The question of when a child is ready to read–and why so many children have difficulty with this fundamental academic skill–has long been a focus of education debates in this country. Now, for the first time in a concerted way, neuroscience is entering the debate, bringing evidence-based science and the sophisticated tools of modern brain research to bear in pursuit of the answers.
While a high-level cognitive skill such as reading may seem like a natural province of brain research–all learning occurs in the brain, after all, and learning disabilities have clear brain correlates–until recently relatively few neuroscience laboratories have focused efforts on investigating the brain basis for this uniquely human skill. As neuroscience now delves into the neurobiology of reading, one thing is becoming clear: not all children’s brains are “wired” for reading in the same way.
Researchers are finding increasing evidence linking reading ability to subtle differences in the neural pathways. These pathways connect and coordinate brain regions involved in the elemental skills that comprise reading proficiency, from visual recognition of letters and words to phonological processing, to higher-level systems that enable content comprehension.
In particular, new techniques in neuroimaging are beginning to shed light on the neurobiological underpinnings of “reading readiness” and subtle brain differences that may help explain the wide variance of reading proficiency among children. These emerging data are illuminating the neural bases for the longstanding observation that reading abilities run a wide gamut from exceptional to below normal–the latter sometimes dipping to a level that society typically terms as reading-disabled.
But some researchers are wary of labeling kids as such, with all the consequent implications for academic stigmatization, lowered self-esteem, and family strife, and prefer to think of high- and low-achieving readers as merely at opposite ends of a normal distribution of skill proficiencies.
“My perspective, after 40 years of working on this entity we call ‘reading disability’ or ‘dyslexia,’ is that we need to be thinking of it as a variant of normal, rather than an abnormality,” said Martha Bridge Denckla, a research scientist at the Kennedy Krieger Institute, professor at Johns Hopkins School of Medicine, and member of the Dana Alliance for Brain Initiatives. She suggests reading can be better understood as a talent, a biologically bestowed gift that is not doled out equally to everyone.
“There is variability at the highest level of the brain for a whole bunch of different things we call talents,” said Denckla. “We accept completely that there are people who do not have whatever the neurological basis for musical talent may be–we just say they don’t have that ‘ear’ for music. Well, people can also be born with an untalented ‘ear’ for the speech sounds of language, which makes it very difficult to connect with an alphabetic system and be proficient at reading.”
An ‘Inconvenient Difference’
Rather than a disability, Denckla prefers to think of people who have difficulty reading as having an “inconvenient difference in biological organization”–though she is quick to point out that the disability moniker is crucial for enabling struggling kids to get the help they need in the education system because of the way special-education laws are set up.
Stanford University psychologist Brian Wandell echoed this sentiment: “Historically, people have assumed that all children’s brains come adequately equipped and ready to learn to read,” just as with learning to speak, which occurs naturally without much training. But, he said, “Sometimes, there is a natural distribution of capabilities. Reading is probably the hardest thing we teach people to do in the education system. There are some kids who are just going to have a hard time.”
Wandell, who received funding from the Dana Foundation, is the principal investigator on an NIH-funded longitudinal study examining the development of reading skills and associated brain-structure changes in a group of 49 children aged 7 to 12. Preliminary findings from his group, reviewed in 2007, suggest that white matter pathways in specific brain areas are critical to reading fluency1.
White matter refers to the bundles of myelinated axons that connect disparate regions of the brain’s cortex; myelin is the fatty white sheath that envelops axons to facilitate efficient nerve signal transmission. These bundles can be thought of as the brain’s communication cables, shuttling electrical signals from groups of nerve cells that are separated by many centimeters–relatively great distances in the brain. When these white matter pathways breakdown, consequences can be profound, as in paralysis, or subtle, as is now becoming clear in reading difficulties.
Wandell’s findings with collaborators Ben-Shachar and Dougherty extend a small but growing body of scientific literature linking changes in the structural integrity of white matter pathways to reading proficiency. The first report, published in 2000 by Torkel Klingberg and colleagues at Stanford, found white matter abnormalities in temporo-parietal brain regions (near the temples and mid-back of the cortex) in adults with dyslexia, compared to a control group of normal-reading adults.2 In the last few years, other groups–Wandell’s among them–have independently found similar findings in children ages 6 to 13. 3,4,5,6
The majority of these studies have focused on temporo-parietal pathways, particularly on the left side, in areas long associated with language skills. But Wandell’s most recent research has also found axonal tract anomalies in the corpus callosum, the thick bundle of fibers that connect the brain’s left and right hemispheres. Specifically, the area in the back part of the corpus callosum, which seems to link up with a part of the visual system essential for perceiving motion and managing eye movements (across a page of text, for example), is subtly different in children who have difficulty reading.
DTI and ‘Leaky’ Fibers
All of these studies have taken advantage of a new method of magnetic resonance imaging, called diffusion tensor imaging (DTI), which has been used in clinical applications for two decades but has only recently been adapted to interpret the properties of white matter pathways. DTI uses complex algorithms to measure the directional flow of water, which carries nutrients and glucose to fuel brain processes, through axon bundles.
One factor that influences how well water flows through white matter is the cell membrane, which can be thought of as the “skin” of the axon. DTI measures the degree to which water crosses through the axonal membrane. This provides information about the microstructural integrity of the axons within a given tract, an indication of how efficiently nerve signals are transmitted in that tract.
In the area Wandell’s group has studied–the hind end of the corpus callosum–DTI revealed distinct differences in water flow that correlated with the results of behavioral tests measuring phonological awareness. This skill is essential to reading, and entails the ability to understand and manipulate the basic sound structures of language. It turns out, Wandell said, that these specific membranes tend to be a little more “leaky” in poor readers.
The degree of leakiness is quite reliably related to children’s ability to read as measured by phonological awareness tests, he said. “In good readers, water doesn’t tend to cross through the cell membrane. But when you measure those same places in kids who have trouble reading, the water flows through the cell membrane at a noticeably higher rate–by about 20 percent higher.”
Although it’s not clear exactly how this “leakiness” factors into the brain’s distributed system for reading proficiency, Wandell and colleagues postulated one explanation in their report of the findings in 2007. The authors suggested that better readers may have fewer but thicker axons connecting the two hemispheres at this part of the brain, which would mean fewer membranes and less overall leakiness. This hypothesis may also explain other previously reported white matter disturbances.
New research led by Walter Kaufmann at Kennedy Krieger adds further support for the hypothesis that localized disruptions in white matter pathways underlying reading ability contribute to dyslexia. Their most recent findings, which stem from work funded in part by the Dana Foundation and which have recently been accepted for publication, are the first to use two complementary DTI approaches: voxel-based, which homes in on individual bits of tissue and is the standard method for DTI in neuropsychiatric disease, and region-of-interest, a method that enables visualization of the pathways across brain regions. This dual approach has shown promise in increasing our understanding of complex changes in white matter, and as such may be well-suited to the study of reading disabilities.
Taken as a whole, Denckla said, these results suggest that in children who have difficulty reading, “the brain’s ‘wiring diagram’ is just a little bit different, better thought of as an anomaly than an abnormality. For all we know, it may have some benefit for some other activities, but it appears to be somewhat disadvantageous for reading.”
There are many examples, she pointed out, of highly successful people who are not so great at basic academic skills like reading, from Albert Einstein to Charles Schwab. “It turns out that the talent for reading–and particularly for reading quickly, which makes it useful–is not distributed to every human being, and not in direct proportion to one’s other talents,” Denckla said.
Foreseeing the Implications
While the research linking white matter integrity with reading proficiency is still in its infancy, the potential implications of the findings are far-reaching. Wandell’s team is already designing interventions that might help children compensate or circumvent the leaky pathways in the corpus callosum. Because these pathways seem to be essential for managing eye movements while reading, he believes it may be possible to use computer displays to “redesign text in a way that might increase the signal on those pathways or otherwise be particularly powerful in driving that part of the brain.”
“If these fibers are weak, it may be that the signals aren’t coming in strong enough, or at the proper rate,” he said. Using larger text or three-dimensional characters, for example, or somehow manipulating the timing of the textual inputs, might “amp up the signal” enough to make a difference, he said. A similar principle has been used successfully to remediate language-based learning disabilities.
At the least, Wandell said, understanding these pathway deficiencies could enable better identification of children who simply may not be equipped with the right neural hardware to enable high-level reading proficiency. “If there is a physical cause that you can understand and that you know is going to be a limiting factor for the child, you don’t want to put the child and the family through something that they may be destined to fail at. You don’t want the school system telling the child ‘you’re stupid’ or ‘you’re not trying hard enough,’ when the child knows perfectly well he’s trying and it’s just not working out.
“If you could say, ‘Look, this child just doesn’t have the pathways in the brain that will enable him or her to read well,’ then you could get the child the information in another way, or encourage the child to focus on other skills.”
Published August 2008
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