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Hearing Loss in Aging May Not Be Ear Problem; Just
an Old Brain
 Feb.
22, 2005 – Losing your hearing as you grow older may not be a problem
with your ears. Older people whose ears are in fine shape may have
trouble hearing because their brain is aging, conclude researchers who
are finding problems in the way our brain processes information as we
age.
In addition to earlier findings of a specific type
of "timing" problem that limits our hearing as we age, the group is now
finding increasing evidence of a "feedback" problem in the brain that
diminishes our ability to hear.
This week at the annual meeting of the Association
for Research in Otolaryngology in New Orleans, researchers are
discussing the results so far of the hunt for genes that play a role in
the aging brain's plummeting ability to organize the information our
ears record.
"Traditionally, scientists studying hearing
problems started looking at the ear," says Robert D. Frisina, Ph.D.,
professor of Otolaryngology at the University of Rochester Medical
Center and an adjunct professor at Rochester Institute of Technology.
"But we are finding patients with normal ears who still have trouble
understanding a conversation. There are many people who have good inner
ears who just don't hear well. That's because their brains are aging."
The findings come from researchers at the
International Center for Hearing and Speech Research (ICHSR), an
NIH-funded group of scientists in Rochester, N.Y., that is recognized as
a leader in research in age-related hearing loss. The center includes
scientists from the National Technical Institute for the Deaf at
Rochester Institute of Technology and neuroscientists from the
University of Rochester.
Sophisticated tests that measure how well the brain
processes information that the ear detects are helping scientists sort
out the findings. Normally the brain does a masterful job of filtering,
sorting, and making sense of the information that flows through our
senses every day – the colors and shapes we see, the textures of the
objects we feel, the sounds ranging from the cooing of children to the
screech of tires on pavement that we hear morning to night. Our brain
stem sorts the bluster of information in ways that make it easy for us
to carry on our lives.
Oftentimes it's this ability of the brain, not
hearing itself, that is diminished in older people who say they don't
"hear" well. The loss is detected most markedly in tests that measure a
person's ability to hear a sentence amid a background of babble, much as
one might hear at a party while trying to speak to an individual nearby.
The recently discovered feedback problem is central to this problem,
says Frisina. His team has found that in mice, the brain problems
usually precede actual hearing difficulties, and that early problems
with the brain's feedback system make the ears more vulnerable to damage
– without the brain's filtering capacity, the ears are more likely to be
exposed to damaging noise.
The brain's ability to provide proper feedback to
the ear, by filtering out unwanted and unnecessary information, declines
beginning in our 40s and 50s, Frisina says. Without that filter, a
person is quickly overcome by a barrage of information that is difficult
to sort. It's a little bit like a computer user who would be overwhelmed
by input if the spam filter suddenly failed and all sorts of bogus
messages started streaming into the "important documents" folder. When
it comes to hearing, the increase in sensory information making its way
to the brain actually hurts the person's ability to hear well.
"The number-one hearing complaint among the elderly
is that they have trouble hearing speech because of background noise.
Someone might hear fine in a quiet environment like their home, but when
they go to a restaurant or a meeting or a party, it sounds like chaos to
them," Frisina says. "That's partly because the feedback system is
failing."
To get to the root of the feedback problem,
Frisina's neuroscience team is investigating the possible role of a
breakdown in calcium regulation in the brain stem, throwing askew the
way nerve cells talk to each other and possibly resulting in a toxic
buildup of calcium in some brain cells.
Recently the team used gene-chip activity to chart
the activity of more than 22,000 genes in mice, comparing activity
levels of genes in young mice and their older counterparts. While dozens
of genes in humans and mice are known to contribute to congenital
deafness, none has been linked to age-related hearing loss in humans.
The latest studies offer several promising leads in genes that affect
the functioning of brain chemicals like glutamate and GABA, important
neurotransmitters that allow nerve cells in the ear and brain to talk to
each other.
The difficulties can isolate people from friends
and family, beginning when people first have difficulty with age-related
hearing loss in their 50s and 60s. "This problem is especially tragic
because just when people have time to spend with their children and
grandchildren, they can't understand what is going on," says Frisina.
"They're losing something they had. People respond to this isolation by
either clamming up or aggressively dominating conversation." The
estrangement can be severe and can even result in depression.
While there is no cure for age-related hearing
loss, or presbycusis, some simple steps can lessen its effects. Speaking
loudly is an instinctual reaction when talking to a hearing-impaired
person, but that won't help when talking to someone with age-related
hearing loss. "Speaking slightly slower than usual will help," says
Frisina, "as if you were talking to someone who speaks a foreign
language.
"Many older people are actually especially
sensitive to loud sounds, so the worst thing you can do is raise your
voice. What you need to do is look at the person and speak slowly and
clearly. Speaking loudly is like turning up the volume on a cheap stereo
– it's only going to distort your speech and add to the confusion."
Six years ago the same team of researchers reported
finding a closely related brain "timing" problem where people are not as
adept as they once were at detecting slight gaps in speech. While the
average person can hear sound gaps of about 2 milliseconds apart,
someone with a timing problem may be anywhere from 2 to 50 times worse
detecting such gaps, which are crucial – though unconscious – for
properly hearing consonants and vowels.
"To a person with a timing problem, conversation
sounds like everything is spoken through a drainpipe," says James Ison,
professor of brain and cognitive science. "One sound leads into the
next, smearing words together." For instance, most people know that in
the English alphabet, the letter that follows "K" is "L," not "Elamenopee."
To a person with a timing problem, short pauses are imperceptible,
blurring words together. The problem has the most effect on a listener's
ability to hear the first consonant of a word – cat, hat, bat, fat, and
rat may sound remarkably similar, for instance.
While most people gradually lose the ability to
hear high frequencies as they age, the feedback and timing problems
account for many of their complaints about hearing, Frisina says.
"These problems with the aging brain, which nearly
everyone experiences, are on top of problems with our ears, which you
may or may not have as you get older. For many people, even if they can
still hear sounds as they get older, they still lose the ability to hear
and understand speech, because of these brain problems," Frisina says.
Frisina and Ison are part of a center that brings
together applied research on hearing at RIT with basic neuroscience and
aging research from the University of Rochester. Frisina is the
associate director of the center; the director is his father, D. Robert
Frisina, founding director of NTID and an adjunct professor at the
University. Other faculty members at the center include William O'Neill,
associate professor of neurobiology and anatomy, and Joseph Walton,
associate professor of otolaryngology, both at the University. The
center is currently funded by a five-year, $6.3 million grant from the
National Institutes of Health.
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