No abstract

Previous investigations of the default-mode network (DMN) in persons with attention-deficit/hyperactivity disorder (ADHD) have shown reduced functional connectivity between the anterior and posterior aspects. This finding was originally demonstrated in adults with ADHD, then in youth with ADHD, and has been tentatively linked to ultra low frequency oscillations within the DMN. The current study evaluates the specificity of DMN abnormalities to neuronal oscillations in the ultra low frequency range, and examines the regional specificity of these DMN aberrations in medicated and unmedicated adults with, and those without ADHD. An individually matched sample of adults with and without ADHD completed 6-minute sessions of resting-state magnetoencephalography (MEG). Participants with ADHD were known responders to stimulant medications and completed two sessions (predrug/postdrug). MEG data were coregistered to the participant's MRI, corrected for head motion, fitted to a regional-level source model, and subjected to spectral analyses to extract neuronal population activity in regions of the DMN. The unmedicated adults with ADHD exhibited broadband deficits in medial prefrontal cortices (MPFC), but not other DMN regions compared to adults without ADHD. Unmedicated patients also showed abnormal cross-frequency coupling in the gamma range between the MPFC and posterior cingulate areas, and disturbed balance within the DMN as activity in posterior regions was stronger than frontal regions at beta and lower frequencies, which dissipated at higher γ-frequencies. Administration of pharmacotherapy significantly increased prefrontal alpha activity (8-14 Hz) in adults with ADHD, and decreased the cross-frequency gamma coupling. These results indicate that neurophysiological aberrations in the DMN of patients with ADHD are not limited to ultra slow oscillations, and that they may be primarily attributable to abnormal broadband activity in the MPFC.

No abstract

The ability to attend to particular stimuli while ignoring others is crucial in goal-directed activities and has been linked with prefrontal cortical regions, including the dorsolateral prefrontal cortex (DLPFC). Both hyper- and hypo-activation in the DLPFC has been reported in patients with attention-deficit/hyperactivity disorder (ADHD) during many different cognitive tasks, but the network-level effects of such aberrant activity remain largely unknown. Using magnetoencephalography (MEG), we examined functional connectivity between regions of the DLPFC and the modality-specific auditory cortices during an auditory attention task in medicated and un-medicated adults with ADHD, and those without ADHD. Participants completed an attention task in two separate sessions (medicated/un-medicated), and each session consisted of two blocks (attend and no-attend). All MEG data were coregistered to structural MRI, corrected for head motion, and projected into source space. Subsequently, we computed the phase coherence (i.e., functional connectivity) between DLPFC regions and the auditory cortices. We found that un-medicated adults with ADHD exhibited greater phase coherence in the beta (14–30Hz) and gamma frequency (30–56 Hz) range in attend and no-attend conditions compared to controls. Stimulant medication attenuated these differences, but did not fully eliminate them. These results suggest that aberrant bottom-up processing may engulf executive resources in ADHD.

Amphetamine-based medications robustly suppress symptoms of attention-deficit/hyperactivity disorder (ADHD), but their exact mechanisms remain poorly understood. Recent hemodynamic imaging studies have suggested that amphetamines may modulate the prefrontal and anterior cingulate brain regions, although few studies have been published and the results have not been entirely consistent. Meanwhile, several electrophysiological studies have shown that abnormal fast oscillations (in the γ range) may be closely linked to inattention and other cardinal symptoms of ADHD. In this study, we utilized magnetoencephalography to examine how amphetamines modulate high-frequency brain activity in adults with ADHD. Participants performed an auditory attention task, which required sustained attention in one block and passive listening in a separate block. Participants completed the task twice in the on-medication and off-medication states. All data were analyzed using beamforming techniques to resolve cortical regions showing event-related synchronizations and desynchronizations. Our primary findings indicated that oral administration of amphetamine decreased γ-band event-related desynchronization activity significantly in the medial prefrontal area and decreased event-related synchronization in bilateral superior parietal areas, left inferior parietal, and the left inferior frontal gyrus. These results suggest that psychostimulants strongly modulate γ activity in frontal and parietal cortical areas, which are known to be central to the brain's core attentional networks.