Tracking the roots of reading ability: white matter volume and integrity correlate with phonological awareness in prereading and early-reading kindergarten children

Abstract

Developmental dyslexia, an unexplained difficulty in learning to read, has been associated with alterations in white matter organization as measured by diffusion-weighted imaging. It is unknown, however, whether these differences in structural connectivity are related to the cause of dyslexia or if they are consequences of reading difficulty (e.g., less reading experience or compensatory brain organization). Here, in 40 kindergartners who had received little or no reading instruction, we examined the relation between behavioral predictors of dyslexia and white matter organization in left arcuate fasciculus, inferior longitudinal fasciculus, and the parietal portion of the superior longitudinal fasciculus using probabilistic tractography. Higher composite phonological awareness scores were significantly and positively correlated with the volume of the arcuate fasciculus, but not with other tracts. Two other behavioral predictors of dyslexia, rapid naming and letter knowledge, did not correlate with volumes or diffusion values in these tracts. The volume and fractional anisotropy of the left arcuate showed a particularly strong positive correlation with a phoneme blending test. Whole-brain regressions of behavioral scores with diffusion measures confirmed the unique relation between phonological awareness and the left arcuate. These findings indicate that the left arcuate fasciculus, which connects anterior and posterior language regions of the human brain and which has been previously associated with reading ability in older individuals, is already smaller and has less integrity in kindergartners who are at risk for dyslexia because of poor phonological awareness. These findings suggest a structural basis of behavioral risk for dyslexia that predates reading instruction.

Figure 1.

Illustration of the tracts of interest. Tracts of interest were estimated in each individual's native diffusion space, extracted from an example participant, and registered to MNI template space for visualization here (sagittal view on the left, axial on the right). The ILF (cyan) spans the occipital and temporal cortices. The SLFp (magenta) connects frontal and parietal regions. The other component of the SLF is the arcuate fasciculus (yellow), which departs from the main SLF tract to branch into temporal cortices and is classically posited to facilitate communication between Broca's and Wernicke's areas.

Figure 2.

Greater volume and FA of the left arcuate fasciculus are associated with superior phonological awareness. a, Volume of the left arcuate fasciculus plotted against individual raw scores of the Blending Words subtest. b, Phonological awareness, illustrated by Blending Words raw scores, was significantly correlated with average FA of the left arcuate fasciculus. In a and b, thick lines represent the best fits and thin outer lines represent the 95% confidence intervals. c, To illustrate the relation between the behavioral phonological awareness scores with left arcuate volume and FA, this tract was rendered from example participants (filled red circles in a and b) and colored according to FA. The tracts are ordered by Blending Words raw score increasing from left to right.

Figure 3.

Greater volume and FA of the left arcuate fasciculus are associated with superior phonological awareness in prereading children. In prereading children (who could read 0 or 1 word), the Blending Words subtest scores were significantly correlated with the volume (a) and the average FA (b) of the left arcuate.

Figure 4.

Whole-brain voxelwise analyses of the relation between FA and behavioral predictors of dyslexia. TBSS analyses revealed that FA values were associated only with phonological awareness scores and only in a cluster of voxels (red) at the bend of the SLF near Wernicke's area, where the arcuate elbows toward the temporal lobe (p < 0.05 corrected; cluster size = 22). All statistics were run on the standard skeleton obtained using TBSS and displayed on thickened skeletons for ease of viewing. (L, left; P, posterior).