*2.4. Analyses*

Behavioral measures included accuracy and mean reaction times (RT) for each participant. Premature responses faster than 150 ms, slow responses longer than 3000 ms, and reaction times

beyond three standard deviations from the mean were not considered in the ERP and behavioral analyses. Intra-individual reaction times variability was estimated as the standard deviation across congruen<sup>t</sup> and incongruent trials. Reaction times after errors were evaluated to determine if there were group di fferences in post-error behavioral adjustments. Accuracy and mean reaction times on correct trials were further analyzed using analysis of variance (ANOVA) with group (ADHD vs. HC) as a between-subject factor and stimulus type (congruent vs. incongruent) as a within-subject factor.

The ERN was quantified using mean amplitude relative to a pre-response baseline (of −200 to −50 msec). The mean amplitude of the ERN was computed on error trials in a window from 0 to 80 msec following the incorrect response. The correct response negativity (CRN) consisted of the same measure computed on correct trials. The ERN e ffect was defined as the di fference between the ERN and CRN (dERN), calculated by subtracting the CRN from the ERN, since it may isolate activity unique to error processing from activity more broadly related to response monitoring [50]. The Pe was quantified using mean amplitude computed on error trials in a window from 200 to 400 msec following the incorrect response, relative to a pre-response baseline of −200 to −50 msec. The correct Pe consisted of the same measure computed on correct trials. The Pe e ffect was defined by di fference between Pe and Pc (dPe), calculated by subtracting the correct waveform (Pc) from the incorrect (Pe). Both N2 and P3 were quantified using mean amplitude relative to a pre-stimulus baseline of −100 to 0 msec. The mean amplitude of the N2 and P3 were computed on congruen<sup>t</sup> and incongruent correct trials in a window from 300 to 400 ms, and from 400 to 600 msec respectively, following the stimulus onset. N2 and P3 congruency e ffect were defined by ERP di fference between incongruent and congruen<sup>t</sup> trials within corresponding windows. Statistics of ERN, Pe, N2, and P3 were reported at FCz, CPz, FCz, and Pz, respectively, where the maximal mean amplitudes were found, and which were also consistent with previous literature. ERN and Pe amplitude were analyzed respectively with group as a between-subject factor and response type (error vs. correct) as a within-subject factor. N2 and P3 amplitude on correct trials were analyzed with group as a between-subject factor and stimulus type (congruent vs. incongruent) as a within-subject factor.

One-way ANOVAs were conducted to compare group di fference on behavioral and ERP measures. Regression analyses were used to examine (1) correlations of behavioral, ERP measures, and age; (2) group di fference on correlations of behavioral, ERP measures and age; (3) behavioral and ERP predictors of ADHD symptom severity. All statistical tests were two-tailed with the alpha level set at 0.05 if not otherwise specified.
