Instead, it is likely that some of these differences come about because typical readers grow brain areas as they acquire reading skills, whereas those with dyslexia do not grow them at the same pace.
“It makes sense that typically reading children are experiencing reading-induced brain matter increases, and these types of increases are likely to be reduced in children with dyslexia who read poorly and, most likely, engage in reading less frequently,” senior author Guinevere F. Eden, PhD, director of the Center for the Study of Learning at Georgetown University Medical Center in Washington, DC, told Medscape Medical News.
The study was published in the January 15 issue of the Journal of Neuroscience.
Whether reductions in GMV are the cause of reading problems or a consequence of the disordered reading experience of people with dyslexia has been an unresolved question.
The investigators attempted to shed more light on the subject by looking at anatomic differences in dyslexia by using a reading level–matched design in which dyslexic children were compared not only with age-matched control individuals but also with younger control individuals who read at the same level as the dyslexic patients.
The researchers used functional magnetic resonance imaging (fMRI) and structural MRI scans of the study participants, and they used voxel-based morphometry, an automated software program, to estimate the amount of brain tissue volume.
“Going into the study, we fully expected to show that the differences in brain anatomy in the dyslexics would be observed when compared to both control groups, those matched on age and those matched on reading level,” said Dr. Eden.
“This was because the reported differences in brain anatomy have been localized to brain areas involved in language, and this is consistent with theories that dyslexia is a language-based learning disability,” she added.
The study included 6 boys and 9 girls, 13 of whom were recruited from a private school that specializes in teaching students with dyslexia, and 2 from a public school, as well as 30 typically reading children (17 boys and 13 girls), who were recruited from the general population to serve as control participants.
The children with dyslexia had to have a documented history of dyslexia as reported by the school; a single real-word reading standard score of less than 92 on the Letter-Word Identification subtest of the Woodcock-Johnson III Tests of Achievement; and a full-scale IQ score of less than 80 on the Wechsler Abbreviated Scale of Intellligence. Each participant also had to be a monolingual English speaker with no significant medical, neurologic, or psychiatric illness.
The control children were similar, and in addition, they had no history of learning disabilities and had real-word reading standard scores of greater than 92.
The researchers looked at the volume of brain tissue in children with and without dyslexia. Consistent with previous reports, they found that the dyslexic children had less GMV in multiple left- and right-hemisphere regions, including the left superior temporal sulcus, when compared with age-matched control participants.
To test whether these differences caused the reading problems in dyslexia or were a consequence of these reading problems, the researchers also did a comparison between the dyslexic children and younger children without dyslexia who were matched on a reading level to the group with dyslexia.
“Surprisingly, we did not find that all of the areas in the brain that came out from the first comparison also differed in this second comparison,” Dr. Eden said.
A significant difference between the reading level–matched groups of children was found only with regard to the right precentral gyrus (P < .5 Bonferroni-corrected for multiple comparisons).
“This means the explanation of these findings being causal is ambiguous,” Dr. Eden noted. “It raises the possibility that some of the differences are in part likely due to the increases in brain anatomy that occur in typical readers when they learn to read.”
But although the results came as a surprise, the investigators recognized other research that explains their findings.
“As we discuss in our paper, it has been shown that when adult illiterates learn to read and are compared to adult illiterates who did not have the opportunity to learn to read, there are profound differences in gray matter between these 2 groups, showing that learning to read as an adult increases brain tissue. This is from Carreiras et al, Nature 2009. Learning-induced increases in gray matter have been shown for a wide range of skills, from studying for college exams to learning how to juggle balls, a finding from Draganski et al, Nature, 2004,” Dr. Eden said.
Currently, the findings do not have any clinical applicability, but they do raise the possibility of whether MRI data have the potential for diagnosing dyslexia.
“We use MRI as a research tool to better understand dyslexia. It does not affect clinical practice at this time, but it does raise an interesting discussion about whether MRI data can be used to diagnose dyslexia at any time in the future,” Dr. Eden said.
“At this time, MRI is not used diagnostically, and people with reading problems should seek a careful psychoeducational evaluation to determine the nature of their reading problems, and based on the outcome, seek the appropriate behavioral treatment, such as intensified reading instructions and tutoring,” she added.
Two Important Take-homes
Asked to comment on this research, Judy Willis, MD, from the University of California, Santa Barbara, and American Academy of Neurology spokesperson, told Medscape Medical News that the noteworthiness of this study comes from the “powerful” reminder that correlation is not the same as causation.
Dr. Willis noted that 2 important messages emerge from the study’s findings.
“With regard to learning disabilities and developmental disorders, brain volume change is not adequate support for causal interpretation. The other message regards interventions for dyslexia and strategies to build reading skills in all children,” she said.
“A decade ago, the changes in density of a ‘brain glitch’ area on scans was hailed as support for a particular phonics-heavy reading instruction. This study reveals that most of the GMV differences associated with dyslexia appear to be consequences of the decreased experiences with reading resulting from the dyslexia rather than the cause of the dyslexia,” Dr. Willis added.
Although the study should not be overinterpreted as proof that more reading experiences will eliminate dyslexia, it is supportive of promoting more reading experiences to improve reading skills, she said.
“The most powerful booster for the amount a child reads is the opportunity to read enjoyable and comprehensible material. As additional studies about dyslexia causes and interventions are done, let’s follow the research suggestions and the smiles of joyful young readers and be sure all children have access to lots of books they want to read and enough time to read them for pleasure and not just to prepare for tests,” Dr. Willis said.
The study was supported by the National Institutes of Child Health and Human Development. Dr. Krafnick, Dr. Eden, and Dr. Willis report no relevant financial relationships.