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.