Guest Post by Priya Kalra, Harvard University
Although scientists now understand dyslexia better than ever before, it is still a condition shrouded in myth and misunderstanding. I first came to see our flawed perceptions of dyslexia while tutoring a 4th grader. Despite normal intelligence and effort, he could not read. I saw how the frustration this caused him affected his general behavior and attitude toward school. At the same time, I read a story in Time magazine about the work of Mike Merzenich and Paula Tallal showing that their behavioral intervention seemed to cause measurable differences in brain activity among people with dyslexia. This got me thinking about the possibility of using neuroscience to better understand dyslexia and help students with reading disabilities.
For at least the past 20 years, neuroscientists have been working on just that: furthering understanding of the neural underpinnings of dyslexia and other reading problems to offer possible solutions. Before fully understanding how dyslexia works, it is important to first look at what it is not. So let’s first do some myth-busting:
1. Dyslexia is a phase that children grow out of.
False. While many people with dyslexia may develop effective work-arounds or strategies for dealing with their reading problems, dyslexia is a lifelong condition. Neuroscience research includes many studies of adults with dyslexia.
2. Dyslexia is a euphemism for “lazy” or “stupid.”
False. While dyslexia can sometimes be difficult to diagnose precisely, it is defined as difficulty in learning to read despite normal intelligence and effort. Studies have shown differences in the structure and activity of people with dyslexia compared to those without the disorder.
3. Dyslexia is a “different way of thinking.” Trouble reading means gifted in other areas.
False. While some people with dyslexia are indeed highly gifted in art, architecture, music, or other areas, these gifts may not be a related to dyslexia, and not all people with dyslexia show these traits.
4. Dyslexia consists primarily of visual-perceptual issues, such as reversing the order of letters.
False. While there are visual-perceptual conditions that can lead to difficulty reading despite normal intelligence (such as Meares-Irlen syndrome), the modern scientific understanding of dyslexia focuses on the concept of phonological processing—trouble perceiving and processing phonemes, the smallest units of sound that differentiate word meanings in a language, as well as trouble mapping phonemes onto the letters that represent them. This can be particularly difficult in languages like English, German, or French in which the phoneme-letter mapping is complicated and often inconsistent, but easier in languages like Italian or Spanish in which certain letters consistently refer to certain sounds.
Structural and functional neuroimaging, as well as electrophysiology, have all contributed to this modern understanding of phonological processing deficits in dyslexia and their possible causes.
Functional neuroimaging (fMRI and PET) looks at patterns of blood flow in the brain that reflect brain activity. Electrophysiology (EEG/ERP) uses changes in the overall electromagnetic field of the brain to detect millisecond-level reactions to stimuli. Both functional neuroimaging and electrophysiology studies have shown that the brains of people with dyslexia show patterns of reaction and activity when they are performing phonological processing tasks that differ from those without the disorder.
Structural neuroimaging uses “still pictures” of the brain. Comparing structural neuroimaging data between people with and without a disorder can reveal differences in size, shape, or organization in particular brain areas. Researchers also can use a relatively recent type of structural neuroimaging called diffusion tensor imaging (DTI) to specifically identify white matter tracts that connect different areas of the brain to each other and allow them to communicate.
Structural neuroimaging has shown both grey matter and white matter differences between people with and without dyslexia, as well as a surprising cause of some cases of dyslexia. Grey matter comprises cell bodies that make up the surface of the cerebral cortex, while white matter is made of the axons that connect neurons and allow them to communicate with each other—sometimes across long distances.
A best-case scenario would be where a child at risk for reading difficulties receives support and intervention before she has a chance to fall behind her peers.
One study by Timothy Keller and Marcel Just of poor readers before and after a course of reading intervention showed increases in the white matter of the poor readers whose reading improved. Another set of studies by Bart Boets et al., supports a hypothesis that some researchers have suspected for some time—that dyslexia stems not from problems with the phonological representations themselves, but with other brain areas being able to access those representations. They found that the functional and structural connectivity between brain regions involved in higher-level phonological processing is significantly hampered in dyslexics.
Structural neuroimaging has also revealed periventricular nodular heterotopias (PNHs) in some people with specific reading difficulties. PNHs occur during development when neurons are growing and migrating to where they will be in the mature brain; most of the time, all the neurons arrive at their genetically specified destinations, but the cells that make up PNHs end up in the wrong place. When they occur on the planum temporale, an area of the temporal lobe involved in higher auditory processing, PNHs can cause reading difficulties, possibly by disrupting phonological processing. PNHs are genetic and heritable, which may explain some cases of reading difficulties running in families.
The biggest challenge in understanding dyslexia is “equifinality,” the idea that multiple causes and pathways can lead to the same (or similar) situations. In dyslexia research, that means that at least a handful of underlying problems could result in a specific reading impairment. For example, in addition to phonological processing,researchers have found links with certain vision disorders, rapid naming, and cerebellar deficits. When a researcher recruits a group of participants with specific reading impairments to investigate one particular theory of dyslexia, their theory might be correct for some of the participants but not others, which is problematic for statistical power among other things.
Despite this challenge, scientists have been able to glean valuable insight into the possible causes and neurological bases of dyslexia. The question still remains, however, of what neuroscience can do to help people with the disorder. Recently, John Gabrieli and colleagues found that brain scans of children performing phonological tasks can predict later reading difficulties better than traditional tests of phonological processing alone.
The challenge for the researchers was to see if they could predict which students would develop reading problems based on information from before they began to read. By combining neuroimaging and traditional behavioral tests, they were able to predict reading difficulties better than behavioral tests alone.
Traditionally,when a student is suspected to have a specific reading disability, he or she is tested using a variety of assessments for phonological processing, rapid naming, and reading accuracy, fluency, comprehension. These behavioral tools are helpful for diagnosing reading problems, but they are usually not used until a student shows signs of difficulty in school; furthermore, some aspects of these tests such as reading comprehension require students to have received instruction in reading and often cannot differentiate between novice readers who will improve and those who will go on to have persistent difficulties.
Most interventions for dyslexia focus on improving phonological processing and phonological awareness. These programs seem to be most effective when provided early in a student’s academic career and also seem to be more effective with younger children. The repetitive nature of some of these programs can make them very frustrating for older students and almost completely impractical for teenagers. Therefore, early diagnosis could be extremely beneficial. A best-case scenario would be where a child at risk for reading difficulties receives support and intervention before she has a chance to fall behind her peers.
Even when they are diagnosed early and receive support and intervention early, some students don’t improve in their phonological processing and reading abilities. This may be due to the problem of equifinality mentioned earlier—their specific reading difficulty may be due to something other than phonological processing problems, and therefore phonological processing-based interventions don’t help them. Fortunately, recent research also supports the idea that neuroimaging can help to predict which students will and won’t respond to intervention.
While it is impractical to use fMRI as a screening tool on a large scale, for children who have other risk factors for dyslexia, a clear early diagnosis could lead to early intervention and spare them potentially years of frustration and setbacks. Although many questions remain unanswered about dyslexia and the role of neuroscience in education, this approach promises future benefits for the field and for people with dyslexia.
Priya Kalra is a doctoral student in the Harvard Graduate School of Education. Her dissertation research was carried out in collaboration with Amy S. Finn at the Gabrieli Lab at MIT.
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