What's the science?
Intelligence has been associated with higher gray matter volume (the outer layer of neurons in the brain composed of cell bodies), in particular in the frontal and parietal regions of the brain, suggesting that people with higher intelligence have a greater number of neurons and greater computational power. Other research suggests that neural efficiency may be more important for intelligence, and that higher intelligence is associated with lower rates of brain activity when reasoning. The neural structure contributing to efficiency in intelligence remains unclear. This week in Nature Communications, Genc and colleagues use a diffusion tensor imaging technique to examine how neurite (i.e. projections from the cell body of a neuron) density and microstructure contributes to intelligence.
How did they do it?
They scanned two groups of healthy individuals (a test sample and a validation sample) using a diffusion tensor imaging technique called neurite orientation dispersion and density imaging (NODDI). They measured gray matter and white matter volume, neurite density, neurite orientation dispersion (a measure of branching of dendrites), and isotropic diffusion (a measure of the orientation of neurons) in the cortex of each individual. Intelligence was measured using a matrix-reasoning test. The then tested for correlations between these brain structural features and intelligence in both samples to see how intelligence and brain structure are related.
What did they find?
They found a negative correlation between neurite density & orientation dispersion and intelligence, indicating that people with higher intelligence have less neurite density and less orientation dispersion. There was also a positive correlation between gray matter volume and intelligence, suggesting that greater brain volume corresponds to greater intelligence. They ran a multiple regression analysis to ensure that these findings were not due to differences in age or between brain structure in males and females. They also tested for correlations across 180 brain regions to see whether associations were driven by frontal and parietal brain regions. They found that neurite density was negatively correlated with intelligence in several frontal and parietal brain regions, confirming previous research. They confirmed all results in the replication sample.
What's the impact?
This is the first study to demonstrate associations between specific neural architecture and intelligence. This study shows that intelligence is associated with brain volume, however, also with a low neurite density and dispersion, supporting the hypothesis that neural efficiency is important for intelligence. These findings help us to understand how neuron structure contributes to a complex trait like intelligence.
Genc et al., Diffusion markers of dendritic density and arborization in gray matter predict differences in intelligence. Nature Communications (2018). Access the original scientific publication here.