Neural basis of dyslexia: a comparison between dyslexic and nondyslexic children equated for reading ability

Abstract

Adults and children with developmental dyslexia exhibit reduced parietotemporal activation in functional neuroimaging studies of phonological processing. These studies used age-matched and/or intelligence quotient-matched control groups whose reading ability and scanner task performance were often superior to that of the dyslexic group. It is unknown, therefore, whether differences in activation reflect simply poorer performance in the scanner, the underlying level of reading ability, or more specific neural correlates of dyslexia. To resolve this uncertainty, we conducted a functional magnetic resonance imaging study, with a rhyme judgment task, in which we compared dyslexic children with two control groups: age-matched children and reading-matched children (younger normal readers equated for reading ability or scanner-performance to the dyslexic children). Dyslexic children exhibited reduced activation relative to both age-matched and reading-matched children in the left parietotemporal cortex and five other regions, including the right parietotemporal cortex. The dyslexic children also exhibited reduced activation bilaterally in the parietotemporal cortex when compared with children equated for task performance during scanning. Nine of the 10 dyslexic children exhibited reduced left parietotemporal activation compared with their individually selected age-matched or reading-matched control children. Additionally, normal reading fifth graders showed more activation in the same bilateral parietotemporal regions than normal-reading third graders. These findings indicate that the activation differences seen in the dyslexic children cannot be accounted for by either current reading level or scanner task performance, but instead represent a distinct developmental atypicality in the neural systems that support learning to read.

Figures

Figure 1.

Scatter plots of ID scores for the three groups. ID raw scores (ID-raw) are on the left and ID standardized scores [ID-ss(age)] are on right.

Figure 2.

Whole brain activation for the contrast rhyme > rest. The main effect of condition for the three groups is examined and the activation map is rendered on a 3D brain (p = 0.001; ET, 10).

Figure 3.

Comparison between age-matched and dyslexic groups. Results from the two-sample t test of the contrast images rhyme > rest with greater activation in the age-matched compared with dyslexic groups overlaid on a normalized single subject axial, coronal, and sagittal views of the brain and on a 3D rendered brain (p = 0.001; ET, 10). Top, Axial view of Talairach coordinate (TC), z: −6, 38, 44; second row, coronal view TC y: −62, −49, 19, 41; third row, sagittal view TC x: −50, −38, 20, 59, 40; bottom, 3D rendered brain. Brain regions: A, left middle frontal gyrus; B, right superior frontal gyrus; C, left inferior parietal lobule; D, left inferior parietal lobule; E, right inferior parietal lobule; F, right posterior middle temporal gyrus. Activation difference coordinates are shown in Table 3.

Figure 4.

Brain activation patterns of the contrast rhyme > rest comparing three groups. A, Brain activation in the six functionally defined ROIs in the three groups. ROIs were selected based on the comparison age-matched > dyslexic groups in Figure 3. Bar graphs represent the mean contrast values of the six regions for dyslexic (black bars), age-matched (white bars), and reading-matched groups (gray bars). Error bars represent SEM. Brain regions: L Frontal, left middle frontal gyrus (corresponding to region A in Fig. 3); R Frontal, right superior frontal gyrus (corresponding to region B in Fig. 3); L Par-temp 1, left inferior parietal lobule (corresponding to region C in Fig. 3); L Par-temp 2, left inferior parietal lobule (corresponding to region D in Fig. 3); R Par-temp, right left inferior parietal lobule (corresponding to region E in Fig. 3); R Occ-temp, right posterior middle temporal gyrus (corresponding to region F in Fig. 3). All six one-way ANOVAs were significant (p values < 0.05) (see also Table 4). Age-matched versus dyslexic, All p values < 0.05; reading-matched versus dyslexic, all p values < 0.05; age-matched versus reading-matched groups, L Par-temp 2, R Par-temp, p values < 0.05; R Frontal, R Occ-temp, p values < 0.10. B, Brain activation comparing each dyslexic subject and their corresponding control. Each dyslexic child's contrast value of the L Par-temp 2 was subtracted from the corresponding control child (age-matched, white; reading-matched, gray) and is plotted so that zero equals the activation value of each of the 10 dyslexic subjects. Positive values indicate that the individually matched control child had greater activation than their corresponding dyslexic child. Nine of 10 children for each control group showed greater activation than their corresponding dyslexic child.

Figure 5.

Scatter plot showing the correlation between brain activation and reading scores and the correlation between word ID-ss(age) scores and the L Par-temp 2 contrast values in the dyslexic (filled circles), age-matched (open squares), and reading-matched groups (open triangles, n = 30). r = 0.50; p = 0.0023.