USC College neuroscientists Zhong-Lin Lu,
Anne Sperling and Frank Manis
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Tuning in to Dyslexia
Scientists suggest new explanation of common reading disability
By Eva Emerson
January 2005
Most researchers agree that dyslexia 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 these difficulties.
USC College neuroscientists are adding fuel to the debate. 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, says 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, has lately 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.
They 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 did a difference
emerge. 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,” says 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,” says
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 says.
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,” says Manis.
“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 says. “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 of dyslexia during the first grade,
when they’re supposed to be learning to read.”
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