Problems sorting through competing sounds, letters, and other "noise" may cause the reading problems associated with dyslexia, according to a new study.
Researchers say the findings offer new clues about the roots of dyslexia and disprove the theory that problems in visual processing cause the spelling and reading problems suffered by dyslexics.
Instead, the study suggests that a more general problem in sensory perception caused by misfiring neurons in the brain makes it harder for children with dyslexia learn how to read. The misfiring neurons make it more difficult for dyslexics to pick out the relevant visual and sound cues from the surrounding "noise" and read effectively.
"If a child has a hard time ignoring 'noise,' it could distort speech perception and complication [the recognition] of sound segments, which is essential for learning how to read," says researcher Anne Sperling of Georgetown University Medical Center in a news release.
Getting to the Roots of Dyslexia
Dyslexia is a learning disorder that affects about 5 percent-10 percent of U.S. children. The condition has little impact on spoken language but seriously impairs the ability to read and often causes sufferers to spell words or parts of words backwards.
In the 1920s, researches proposed that the problem was caused by vision problems. Fifty years later, researchers increasingly thought that dyslexic reading problems were caused by the inability to blend component sounds within a word.
Researchers say that for some reason, children with dyslexia do not develop the knowledge that spoken words consist of these sound components known as phonemes.
For example, children need to understand that words like "bat" consist of three sounds (buh, aah, and tuh). That knowledge makes it easier to read and pronounce letters.
A New Direction for Dyslexia Research?
Recent advances in the understanding of how the brain processes information has lead some researchers to focus on one of two visual pathways that the brain uses to process motion and brightness, called the magnocellular or M pathway. Some studies have shown that an impaired M pathway may cause children with dyslexia to have trouble seeing rapidly changing or moving stimuli.
The other visual channel, the parvocellular or P pathway, processes detail and color.
In the study, which appears in the current issue of Nature Neuroscience, researchers looked at whether these pathways or something else might play a role in developing the reading problems associated with dyslexia.
Researchers had 28 dyslexic children and 27 nondyslexic children watch patterns of alternating light and dark bars on a computer screen.
One pattern with thick, rapidly flickering bars targeted the children's M pathways, and the other with thin, nonflickering bars activated their P pathways.
The results showed that when the patterns appeared by themselves, the dyslexic children were as good as the others at picking them out. But when the patterns were partially obscured by "noise" or television static-like spots, the dyslexic children had greater problems isolating the patterns.
Researchers say the study shows that problems with ignoring noise plays a more central role in the development of dyslexic reading problems than either the M or P pathways.
They say the findings suggest that teachers should accentuate the differences between sounds, showing the extremes to help dyslexic children build categories of sound components.
SOURCES: Sperling, A. Nature Neuroscience, May 29, 2005 advance online edition. News release, University of Wisconsin-Madison.