mystery, he said. The
connections between hair cells are called synapses, and a given hair cell has
many of them. Animal studies suggest you could lose more than half of your
synapses without any effect on how you score on an audiogram.
But it turns out, Liberman says, that losing enough synapses erodes the
message the nerves deliver to the brain, wiping out details that are crucial
for sifting conversation out from background noise. It's as if there's a big
Jumbotron showing a picture, he says, but as more and more of its bulbs go
black, it gets harder and harder to realize what the picture shows.
The study Garlock noticed is one of the few explorations of the idea in
people. Researchers rounded up 34 college students between ages 18 and 41 who
had normal scores on a standard hearing test. The volunteers were designated
high-risk or low-risk for hidden hearing loss, based on what they said about
their past exposure to loud noise and what steps they took to protect their
hearing,
The higher-risk group reported more difficulty understanding speech in
noisy situations, and they scored more poorly on a lab test of that ability.
They also showed evidence of reduced function for hearing-related nerves. It's
a small study that must be repeated, Liberman says, but it adds to evidence for
the idea.
One encouraging indication from the animal studies is that a drug might
be able to spur nerves to regrow the lost synapses, said Liberman, who holds a
financial stake in a company that is trying to develop such treatments.
In the meantime, he says, the work lends a new urgency to the standard
advice about protecting the ears in loud places. "It isn't awesome to have
your ears ringing. It's telling you (that) you did some damage," he said.
Liberman's own hearing scores are pretty good, but at age 65, he
sometimes can't understand his kids in a loud setting. He figures some of that
may be from his years of handyman chores, like using a belt sander or a table
saw. "I wear earplugs when I mow the lawn now."