The discovery dataset included all possible Human Connectome Project (HCP) participants for whom both diffusion and T1 images were available ( Van Essen et al., 2012). We further investigated the relationship between CC length scaling with fiber composition, functional lateralization, cortical myelin content, and cortical evolutionary and developmental expansion. In the present study, we assessed whether length scaling variations exist according to brain size within the corpus callosum (CC), the principal white matter (WM) bundle supporting the communication between the two brain hemispheres. While the fiber length adjustment to brain size has been assumed uniform so far, heterogeneity in fiber length scaling may reveal adaptive principles underlying brain development and evolution. These adaptations, however, cannot apply systematically to all fiber tracts due to the limit in brain space and energy consumption ( Ringo et al., 1994), leading to alternative biological strategies that might impact fiber length differently. It is well known that brain fibers can increase their myelination and diameter to increase conduction velocity and compensate for this delay ( Waxman et al., 1995 Wang et al., 2008).
In larger brains, fiber length is inevitably increased, accompanied by a longer delay in conduction and slower information transfer. The scaling patterns of specific brain structures provide essential clues for understanding organizational and adaptive principles underlying brain development and evolution ( Zhang and Sejnowski, 2000 Herculano-Houzel et al., 2010 Van Essen, 2018).Īn increase in brain size also has a significant impact on brain circuits ( Kaas, 2000). Large brains’ structures are not simple linearly scaled versions of small brains, e.g., cortical expansion of large brains is disproportionately larger for higher cognitive function regions ( Hill et al., 2010 Reardon et al., 2018). Total brain size varies dramatically across evolution, development, and individuals. These findings highlight an interaction between interhemispheric communication and organizational and adaptive principles underlying brain development and evolution. The variation in such length scaling was biologically meaningful: larger scaling corresponded to larger neurite density index but smaller fractional anisotropy values cortical regions connected by the callosal fibers with larger scaling were more lateralized functionally as well as phylogenetically and ontogenetically more recent than their counterparts. The underscaled callosal fibers mainly connected the precentral gyrus and parietal cortices, whereas the overscaled callosal fibers mainly connected the prefrontal cortices. The results showed substantial variation in length scaling among callosal fibers, replicated in two large healthy cohorts (∼2000 individuals).
We investigated how fiber lengths within the corpus callosum, the most prominent white matter tract, vary according to brain size. Fiber length scaling – the degree to which fiber length varies according to brain size – was overlooked. Brain size significantly impacts the organization of white matter fibers.
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