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    • Rate of change maps for measures of apparent cortical thickn

    2018-11-03

    Rate of change maps for measures of apparent cortical thickness are shown in Fig. 7. Thinning of the purchase ptio appears to be present across the entire cortex and across the entire age range from 3 to 21, contrasting with some previous reports of early childhood cortical thickening. If anything, thinning may be accelerated in the pre-adolescent children. Again, the maps suggest some degree of regional heterochronicity in apparent thinning of the cortex, as highlighted by comparing the smooth age functions from the GAMs for the genetically-derived parcels (note that mean thickness measures in ROIs derived from the surface area genetic parcels are used for consistency with Fig. 6). Shown in Fig. 8 are the trajectories for covariate-adjusted mean cortical thickness measures for the parcels in the dorsolateral prefrontal cortex (blue), dorsomedial frontal cortex (red), and occipital cortex (green); labeled as parcels 2, 3, and 12, respectively, in Fig. 1. The age functions vary slightly, suggesting that unlike in the frontal parcels, where the rate of thinning appears to be fairly constant, thinning may decelerate slightly after age 10 in the occipital parcel. These models describe the modal developmental course of surface area expansion (and contraction) and of apparent cortical thinning, but one might ask: what do we know about individual differences in these processes of cortical development and how might they relate to behavioral differences? In recent years a number of observations in children have associated individual differences in the cortical architecture with individual differences in behavioral phenotypes, and a few suggest that trajectories of biological maturation in specific neural systems may themselves map onto emerging phenotypes. For example, beginning with Sowell et al. (2001) using early global regional measures, and more recently by investigators using surface-based methods (Tamnes et al., 2010; Porter et al., 2011; Squeglia et al., 2013), regressive changes (volume loss or thinning) of the cortex has been associated with better performance on memory, cognitive, and executive functions in developing children and adolescents. These results would seem to suggest that more mature cortical phenotypes are mirrored in more mature performance profiles during development, since thinner cortex is rarely associated with performance improvements in other contexts. In other words, they suggest that phase advance of apparent cortical thinning might be associated with precocious functional development. Cortical surface area phenotypes have only rarely been correlated with behavioral measures in developing children, but in two reports from the PING study, such associations have been found. Fjell et al. (2012) observed, in the 5–12 year old children, an association between greater (relative) cortical expansion in the anterior cingulate region and better performance on a flanker task, the latter measured as reduced effects of incongruent cues on reaction time. This association was independent of age. Since this region continues to expand in surface area over the 5–12 year age range (Figs. 5 and 6), one explanation is that earlier anterior cingulate surface area expansion is associated with greater functional maturity of circuitry involved in response conflict resolution on this task. An alternative explanation is that individuals with relatively larger anterior cingulate, e.g., through regionalization effects, regardless of their developmental status, are more adept at such tasks. That the associations seemed to be absent in older individuals in the study is more consistent with the former than the latter interpretation, but these null effects in older participants could also be due to differences in measurement sensitivity or other factors. A second PING study (Newman et al., 2015) revealed a surface area phenotype related to high self-reported levels of generalized anxiety in 287 PING participants aged 7–20 years. Independent of age, gender, and genetic ancestry factors, anxiety was negatively associated with relative cortical surface area of the ventromedial prefrontal cortex, as well as with global cortical thickness, and these associations significantly diminished with age. The two cortical phenotypes contributed additively to the prediction of anxiety. These findings suggest that higher levels of anxiety, even in typically-developing children, may be characterized by both delayed expansion of the ventromedial prefrontal cortex and an altered trajectory of global cortical thinning.