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Background: Neuroimaging studies have shown structural alterations in several brain regions in children and adults with attention deficit hyperactivity disorder (ADHD). Through the formation of the international ENIGMA ADHD Working Group, we aimed to address weaknesses of previous imaging studies and meta-analyses, namely inadequate sample size and methodological heterogeneity. We aimed to investigate whether there are structural differences in children and adults with ADHD compared with those without this diagnosis., Methods: In this cross-sectional mega-analysis, we used the data from the international ENIGMA Working Group collaboration, which in the present analysis was frozen at Feb 8, 2015. Individual sites analysed structural T1-weighted MRI brain scans with harmonised protocols of individuals with ADHD compared with those who do not have this diagnosis. Our primary outcome was to assess case-control differences in subcortical structures and intracranial volume through pooling of all individual data from all cohorts in this collaboration. For this analysis, p values were significant at the false discovery rate corrected threshold of p=0·0156., Findings: Our sample comprised 1713 participants with ADHD and 1529 controls from 23 sites with a median age of 14 years (range 4-63 years). The volumes of the accumbens (Cohen's d=-0·15), amygdala (d=-0·19), caudate (d=-0·11), hippocampus (d=-0·11), putamen (d=-0·14), and intracranial volume (d=-0·10) were smaller in individuals with ADHD compared with controls in the mega-analysis. There was no difference in volume size in the pallidum (p=0·95) and thalamus (p=0·39) between people with ADHD and controls. Exploratory lifespan modelling suggested a delay of maturation and a delay of degeneration, as effect sizes were highest in most subgroups of children (<15 years) versus adults (>21 years): in the accumbens (Cohen's d=-0·19 vs -0·10), amygdala (d=-0·18 vs -0·14), caudate (d=-0·13 vs -0·07), hippocampus (d=-0·12 vs -0·06), putamen (d=-0·18 vs -0·08), and intracranial volume (d=-0·14 vs 0·01). There was no difference between children and adults for the pallidum (p=0·79) or thalamus (p=0·89). Case-control differences in adults were non-significant (all p>0·03). Psychostimulant medication use (all p>0·15) or symptom scores (all p>0·02) did not influence results, nor did the presence of comorbid psychiatric disorders (all p>0·5)., Interpretation: With the largest dataset to date, we add new knowledge about bilateral amygdala, accumbens, and hippocampus reductions in ADHD. We extend the brain maturation delay theory for ADHD to include subcortical structures and refute medication effects on brain volume suggested by earlier meta-analyses. Lifespan analyses suggest that, in the absence of well powered longitudinal studies, the ENIGMA cross-sectional sample across six decades of ages provides a means to generate hypotheses about lifespan trajectories in brain phenotypes., Funding: National Institutes of Health., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Late positive potentials (LPPs) were found to be decreased during down-regulation and increased during up-regulation of positive and negative emotions. However, previous studies lack ecological validity, since they explicitly instructed their participants to use certain regulation strategies. The goal of our study was to test an ecologically more valid paradigm of emotion regulation. We therefore investigated the effects of freely chosen emotion regulation strategies on LPPs and additionally assessed facial EMG responses and valence and arousal ratings as control variables. Responses to positive IAPS pictures were marked by pleasant valence ratings and high activations of M. zygomaticus major, negative pictures elicited unpleasant valence ratings and high activations of M. corrugator supercilii, and both, positive and negative pictures, went along with increased arousal ratings and LPPs. Importantly, ratings and EMG activity were intensified through up-regulation and attenuated through down-regulation of emotions, while LPPs were increased through both up-and down-regulation. We conclude that LPPs in paradigms with free choice of emotion regulation strategies might be a marker of attentional resources required for the selection of adequate emotion up- and down-regulation strategies, while LPP effects following emotion regulation with specific, instructed strategies reflect modulated arousal processes., (Copyright © 2015 Elsevier B.V. All rights reserved.)