June 4, 2018Neurology
According to an article published in Science, First Release Online (31 May 2018), it was found that some human brains are nearly twice the size of others, and that these differences in size are related to the brain's shape and the way it is organized. Interestingly, it was observed that the bigger the brain, the more its additional area is accounted for by growth in thinking areas of the cortex, or outer mantle; however, at the expense of relatively slower growth in lower order emotional, sensory, and motor areas. According to the authors, this mirrors the pattern of brain changes seen in evolution and individual development -- with higher-order areas showing greatest expansion. The authors also found evidence linking the high-expanding regions to higher connectivity between neurons and higher energy consumption.
For the study, the authors analyzed magnetic resonance imaging brain scans of more than 3,000 youth from the Philadelphia Neurodevelopmental Cohort, a NIMH IRP sample, and the Human Connectome Project. Results showed that cortex areas showing relatively more expansion in larger brains sit at the top of a network hierarchy and are specialized functionally, microstructurally and molecularly at integrating information from lower order systems. Since this theme holds up across evolution, development and inter-individual variation, the authors suggested that it appears to be a deeply ingrained biological signature. The authors added that since not all cortex regions are created equal, the high-expanding regions seem to exact a higher biological cost, and that these regions seem to be greedier in consuming energy and that they use relatively more oxygenated blood than low-expanding regions. Gene expression related to energy metabolism is also higher in these regions.
Since people with certain mental disorders show alterations in brain size related to genetic influences, the new cortex maps may improve understanding of altered brain organization in disorders. The higher expanding regions are also implicated across diverse neurodevelopmental disorders, so the new insights may hold clues to understanding how genetic and environmental changes can impact higher mental functions. The authors concluded that the study shows there are consistent organizational changes between large brains and small brains, and that observing that the brain needs to consistently configure itself differently as a function of its size is important for understanding how the brain functions in health and disease states.