The Stop-Signal Task (SST) is a procedure that can provide a measure of inhibitory control of an ongoing motor response. We used the stop-signal paradigm to determine whether deficient inhibitory control distinguishes children with a diagnosis of attention deficit hyperactivity disorder combined type (ADHD-Com) from normally developing children, matched on age and sex. Participants performed a standard visual two-choice task with an auditory stop-signal stimulus, while an EEG was recorded. The behavioral results indicated that the ADHD group had significantly impaired inhibitory control compared to the performance of normal children. Relative to controls, the go stimulus reaction time (RT) and the stop-signal reaction time (SSRT) were prolonged in children with ADHD. The amplitudes of P2 and P3 components to the auditory stop-signal were more pronounced for successful than for unsuccessful stop-signal trials in both groups. However, the successful-unsuccessful difference was larger in control subjects. In contrast, the amplitude of the N2 component to auditory stop-signal was more pronounced for unsuccessful than for successful stop-signal trials in both groups. The comparison of the N2 component between control and ADHD groups revealed a greater amplitude and longer latency in the latter group, in successful stop-signal trials only. Additionally, the amplitude of response-locked ERPs, containing the ERN-Pe complex related to error-detection, was smaller in ADHD children. These results support the hypothesis of a complex deficit of inhibitory control, conflict monitoring, and error recognition mechanisms in ADHD and corroborate the utility of the stop-signal task in distinguishing hyperactive from normal children.
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