Brain Structural Correlates of Intelligence in Attention Deficit Hyperactivity Disorder (ADHD) Individuals

Neuroimaging evidence has shown the relationship of intelligence with several structural brain properties in normal individuals. However, this relationship with attention deficit hyperactivity disorder (ADHD) needs to be investigated.

We estimated grey matter (GM) density of the brain using magnetic resonance imaging (MRI) scan on 56 ADHD individuals, including 30 combined individuals (Mean±SD age: 10.44±2.41, intelligence quotient: [IQ]=112.13±13.15, male, 24 right hands) and 26 inattentive individuals (mean age =11.39±2.1, IQ=107.44±13.98, male, 28 right hands) as well as 30 IQ matched healthy control group (mean age=11.08±2.15, IQ=115±13.56, male, 23 right hands).

In this study, two statistical approaches were used. In the first approach, region-based as well as the whole association patterns between full-scale IQ and GM were computed and compared between groups. The second approach was to examine the differential pattern of GM density without considering IQ in three groups.

Results showed significant differences between the ADHD group and the control. This finding could indicate that intelligence is not purely based on the density of GM in certain brain regions; it is a dynamic phenomenon and drastically changes neurodevelopmental disorders.

In ADHDs as compared to healthy controls the relation between GM and IQ was decreased at right hemisphere;In ADHDs as compared to healthy controls the relation between GM and IQ was increased at left hemisphere;Differenceses of the observed relation between control group and IQ matched ADHDs suggest a compensatory mechanism in ADHDs to maintaine an adequate cognitive performance;GM is not the only determiner of intelligence. IQ score may be affected by neural dynamic of the brain; therefore, the structural covariate could be a better alternative for GM density.

In this study, we estimated the relation between GM density and IQ score in 2 subtypes of ADHD (combined and inattentive) and IQ matched healthy control group. We compared the association between groups and found that the pattern of association in ADHDs were different from controls. In the other words, the decreased association at right hemisphere, were compensated by increased association at left hemisphere in ADHDs to maintaine adequate performance. We conclude that, the brain structure is not the single determiner of intelligence, rather intelligence may underpine by neural dynamics of the brain. Therefore, the structural covariate may be a better alternative for GM density.