Gray Matter Thinning in Ventral Temporal Cortex from Childhood to Adulthood is Associated with Increased Myelination

Abstract

Cortical thickness (CT) decreases from childhood to adulthood. However, the underlying mechanisms remain unknown. We tested two hypotheses using fMRI, quantitative MRI (qMRI), and diffusion MRI (dMRI) in 26 children (ages 5-12) and 27 adults. Pruning hypothesis: CT decreases from childhood to adulthood due to tissue reduction. Pruning predicts higher T₁ relaxation time, measured with qMRI, and higher mean diffusivity (MD), measured with dMRI, in adults than children. Growth hypothesis: white/gray boundary shifts deeper into cortex due to increased myelination, making the cortex whiter in adulthood. Myelin growth predicts lower T₁ and lower MD in adults than children. To test these predictions, we measured CT, T₁, and MD in face-, character-, and place-selective regions in ventral temporal cortex of each participant and in the white matter neighboring each region. CT decreased from age 5 to adulthood in all category-selective regions. In white matter neighboring face- and character-selective regions T₁ and MD also decreased with age. Similar decreases in T₁ and MD were found in gray matter of face- and character-selective regions, with greatest changes in T₁ in mid-cortical depths and in MD near the white/gray boundary. Importantly developmental decreases in T₁ and MD were correlated with cortical thinning. In contrast, cortical thinning in place-selective cortex, was associated with developmental shifts in its location deeper into the sulcus, rather than changes in T₁ or MD. Finally, we validated in-vivo measurements using Silver staining for myelin in adult ex-vivo brain slices. Consistent with in-vivo data showing lower T₁ in face- than place-selective cortex, we found higher myelin content in face- than place-selective cortex in ex-vivo slices. Together, results suggest differential mechanisms of cortical thinning: development of face- and character-selective regions is associated with increased myelination, but development of place-selective cortex is associated with a shift in the region's location deeper into the sulcus.

Meeting abstract presented at VSS 2018