State dependent changes in resting electroencephalographic activity in children with and without Attention Deficit Hyperactivity Disorder

• ¹ Ghent University, Belgium

Introduction: In the last decades, electroencephalographic (EEG) measures have been considered useful to examine the underlying neurophysiology of attention deficit hyperactivity disorder (ADHD). Although not always consistent, eyes closed resting EEG studies show enhanced theta activity, reduced beta activity and an elevated theta/beta (t/b) ratio in children with ADHD relative to typically developing children. Research procedures in EEG studies on ADHD often differ in scheduling EEG recording. That is, EEG activity is recorded at the start or at the end of a test session, representing EEG power respectively before and after cognitive effort. Situations that require sustained mental effort are believed to attribute to a state of fatigue, which has been shown to affect EEG. So far, EEG research hardly concentrated on such state dependent changes in ADHD. Therefore, in the present study resting EEG before and after cognitive effort was evaluated in children with ADHD and typically developing children.
Method: 128 channel EEG was recorded in 21 children with ADHD and 30 typically developing children during three minutes eyes closed resting conditions before and after N-back task execution. Groups did not significantly differ in age and IQ. Absolute power estimates were derived for theta (4-7,5 Hz) and beta (12,5-21 Hz) frequencies at frontal (Fz, F3, F4), central (Cz, C3, C4) and parietal (Pz, P3, P4) sites. T/b power ratio was calculated by dividing the power of the theta band by the power of the beta band. Separate repeated measures analyses of variance (ANOVAs) were used to compare theta and beta power and t/b ratio before and after cognitive effort in the ADHD and control group.
Results: No significant main group effects were found for theta power, nor for beta power and t/b ratio. Theta and beta power significantly reduced after task execution in both groups, whereas t/b ratio did not significantly change after cognitive effort across groups. However, analyses revealed a group difference for t/b ratio at parietal sites after task execution, which was due to a reduction of t/b ratio in typically developing children together with an increment of t/b ratio in children with ADHD.
Conclusion: Children with ADHD as well as typically developing children show a reduction in absolute theta and beta power after cognitive effort, confirming state dependent changes in EEG activity in both groups. Despite the lack of significant main group differences for theta and beta power and t/b ratio across conditions, unlike typically developing children, children with ADHD showed EEG slowing in the transition from before to after cognitive effort, most apparent at parietal sites. This may suggest that differences in EEG activity in children with ADHD compared to typically developing children may be better explained by deviances in regulation of EEG activity than in stable discrepancies in EEG power. This may also partly explain inconsistencies in resting EEG findings across ADHD studies.