+ Dyslexic Brain Hears Blurred Sounds

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In findings published September 19th in the journal PLoS ONE, scientists found that dyslexia may be rooted in the brain’s diificulty teasing out distinct sounds from the incoming babble.

Charles Choi reported the story in LiveScience online.

Dyslexia is considered a learning disability which makes it difficult to read and spell.  It is thought to impact 15 percent of Americans.  The disorder is frequently linked to subtle difficulties with spoken language, as in trouble distinguishing rhyming syllables (“ba” and “pa,” for example).

Those deficits are seen in those who are at high risk for dyslexia, even infants.

The brain perceives speech in at least two linguistic systems: the phonetic system, which extracts distinct units of sound from speech such as vowels and consonants, and the phonological system, which combines these units to form specific words.

It has been frequently thought that dyslexia is caused by an  impaired phonological system. Researcher Iris  Berent is a cognitive scientist at Northeastern University in Boston, who says

It has become commonplace to assume that dyslexia results from a phonological deficit.  But when one looks at the facts more closely, there is really no firm evidence that such a deficit exists, and some suggestion that the deficit might actually result from lower-level impairments.

For instance, recent research implied that dyslexics had difficulty perceiving not just speech but also musical tones.

This new research suggests the phonetic system may be to blame.

Researchers analyzed Hebrew-speaking dyslexic students, and chose the Hebrew language because of its rules.  Hebrew bans repeated consonants depending on where they might occur within a base word (a word without prefixes of suffixes added on to it).

For example, Hebrew “simum” has a repeated <m> on its right side. Identical consonants are not allowed on the left edge of a word’s base, so Hebrew would not have the word “sisum” because it would repeat <s> on its left side.  Participants would be aware of this.

For this reason, the results would tease out the phonetic from the phonological systems.

The scientists found these volunteers had problems telling apart similar speech sounds.  However, they had no difficulty tracking patterns in how these words were strung together, even when it came to novel words.

That is, the participants knew how Hebrew places consonants within words.

Berent says she was “astonished to find out that the dyslexic individuals in this study showed no hint of a phonological deficit.  This was unexpected in light of the existing literature.”

The findings suggest the phonological system is intact, but the phonetic system is compromised.

According to Berent, “A closer analysis of the language system can radically alter our understanding of the disorder, and ultimately, its treatment.”

Berent cautioned that these findings don’t address how reading should be taught and what methods might best help people with dyslexia.

Certainly these results should not be taken as a challenge to the demonstrable significance of teaching phonics to beginning readers. [One limitation of these results] is that we assess the phonological abilities of dyslexics on the basis of a single phonological rule in a single language, so findings that these dyslexic individuals have an intact sensitivity to this rule does not mean that their ability to encode all phonological rules in all languages is intact, and that this is the case for every dyslexic individual.

Even so, Berent says the type of phonological restriction studied here — a restriction on the repetition of phonological elements — is likely to play a role in many languages.  As such, this phenomenon is likely to speak to the core of the phonological grammar rather than to some esoteric property of this single language.

Future research will analyze other linguistic rules and languages in relation to dyslexia.  Berent and her colleagues would like to understand the brain mechanisms that support this dissociation; they want  to find out why the dyslexic brain has developed in a manner that is different from those of typical readers as well as what is the genetic basis of those differences.

source: online article by Chris Choi in LiveScience, 9/20/12. http://www.livescience.com

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