Popular Science - USA (2019-10)

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, chinchil-
las, 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 work-
ing 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 in-
jury, 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 Bas-
sett’s head, they looked at her brain activity
when she listened to sounds while sleeping.
The assessment—called an auditory brain-
stem 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. Bas-
sett 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 under-

went could potentially contribute to

future diagnoses. But right now, the re-

sponse 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 hear-

ing 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 mi-

croscope 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 plas-

tic 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

THERE’S

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THE IMPORTANCE OF

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