Dyslexia marked by poor reading fluency — slow and choppy reading — may be caused by disorganized, meandering tracts of nerve fibers in the brain, according to researchers at Children’s Hospital Boston and Beth Israel Deaconess Medical Center (BIDMC). Their study, using the latest imaging methods, gives researchers a glimpse of what may go wrong in the structure of some dyslexic readers’ brains that makes it difficult to integrate the information needed for rapid, “automatic” reading.
“We looked at dyslexia caused by a particular genetic disorder, but what we found could have implications for understanding the causes of dyslexia in other populations as well,” says Walsh, who is also a Howard Hughes Medical Institute investigator at BIDMC.Dyslexia, which affects 5 to 15 percent of all children, has different forms. Subjects in this study had reading problems caused by a rare genetic disorder known as periventricular nodular heterotopia, or PNH. Although their intelligence is normal, people with PNH have trouble reading fluently, or smoothly, because they lack the rapid processing necessary for this aspect of reading.
The genetic mutation that causes PNH disrupts brain structure. In a normal brain, much of the gray matter (consisting mostly of nerve cells) appears on the brain’s surface, while white matter (consisting mostly of nerve fibers, or “wiring,” connecting areas of gray matter) runs deeper in the brain. In PNH, nodules of gray matter sit deep in the brain’s core, in the white matter; they failed to migrate out to the surface as the brain was developing.To learn more about how these developmental changes in the brain might lead to reading problems, the researchers tested cognitive skills needed for reading in three groups: 10 patients with PNH, 10 individuals with dyslexia without neurological problems, and 10 normal readers. They used a specialized form of MRI called “diffusion tensor imaging” to look at the structure of the white matter in the brain.
In PNH patients, unlike in normal readers, white matter fibers took circuitous routes around the misplaced gray matter and in some cases didn’t organize into uniform bundles, which could leave regions of gray matter poorly connected. Importantly, the more disorganized the PNH patients’ white matter, the less fluent their reading.“This makes sense,” says Chang. “When we read, we need to take in information visually, hook it up with our inner dictionary of what letters and words mean, and when we’re reading aloud, connect that with the region that gives us our ability to speak.” For smooth, automatic reading, “the white matter is there to connect different regions of gray matter and allow them to function seamlessly.