The emergence of VWFA laterality: examining the role of white matter connectivity in early childhood

The visual word form area is a brain region that selectively responds to visual words as people become literate. Interestingly, while face selectivity is typically dominant on the right hemisphere, word selectivity is dominant on the left. What factors contribute to the development of left laterality for words? Here we tested three potential sources of this word laterality: 1) the development of face laterality, 2) cross-hemispheric structural connectivity, and 3) connectivity with ipsilateral language regions. We scanned children (3-9 years, prereaders and readers) and adults on an fMRI task to extract functional activation to words and faces and diffusion-weighted imaging (DWI) in the same participants to examine white matter connectivity. In children, even though word selectivity became increasingly left-lateralized and face selectivity more right-lateralized over development, they were not directly related: while face laterality increased with age in both readers and prereaders, word laterality increased with age only in readers. Word laterality additionally showed a significant correlation with cross-hemispheric connectivity between left VWFA and its right homotope, especially in readers. Furthermore, we found that in readers, the connectivity of VWFA with ipsilateral frontal language regions was positively related to its laterality. Interestingly, in adults, neither face laterality nor connectivity was correlated with individual differences in word laterality. These results demonstrate how anatomical and developmental factors contribute to changes in laterality of high-level visual cortex, and highlight the role of both cross-hemispheric and ipsilateral white matter connectivity in developing word laterality.