July 23, 2007

Ability to Listen to Two Things at Once Is Largely Inherited

Picture of a man listening to two phones at the same time

Your ability to listen to a phone message in one ear while a friend
is talking into your other ear and follow what both are saying
is heavily influenced by your genes, according to a new study.

Your brain analyzes the sounds you hear so you can make sense of them. This "auditory processing" enables you to, among other things, tell where a sound is coming from, the timing and sequence of a sound, and whether a sound is a voice you need to listen to or background noise you should ignore. Auditory processing skills play a role in a child's language acquisition and learning abilities. Disorders in this system may affect as many as 7% of school-aged children in the U.S. and often appear alongside language and learning disorders, including dyslexia. Auditory processing disorders also affect older adults and stroke victims and can limit the successfulness of hearing aids.

To learn whether our auditory processing skills are inherited, researchers at NIH's National Institute on Deafness and Other Communication Disorders (NIDCD) 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, ages 12 through 50, participated. DNA tests confirmed that 138 of them were identical twins and 56 fraternal, and hearing tests made sure they had normal hearing.

The volunteers took 5 tests that are frequently used to identify auditory processing difficulties. In 3 of the tests, 2 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 they were asked to name both. In 2 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 tends to obscure consonants, while the other sped up the word.

The researchers reported in the August 2007 issue of Human Genetics that in all but the filtered-words test there was a significantly higher correlation among identical twins than fraternal twins, demonstrating a strong genetic component in auditory processing. Participants showed the widest range of abilities when they were asked to identify competing words entering each ear — called dichotic listening ability. As much as 73% of the variation in dichotic listening ability was due to genetic differences. Conversely, the ability to understand the filtered words showed high correlation among all twins, indicating that variation in that skill is primarily influenced by the environment.

"Our auditory system doesn't end with our ears," says Dr. James F. Battey, Jr., director of 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 finding will help researchers further understand auditory processing and the role it plays in the development of language and learning disorders.