| Dysfunctional attention systems in children and adults with Attention Deficit Hyperactivity Disorder (ADHD) involve an array of neural abnormalities and have been extensively investigated with behavioral, electrophysiological, neurochemical and neuroimaging techniques. However, their is a paucity of studies that have investigated differences in neural abnormalities among subtypes of ADHD. The present study examined behavioral performance criteria and scalp distributions of electrophysiological waveform components (performance accuracy, N1, N2, and P3 event related potential components) elicited during a simple auditory discrimination task by normal control children and children with subtypes of ADHD. ADHD subtypes were characterized by the presence of either attention deficit disorder without impulsivity (ADD/wo) or attention deficit disorder with impulsivity and/or overactivity (ADD/h). Overall performance accuracy for this simple task did not differ between groups; although ADHD children demonstrated more behavioral performance variability. ADD/h children displayed significant increases in cerebral activity, relative to controls and ADD/wo, at anterior scalp locations for the early fronto-central negative N1, and centrally distributed N2 components during directed attention to a Target stimulus. ADD/h children also exhibited enhanced anterior late positive P3 voltages, relative to controls and ADD/wo subjects. Significant amplitude increases at anterior scalp locations by ADD/h subjects may reflect dysfunctional frontal attention mechanisms which regulate cerebral excitation, habituation, and/or orientation to deviant stimuli. ADD/wo children, however, displayed maximum P3 amplitudes at the typical parietal scalp distribution which were significantly larger than those of ADD/h children, but did not differ from controls. Enhanced amplitudes generated by the ADHD groups may, paradoxically, indicate reduced cerebral processing efficiency necessitating an increased allocation of available cognitive resources. Moreover, compared to normal children, ADD/wo, but not ADD/h children, demonstrated significantly faster P3 latencies associated with maximum amplitudes. In conclusion, the investigative results of ERP components and their distribution, in association with the employment of stringent diagnostic subtyping, indicates the presence of several neurophysiological distinctions between ADD/h and ADD/wo children that may be associated with neuroanatomical, neurochemical and/or behavioral group differences. |
