Parcellations and Hemispheric Asymmetries of Human Cerebral Cortex Analyzed on Surface-Based Atlases

Abstract

We report on surface-based analyses that enhance our understanding of human cortical organization, including its convolutions and its parcellation into many distinct areas. The surface area of human neocortex averages 973 cm(2) per hemisphere, based on cortical midthickness surfaces of 2 cohorts of subjects. We implemented a method to register individual subjects to a hybrid version of the FreeSurfer "fsaverage" atlas whose left and right hemispheres are in precise geographic correspondence. Cortical folding patterns in the resultant population-average "fs_LR" midthickness surfaces are remarkably similar in the left and right hemispheres, even in regions showing significant asymmetry in 3D position. Both hemispheres are equal in average surface area, but hotspots of surface area asymmetry are present in the Sylvian Fissure and elsewhere, together with a broad pattern of asymmetries that are significant though small in magnitude. Multiple cortical parcellation schemes registered to the human atlas provide valuable reference data sets for comparisons with other studies. Identified cortical areas vary in size by more than 2 orders of magnitude. The total number of human neocortical areas is estimated to be ∼150 to 200 areas per hemisphere, which is modestly larger than a recent estimate for the macaque.

Motion‐related artifacts in structural brain images revealed with independent estimates of in‐scanner head motion

“…Surface‐based thickness maps were generated by first bringing the fsaverage‐registered left and right hemisphere anatomical surfaces into register with each other using deformation maps from a landmark‐based registration of the left and right fsaverage surfaces to a hybrid left–right fsaverage surface [fs_LR; Van Essen et al, 2012a] and resampled to a resolution of 163,842 vertices per hemisphere (164k fs_LR) using Caret tools [Van Essen et al, 2001]. Each individual's “native” FS‐generated left and right hemisphere surfaces were deformed to the left and right 164k fs_LR surface meshes using single deformation maps, allowing for minimal resampling of anatomical data.…”

Parcellations and Connectivity Patterns in Human and Macaque Cerebral Cortex

“…The average number of neurons underneath each mm 2 of the cortical surface is lower in humans (8 Â10 3 ) than in the macaque (1.4 Â 10 4 ). Neuronal density is even lower in humans because the neocortex is on average slightly thicker in humans (2.44 mm, from Van Essen et al 2012c; 2.68 AE 0.40 mm for 196 HCP subjects- Glasser et al 2013b) than in macaques (1.86 AE 0.40 mm from 19 macaques- Glasser et al 2013b). …”

“…The cerebral neocortex is a sheet-like structure that in the macaque contains~1.4 billion neurons/hemisphere deployed over a surface area of~105 cm 2 per hemisphere-equivalent to a pair of~12 cm diameter cookies (Collins et al 2010;Van Essen et al 2012b). Human cortex has about fourfold more neurons (~8 billion/hemisphere) and ninefold greater surface area (~973 AE 88 cm 2 /hemisphere), equivalent to a pair of 35 cm pizzas (Azevedo et al 2009;Van Essen et al 2012c). The average number of neurons underneath each mm 2 of the cortical surface is lower in humans (8 Â10 3 ) than in the macaque (1.4 Â 10 4 ).…”

“…In contrast, the ninefold larger human cortical sheet is far more convoluted, with many primary, secondary and tertiary folds. It is also far more variable in the pattern of convolutions and in the relation of areal boundaries to cortical folds (Amunts et al 2007;Fischl et al 2008;Van Essen et al 2012c). Figure 1 illustrates the variability of human cortical folding in six exemplar right hemispheres, shown on the left as 3D 'midthickness' surfaces and on the right as FreeSurfer 'sulc' (depth) maps on inflated surfaces.…”

Decreased segregation of brain systems across the healthy adult lifespan

“…As this may confer particular benefits toward minimizing variation in anatomical registration in the present adult-lifespan sample, FreeSurfer 5.0 was used to process volumetric images into fsaverage surface images. The left and right hemisphere fsaverage surfaces were brought into register with each other by mapping onto a hybrid left-right fsaverage atlas ("fs_LR") (66). See SI Appendix, Supplemental Experimental Procedures for details.…”