New evidence for cause of dyslexia

May 29 2005

Addressing a persistent debate in the field of dyslexia research, scientists at the University of Wisconsin-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 dyslexics.

Rather, a more general problem in basic sensory perception may be at the root of the learning disorder, the scientists report 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, alongside 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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