White matter and literacy: a dynamic system in flux

Past studies have linked differences in white matter tissue properties to reading skills. However, these studies have reported a range of, sometimes conflicting, results. Some studies suggest that white matter properties act as individual-level traits predictive of reading skill, whereas others suggest that reading skill and white matter develop as a function of an individual’s educational context. In the present study, we tested two hypotheses: a) that diffusion properties of the white matter reflect stable brain characteristics that relate to reading skills over development or b) that white matter plays a part in a dynamic system, linked with learning over time. To answer these questions, we examined the relationship between white matter and reading in a series of large-scale, single-observation datasets and a four-year longitudinal dataset. We find that gains in reading skill correspond to longitudinal changes in the white matter. However, in the single-observation datasets, we find no evidence for the hypothesis that individual differences in white matter predict reading skill. These findings highlight the link between dynamic processes in the white matter and learning. Significance statement Both cross-sectional and longitudinal studies have identified a relationship between white matter properties and reading skill. However, these effects are inconsistent across studies and lead to sometimes conflicting conclusions about the relationship between white matter and reading skills. In a longitudinal dataset, we found that gains in reading skill correspond to changes in white matter properties of the left arcuate over time. However, we did not identify a stable link between white matter and reading skill across three large-scale, cross-sectional datasets. These findings suggest that the relationship between reading skill and white matter should be considered longitudinally. These findings further suggest that within-subject approaches are critical for understanding the dynamic interplay between white matter development and reading skills.