Functional MRI helps reveal brain abnormalities in dyslexia

MIndividuals with dyslexia not only have difficulty reading, but they also exhibit unusually high activity in some regions of the brain and diminished activity in others. The exact relation of these phenomena to the disorder has remained largely undetermined.

Fumiko Hoeft and researchers from Stanford University in California have used functional MRI to compare brain function in children with dyslexia with that of nondyslexic children of the same age and of younger individuals who are at an equivalent reading level. The researchers also examined the subjects’ brain morphology.

In dyslexic individuals, decreased activation and volume were found in a region of the left hemisphere, shown as a blue-green speck in the last row of the diagram. Courtesy of Fumiko Hoeft.

Employing a GE Medical Systems magnetic resonance imager, the researchers examined the children’s brain activity during a word-rhyme task. Relative to both age- and reading-matched subjects, individuals with dyslexia showed hypoactivation in the left parietal and fusiform regions. Compared with age-matched children, they also showed hyperactivation in the caudate, thalamus and left inferior and middle frontal gyri. Activation in these regions was found to be equally high in reading-matched children, however.

Voxel-based morphology analysis conducted on images showed that children with dyslexia had significantly lower gray matter volume in the left inferior parietal lobule than control group members but showed normal volume elsewhere. These results — which aligned with the researchers’ hypotheses — implicate the region as the structural cause of dyslexia. With the brain activity data, they point to the left inferior frontal region as the site of compensatory activity, not abnormality. A summary of the work appeared in the Mar. 6 issue of PNAS.

The first to combine functional and structural neuroimaging, the investigation successfully defined the abnormalities associated with dyslexia. Hoeft believes that brain-based interventions that target the left inferior frontal region could help dyslexic individuals improve their reading ability.

The team suggests that further studies also could explore morphological changes that might accompany educational interventions for the condition.