In people with dyslexia, less grey matter in the brain has been linked to reading disabilities, but new evidence suggests this is a consequence of poorer reading experiences and not the root cause of the disorder.
A study compared a group of dyslexic children with two different control groups - an age-matched group included in most previous studies, and a group of younger children who were matched at the same reading level as the children with dyslexia.
"This kind of approach allows us to control for both age as well as reading experience," explained neuroscientist Guinevere Eden, professor of paediatrics at Georgetown University Medical Centre (GUMC).
"If the differences in brain anatomy in dyslexia were seen in comparison with both control groups, it would have suggested that reduced grey matter reflects an underlying cause of the reading deficit. But that's not what we observed," Eden said.
The dyslexic groups showed less grey matter compared with a control group matched by age, consistent with previous findings, said the study published in the Journal of Neuroscience.
However, the result was not replicated when a control group matched by reading level was used as the comparison group with the dyslexics.
"This suggests that the anatomical differences reported in left hemisphere language processing regions appear to be a consequence of reading experience as opposed to a cause of dyslexia," said Anthony Krafnick, lead author of the publication.
"These results have an impact on how we interpret the previous anatomical literature on dyslexia and it suggests the use of anatomical MRI would not be a suitable way to identify children with dyslexia," Krafnick added.
The work also helps to determine the fine line between experience-induced changes in the brain and differences that are the cause of cognitive impairment.