Neurobiology of Adult ADHD: Emerging Evidence for Network Dysfunctions

Abstract

FIGURE 1. Changes in cortical thickness provide one measure of brain maturation. A large longitudinal study found that for most areas of cortex, children with attention deficit hyperactivity disorder (ADHD) reach peak cortical thickness several years later than typically developing children, supporting presence of developmental delay. 1 The rate of cortical thinning also differed between the group who continued to meet diagnostic criteria into adulthood (persistent ADHD) and those who did not (remitted ADHD). 2 Areas of cortex in which the rate of thinning correlated with adult symptom level (green, more symptoms associated with more thinning) are approximated on medial and lateral simplified representations of cortex. 2 An earlier study also identified multiple areas in which cortex was thinner in adults with persistent ADHD compared with controls (orange). 3 In addition, this study noted some areas of thicker cortex in remitted ADHD when compared with persistent ADHD (blue). FIGURE 2. Two recent functional magnetic resonance imaging (fMRI) studies have used cued reaction time tasks to assess differences between adults with a childhood diagnosis of ADHD and controls in task-related activations and functional connectivity. 4,5 Left. One study used the area with the highest level of activation associated with cognitive control (right dorsolateral prefrontal cortex) as the seed to assess functional connectivity associated with cognitive control. 5 The ADHD group had lower connectivity than the control group to multiple areas (orange, left putamen, left inferior frontal gyrus, and bilateral subgenual cingulate cortex), but connectivity was not correlated with symptom severity. Right. The other study assessed thalamocortical functional connectivity and brain activations associated with response preparation. 4 The persistent ADHD group had lower connectivity compared with the remitted ADHD group from the seed region in thalamus to several areas in prefrontal cortex. The locations of the cluster centers (blue, bilateral frontopolar cortex and left dorsolateral prefrontal cortex) are approximated onto a dorsal view of the cortical surface. An unexpected finding was that there were no significant differences between the remitted ADHD group and persistent ADHD in brain activations associated with response preparation. COVER AND FIGURE 3. Left. A resting state fMRI study in adults with a childhood diagnosis of ADHD reported significantly reduced functional connectivity within the default mode network in persistent ADHD. 6 The areas in which either remitted ADHD (blue) or controls (orange) had greater connectivity from the seed region (black, posterior cingulate cortex) compared with persistent ADHD are approximated on a midline MRI. 6 This study also reported that neither ADHD group exhibited the expected negative functional connectivity between the other default mode network seed region (medial prefrontal cortex) and dorsolateral prefrontal cortex . Right. A resting state fMRI study in medication naive adults with ADHD re...

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