+ Abnormal Auditory Processing Underlies Dyslexia

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From Science Daily, a report indicating that people with dyslexia may be impacted by an abnormality in auditory processing.

Experts have long known that the inability to accurately decode and identify what they read is a result of speech processing problems.  But the basis of that disruption and how it interferes with reading comprehension had not been fully explored.

But now, new reasearch published in the December issue of the journal Neuron suggests that a specific abnormality in the processing of auditory signals accounts for the main symptoms of dyslexia.

Senior study authors are Dr. Anne-Lise Giraud and Frank Ramus of the Ecole Normale Superieure in Paris, France.

According to Giraud, everyone has been in agreement that for a majority of dyslexic children, the main cause is related to a deficit in the processing of speech sounds.  And also well established is that  there are three main symptoms of this deficit:

  • difficulty paying attention to individual speech sounds,
  • a limited ability to repeat a list of pseudo-words or numbers,
  • and a slow performance when asked to name a series of pictures, colors, or numbers as quickly as possible. 

However, the underlying basis of these symptoms had not been elucidated.

Giraud and her colleagues examined whether an abnormality in the early steps of auditory processing in the brain, called “sampling,” is linked with dyslexia.  They focused on the idea that an anomaly in the initial processing of phonemes — the smallest units of sounds that can be used to make a word — might have a direct impact on the processing of speech.

What they found is that typical brain processing of auditory rhythms associated with phonemes was disrupted in the left auditory cortex of dyslexics.  This deficit correlated with measures of speech sound processing.

Further, they found that dyslexics exhibited an enhanced response to high-frequency rhythms that indirectly interfered with verbal memory.

It is possible that this “oversampling” might result in a distortion of the representation of speech sounds.

Girard says

Our results suggest that the left auditory cortex of dyslexic people may be less responsive to modulation at very specific frequencies, which is potentially detrimental to their verbal short-term memory abilities.  Taken together, our data suggest that the auditory cortex of dyslexic individuals is less fine-tuned to the specific needs of speech processing.

Visit the Science Daily article, which is aggregated and has no byline, to locate citations:  http://www.sciencedaily.com/releases/2011/12/111221140340.htm

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