Modeling the 3D Geometry of the Cortical Surface with Genetic Ancestry

Highlights

• Geometry of the human cortical surface contains rich ancestral information
• The most informative features are regional patterns of cortical folding and gyrification
• This study provides insight on the influence of population structure on brain shape

Summary

Knowing how the human brain is shaped by migration and admixture is a critical step in studying human evolution [ 1, 2 ], as well as in preventing the bias of hidden population structure in brain research [ 3, 4 ]. Yet, the neuroanatomical differences engendered by population history are still poorly understood. Most of the inference relies on craniometric measurements, because morphology of the brain is presumed to be the neurocranium’s main shaping force before bones are fused and ossified [ 5 ]. Although studies have shown that the shape variations of cranial bones are consistent with population history [ 6–8 ], it is unknown how much human ancestry information is retained by the human cortical surface. In our group’s previous study, we found that area measures of cortical surface and total brain volumes of individuals of European descent in the United States correlate significantly with their ancestral geographic locations in Europe [ 9 ]. Here, we demonstrate that the three-dimensional geometry of cortical surface is highly predictive of individuals’ genetic ancestry in West Africa, Europe, East Asia, and America, even though their genetic background has been shaped by multiple waves of migratory and admixture events. The geometry of the cortical surface contains richer information about ancestry than the areal variability of the cortical surface, independent of total brain volumes. Besides explaining more ancestry variance than other brain imaging measurements, the 3D geometry of the cortical surface further characterizes distinct regional patterns in the folding and gyrification of the human brain associated with each ancestral lineage.

[Editor’s Note: Below are comments on this study by Dr. James Thompson. They are taken from here.]

The authors are a San Diego team, and have worked on 562 individuals scanned when older than 12 years, by which time brain development is fairly stable. Overall, can explain about 50% of surface variability, though up to 66% for the African group.

For example, as the proportion of the African component increases, the temporal surfaces move posteriorly and inward. The proportion of the European component is associated with protrusion of the occipital and frontal surfaces. Increases in the proportion of the East Asian component are accompanied by variations in temporal-parietal regions. The Native American component is associated with flattening of the frontal and occipital surfaces.

The authors say:

Our data indicate that the unique folding patterns of gyri and sulci are closely aligned with genetic ancestry. The geometry robustly predicts each individual’s genetic background even though the population has been shaped by waves of migration and admixtures. A previous study, using only facial features, achieved 64% explained variance in YRI ancestry among African Americans. Our 3D representation of cortical surface geometry performs similarly in predicting YRI ancestry and also performs well for the other three continental ancestries. As data in Table 1 show, the explanatory power is not due to the differences in total brain volumes, nor to the differences in areal expansion of the cortical surface. Instead, regional folding patterns characterize each ancestral lineage.

On the other hand, the global shapes of the reconstructed cortical surface geometry match W.W. Howells’ description of craniometry of 2,524 ancient human crania from 28 populations [20]. Crania of African ancestry tended to have a narrower cranial base, and those of Northern European ancestry had elongated occipital and frontal regions. Crania of East Asian ancestry had a high cranial vault, and crania of Native American ancestry were flatter. Regarding the morphing differences of YRI, EA, and NA, all had high magnitude and variations in the posterior-temporal regions (Figure 3).These findings are consistent with the notion that temporal bones contain more variations across ancestral groups [6].

I would have expected the authors to show the total brain volumes for each group, but I cannot find those in the paper, nor in the supplemental materials. A pity, because it would help resolve some debates about brain sizes.

It may be being held over for the next paper, but of course I would like to see to what extent the 3D model predicts the intelligence measures for the children.

The authors point out that brain studies will have to bear in mind that individuals of different racial groups can affect the overall results. Of larger importance is that these brain differences in folding patterns could have links to intelligence, personality and other behaviours.

So, you can spot the difference between races in adolescence by looking at the cortical surface of the brain . When discussing this, just remember to call it “population stratification”, or you will be troubled by fools getting in the way of your research.