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Study Unravels Mystery of Dyslexia

Children With Dyslexia Can't Focus on Repeated Speech Sounds, Researchers Say
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WebMD Health News
Reviewed by Louise Chang, MD

Nov. 11, 2009 -- New research may provide an answer as to why children with dyslexia often have difficulty hearing someone talk in a noisy room.

Dyslexia is a common, language-based learning disability that makes it difficult to read, spell, and write. It is unrelated to a person's intelligence. Studies have also shown that patients with dyslexia can have a hard time hearing when there is a lot of background noise, but the reasons for this haven't been exactly clear.

Now, scientists at Northwestern University say that in dyslexia, the part of the brain that helps perceive speech in a noisy environment is unable to fine-tune or sharpen the incoming signals.

"The ability to sharpen or fine-tune repeating elements is crucial to hearing speech in noise because it allows for superior 'tagging' of voice pitch, an important cue in picking out a particular voice within background noise," Nina Kraus, director of Northwestern University's Auditory Neuroscience Laboratory, says in a news release.

The brainstem is the first place in the brain to receive and process auditory (hearing) signals. It is supposed to automatically focus on the information, such as repeated bits of speech, and sharpen it so you can discern someone's voice from, say, the noise of a chaotic classroom. The new study, however, provides the first biological evidence that children with dyslexia have a deficit in this auditory process. As a result, the brainstem cannot focus on relevant, predictable, and repeating sounds.

The new evidence is based on a brain activity study of children with both good and poor reading skills. The children wore earphones that repeated the sound "da" in different intervals while watching an unrelated video. The first time, "da" repeated over and over again in a repetitive manner. In a second session, the sound "da" occurred randomly along with other speech sounds, in a variable manner. Electrodes taped to each child's scalp recorded the brain's response to the sounds.

The children also underwent standard reading and spelling tests and were asked to repeat sentences provided to them amid different noise levels.

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