Multiphasic myelination and dendritic growth modulate qMRI signals in human visual cortex

How does myelin develop in human visual cortex? By combining immunohistochemistry with in vivo and postmortem magnetic resonance imaging of longitudinal relaxation rate (R ₁ ), which increases with myelin content, we find that myelin and R ₁ increase across development but follow distinct trajectories. Immunohistochemistry reveals two phases of myelination: an infant phase of limited oligodendrogenesis, with myelin restricted to deep cortical layers, followed by widespread myelination across all layers during childhood. Cortical R ₁ also increases across development and correlates with myelin by childhood. However, in infancy, R ₁ increases outpace myelin growth and instead tracks dendritic arborization, indicating that the microstructural drivers of R ₁ change across development. We hypothesize that deep layer myelination in infancy contributes to early visual function whereas later myelination of superficial layers enables prolonged cortical plasticity and learning of complex visual behaviors.