Myelin contributes to microstructural growth in human sensory cortex during early infancy

The infant brain undergoes rapid physical changes after birth. However, how cortex develops remains unknown. Using in vivo biomarkers of tissue microstructure, we find that infants' sensory-motor cortices undergo profound microstructural growth during the first six months of life. Using visual cortex as a model system, we find hierarchical microstructural growth: higher-level visual areas have less mature tissue at birth than earlier visual areas but grow at faster rates. Comparison of postnatal to prenatal gene expression data suggests that myelination and synaptic processes are dominant contributors to postnatal development. These data suggest a rethinking of developmental hypotheses and highlight the significance of cortical myelination in the development of brain function. Our findings have important implications for diagnosis of neurodevelopmental disorders and delays affecting life-long outcomes.