All participants received a T1-weighted high-resolution structural MRI scan on a 3T Allegra scanner (Siemens, Erlangen, Germany) at the Cape Universities Brain Imaging Centre (CUBIC). Children were scanned in sagittal orientation using a 3D echo planar imaging (EPI) navigated (Tisdall et al. 2012 (link)) multiecho magnetisation prepared rapid gradient echo (MEMPRAGE; van der Kouwe et al. 2008 (link)) sequence (FOV 224 × 224 mm2, TR 2530 ms, TI 1160 ms, TE's = 1.53/3.19/4.86/6.53 ms, bandwidth 650Hz/px, 144 slices, 1.3 × 1.0 × 1.0 mm3) that prospectively corrects for motion during the scan. To limit motion due to restlessness, children watched a movie via a mirror and rear projection screen during scanning. Scans were performed without sedation according to protocols approved by the Faculty of Health Sciences Human Research Ethics Committees of both the Universities of Cape Town and Stellenbosch. Parents/guardians provided written informed consent and the children gave oral assent.
FreeSurfer version 6.0 (https://surfer.nmr.mgh.harvard.edu) was used for automated cortical reconstruction and to measure cortical thickness and local gyrification index (LGI) across the cortical surface, as well as regional and total brain volumes. FreeSurfer outputs were manually checked for errors in cortical and sub-cortical segmentations. Minor pial edits (skull strip corrections) were required for some of the outputs, but no white matter correction. There were no failures in the LGI computation. Subjects were excluded from particular analyses if their mean values on these measures were extreme outliers (removed from the median of the sample by more than 3 times the interquartile range (IQR)).