14 DAY TRIAL // You could be a little Napoleon that can listen to a phone message while at the same time talking with a friend and understand what both are saying and perhaps writing something at your desk. In this case, you have a good genetic package, as signaled by a team at the National Institute on Deafness and Other Communication Disorders (NIDCD). This could explain many auditory processing disorders (APDs), when overall healthy individuals display problems in making sense of the sounds around them.
"Our auditory system doesn't end with our ears. 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," said Dr. James F. Battey, Jr., director of the NIDCD.
Auditory processing is the brain's function of interpreting sounds. It enables us to detect the origin and direction of a sound, its timing and sequence and whether the sound is a voice we should listen to or just background noise. These skills are important in a child's language acquisition and learning abilities.
The NIDCD team investigated this ability in 194 pairs of identical (138) and fraternal (56) twins, aged 12 -50, who attended a national twins festival in Twinsburg, OH, between 2002 and 2005. All twins were DNA tested to check if they were identical or fraternal and followed a hearing to verify if they had normal hearing.
If a characteristic feature is purely genetic, identical twins, who have identical DNA (100 %), will be alike nearly 100 % of the time, while fraternal twins, which are just regular brothers born at the same moment, share between almost 0 and almost 100 % of their DNA (80 % of them 40-60 % of their DNA) and will be less similar.
If a characteristic feature is primarily due to a person's environment, both identical and fraternal twins should have about the same degree of similarity, since twins usually grow up in similar conditions.
The subjects made five common tests employed to detect auditory processing difficulties. In three of them, the volunteers listened to the way 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 they tried to name both words and syllables. In the other two tests, subjects listened to digitally altered one-syllable words played into the right ear and attempted to track down the word.
One test artificially eliminated high-pitched sounds, which usually hide the consonants, while the other sped up the word. Except for the filtered-words test, there was a much higher correlation among identical twins than fraternal ones, pointing strongly to a genetic component. The widest array of abilities were found on those tests demanding the identification of competing words or nonsense syllables entering each ear, the dichotic listening ability.
The tests of one-syllable words played simultaneously into each ear had the widest degree of variation and the strongest correlation to identical twins.
73 % of the variation in dichotic listening ability was connected to heredity, like well-known inherited traits such as type 1 diabetes and height. But the ability to understand the filtered words proved high correlation among all twins, thus this skill is rather linked to experience.
Dichotic listening ability is believed to be linked to a lesion or disconnection between the brain's right and left hemispheres. Sounds received by the right ear are transmitted to the left side of the brain, where language is processed. Those reaching the left ear move first to the right side of the brain before reaching the brain's language center on the left side through the corpus callosum, a brain zone connecting the brain's right and left hemispheres.
Auditory processing disorders affect up to 7 % of school-aged children in the US and are often accompanied by language and learning disorders, like dyslexia. APDs also appear in older adults, affect victims and impede the treatment of hearing loss.