Brain Research Produces Clues on
Possible Causes of Dyslexia
April 2020 - Brain imaging research at Georgetown University Medical Center continues to clarify the functional causes of dyslexia. A report published in Human Brain Mapping in 2019 challenges a theory that the cerebellum is involved in developmental dyslexia - a common reading disability.
While it is well known that the left cortex of the brain is involved in mapping of spoken words onto their written equivalents, there have been arguments that the cerebellum is also involved in phonological processing. The cerebellum is a brain structure outside and below the back of the cortex considered to be involved in motor function. While the "cerebellar deficit hypothesis" has always been seen as controversial, new Georgetown research has found that the cerebellum is not engaged when typical readers are reading and that is also the case in children with dyslexia.
Sikoya Ashburn, a Georgetown PhD candidate in neuroscience said:
"Prior imaging research on reading in dyslexia had not found much support for this theory called the cerebellar deficit hypothesis of dyslexia, but these studies tended to focus on the cortex. Therefore, we tackled the question by specifically examining the cerebellum in more detail. We found no signs of cerebellar involvement during reading in skilled readers nor differences in children with reading disability."
fMRI was used to look for brain activation when subjects were reading. The researchers also tested for functional connections between cerebellum and cortex when reading. According to Sikoya Ashburn:
"Functional connectivity occurs when two brain regions behave similarly over time; they operate in sync. However, brain regions in the cortex known to partake in the reading process were not communicating with the cerebellum in children with or without dyslexia while the brain was processing words."
The researchers found that when communication between brain regions 'at rest' (ie when reading was not considered), communication between cerebellum and cortex was stronger in the children with dyslexia. Sikoya Ashburn explained that:
"These differences are consistent with the widely distributed neurobiological alterations that are associated with dyslexia, but not all of them are likely to be causal to the reading difficulties."
According to Professor Guinevere Eden, senior author:
"The evidence for the cerebellar deficit theory was never particularly strong, yet people have jumped on the idea and even developed treatment approaches targeting the cerebellum. Standing on a wobble board - one exercise promoted for improving dyslexia that isn't supported by the evidence - is not going to improve a child's reading skills. Such treatments are a waste of money and take away from other treatment approaches that entail structured intervention for reading difficulties, involving the learning of phonologic and orthographic processing."
Clinician and prominent dyslexia researcher, Dr. Samuel Orton, hypothesized in 1925 that normally developing readers suppress visual images reported by the right hemisphere of the brain that could potentially interfere with input from the left. Researchers at Georgetown University Medical Center using functional magnetic resonance (fMRI) reported in 2003 found that children do in fact "turn off" the right side of the visual parts of the brain as they become accomplished readers. This finding confirms an aspect of Orton's theory - born out of observations of individuals with reading disability - is correct.
For the first time, they also were able to show that different phonological skills are related to activity in different parts of the brain when children read. Phonological skills allow readers to sound out words by correctly associating sounds with written symbols. They are critical for children learning to read and are often found to be impaired in children with developmental dyslexia.
The observation adds support to the theory that there may be several neurobiological profiles corresponding to different subtypes of dyslexia - each associated with varying deficits in one or more of these different phonological skills.
"Reading is the single most important skill our children learn - it impacts virtually every aspect of a child's life," said Dr. Guinevere Eden, associate professor of pediatrics and director of Georgetown University's Center for the Study of Learning. "Despite the extraordinary effort that goes into teaching children to read, very little is known about the neurobiology of reading acquisition in children. This study is important because we need to understand the brain basis of learning in kids who read well in order to understand why some children, like those with dyslexia, don't."
Dr. Eden and colleagues Peter Turkeltaub, Lynn Gareau, and Dr. Tom Zeffiro of Georgetown, and Dr. Lynn Flowers of Wake Forest University, studied 41 people aged between six and 22 using fMRI to examine which parts of the brain were employed when they saw words. Using a method where subjects were asked to locate tall letters within a word - forcing them to read the words implicitly - the researchers correlated brain activity with scores on reading tests to see if more advanced readers had more activity in certain brain areas than less experienced readers, and vice versa. Then they studied brain activity during reading related to scores on tests of phonological skills.
The fMRI scans showed that young children just learning to read used the left temporal regions of their brains; increases in age and the associated gains in reading, were characterized by a suppression of the visual areas of the right hemisphere - supporting Orton's theory.
The study also showed that the same locus in the left temporal lobe engaged during reading in younger children is also more active if children are good at phonemic awareness, such as understanding that "pop" without "p" is "op." These measures are frequently employed for behavioral evaluation of children at risk for developing reading problems and these new findings provide an anatomical correlate of this ability.
"Work like this can provide important background information to develop new research-based teaching programs that can ultimately help all children to become proficient readers and identify those who are need of specific interventions," said Peter Turkeltaub, primary author of this study. "This is an exciting area of research in which scientists converge with educators and parents to achieve the common goal of helping children achieve the reading skills they need to succeed in life."
Ashburn SM, Flowers DL, Napoliello EM, Eden GF. "Cerebellar function in children with and without dyslexia during single word processing." Human Brain Mapping, 2019
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