Neuroscientists in the USC College of Letters, Arts and Sciences are adding fuel to the debate over dyslexia.

Most researchers agree that the reading disability stems from an inability to link specific sounds to written letters and words. But any consensus quickly breaks down when scientists discuss the underlying biological causes that lead to those difficulties.

In work presented at a meeting of the Society for Neuroscience, Zhong-Lin Lu, Frank Manis and Anne Sperling showed that dyslexic children have a harder time than non-dyslexics filtering out a signal from background “noise” during tests of visual perception.

The team, which also includes the University of Wisconsin’s Mark Seidenberg, speculates that the deficit may affect the whole brain.

If confirmed, the research could lead to a better understanding of the physical basis of the learning disability as well as improved identification of children with dyslexia. That could lead to earlier interventions.

The most common reading disability, dyslexia affects between 5 and 15 percent of Americans, with an estimated 14 million suffering from a severe form of the reading disability. Dyslexia may show itself as problems in learning to read, writing legibly, spelling or calculating math problems.

Even in reading a simple word like “cat,” the brain must recognize the three distinct sounds represented by each letter (“cah,” “ah” and “tuh”); recognize the sounds in the proper order (cat vs. tac vs. act); blend the sounds into a word; and mentally map the sounds onto the letters written on the page.

People with dyslexia may run into trouble during any of these steps that, in strong readers, become automatic, said Manis, a professor of psychology and a dyslexia expert.

Its results can be devastating, especially in the Information Age, when reading is critical to success in school and careers.

Although most dyslexics can and do learn to read with special training, many dyslexic children and adults are never identified.

A number of studies have suggested that people with dyslexia have a deficit in one of the brain’s visual processing pathways, the M (magnocellular) pathway that processes motion and brightness.

The same studies showed no deficits in the overlapping but distinct P (parvocellular) visual pathway, which processes colors and fine details.

The M processing deficit explanation of dyslexia, however, lately has come under increased scrutiny by scientists.

In the recent study, the USC team recruited 55 children aged 8 to 12, with 28 identified as dyslexics and 27 as non-dyslexics. The children were asked to complete a series of tasks testing their language and reading skills before completing a number of visual tasks. The children hit a button when they saw a rectangle of black-and-white stripes appear on a computer screen.

Adjusting how much the stripes contrasted with the background, the team compared the ability of the children to detect two different patterns, a flashing pattern that stimulates the M pathway and a stationary one processed by the P pathway.

The team found that dyslexics and non-dyslexics were equally able to detect both M- and P-type patterns. It was only when researchers added visual noise—in the form of TV “snow”—on top of the pattern that a difference emerged.

Under the “noisy” conditions, both the M- and P-type patterns had to be 10 percent more contrasting for children with dyslexia to detect them compared with non-dyslexics.

“People with dyslexia may have a harder time distinguishing a signal from the noise—not because they can’t perceive the signal, as had been thought, but because they are not as adept as filtering out the background, non-essential sensory information,” said Lu, associate professor of psychology and an expert on vision and attention.

“In fact, the deficit we found is not specifically visual—we think of it as a sign of a basic problem in sensory perception. Next, we want to look for this same deficit in the brain’s auditory pathways,” said Sperling, whose doctoral dissertation was based on the study. She earned a Ph.D. in neuroscience from the college in August and is now a postdoc in neurology at Georgetown University.

In terms of impact on reading, poor filtering ability could distort speech perception in infancy, complicating the development of categories of phonemes [speech sounds] and later, letter recognition and the child’s appreciation of spelling-sound links, Sperling said .

The team has planned a number of follow-up studies to further test the hypothesis.

“The study needs to be replicated, with different kids, different types of vision tasks and with auditory processing tasks. I’d like to do this in younger kids as well—if it’s there, we should be able to pick up on this difference even in kindergartners,” Manis said.

“The biggest gain that we who work on dyslexia could get right now would be identifying children with dyslexia as early as ages 4 to 5, before the reading problem becomes acute,” he said.

“If a student is failing to learn to read by the end of first grade, to me that’s an emergency. Early identification would allow us to begin aggressive interventions for those at-risk.”