The inability to rapidly and smoothly process serial words has long been a bane to many students and remediation tutors. Now Harvard researchers are on the cusp of understanding how specific brain malformations may lead to poor reading fluency.
The scientists used diffusion tensor imaging, a form of MRI that captures how water diffuses around myelin sheaths, to find a correlation between disorganized white matter tracts and difficulty in reading, which they report in the Dec. 4 Neurology.
The scientists studied two groups of patients who had brain disorders preventing normal reading, the first including people with dyslexia. The second group, patients with periventricular nodular heterotopia (PNH), were born with brain malformations. In the fetal life of PNH patients, specific neurons fail to migrate to the periphery of the brain and instead stay deep in the middle.
“The amazing thing about PNH is that even though patients have large nodules of misplaced gray matter, they essentially, on the surface, seem quite normal,” said Bernard Chang, an HMS assistant professor of neurology at Beth Israel Deaconess Medical Center who co-led the study with Tami Katzir, a dyslexia specialist who was at the Harvard Graduate School of Education when the research was performed. Many PNH patients have ordinary intelligence, but begin experiencing seizures during adolescence. Their characteristically placed brain nodules allow them to be easily diagnosed via MRI.
In a 2005 paper, Chang, with senior author Christopher A. Walsh, Howard Hughes investigator and the Bullard professor of neurology at BID, and colleagues, described their study of 10 PNH patients, finding that they had difficulty with the processing speed of reading.
“That made us wonder, why would misplaced gray matter like this lead to a specific problem with reading?” said Chang, “and was this in any way related to dyslexia?”
In their current study, the researchers compared reading skills in 10 patients with PNH, 10 with dyslexia, and 10 normal subjects. After a battery of reading tests and brain imaging, the scientists found that the dyslexics and the PNH patients shared a specific problem relating to both reading fluency and disorganized white matter tracts.
The more disorganized the white matter tracts, the poorer the reading fluency. Chang said that until they learned that PNH patients had problems with reading, he never thought to use the disease as a model to understand reading fluency in dyslexia.
“Because dyslexia is so common and has so many different subtypes and is so heterogeneous, it can make it hard to identify specific features that tie into specific aspects of reading,” he said.
Indeed, the reading tests showed that dyslexics had more trouble with phonological tests—breaking down words into sound segments—in addition to reading-fluency exams. When given more time, both the PNH patients and the dyslexics performed nearly as well as the normal readers.
If students with dyslexia and other reading disorders are found to have unorganized white matter tracts, then tutors might, for example, use a variety of approaches to specifically improve processing speed, said Walsh, also an HMS professor of pediatrics at Children’s Hospital Boston.
“The clinical implications of this study are that assessment and treatment of reading disabilities should include not just measures of reading accuracy but also measures of reading fluency,” said Katzir.
