Correlation of intelligence and human brain structure

Objective: Biological correlates of intelligence have been discussed since a long time. Structural and functional neuroimaging studies mostly reported correlation findings in frontoparietal cortical areas. However, the relationship between intelligence and ultrastructural properties of the brain has hardly been investigated.

Methods: High-resolution T1-weighted magnetic resonance imaging (MRI) and diffusion tensor imaging (DTI) data sets were acquired in N=26 unrelated healthy subjects (age: 22.7±1.8 years). IQ was assessed by the HAWIE-R (Hamburg-Wechsler Intelligenztest) Scale. Voxel-wise t-statistic regression analysis was used to correlate fractional anisotropy (FA) and mean diffusivity (MD) values with full-scale IQ (FSIQ), verbal IQ (VIQ), and performance IQ (PIQ). In addition, voxel based morphometry (VBM) regression analyses of regional brain volume with IQ values was performed.

Results: Our analyses revealed significant negative correlation between VIQ and FA as well as positive correlation between VIQ and MD in the left Broca area. To a lesser extent, significant negative correlations were found between PIQ and FA, predominantly in parietal cortical areas. VBM regression analyses demonstrated positive correlation of gray matter volume and VIQ in the left Broca area as well as negative correlation with adjacent cerebrospinal fluid (CSF) volume.

Conclusion: These findings suggest that cortical (ultra-) structural properties in left Broca area contribute essentially to verbal intelligence, while right parietal association areas predicted performance IQ. Further studies are needed, combining DTI and functional imaging/electrophysiology, to better understand the precise functional impact of FA and MD variations.