Dyslexia :: New dyslexia theory blames ‘noise’

Kids with dyslexia can’t block out distractions, say a group of new studies. The results cast doubt on an influential neurological model of dyslexia developed in the 1970s.

The dyslexic brain struggles to read because even smalldistractions can throw it off, according to a new model ofdyslexia emerging from a group of recent studies.

The studies contradict an influential, 30-year-old theory thatblamed dyslexia on a neural deficit in processing the fastsounds of language.

Instead, the studies suggest that children with dyslexiahave bad filters for irrelevant data. As a result, they struggleto form solid mental categories for identifying letters andword sounds.

Such children may benefit from intensive training under”noisy” conditions to strengthen their mental templates,said University of Southern California neuroscientistZhong-Lin Lu.

Lu was a co-author on three studies, along with lead authorand former USC graduate student Anne Sperling (now atthe National Institute of Mental Health), USC psychologistFranklin Manis and University of Wisconsin, Madisonpsychologist Mark Seidenberg.

The most recent study is due to be published later thismonth in Psychological Science.

Confusion about dyslexia rivals the confusion of dyslexia.Many still think that to have dyslexia means to mix up yourletters (one of many possible symptoms having to do withword recognition, directional ability and decoding ofsymbols).

What is known is that dyslexia affects millions of children,with estimates of its incidence ranging from 5 to 15percent.

Sperling, who conducted her research as a doctoralstudent at USC, said the new findings point to a deeperproblem – not just a visual deficit – affecting all areas ofperception.

Sperling said people with dyslexia appear to have shakymental categories for the essential sounds that make uplanguage.

“It’s harder to make a [language] task automatic when yourcategories are fuzzier than they ought to be to begin with,”she said.

“In terms of treatment, the results suggest that programsthat foster the development of sharper perceptualcategories for letters and letter sounds might be a goodway to supplement existing dyslexia interventions,” sheadded.

Lu said, “Train them in noise.”

The new study in Psychological Science builds on similarresults published by the team of Sperling, Lu, Manis andSeidenberg last year in Nature Neuroscience.

In addition, the same authors previously showed that poorreaders also have trouble figuring out categories in simplecard games.

Other recent studies lend support to the noise exclusiontheory.

Johannes Ziegler of the Universite de Provence inMarseille, France, was the lead author on a study ofdyslexia and auditory noise published this year inProceedings of the National Academy of Sciences.

Ziegler said his results suggest that dyslexia stems fromshaky categories for phonemes (the basic sounds oflanguage).

“In silence, information is often redundant and dyslexics getaway with the perception deficit,” Ziegler said in an e-mail.”In noise, however, they can no longer compensate.

“What is important is that noisy environments are the ruleand not the exception,” he added, citing a study from SouthBank University in the U.K. that found average noise levelsin primary classrooms to be as high as near a busyintersection.

“What Sperling and Lu’s data suggest is that themechanism responsible for faulty phonologicaldevelopment is quite general and has to do with attentionin a broad sense….

“This is a great paper of very high significance… As peoplelike Steve Grossberg [of Boston University] have argued formany years, attention … is crucial for stable learning ofcategories.”

Ziegler called for preventive training for children with weakspeech perception in noise in kindergarten or early primarygrades, saying they are at greater risk for developingdyslexia.

He also cited a Northwestern University study from 2003that documented negative effects from noise on childrenwith learning deficits.

Lu said there is a “lot of evidence” of learning problemsfrom ambient noise. In one such study, Manis and acollaborator from UCLA found that children with dyslexiastruggled to discriminate similar sounds, like “spy” and”sky,” because they weighed irrelevant differences insounds equally with key distinctions.

Manis also cited research from Finland and the UnitedStates showing that infants with dyslexic parents lagbehind their peers in forming categories for speechsounds.

In the conclusion to their study in Psychological Science,the authors speculate that the deficit in noise exclusionmay have biochemical roots in abnormal levels of GABA, aneurotransmitter that helps the brain to filter out irrelevantinformation.

“This may become interesting for drug development,” saidLu, who is testing this hypothesis with functional magneticresonance imaging trials.

Lu and his collaborators interpret the new results as arejection of the “magnocellular hypothesis” – named for atype of neuron involved in processing fast visualinformation – that influenced dyslexia research fordecades.

The researchers found that the magnocellular pathwayworks normally both in children with dyslexia and in adultpoor readers – as long as visual or aural noise is low.

As external noise goes up, the same subjects begin toscore poorly on visual pattern tests.

The deficit persists even when the task requires only slowprocessing.

“The findings, and particularly the [slow processing] ones,are consistent with the hypothesis that … dyslexic childrenhave difficulty setting their signal filters to optimum andignoring distracting noise,” Lu said at the time of the NatureNeuroscience study.

The new study in Psychological Science was designed toreplicate visual tests on motion perception from seminalexperiments in the 1970s, with the addition of variableexternal noise. It also found no magnocellular deficit.

“These were the stimuli people used to establish themagnocellular hypothesis,” Lu said. “This is a more directtest of what we said before [in the Nature Neurosciencestudy], which used different spatial and temporal patterns.”

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