and the definition of hearing loss. In the traditional view, the or-
gan 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 vol-
umes. 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—com-
plete 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 Wash-
ington 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 pri-
marily 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 con-
tinuing 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. Scien-
tists had thought that after, say, a truck backfired near your head,
you would either immediately suffer the ill effects or quickly re-
cover. 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 estab-
lished 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. “Lit-
erally 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 hid-
den hearing loss. “Hidden” because in humans, there’s no simple
way to see if those synapses snap, and the deficiency doesn’t di-
rectly 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 functioningINSIDE YOUR EARPG 46EAR CANALCOCHLEA(STEREOCILIAHAIR ) HAIR^ CELLSYNAPSESUPPORTING NUCLEUS
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