This growth increases the efficiency of saltatory conduction and is thought to lead to faster and more reliable information transmission. Myelination suggests that the myelin sheath wrapping axons grows during development ( 2, 10– 13). Pruning is hypothesized to produce thinner cortex in adulthood and improve neural processing by optimizing brain circuits for particular operations. Pruning, evaluated using cross-sectional histological studies on postmortem brains, suggests that the removal of inefficient synapses, dendrites, and neurons leads to cortical tissue loss ( 7– 9). Three developmental theories have been proposed to explain apparent cortical thinning across development: Pruning, myelination, and cortical morphology. Our data have broad ramifications for understanding both typical and atypical brain development using advanced in vivo quantitative measurements and clinical conditions implicating myelin. These findings are important because they suggest that VTC does not thin during childhood but instead gets more myelinated. These findings suggest that lateral VTC likely becomes more myelinated from childhood to adulthood, affecting the contrast of MR images and, in turn, the apparent gray–white boundary. In contrast, in place-selective cortex in medial VTC, we found no development of T 1 or MD after age 5, and thickness was related to cortical morphology. T 1 and MD decreases 1) were consistent with tissue growth related to myelination, which we verified with adult histological myelin stains, and 2) were correlated with apparent cortical thinning.
T 1 reduction also occurred longitudinally in children’s brain regions. In face- and character-selective regions in lateral VTC, T 1 and MD decreased from age 5 to adulthood in mid and deep cortex, as well as in their adjacent white matter. T 1 relaxation time from qMRI and mean diffusivity (MD) from dMRI provide independent and complementary measurements of microstructural properties of gray and white matter tissue. Using functional magnetic resonance imaging (fMRI), quantitative MRI (qMRI), and diffusion MRI (dMRI) in children and adults, we tested what quantitative changes occur to gray and white matter in ventral temporal cortex (VTC) from childhood to adulthood, and how these changes relate to cortical thinning. However, the underlying microstructural mechanisms are unknown. Human cortex appears to thin during childhood development.
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