Cortical and white matter T1w/T2w development proceed in concert during early infancy.

The infant brain undergoes rapid myelin growth that is critical for healthy brain function. This development has been characterized for gray and white matter independently, but the link between gray and white matter myelination remains unexplored. To close this knowledge gap, we processed large-scale (N=279) Developing Human Connectome Project data with automated software to identify 26 white matter bundles and map their cortical terminations, before evaluating T1w/T2w development in individuals. We found that mean T1w/T2w as well as the slope of T1w/T2w development are correlated across tissues. This synchrony of brain T1w/T2w is impacted by birth age and postnatal experience, whereas inter-individual differences in this synchrony predict motor outcomes at 17 - 25 months of age. As T1w/T2w is associated with myelin content, our results reveal an intricate relationship between gray and white matter myelination, highlighting the importance of conjointly evaluating both tissues.