3.2.2. Response-Locked Pe

The two (response type: error vs. correct) by two (group: ADHD vs. HC) repeated-measures ANOVA revealed a main effect of response type (F (1, 149) = 502.62, *p* < 0.001) and interaction between response type and group (F (1, 149) = 8.37, *p* = 0.004). Error trials elicited a larger Pe than did correct trials (Pc); the Pe effect (i.e., dPe) was smaller in participants with ADHD than in HC (Table 1; Figure 1). There was no main effect of group (F = 1.826, *p* = 0.179).

Age was positively correlated with Pc on correct trials (*r* = 0.182 *p* = 0.026); there was no significant correlation of age with the Pe or the Pe effect, and there was no group difference in these correlations with age. When covarying age, the Pe and Pe effect were negatively correlated with overall RT (Pe: *r* = −0.471, *p* < 0.001; dPe: *r* = −0.386, *p* < 0.001), RTV (Pe: *r* = −0.431, *p* < 0.001; dPe: *r* = −0.413, *p* < 0.001), positively correlated with accuracy (Pe: *r* = 0.213, *p* = 0.009; dPe: *r* = 0.313, *p* < 0.001) and post-error slowing (Pe: *r* = 0.249, *p* = 0.002; dPe: *r* = 0.202, *p* = 0.013). There was no Pc correlation with behavioral performance or group difference in correlation between Pe/Pc/Pe effect and behavioral performance.

#### 3.2.3. Stimulus-Locked N2

The two (congruency: congruen<sup>t</sup> vs. incongruent) by two (group: ADHD vs. HC) repeated-measures ANOVA revealed a main effect of congruency (F (1,152) = 46.23, *p* < 0.001) and an interaction between congruency and group (F (1,152) = 4.99, *p* = 0.027). Incongruent trials elicited a larger N2 (more negative) than congruen<sup>t</sup> trials; the N2 congruency effect was smaller in participants with ADHD than in HC (Table 1; Figure 2). There was no main effect of group (F < 1, *p* = 0.992).

**Figure 2.** N2 and P3 waveforms for participants with ADHD and HC, and topography for incongruent correct trials (N2: 300–400 ms mean amplitude; P3: 400–600 ms mean amplitude; Baseline: −100–0 ms) in all participants. Stimuli onset occurred at 0 msec. Con, congruen<sup>t</sup> correct trials; Inc, incongruent correct trials; ADHD, attention deficits/hyperactive disorder; HC, healthy controls.

Age was positively correlated with N2 on both congruen<sup>t</sup> (*r* = 0.395, *p* < 0.001) and incongruent trials (*r* = 0.338, *p* < 0.001), and negatively correlated with the N2 congruency effect (*r* = −0.194, *p* = 0.016). There was no significant group difference in correlations with age. With age increasing, participants showed larger N2 congruency effect (more negative), which was evident in HC but not in ADHD cases (HC, *r* = −0.297, *p* = 0.009; ADHD, *r* = −0.084, *p* = 0.469; ADHD vs. HC, *p* = 0.118). When covarying age, there was no significant correlation or group difference in the correlation between N2 and behavioral performance (all *p*s > 0.09).

## 3.2.4. Stimulus-Locked P3

The two (congruency: congruen<sup>t</sup> vs. incongruent) by two (group: ADHD vs. HC) repeated-measures ANOVA revealed main effects of congruency (F (1,152) = 84.24, *p* < 0.001) and group (F (1,152) = 13.12, *p* < 0.001), a significant interaction between congruency and group (F (1,152) = 6.26, *p* = 0.013). Incongruent trials elicited larger P3 than congruen<sup>t</sup> trials. Participants with ADHD hadsmallerP3amplitudes,andalsoshowedsmallerP3congruencyeffectsthanHC(Table1;Figure2).

Age was not correlated with congruen<sup>t</sup> (*r* = −0.151, *p* = 0.062) or incongruent P3 (*r* = 0.049, *p* = 0.544) amplitudes, but was positively correlated with the P3 congruency effect (*r* = 0.375, *p* < 0.001). There was no significant group difference in the correlation of age with P3 congruency effect. With age increasing, participants showed larger P3 congruency effects. When covarying age, RT on congruen<sup>t</sup> and incongruent trials were negatively correlated with P3 at congruen<sup>t</sup> (*r* = −0.404, *p* < 0.001) and incongruent trials (*<sup>r</sup>*<sup>=</sup>−0.432, *p*<0.001) respectively. The RT congruency effect was negatively correlated with the P3 congruency effect (*r* = −0.171, *p* = 0.034). There was no significant association between accuracy and P3, and there was no group difference in the correlation of P3 with behavioral performance.

#### *3.3. The Association of Behavioral and ERP Measures with ADHD Symptoms and ADHD Subtype*

#### 3.3.1. The Association of Behavioral and ERP Measures with K-SAD-PL ADHD Symptoms

In ADHD, age was negatively correlated with ADHD total score (*r* = −0.385, *p* = 0.001) and hyperactivity/impulsive score (*r* = −0.443, *p* < 0.001). Older youth with ADHD in our sample had reduced symptom severity in total and hyperactivity/impulsive scores; there was no correlation of age with the inattention score (*r* = 0.043, *p* = 0.710). When covarying age, there was no correlation between any ADHD symptom scores (including ADHD total score, inattention score, or hyperactive/impulsive score) and behavioral performance (all *p*s > 0.1). There was no correlation between ADHD symptom scores and ERP measures, except that the P3 congruency effect was found positively correlated with the inattention subscale (*r* = 0.336, *p* = 0.003).

#### 3.3.2. The Association of Behavioral and ERP Measures with CBCL ADHD Problems Scale Scores

Across all subjects, age was negatively correlated with CBCL ADHD Problems Scale scores (*r* = −0.179, *p* = 0.028). The age correlation with symptom severity was more significant in ADHD patients than in HC (ADHD: *r* = −0.329, *p* = 0.004; HC: *r* = 0.194, *p* = 0.090; ADHD vs. HC: *t* = 2.13, *p* = 0.035). When covarying age, ADHD Problems Scale scores were correlated with overall RT (*r* = 0.330, *p* < 0.001), RTV (*r* = 0.329, *p* < 0.001), post-error slowing (*r* = −0.218, *p* = 0.007) and the RT congruency effect (*r* = 0.161, *p* = 0.049). When age and these significant behavioral measures were included in a backward stepwise regression model, only RTV was found positively correlated with ADHD Problems Scale scores (*b* = 0.031, *p* < 0.001). When covarying age, ADHD Problems Scale scores were correlated with the CRN (*r* = −0.205, *p* = 0.013), ERN effect (*r* = 0.179, *p* = 0.030), Pe (*r* = −0.208, *p* = 0.011), Pe effect (*r* = −0.176, *p* = 0.033), and P3 on congruen<sup>t</sup> (*r* = −0.266, *p* = 0.001) and incongruent (*r* = −0.291, *p* = 0.001) trials. When age and these significant ERP measures were included in a backward stepwise regression model, only P3 on incongruent trials was found negatively correlated with ADHD Problems Scale scores (*b* = −0.339, *p* = 0.004). In the ADHD or HC group alone, there was no correlation of ADHD Problems Scale scores with any behavioral or ERP measures when covarying age.

#### 3.3.3. Behavioral and ERP Measures Among Different ADHD Subtypes

For patients included in our sample, participants with ADHD inattentive type (14.7 ± 2.8 years) were older than patients with combined type (11.6 ± 2.9, *p* < 0.001), and patients with the hyperactive/impulsive type (12.6 ± 3.9, *p* < 0.001). When controlling age, there were no group differences on any behavioral or ERP measures among the three subtypes of ADHD (all *p*s > 0.1).
