Diffusion markers of dendritic density and arborization in gray matter predict differences in intelligence

Previous research has demonstrated a consistent link between cortical macrostructure and intelligence. Individuals with better abstract problem solving abilities are more likely to have larger gray matter volume in brain areas... [ view full abstract ]

Previous research has demonstrated a consistent link between cortical macrostructure and intelligence. Individuals with better abstract problem solving abilities are more likely to have larger gray matter volume in brain areas predominantly located in parieto-frontal regions. These findings were usually interpreted such that individuals with more cortical brain volume had more neurons and thus more computational capacity during reasoning. Interestingly, this working hypothesis which finds that "bigger is better" has been contradicted by neuroimaging studies demonstrating that intelligent individuals, despite their large and neuron-rich brains, tend to show lower rates of cortical activity during reasoning. However, the exact microstructural architecture underlying both observations remains unknown.

For the purpose of analyzing the microstructural properties of the human brain in vivo, we utilized an advanced multi-shell diffusion tensor imaging protocol and computed various diffusion markers representing neurite density and arbotization. We measured intelligence by using a culture-fair matrix-reasoning test and obtained data in a sample of in 222 healthy individuals. We found that high intelligence was related to low values of dendritic density and arborization as well as high values of neuron density in the cerebral cortex. Subsequent analyses demonstrated that this structure-function relationship was particularly apparent in brain areas predominantly located in parieto-frontal regions.

We were able to show that highly intelligent individuals are likely to possess more cortical neurons than less intelligent individuals. More importantly, our results suggest that the cortical neurons in brains of highly intelligent individuals are wired up in a sparse but very efficient fashion. On a functional level, such distinctly tuned circuits might foster a less redundant and a more elaborative processing of information resulting in less cortical activity during reasoning. Hence, the data presented in this study provide first evidence for a structural foundation of neural efficiency in intelligent individuals.