Neurophysiological studies yield contrary results whether attentional problems of patients with attention-deficit/hyperactivity disorder (ADHD) are related to early visual processing deficits or not. Evoked gamma-band responses (GBRs), being among the first cortical responses occurring as early as 90ms after visual stimulation in human EEG, have been assigned a pivotal role in early visual processing. In particular, they are involved in memory matching processes and are enhanced when known stimuli are processed. The current study examined whether evoked GBR patterns during early memory matching processes could be indicative of an early visual processing deficit in ADHD patients. EEG was recorded from 13 young ADHD patients as well as 13 age-matched healthy participants. Both groups performed a simple forced choice reaction task employing line drawings of either known real-world items with representations in long-term memory or physically similar unknown items without such representations. Evoked GBRs of ADHD patients did not differentiate between known and unknown items. However, in healthy children, evoked GBRs were enhanced when stimuli matched a representation stored in memory. This finding indicates disadvantages at early visual processing stages in ADHD patients: In contrast to healthy participants, ADHD children lack an early memory based classification, possibly resulting in an impaired ability to rapidly reallocate attentional resources to relevant stimuli. These findings suggest that impaired early automatic stimulus classification in ADHD patients could be involved in deficits of selective and sustained attention.
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