Characterization of late structural maturation with a neuroanatomical marker that considers both cortical thickness and intracortical myelination

The cortical ribbon changes throughout a persons lifespan, with the most significant changes occurring during crucial development and aging periods. Changes during adulthood are rarely investigated due to the scarcity of neuroimaging data during this period. After childhood, the brain loses gray matter, which is evidenced by an apparent reduction in cortical thickness (CT); one factor of this thinning process is intense ongoing intracortical myelination (MYEL). Here, we report age-related changes in CT, MYEL, and their ratio in 447 participants aged 18 to 57 years (BIL&GIN cohort). We propose the CT/MYEL ratio to be a multimodal cortical maturation index (MATUR) capable of reflecting 1) stages during which CT and MYEL patterns diverge and 2) the regional differences in cortical maturation that occur in adulthood. Age mainly decreased CT in all cortical regions, with larger reductions occurring in the bilateral insular lobes, temporal and frontal poles, and cingulate cortices. Age led to a linear increase in MYEL in the entire cortex and larger increases in the primary motor, auditory, and visual cortices. The effects of age on the MATUR index were characterized by both linear and quadratic components. The linear component mimicked the pattern found in CT, with 1) a robust amplification of the global and regional effects of age on CT and 2) evidence of new bilateral linear decreases in the frontal and cortical cortices. Most importantly, age exhibited additional large quadratic effects on the MATUR index in the bilateral frontal (more prominent in the right hemisphere), parietal, temporal, and cingulate regions that were not highlighted by the CT metric. Thus, the MATUR index was more sensitive to age-related cortical structural changes during adulthood than was either CT or MYEL alone. As evidenced by the large quadratic component of the effect of age, the newly proposed maturation index dramatically improved the characterization of the regional cortical territories, uncovering the latest brain maturation steps that occur before stabilization and deterioration occur in mid- and late adulthood.