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The building blocks of dyslexia

by Angela Herring of news@Northeastern

While dyslexia is most often clas­si­fied as a reading dis­order, it is also well known to affect how indi­vid­uals process spoken lan­guage. Even in infancy, people at high risk for dyslexia seem to have dif­fi­culty pro­cessing speech sounds, according to Iris Berent, pro­fessor of psy­chology in the Col­lege of Sci­ence at Northeastern.

While more than 5 per­cent of the global pop­u­la­tion suf­fers from dyslexia, Berent said much remains unclear about what causes this dis­order because we don’t com­pletely under­stand how the brain decodes printed lan­guage. There are also mys­teries sur­rounding how the brain deals with spoken lan­guage, but lin­guis­tics tells us it involves at least two dif­ferent sys­tems: the pho­netic system and the phono­log­ical system.

“The pho­netic system extracts the dis­tinct building blocks from con­tin­uous acoustic sound,” Berent said. She fur­ther explained that the phono­log­ical system takes those blocks and builds pat­terns with them.

“Think of the metaphor of Lego blocks,” she con­tinued. “The pho­netic system gets the Legos from the plastic stuff; the phono­log­ical system builds pat­terns with them.”

Researchers have long believed that the phono­log­ical system was impaired in people with dyslexia. Yet, sur­pris­ingly, very few studies ever both­ered to check. In a new paper in the journal PLOS ONE, Berent and her col­leagues — Vered Vaknin Nus­baum of the Uni­ver­sity of Haifa, Evan Bal­aban of McGill Uni­ver­sity and Albert Gal­aburda of Har­vard Med­ical School — show that the phono­log­ical system of dyslexics is intact. It is actu­ally the pho­netic system that is failing, she said.

To test the phono­log­ical system, Berent and her team pre­sented both skilled and dyslexic readers with the sounds of fake words in Hebrew, the lan­guage used in the study. Of those fake words, some had sound pat­terns that are pos­sible in Hebrew while others are not. The two groups were equally skilled at dis­tin­guishing between them, indi­cating that dyslexic par­tic­i­pants could just as easily iden­tify the Lego pat­terns as those without dyslexia.

Given the existing lit­er­a­ture, this was extremely sur­prising. Indeed, when Berent designed this research, she said she “fully expected to val­i­date the phono­log­ical hypoth­esis.” But the results showed no hint of a phono­log­ical deficit.

Instead, she found a host of subtle prob­lems in the per­cep­tion of speech sounds. For example, in one exper­i­ment, dyslexic indi­vid­uals had a harder time dis­tin­guishing real words from fake words. A second exper­i­ment showed that dyslexics had a hard time dis­tin­guishing real human speech from dig­ital sounds mim­ic­king speech.

“So maybe it’s get­ting the Lego blocks, not pat­terning the Lego blocks, that is impaired,” she said. That is, the problem seems to reside in their pho­netic systems.

To test this hypoth­esis, they also asked lis­teners to dis­tin­guish between dis­crete acoustic sounds (such as “ba” or “pa”). Once again, “there were some reli­able dif­fer­ences in the two groups in iden­ti­fi­ca­tion, and even more so in dis­crim­i­na­tion,” she said.

In the past, dyslexia researchers accepted what Berent calls a “mushy def­i­n­i­tion” of phonology. “Some researchers iden­tify phonology as any process related to speech pro­cessing, whether it is speech per­cep­tion, or the map­ping of let­ters to speech sounds,” she said. “I think the con­tri­bu­tion of our work, is saying, ‘Look at the lin­guis­tics, look at what the two sys­tems really are doing in human lan­guages and maybe that will help you under­stand dyslexia.’”

Berent’s find­ings sug­gest that the dis­order may reside in a lower-​​level com­po­nent of speech per­cep­tion, such as the audi­tory system. Other research sug­gests these dif­fi­cul­ties might arise in the early devel­op­ment of the human brain, per­haps even before birth.

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