Distinct information processing characteristics in dyslexia and ADHD during a covert orienting task: an event-related potential study.

A visuo-spatial orienting task was used to investigate the individual and joint contribution of the presence of dyslexia and attention-deficit hyperactivity disorder (ADHD) to information processing. Sixteen control, 17 dyslexic, 16 ADHD, and 15 comorbid adults performed the task, comprising a valid, invalid, and no-cue condition. Performance measures were errors and reaction time (RT). A negative potential in response to cues and targets (N2), and a positive potential in response to targets (P3) were derived from the EEG. A 2x2 design was used with the factors dyslexic/non-dyslexic, and ADHD/non-ADHD. Dyslexic participants demonstrated a smaller cue-related N2, yet a greater target-related N2 in the valid condition. ADHD participants were discriminated by the P3 difference between the invalid and valid conditions. Comorbids differed from ADHD mainly in invalid-valid RT, and were similar to dyslexics in target N2 processing. Dyslexics were impaired in early information processing, and participants with ADHD differed for later processing stages. This is the first ERP study of attentional processes in dyslexia to incorporate an ADHD and a comorbid group. Its results may contribute to differentiation of these clinical groups.