The individual brain: mapping variability in hemispheric functional organization across cognitive domains in a representative sample

Several core cognitive networks in the human brain show marked left–right differences in their functional organization. While these asymmetries are well described at the level of individual functions, overarching patterns of variability in hemispheric functional organization across multiple domains have not been mapped in a representative sample. To address this gap, we conducted a large-scale neuroimaging study with 200 participants (100 left-handers and 100 right-handers) to map hemispheric phenotypes across four distinct cognitive domains: language, tool use, spatial attention, and face perception. We challenge the traditional one-size-fits-all view of hemispheric organization by showing that deviations from the typical pattern of functional segregation are more prevalent than generally assumed, in both left- and right-handers. As predicted, variation in asymmetry was more pronounced in the left-handed sample. Critically, we found no evidence that the prototypical, “textbook”, pattern of brain organization confers any advantage in general cognitive ability or IQ. These results challenge the assumption of a single optimal brain organization and demonstrate that hemispheric functional segregation in humans is much more variable than anticipated.