Reduced power in fronto-parietal theta EEG linked to impaired attention-sampling in adult ADHD.

Attention-deficit hyperactivity disorder (ADHD) in adults is understudied, especially regarding neural mechanisms such as oscillatory control of attention sampling. We report an EEG study of such cortical mechanisms, in ADHD-diagnosed adults during administration of TOVA (Test of Variables of Attention), a gold-standard continuous performance test for ADHD that measures the ability to sustain attention and inhibit impulsivity.We recorded 53 adults (28f, 25m, aged 18-60), and 18 matched healthy controls, using 128-channel EEG. We analysed sensor-space features established as neural correlates of attention: timing-sensitivity and phase-synchrony of response activations, and event-related (de)synchronisation (ERS/D) of alpha and theta frequency band activity; in frontal and parietal scalp regions.TOVA test performance significantly distinguished ADHD adults from neurotypical controls, in commission errors, RT variability and d' (response sensitivity). The ADHD group showed significantly weaker target- and response-locked amplitudes, that were strongly right-lateralised at the N2 wave, and weaker phase synchrony (longer reset post-stimulus). They also manifested significantly less parietal pre-stimulus 8Hz theta ERS, less frontal & parietal post-stimulus 4Hz theta ERS, and more frontal & parietal pre-stimulus alpha ERS during correct trials.These differences may reflect excessive modulation of endogenous activity by strong entrainment to stimulus (alpha), combined with deficient modulation by neural entrainment to task (theta), which in TOVA involves monitoring stimulus spatial location (not predicted occurence onset which is regular and task-irrelevant). Building on the hypotheses of theta coding for relational structure and rhythmic attention sampling, our results suggest that ADHD adults have impaired attention sampling in relational categorisation tasks.Significant StatementThis study identifies one factor potentially contributing to difficulty of paying sustained attention among adults with attention-deficit/hyperactivity disorder (ADHD). We recorded EEG from a good-sized sample of adults with ADHD diagnosis (N=53), while they performed a gold-standard computerised performance test, TOVA (Test of Variables of Attention). We found that, compared to matched healthy controls (N=18), ADHD participants showed reduced EEG indices of strength of rhythmic attention-sampling. The primary such index was parietal theta-rhythmic event-related synchronisation (ERS), while other indices related to amplitude and phase synchrony of neural responses to stimuli. Characteristics of the task and brain-signal analysis suggest that ADHD participants may be deficient in relational processing, which (to our knowledge) has not been shown before.