The Brains of People With Dyslexia May Have Trouble Processing Sounds, Especially Speech

By Brenda Goodman, MA
WebMD Health News

Reviewed by Laura J. Martin, MD

Dec. 21, 2011 — There may be more to dyslexia than trouble with reading.

People with dyslexia sometimes see words and letters as scrambled, making reading a difficult task. Now a new study shows that dyslexia isn’t just a visual disturbance. It also appears to be a problem with the way the brain interprets sounds, particularly speech.

The study appears in the journal Neuron.

French researchers mapped the brain activity of 23 people with dyslexia and 21 people without the disorder as they listened to a white noise.

In order to understand the information in speech, the brain needs to be able to sync to the same frequency as the sounds it hears. The syncing of brain waves with sounds is called entrainment.

When the brain is properly entrained with a sound, it can correctly separate and interpret the signal, almost like breaking a code.

Researchers found that people without dyslexia had no trouble tuning their brains to the same frequencies as they heard in the white noise.

People with dyslexia, on the other hand, could not. Their brains had trouble syncing with sounds in the range of about 30 hertz, a frequency that’s important for understanding and decoding speech.

The dyslexic brain also appeared to be hyper-responsive to higher-frequency sounds.

This disrupted sound processing may help explain why people with dyslexia have trouble remembering and processing words and speech, says researcher Anne-Lise Giraud, a scientist with the Auditory Language Group at the Ecole Normale Superieure in Paris.

Study Signals ‘Progress’

Other experts who reviewed the study for WebMD said it offered important clues about the brain’s role in the often frustrating and debilitating condition.

“This is important information,” says Ken Pugh, PhD, who directs the Yale Reading Center in New Haven, Conn. “It’s progress.”

“This suggests a problem in the auditory cortex on the left side of the brain is making it difficult to perceive speech,” says Pugh, who is also the president and director of research at Yale’s Haskins Laboratories, which focuses on the biology of speech and language.

“That, in turn, might make it difficult to build an understanding of speech sounds that you need to have in order to learn to read,” he says.

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