Are You Listening as Well as You?re Hearing?
26 Jul, 2007 07:03 pm
When we listen, we do more than just hear. We make sense of a complex sound environment. We?re so good at it that we take our auditory processing skills for granted ? until there?s a problem.
You’re able to do this because, in addition to hearing sound, you are processing it in your brain. “Auditory processing” describes the many ways that our brains analyze the sound we hear to make sense of it all. We do it all the time, every day. Most of us are remarkably good at it. But there are differences among individuals in some of these skills. And when a geneticist sees differences in some skill or trait among people, he naturally wonders: “How much of that variation is because of differences in genes?” For some of our auditory processing abilities, the answer to that question is “Quite a lot”.
My colleagues and I investigated this issue in a study we conducted on twins (published in the August 2007 issue of Human Genetics). We gave different tests of auditory processing to identical and fraternal twins who attended the annual Twins Festival in Twinsburg, Ohio. All of our twin volunteers were first given the kind of hearing test you’re already familiar with: Pure tones played quieter and quieter until you can no longer hear them at all. All of our twins passed this test with average or better than average hearing. Then they took tests in which sounds were presented in ways that challenged their abilities to understand what they heard. In one of the tests they heard words that sounded “muffled.” In another, the words were artificially speeded up. In yet another, they heard two different words presented simultaneously, one to each ear, through their headphones. For example, in the right headphone the word “book” was played, while in the left was the word “dug.” The challenge was to identify both the words. This type of test is called a “dichotic listening” test. Most people did better than they thought on this and other dichotic listening tests. However, there was considerable variation in people’s test scores. More interesting was the fact that twins in a twin pair tended to have similar scores.
The reasons twins might have similar scores on dichotic tests are many. Among other things, the twins are the same age, are raised in the same household, eat similar foods, and are educated in the same schools. (In our study, we also selected only same-sex twins.) That’s true for all twins, whether they are identical or fraternal. Twins also share genes they inherited from their parents. But identical twins share 100 percent of their genes, while fraternal twins share approximately 50 percent. So, if there is a higher correlation among the identical twins than there is among fraternal twins, the increase in correlation can be attributed to an increase in their percentage of shared genes. That is exactly what we found with several of the tests: For example, the scores of fraternal twins were 40 percent correlated on one dichotic listening test, while the correlation among identical twins was 70 percent. We estimate that about 74 percent of the variation seen on this test can be explained by variation in the genes of the people who took it.
In other tests, there was less evidence for a genetic contribution. In fact, our data suggest that performance on the test in which the words sounded “muffled” is slightly more likely to be due to the similar environment and upbringing that twins who are raised together experience.
So why do we care how much our auditory processing abilities are heritable? It’s important to know because some people have “auditory processing disorder” or APD. Often, people with APD are diagnosed when they are children and they are having difficulty learning in school, have dyslexia, or are delayed in developing language. The prevalence, and the consequences, of APD are not well known. But knowing that much of the normal variation in auditory processing is due to genes suggests to us that we might look for genetic causes of APD.
Robert J. Morell, et al, A twin study of auditory processing indicates that dichotic listening ability is a strongly heritable trait, Human Genetics, Volume 122, Number 1/August, 2007, p. 103-111