Neuroanatomical precursors of dyslexia identified from pre-reading through to age 11

Abstract

Developmental dyslexia is a common reading disorder that negatively impacts an individual's ability to achieve literacy. Although the brain network involved in reading and its dysfunction in dyslexia has been well studied, it is unknown whether dyslexia is caused by structural abnormalities in the reading network itself or in the lower-level networks that provide input to the reading network. In this study, we acquired structural magnetic resonance imaging scans longitudinally from 27 Norwegian children from before formal literacy training began until after dyslexia was diagnosed. Thus, we were able to determine that the primary neuroanatomical abnormalities that precede dyslexia are not in the reading network itself, but rather in lower-level areas responsible for auditory and visual processing and core executive functions. Abnormalities in the reading network itself were only observed at age 11, after children had learned how to read. The findings suggest that abnormalities in the reading network are the consequence of having different reading experiences, rather than dyslexia per se, whereas the neuroanatomical precursors are predominantly in primary sensory cortices.

Keywords: cortical thickness; development; neuroimaging; paediatric; reading.

Figure 1

Early signs of dyslexia. Pre-reading differences in cortical thickness between children who later went on to develop dyslexia (Dys) and those who did not (Ctrl). Images: regions in which Dys

Figure 2

Neuroanatomical signature of dyslexia. Regions of thinner cortex in the left hemisphere observed in children diagnosed with dyslexia (Dys) compared to those who were not (Ctrl). These data are cross-sectional from MRI 3, when the children were in the sixth grade. The left panel shows the whole group differences, whereas the right panel shows the differences when only the males were considered. IFG = inferior frontal gyrus; MTG = middle temporal gyrus; STG = superior temporal gyrus; VWFA = visual word form area.

Figure 3

Developmental trajectories of regions that showed pre-reading differences in cortical thickness. Longitudinal cortical thickness values are shown for the five regions that were different before reading: (A) left Heschl’s gyrus, (B) left lingual, (C) left cingulate, (D) left medial frontal, and (E) right orbitofrontal. The lingual gyrus (B) starts out thicker in the controls and thins over time, while all other regions thicken or stay the same over time. Cross-sectional post hoc t-tests between the two groups are indicated by *P < 0.05, ***P < 0.001.