**4. Discussion**

We investigated behavioral and electrophysiological indices of performance monitoring and the association of these indices with age in children and adolescents with ADHD and healthy controls, using an EEG arrow-flanker task. Overall, with age increasing, participants responded faster, more accurately, and less variably, and they showed an increased post-error slowing e ffect. In addition, they exhibited an increased ERN e ffect and increased N2 and P3 congruency e ffects. Children and adolescents with ADHD showed impaired behavioral performance, attenuated error awareness and conflict monitoring. Specifically, participants with ADHD responded more slowly, more variably and had reduced post-error slowing; they showed reduced ERN and Pe e ffects in error monitoring, and reduced N2 and P3 congruency e ffects. Impaired behavioral and ERP indices of performance monitoring are consistent with the pattern associated with younger age across groups. Moreover, increased reaction time variability and reduced P3 amplitude in incongruent trials were associated with increased ADHD Problems Scale scores measured by CBCL across all participants.

The developmental e ffects on behavioral performance across groups, including faster reaction times, increased accuracy, decreased reaction time conflict, and decreased reaction time variability, are consistent with the literature [51,52]. With increased age and brain maturation, children increase their response speed and improve their attention and ability to resolve conflict. As expected, relative to HC, participants with ADHD responded more slowly, with increased reaction time variability; they also tended to show a larger RT congruency e ffect. Impairments in these behavioral measures are thought to result from lapses in attention and failures in executive control [17]; the pattern of performance deficits in ADHD is in line with the pattern shown in younger ages across study participants, consistent with the developmental lag model for ADHD [53]. Regarding post-error processes, there was a lack of post-error slowing in ADHD, while HC slowed their response after errors. However, the post-error slowing was positively correlated with age in ADHD, suggesting that the post-error slowing e ffect in ADHD was not developed until later in adolescence, which further suggests that impairment of performance monitoring in ADHD may be associated with a developmental delay. We did not observe a reduced accuracy among children with ADHD, which may be related to our speed-accuracy instructions to respond quickly enough to maintain a certain number of error trials. Consistently with the previous literature, RTV is found to be larger in ADHD than in HC; among several behavioral indices showing group di fferences between ADHD and HC, RTV is uniquely associated with continuous measures of ADHD Problems Scale scores, suggesting RTV is a robust marker of ADHD symptoms.

Error responses elicited a larger ERN and Pe than did correct responses. ERN and Pe e ffects were larger with faster reaction times, reduced RTV and greater post-error adjustments, suggesting that increased ERN and Pe e ffects may reflect better performance monitoring and compensatory processing. Consistent with the literature, ERN e ffects increased with age and Pe e ffects did not change with age in children and adolescents [54,55]. The reduced ERN e ffect found in ADHD compared to healthy controls is consistent with most pediatric and adult studies on ADHD [22,56], suggesting an error detection deficit in ADHD. Following early error detection, participants with ADHD showed a reduced Pe e ffect compared to healthy controls, suggesting alterations in the evaluation of error responses and their motivational significance. Individuals with ADHD may fail to initiate adaptive control processes after errors to make adjustment in the next trial, as demonstrated by diminished post-error slowing. Together with evidence of a larger ERN e ffect with increasing age, and larger ERN and Pe e ffects with better performance, the alteration of ERN and Pe e ffects in ADHD may be associated with a developmental delay in ADHD.

Incongruent compared with congruen<sup>t</sup> stimuli yielded the typical N2 and P3 amplitude enhancement across groups. Both e ffects were stronger when participants were older across groups, suggesting larger N2 and P3 e ffects may reflect enhancements of conflict monitoring with age. While N2 was not associated with behavioral performance, a larger P3 e ffect was associated with a reduced RT congruency e ffect. Participants with ADHD showed reduced N2 and P3 e ffects, indicating problems with conflict monitoring and attention resource allocations. Specially, the attenuated N2 congruency effect in ADHD is consistent with previous findings that N2 is reduced in ADHD and una ffected siblings [25], and in line with the notion that N2 is an index for a general conflict monitoring process. In ADHD, the reduced P3 congruency e ffect and the reduced P3 amplitude on both congruen<sup>t</sup> and incongruent trials are consistent with previous findings of a general deficit in attention resource allocation [37]. Meanwhile, P3 amplitude on incongruent trials was negatively correlated with CBCL ADHD Problems Scale scores as a continuous measure across groups, further suggesting the cognitive process underlying P3 may serve as a target of intervention to reduce ADHD symptoms. Reduced congruency e ffects indicated by N2 and P3 in ADHD, together with stronger N2 and P3 e ffects with age increasing across groups, could also imply a developmental delay in ADHD.

The current study investigated performance monitoring and its development in ADHD. Our study used a larger sample size (*n* = 77 per group) and broader age range (ages 8–18) than has been typical in the literature, and in addition, our control group was closely matched in age and matched in gender. While the broad age range allowed us to investigate age e ffects on performance monitoring, the sample was less homogenous and sample size was small for any given age, preventing us from analyzing the effects of other factors, such as gender, comorbidity and medication [6]. Moreover, the current study included participants ranging in age from 8 to 18 years old, though the development of performance monitoring begins earlier and continues later into the life course [49]; further work should evaluate the relationship between brain activation and ADHD across a wider age range. We have interpreted developmental results using a cross-sectional design, which needs to be verified through a longitudinal design. Another limitation of our study is that we included patients with comorbid anxiety, depression and ODD in ADHD group to increase sample size. While comorbid anxiety may have increased ERN in the ADHD group [57], e ffects of anxiety, depression and ODD comorbidity on di fferent measures of ERPs call for further investigation.
