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Elizabeth Huber, Emily C Kubota, Jason D Yeatman; Linking occipital callosal white matter to cortical responses and reading skill. Journal of Vision 2019;19(10):172a. doi: https://doi.org/10.1167/19.10.172a.
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A counterintuitive finding in the diffusion MRI literature is the presence of higher radial diffusivity and lower fractional anisotropy in the posterior callosal connections of individuals with greater reading proficiency. These data suggest increased density of inter-hemispheric connections in struggling versus skilled readers, and they have been linked to the hypothesis that reading related functions are not as strongly left-lateralized in struggling readers. However, this relationship has not been tested directly in a single participant group. Further, since callosal connections continue to mature throughout childhood and adolescence, it is unclear how reading experience and maturational factors shape the observed anatomy-behavior relationships. Here, we used diffusion MRI to examine the structure of posterior callosal connections in 55 children (age 6–12) with a range of reading levels. Two modeling approaches were used to estimate axonal water fraction (AWF) from the diffusion MRI data. Participants also completed functional MRI scans designed to localize word-, face- and object-selective responses. A subset (n=36) participated in an intensive reading intervention, with longitudinal behavioral and MRI measurements taken at regular 2-week intervals. In a cross-sectional analysis (n=55), AWF was found to increase with age, consistent with previous work showing maturational changes in myelination. Posterior callosal AWF predicted reading skill, after controlling for age effects: Struggling readers had higher AWF than skilled readers, consistent with increased axonal density and/or greater myelination of inter-hemispheric connections. Word-selective regions localized with fMRI were strongly left lateralized in skilled readers and more bilateral or right-lateralized in struggling readers. Finally, over intervention, AWF remained stable, even as reading skills improved. We therefore suggest that differences in the structure of axons connecting left and right visual cortex emerge early in life and shape subsequent reading development. Other portions of the network remain more malleable, perhaps supporting compensatory mechanisms that can emerge with educational experience.
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