After decades of research, it's become obvious that one of the main causes of dyslexia is a disruption in phonological processing--people have trouble breaking words into their component sounds. But what's still unclear are the factors that underlie this processing problem and whether other, equally important deficits impair a person's ability to read.
Probably the hottest debate revolves around the question of whether people with dyslexia have, in addition to phonological problems, a general processing speed deficit: Their brains appear to deal with information that requires rapid rates of processing more slowly than normal, and this speed deficit inhibits their ability to analyze letter patterns and text rapidly enough to become fluent readers.
Several lines of research, mostly from the world of neuroscience, provide evidence that this may be the case for some people with dyslexia. But several mainstream dyslexia researchers--many of whom have a more behavioral bent--aren't convinced that the data to date support a general processing-speed problem. Instead, they believe the problem lies specifically within language-related brain circuits and particularly within the phonological processing systems of the brain.
In fact, most current research and theorizing on dyslexia has focused on phoneme awareness--people's abilityto break written words into individual speech sounds, known as phonemes, as when "bat" is broken into "b," "a" and "t."
No one denies the power of this line of research in helping diagnose most people with dyslexia, or that teaching them how to pull words apart into phonemes helps them learn to read. But testing for phoneme awareness alone does not identify everyone with a reading disability. And teaching it doesn't solve everyone's problems.
Part of the controversy over whether people with dyslexia have a general problem rapidly processing information or a problem specific to language stems from researchers' different interpretations of behavioral data.
One example is the interpretation of what Tufts University psychologist Maryanne Wolf, PhD, and the University of Waterloo's Patricia Greig Bowers, PhD, call a double deficit in children with dyslexia. Not only do these children often have trouble distinguishing phonemes, but they also score poorly on tests of "rapid automatic naming," or "rapid naming" for short. The traditional rapid-naming task requires people to look at a grid of letters, numbers or pictures and sequentially name them as quickly as possible. People with dyslexia often perform such tasks much more slowly than people who have no reading problems, find Wolf and others.
In fact, rapid-naming tasks appear to measure something separate from phoneme awareness, says Wolf. The measure itself correlates only weakly with phoneme awareness. Also, Wolf and others find that children with dyslexia can have problems with both phoneme-awareness tasks and rapid-naming tasks or with just one or the other task, indicating the two are mutually exclusive. Children who have problems with both tasks tend to be at the lowest end of the reading continuum, suggesting a "double-whammy" of sorts and something beyond what is explained by phonological processes, says Wolf.
Although others agree that these rapid-naming tests appear to measure something different from phoneme awareness, they aren't convinced that those tests tap a distinct underlying problem. The rapid-naming task itself, they say, is inherently phonological--that is, it involves the brain's phonological processing system, which processes, manipulates and generates language sounds. Rapid naming involves this system because it requires people to retrieve and produce word sounds, whether it's the name of a letter, a number or an object. So the underlying problem for both tasks is likely a glitch in the brain's ability to process sounds specifically related to language.
"You've got to use the phonological system for both tasks," says dyslexia researcher Susan Brady, PhD, of Haskins Laboratory and the University of Rhode Island. "One of them is a metacognitive task--phonological awareness--and one of them is a basic phonological task involving speech."
Responding to such claims, Wolf and Bowers don't deny that one aspect of the rapid-naming task is phonologically based. However, they believe that other equally important parts of the task are tapping into processes discrete from phonological processing, indicating a potentially distinct set of underlying causes.
"I argue that the problems underlying dyslexia are multiple," says Wolf. "One possibility is surely phonological for some children, but there are others that must be understood. One may well be something more general that crosses many domains including visual, auditory and motor processing. We're saying that this speed deficit in naming is but the tip of an iceberg that needs explanation."
Wolf and Bowers believe that poor performance on the naming speed test--which uses many of the same underlying skills needed for reading--provides a clue that, in many people with dyslexia, brain circuits that connect visual information with the verbal system fail to operate efficiently. This could indicate a deficit specific to language or, more likely according to Wolf, a problem of varying degrees of generality, with some people with dyslexia having difficulty rapidly processing just visual language and others having difficulties rapidly processing several kinds of information.
Evidence from neuroscience
The concept of a general processing-speed deficit in people with dyslexia is at the core of several findings coming out of the brain-imaging and neuroscience communities. Likely the best known findings of this type come from Rutgers University psychologist Paula Tallal, PhD, and her colleagues, who primarily study children with language-learning disabilities rather than dyslexia per se.
They posit that children with language-learning disabilities--which may lead to dyslexia--process sounds more slowly than the average child, and that this diminishes their ability to distinguish phonemes.
Evidence for this theory comes from studies that measure how much time children need between two sounds before recognizing that there is more than one sound. This timing threshold, as Tallal calls it, is the time it takes nerve cells to fire, process a sound's acoustic features and then recover enough to pick up the next sound. While the average child has an average timing threshold in the tens of milliseconds range for simple tones, children with language-learning problems have timing thresholds measuring hundreds of milliseconds.
Because the differences between many phonemes--such as "ba" and "da"--occur within tens of milliseconds, children who need longer to detect changes may not hear the difference, the theory posits. And if they can't hear the difference between certain phonemes, they may develop problems mapping phonemes to words. In fact, Tallal and her colleagues find that as many as 85 percent of children with language-learning problems go on to develop dyslexia.
While some dyslexia researchers contend that they have yet to replicate Tallal's findings in children diagnosed with dyslexia, many other published studies have. And, Tallal argues, by the time children are diagnosed with dyslexia at around age 9, their brains may have compensated for the auditory deficit. She theorizes that the auditory problem might have been present earlier in life and laid the groundwork for trouble with phoneme awareness. In fact, in her longitudinal samples Tallal does find that easily measurable auditory processing problems disappear as children age, requiring more subtle assessments to detect these problems.
Some brain-imaging studies also find evidence of a processing-speed deficit in people with dyslexia, though most of the work has been done with visual processing rather than auditory processing (see sidebar, page 37).
Does the theory fit the data?
These findings taken together--along with others from the neuroscience literature--point to a global processing-speed deficit at the root of some, if not all, reading problems, say proponents of the processing-speed theory.
But others aren't convinced by the evidence, at least not yet.
"The theory doesn't fit well with what we know about poor readers," says Haskins Laboratory's Brady.
Work by Brady and her colleagues does not find evidence that children with dyslexia have trouble tracking rapid transitions in speech signals, she says.
"The children have trouble identifying speech signals that are very similar to each other because of a weakness in their phonological system, but they seem to track the information in the same way good readers do," says Brady. "When we've compared perception of speech and nonspeech signals that are carefully matched, poor readers only have difficulty with the speech."
Proponents of the processing-speed deficit theory, including Stanford University neuroscientist David Heeger, PhD, aren't swayed, pointing to studies from other labs that do find the deficits. And arguments by some that people with dyslexia don't show deficits in other tasks that require rapid processing, such as music and athletics, aren't convincing, either. Reading and language processing are likely the most difficult and fast-paced tasks people routinely do, says Heeger. Although music and athletics require fast temporal processing, they're not obviously equivalent to the speed with which the brain has to process words to read and comprehend a sentence, he says. Besides, we don't expect everyone to be great athletes and musicians, but we do expect everyone to be reading experts.
In June, researchers will have a chance to debate these issues at a conference in Crete, sponsored by the National Dyslexia Research Foundation.
"The debate is so healthy for the field," says Tufts' Wolf. "One of the most exciting aspects of this work is that we don't know all the answers. Even if we discover, in the end, the problem is only within the language domain, we'll have done the work necessary to prove it."
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