Brain electrical activity topography in attention-deficit/hyperactivity disorder

Attention-deficit/hyperactivity disorder (ADHD) is a common childhood psychiatric disorder characterized by developmentally inappropriate levels of inattentiveness, impulsivity and hyperactivity. Current theories of ADHD cite evidence from neuropsychological and brain imaging studies suggesting that abnormalities in the structure and function of the frontal lobes and connected brain regions are associated with impaired behavioural inhibition, constituting the primary deficit in ADHD. While most reviewers conclude that neuropsychological studies have failed to find specific deficits in various aspects of attention in ADHD, poor performance on attentional tasks, including the continuous performance task (CPT), is a common finding and previous electrophysiological studies suggest evidence of impaired attentional processing. This study aimed to investigate the cortical activity associated with attentional processes in children with and without ADHD, using steady-state probe topography (SSPT). Seventeen boys diagnosed with ADHD and seventeen age matched control boys participated. Changes in the amplitude and latency of the steady-state visually evoked potential (SSVEP) associated with correct responses to targets in the ‘X’ and ‘AX’ versions of the CPT were examined. At critical time points in both tasks, the control group demonstrated SSVEP changes suggesting increased activation and increased speed of neural processing. These effects occurred predominantly in medial frontal, right prefrontal, right parietal and occipital regions, suggesting enhanced activity in regions previously shown to be involved in attentional processes. The ADHD group demonstrated much smaller increases in activation and processing speed in frontal regions and predominantly reduced activation and slower processing in parieto-occipital regions. Group differences suggesting reduced activity in the ADHD group were observed in response to the presentation of both cues and targets, as well as in the intervals leading up to target presentation, especially in the cued CPT-AX. These results suggest that processing of task relevant stimuli as well as preparatory and motor processes may be associated with dysfunctional activation of brain networks of attention in ADHD, involving deficits in both frontal and parietal cortical regions. These regions may also be involved in the maintenance of information required for correct task performance and the results also suggest possible deficits in these processes in ADHD. The findings are consistent with others of reduced activation and cognitive deficits in ADHD involving these brain regions and networks, and with the idea that ADHD may be associated with a diminished ability to regulate levels of arousal and activation appropriate to task demands