When compared with adults without dyslexia, anatomical brain studies performed on adults with this reading disorder have shown a difference in the amount of gray matter. Specifically, people with dyslexia show less gray matter, which includes regions of the brain involved in speech, sensory perception, muscle control, memory, decision making, and emotions. Now, new evidence suggests that less gray matter in the brains of people with dyslexia is a consequence of poorer reading experiences and is not the root cause of the disorder, as has been suggested by scientists in the past.
“Our results show that the brain anatomical aberrations observed by investigators with MRI do not clearly precede these difficulties and are therefore not likely the root of their cause. Rather they in part follow poor reading experiences,” Dr. Guinevere Eden, neuroscientist, Georgetown University Medical Center, told Medical Daily in an email. “It is a chicken-and-egg-problem that has puzzled investigators for years and is important because it should caution us from thinking about these areas being the culprit; rather they may be innocent bystanders that are detracting us from the areas that are truly driving the reading problems.”
Dyslexia is a learning disability characterized by difficulties with accurate word recognition, decoding, and spelling. Many scientists, including Eden, believe such poor reading fluency and comprehension in people who otherwise display average to above-average intelligence may be due to a struggle in understanding how sounds make up words and how these are mapped onto their written counterpart. Results from past brain studies involving adults with dyslexia have shown less gray matter when compared to age-matched adults without the disorder. This outcome led some scientists to hypothesize this difference might explain, at least in part, why dyslexic children have difficulties correctly relating the sounds of words to their written equivalent. Yet Eden and her colleagues sensed a possible oversight in previous study methods. “It is noteworthy that the majority of dyslexia studies have been conducted in adults, which means the control groups in these studies have experienced many years of reading, presumably leading to experience-dependent neuroanatomical change,” wrote Eden and her co-authors in The Journal of Neuroscience. “It is this anatomical outcome that could be driving the results reported between adult dyslexics and controls.”
To investigate a possible relationship between reading experience and brain development, the team of researchers recruited 15 children with dyslexia (6 boys, 9 girls) and 30 typically-reading children (17 boys, 13 girls). Next, they compared brain scans of the dyslexic children with those of two separate control groups: a group of age-matched children, and a group of younger children who tested at the same reading level as those with the reading disorder. "This kind of approach allows us to control for both age as well as reading experience," Eden explained in a press release. "If the differences in brain anatomy in dyslexia were seen in comparison with both control groups, it would have suggested that reduced gray matter reflects an underlying cause of the reading deficit. But that's not what we observed."
In fact, most brain regions identified as different in the analysis of the age-matched group (and therefore potentially a root cause of dyslexia) did not emerge as dissimilar in analysis of the younger, reading-level-matched group. "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," Dr. Anthony Krafnick, lead author, said in a press release, adding, "…It suggests the use of anatomical MRI would not be a suitable way to identify children with dyslexia."
In addition to expanding the scientific literature on dyslexia, this new work may also help researchers better understand and identify the differences in brain structures caused by experience and those caused by cognitive impairment. Going forward, in fact, the team of researchers will further explore learning-induced changes to the brain. “We have just completed a longitudinal study of typical readers (tracking 1st graders into 3rd grade) to see the brain mechanisms unfold as they learn to read, and hope to do similar work in children with dyslexia in the near future,” Eden told Medical Daily.
Source: Krafnick AJ, Flowers DL, Luetje MM, Napoliello EM, Eden GF. An Investigation into the Origin of Anatomical Differences in Dyslexia. The Journal of Neuroscience. 2014.