Intracortical myelin links with performance variability across the human lifespan: results from T1- and T2-weighted MRI myelin mapping and diffusion tensor imaging.

Cerebral myelin maturation and aging-related degradation constitute fundamental features of human brain integrity and functioning. Although mostly studied in the white matter, the cerebral cortex contains significant amounts of myelinated axons. However, how intracortical myelin content evolves during development, decays in aging, and links with cognition remain poorly understood. Several studies have shown the potential of mapping myelin in the cortex by use of T1-weighted (T1w) and T2-weighted (T2w) magnetic resonance imaging signal intensity, which show inverse sensitivity to myelin. Here, we characterized cortical myelin in 339 participants 8-83 years of age using a recently introduced T1w/T2w ratio myelin mapping technique and mean diffusivity (MD) from diffusion tensor imaging. To test for cognitive correlates, we used intraindividual variability (IIV) in performance during a speeded task, a measure recently associated with white matter integrity. The results showed that intracortical myelin maturation was ongoing until the late 30s, followed by 20 relative stable years before declining from the late 50s. For MD, U-shaped paths showing similar patterns were observed, but with fewer maturational effects in some regions. IIV was correlated with both T1w/T2w ratio and MD, mainly indicating that the higher degree of intracortical myelin is associated with greater performance stability. The relations were more prominent with advancing age, suggesting that aging-related cortical demyelination contributes to increased IIV. The T1w/T2w ratio myelin-mapping technique thus seems sensitive to intracortical myelin content in normal development and aging, relates to cognitive functioning, and might constitute an important future tool in mapping normal and clinical brain changes.