Study spells out new evidence for roots of dyslexia
Wisconsin Week
University Communications
Posted: 5/31/2005
Addressing a persistent debate in the field of dyslexia research, scientists at
UW-Madison and the University of Southern California (USC) have disproved the
popular theory that deficits in certain visual processes cause the spelling and
reading woes commonly suffered by people with dyslexia.
Rather, a more general problem in basic sensory perception may be at the root of
the learning disorder, the scientists reported May 29 in the journal
Nature Neuroscience. The work
suggests new ways to identify dyslexics and to assess the many unevaluated
techniques teachers use to help dyslexics in the classroom.
 Misfiring
neurons perhaps make it difficult for dyslexics to pick out relevant visual and
auditory cues from the expanse of surrounding sounds and patterns, or "noise";
it is this inability that may bear heavily on how easily a child can read, says
lead author Anne Sperling, who conducted the research as a USC graduate student,
in collaboration with USC professors Frank Manis and Zhong-Lin Lu and co-author
Mark Seidenberg, a UW-Madison
psychology professor who left USC in 2001.
"We really want to understand what is going on at the neurological level that's
leading to reading problems," says Sperling. "[We think] that if a child has a
hard time ignoring 'noise,' it could distort speech perception and complicate
[the recognition] of sound segments, which is essential for learning how to
read."
A learning disorder with neurological underpinnings, dyslexia affects between 5
to 10 percent of children in the U.S. Sperling calls the condition a "spiraling
problem" because poor reading interferes with many types of learning.
Researchers first proposed during the 1920s that dyslexic children sometimes
spell words backwards because they have trouble seeing straight. Five decades
later, that idea out of favor as researchers increasingly believed that dyslexic
reading problems are directly linked to the inability to blend phonemes, or the
component sounds in any word.
A child needs to understand that spoken words consists of such sounds-that "bat"
for example, includes three sounds ("buh," "aah" and "tuh") while the word
"splat" has five. The knowledge makes it easier to learn how to pronounce
letters, explains Seidenberg.
"For some reason [dyslexic children] are not developing knowledge of phonemes,"
says Seidenberg. "This has little impact on their spoken language, but really
interferes with learning to read."
Scientists have long tried to understand why dyslexics stumble with phonemes.
With recent advances in the understanding of the brain and visual processes,
dyslexia researchers again turned in the 1990s to vision as the likely root of
the learning disorder. In particular they focused on the magnocellular (M)
pathway, one of two visual pathways in the brain that processes motion and
brightness. The other visual channel, the parvocellular (P) pathway, processes
detail and color.
Some studies implicated an impaired M channel, showing that dyslexic children
have trouble seeing rapidly changing or moving stimuli. But the findings have
not been readily replicated and there was little consensus among experts, says
Sperling. "We wanted to know decisively once and for all whether it is the M
pathway or not," she says.
Devising a new approach, Sperling gathered 28 dyslexic and 27 non-dyslexic
children, and showed them a pattern on a computer screen showing alternating
light and dark bars. One type of pattern, with thick, rapidly flickering bars,
targeted study participants' M pathways. The other type of pattern, with thinner
non-flickering bars activated participants' P pathways. The patterns appeared
either on the left or right side of the screen, and the children's task was to
indicate which side they saw them.
When only the patterns appeared, the dyslexic children were as able as their
peers to pick out both the M and P displays. But when Sperling partially
obscured the patterns with patches of "noise," or television static-like bright
and dark spots, the dyslexic children struggled to isolate both M and P
patterns.
The work confirms that problems with "ignoring noise" play a more central role
in the onset of dyslexia than the M and P pathways, Sperling says. An immediate
classroom application, she suggests, could be for teachers to "accentuate
differences between sounds, showing the extremes to help [dyslexic children]
build categories."
Future studies should examine additional sensory systems, Seidenberg adds, to
see if the noise idea holds for all senses and to seek connections between
auditory and visual processes in dyslexia.
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