The dyslexic brain may have a general problem forming perceptual categories, including the templates for printed letters and speech sounds, say University of Southern California (USC) neuroscientists.
This is reflected in a reduced ability to filter out visual "noise" that can obscure a pattern, the researchers suggest.
Their novel hypothesis, published in the current issue of Nature Neuroscience, raises broader questions: Does the dyslexic brain's trouble with patterns and noise extend to other senses? Does poor filtering inhibit the formation of perceptual categories? Or is poor formation of categories the root cause of dyslexics' problem with noise?
Dyslexia is the most common and perhaps least understood reading disability. Affecting millions of Americans, it has a history of uncertain explanations.
An old, discredited, but persistent view is that dyslexics jumble their letters.
In the 1980s, the subtler "magnocellular hypothesis" gained favor with some scientists. Named for a type of neuron, the hypothesis held that dyslexics struggle to process rapid visual signals. Language comprehension also requires rapid processing ability.
The Nature Neuroscience study casts doubt on the magnocellular hypothesis. The lead author was Anne Sperling, a graduate of USC's neuroscience program whose Ph.D. thesis was based on the study.
The research team, which included Zhong-Lin Lu and Franklin Manis, professors of psychology in the USC College of Letters, Arts and Sciences, and Mark Seidenberg of the University of Wisconsin, Madison, asked dyslexic and non- dyslexic children to identify patterns presented with and without visual noise.
"The dyslexic children performed the same as the non-dyslexic children when there was no noise," Sperling and colleagues wrote.
With noise, the dyslexic children needed more brightness contrast to perform the same tasks as their non-dyslexic peers. This was true whether the patterns required fast or slow processing.
"The findings, and particularly the (slow processing) ones, are consistent with the hypothesis that rather than having an M- or rapid- processing deficit, dyslexic children have difficulty setting their signal filters to optimum and ignoring distracting noise," Lu stated.
"Kids with dyslexia ... have a hard time focusing in on what's relevant and ignoring what is irrelevant," Sperling said. The study offers guidance for parents and teachers. On a basic level, any method that helps children to concentrate on a reading task and excludes distractions should be helpful, Sperling said.
Specifically, programs that help children to form sharper perceptual categories for sounds and letters could supplement existing dyslexia interventions, said the authors, who questioned the cottage industry of visual tools that has sprung up around the magnocellular hypothesis and related theories.
"Our research provides an explanation for why the child is failing to read and helps to explain why certain methods are effective. It does not support the idea that children need special glasses or visual training or anything specifically related to treating their vision," Seidenberg wrote.
"We do not think this is a specifically visual problem," Seidenberg added. The research team plans to conduct an analogous study of hearing to check whether dyslexics also have trouble separating patterns of sounds from noise. Lu, Manis and Sperling also plan to explore the root cause of dyslexics' problem with noise. Last year, the researchers showed that poor readers also lag in categorical learning, defined in the paper as the ability to deduce rules for sorting geometrical shapes.
Research in Finland and the U.S. has shown that infants with dyslexic parents fare worse than other children in forming categories or templates for speech sounds, Manis said. A child with shaky templates might then be less tolerant of noise.
"Is the reason they have problems with noise because they don't have good categories?" Lu asked.
The National Institutes of Health define dyslexia in part as "a specific learning disability that is neurological in origin. It is characterized by difficulties with accurate and/or fluent word recognition and by poor spelling and decoding abilities."