Attenuated Rightward Hemispheric Asymmetry in ADHD: Structural MRI Evidence from a Normalized Asymmetry Index and Its Association with Cognitive Performance

Abstract

Background Altered hemispheric asymmetry has been proposed as a potential neurodevelopmental feature of Attention-Deficit/Hyperactivity Disorder (ADHD). However, findings remain inconsistent, and the functional relevance of structural asymmetry patterns is not well established. This study examines volumetric and cortical-thickness asymmetries across cortical and subcortical regions in children and adolescents with ADHD compared to typically developing controls and evaluates their association with objective cognitive performance. Methods Forty participants with ADHD and 30 age- and sex-matched controls underwent high-resolution T1-weighted MRI. Bilateral regional volumes and cortical thickness were quantified using the volBrain pipeline, and asymmetry indices (AI = [R-L]/[(R + L)/2]) were computed for lobar and subcortical structures. Group differences were assessed using independent t-tests. Within the ADHD group, associations between asymmetry indices and MOXO-d-CPT performance (Attention, Timing, Impulsivity, Hyperactivity) were examined using Pearson correlations with correction for multiple comparisons. Results ADHD participants showed significantly reduced rightward asymmetry in frontal lobe volume, cerebellar hemispheres, caudate, putamen, and amygdala (ps < 0.05). Cortical-thickness asymmetry was also diminished in the frontal and parietal lobes and the anterior cingulate cortex. Temporal and occipital asymmetries were preserved. Within the ADHD group, greater rightward frontal and ACC thickness asymmetry correlated with better attention performance (r = 0.45 and 0.40), rightward parietal asymmetry associated with more accurate timing (r = 0.38), reduced rightward IFG asymmetry related to greater impulsivity (r = -0.42), and amygdala asymmetry correlated with lower hyperactivity (r = 0.36). Conclusion Children with ADHD exhibit a consistent attenuation of typical right-hemisphere dominance across frontal, striatal, cerebellar, and limbic systems. These altered asymmetry patterns are meaningfully associated with attentional control, timing accuracy, impulsivity, and hyperactivity, suggesting that hemispheric imbalance may serve as a structural may represent a neurodevelopmental characteristic associated with ADHD. Findings support models emphasizing right-hemisphere developmental lag and highlight hemispheric asymmetry as a clinically relevant dimension of ADHD neurobiology.