News Release

Tuesday, July 17, 2007

Ability to Listen to Two Things at Once Is Largely Inherited, Says Twin Study

Your ability to listen to a phone message in one ear while a friend
is talking into your other ear — and comprehend what both
are saying — is an important communication skill that’s heavily
influenced by your genes, say researchers of the National Institute
on Deafness and Other Communication Disorders (NIDCD), one of the
National Institutes of Health. The finding, published in the August
2007 issue of Human Genetics, may help researchers better
understand a broad and complex group of disorders — called
auditory processing disorders (APDs) — in which individuals
with otherwise normal hearing ability have trouble making sense
of the sounds around them.

“Our auditory system doesn’t end with our ears,” says James F.
Battey, Jr., M.D., Ph.D., director of the NIDCD. “It also includes
the part of our brain that helps us interpret the sounds we hear.
This is the first study to show that people vary widely in their
ability to process what they hear, and these differences are due
largely to heredity.”

The term “auditory processing” refers to functions performed primarily
by the brain that help a listener interpret sounds. Among other
things, auditory processing enables us to tell the direction a
sound is coming from, the timing and sequence of a sound, and whether
a sound is a voice we need to listen to or background noise we
should ignore. Most people don’t even realize they possess these
skills, much less how adept they are at them. Auditory processing
skills play a role in a child’s language acquisition and learning
abilities, although the extent of that relationship is not well
understood.

To determine if auditory processing skills are hereditary, NIDCD
researchers studied identical and fraternal twins who attended
a national twins festival in Twinsburg, OH, during the years 2002
through 2005. A total of 194 same-sex pairs of twins participated
in the study (138 identical pairs and 56 fraternal pairs), representing
ages 12 through 50. All twins received a DNA test to confirm whether
they were identical or fraternal and a hearing test to make sure
they had normal hearing.

If a trait is purely genetic, identical twins, who share the same
DNA, will be alike nearly 100 percent of the time, while fraternal
twins, who share roughly half of their DNA, will be less similar.
Conversely, if a trait is primarily due to a person’s environment,
both identical and fraternal twins should have roughly the same
degree of similarity, since most twins grow up in the same household.

The volunteers took five tests that are frequently used to identify
auditory processing difficulties in children and adults. In three
of the tests, volunteers listened as two different one-syllable
words or nonsense syllables (short word fragments such as ba, da,
and ka) were played into their right and left ears simultaneously,
and then tried to name both words or syllables. In two other tests,
volunteers listened to digitally altered one-syllable words played
into the right ear and tried to identify the word. One test artificially
filtered out high-pitched sounds, which tended to obscure the consonants,
while the other sped up the word.

In all but the filtered-words test, researchers found a significantly
higher correlation among identical twins than fraternal twins,
indicating that differences in performance for those activities
had a strong genetic component. Participants showed the widest
range of abilities on those tests in which they were asked to identify
competing words or nonsense syllables entering each ear — called
dichotic listening ability. The tests in which different one-syllable
words were played simultaneously into each ear showed the widest
degree of variation as well as the highest correlation among twins,
especially identical twins. As much as 73 percent of the variation
in dichotic listening ability was due to genetic differences, a
magnitude that is comparable to well-known inherited traits such
as type 1 diabetes and height. Conversely, the ability to understand
the filtered words showed high correlation among all twins, indicating
that variation in that skill is primarily due to differences in
environment.

Scientists believe that problems with dichotic listening ability
are often due to a lesion or disconnect between the brain's right
and left hemispheres. When we listen to someone talking, speech
entering the right ear travels in large part to the left side of
the brain, where language is processed. Speech entering the left
ear travels first to the right side of the brain before crossing
to the brain’s language center on the left side by way of the corpus
callosum, a pathway connecting the brain's right and left hemispheres.

Today’s finding that normal twins show such wide variation in
their dichotic listening abilities, and that the differences are
mostly due to genetic variation, adds a new perspective to our
understanding of auditory processing disorders. These disorders
may affect as many as seven percent of school-aged children in
the United States and often appear alongside language and learning
disorders, including dyslexia. APDs also affect older adults and
stroke victims and can limit the successfulness of hearing aids
in the treatment of hearing loss. The researchers suggest that
scientists may be able to fine-tune their understanding of what
an APD is and the role these disorders play in the development
of language and learning disorders.

The NIDCD supports and conducts research and research training
on the normal and disordered processes of hearing, balance, smell,
taste, voice, speech, and language and provides health information,
based upon scientific discovery, to the public. For more information
about NIDCD programs, see the Web site at www.nidcd.nih.gov.

About the National Institutes of Health (NIH): NIH, the nation's medical research agency, includes 27 Institutes and Centers and is a component of the U.S. Department of Health and Human Services. NIH is the primary federal agency conducting and supporting basic, clinical, and translational medical research, and is investigating the causes, treatments, and cures for both common and rare diseases. For more information about NIH and its programs, visit www.nih.gov.

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