Abstract: Two stop-signal tasks were used to investigate two types of response inhibition, response conflict and response stopping. Subjects were two subtypes of children with ADHD (18 of predominantly inattentive and 11 of combined) and normal controls. Results showed that ADHD children were deficient in both types of response inhibition compared with normal controls. No significant differences were observed between the two subtypes of ADHD children after the effect of age was partialed out. Moreover, children showed the same pattern of effects in response inhibition under condition of either endogenous or exogenous attentional cueing. These findings suggest that the two types of response inhibition employ some common cognitive and brain mechanisms and the two subtypes of ADHD were impaired equivalently on these functions.

Key words: ADHD, subtypes of ADHD, response inhibition, response conflict, response stopping, stop-signal task