In an update to their groundbreaking earlier demonstration that high-IQ children initially show a thinner cortex, and later show an initially thicker one than their average-IQ peers, Shaw et al. have now documented those trajectories of cortical thickening which are invariant to socio-economic status and IQ, but vary between regions of the brain. These videos show the peak in gray matter in cortex between the ages of 5 and 15 years, as assessed from magnetic resonance imaging (MRI) of 375 subjects of varying ages.
And another video below the fold…
Dark red areas indicate those regions that show linear trends, for which peak thickness cannot be meaningfully calculated.
By comparing linear, quadratic and cubic fits to this thickening data, Shaw et al demonstrate that the cellular organization of these different brain regions (with isocortex being the most recently evolved, allocortex being the oldest, and “transitional” cortex somewhere in between) predicts the type of developmental trajectory they show. Specifically, cubic fits (of thickening, thinning and stabilization) characterize recently-evolved regions, quadratic fits characterize transitional cortex, and linear fits characterize allocortical regions. One implication is that the relationship between strong cubic trends and IQ (as demonstrated in previous work) reflects the reliance of IQ tests on the integrity of more recently evolved regions.
This new paper extends previous work by covering a wider age-range, and by showing that the stabilization of gray matter in later life varies by region – in particular, while the anterior cingulate and insula show a pattern of thickening and then thinning, they do not show the stabilization of thinning which characterizes many other areas of more-recently evolved “isocortex”. More strangely, frontal opercular and orbitofrontal areas show only thinning throughout the first 3 decades of life. These regions have all been implicated in the cognitive control of behavior, and linked in particular to so-called “hot” executive functions involving emotional processing, yet the thickening trajectory of the latter two regions is more characteristic of areas involved in primitive functions like olfaction.
And the nitty gritty: A neural network was used to segment these MRIs into gray matter, white matter, and cerebrospinal fluid (the authors note this algorithm performed poorly in the middle frontal gyrus, and five other regions of the brain – but was not significantly different from the estimates of an experienced neuroanatomist in another 30 regions of the brain).