*2.1. Participants*

There were 77 youth with ADHD (30 female) and 77 age- and gender-matched healthy controls (HC), ranging in age from 8 to 18 years. Patients were recruited through the Section of Child and Adolescent Psychiatry within the Department of Psychiatry at the University of Michigan. Comparison subjects were recruited from the surrounding community. After a complete description of the study, written informed consent was obtained from at least one parent of the participant and written informed assent from the participant. Participants were paid for their interviews and psychophysiological recordings. All tasks and procedures were approved by the University of Michigan Medical School Institutional Review Board.

All participants were interviewed with the Kiddie Schedule for A ffective Disorders and Schizophrenia for School-Aged Children-Present and Lifetime Version (K-SAD-PL) [40] and the Schedule for Obsessive-Compulsive and Other Behavioral Syndromes [41]. Parents completed the Child Behavior Checklist/6–18 (CBCL) [42] and Social Communication Questionnaire (SCQ) [43] about their children. Best-estimate diagnoses were made according to *DSM-5* criteria [44] using all sources of information, including two semi-structured interviews, two parent-report rating scales, four self-report rating scales, and all available clinical records [45]. The clinical records often included

outpatient clinic notes, psychological testing results, and teacher rating scales. All participants were evaluated with the Wechsler Abbreviated Scale of Intelligence, Version II (WASI-II), which is normed for individuals ages 6 to 90 [46]. The WASI-II provides a global estimate of overall cognitive abilities with full-scale IQ. The ADHD subtype groups consisted of 23 children with the combined type, six children with the hyperactive-impulsive type, and 48 children with the inattentive type. Eighteen ADHD patients had a comorbid anxiety disorder, eleven patients had comorbid major depression disorder, twelve patients had comorbid oppositional defiant disorder (ODD) and one patient had motor tics that did not interfere with the EEG recording. Participants were excluded if they had a history of intellectual disability, head injury with loss of consciousness, a chronic neurological disorder, or SCQ scores higher than 14. Patients were excluded if they had a lifetime diagnosis of schizophrenia, other psychotic disorder, bipolar disorder, substance-related disorder, conduct disorder, obsessive-compulsive disorder, or anorexia nervosa. HC had no history of a specific axis I disorder. DSM-Oriented ADHD Problems Scale t-scores from the CBCL were used as a continuous measure of ADHD symptoms within and across groups. Symptom counts from the K-SAD-PL (ADHD total score, inattention score, and hyperactive/impulsive score), were used to assess symptom severity in ADHD patients only. Participants with ADHD taking a stimulant (*n* = 43) were asked to stop taking the medication for 48 hours prior to the EEG. Cases and HC were taking no other psychotropic medications. For the error analyses, two participants with ADHD and one healthy control were excluded due to commission of fewer than six errors, leaving a total of 151 participants.
