Reinforcement and Stimulant Medication Ameliorate Deficient Response Inhibition in Children with Attention-Deficit/Hyperactivity Disorder

Abstract

This study examined the degree to which reinforcement, stimulant medication, and their combination impact response inhibition in children with Attention-Deficit/Hyperactivity Disorder (ADHD). Across three studies, participants with ADHD (n = 111, 25 girls) and typically-developing (TD) controls (n = 33, 6 girls) completed a standard version of the stop signal task (SST) and/or a reinforcement-manipulation SST with performance-contingent points. In two of these studies, these tasks were performed under placebo or 0.3 and 0.6 mg/kg methylphenidate (MPH) conditions. Cross-study comparisons were conducted to test hypotheses regarding the separate and combined effects of reinforcement and methylphenidate on response inhibition among children with ADHD relative to TD controls. Baseline response inhibition was worse among children with ADHD compared to controls. MPH produced dose-related improvements in response inhibition in children with ADHD; compared to non-medicated TD controls, 0.3 mg/kg MPH normalized deficient response inhibition, and 0.6 mg/kg MPH resulted in better inhibition in children with ADHD. Reinforcement improved response inhibition to a greater extent for children with ADHD than for TD children, normalizing response inhibition. The combination of MPH and reinforcement improved response inhibition among children with ADHD compared to reinforcement alone and MPH alone, also resulting in normalization of response inhibition despite repeated task exposure. Deficient response inhibition commonly observed in children with ADHD is significantly improved with MPH and/or reinforcement, normalizing inhibition relative to TD children tested under standard conditions.

Keywords: ADHD; Inhibition; Medication; Methylphenidate; Reinforcement.

Figures

Figure 1. Task structure for the standard and reinforcement-manipulation versions of the stop signal task

(A) Standard and (B) reinforcement-manipulation stop signal task structure and (C) feedback stimuli. Trial structure for all tasks included the (a) fixation point, (b) stimulus presentation, (c) response window, and (d) intertrial interval. The (e) reinforcement and (f) and no-reinforcement conditions included feedback stimuli. During the reinforcement condition, an empty square appeared when an error occurred, indicating no points were earned on the trial, and a square with stars of various sizes and colors appeared for a correct response indicating points were earned on the trial. During the no-reinforcement condition, no points could be earned and a yellow box always appeared inside the square.

Figure 2. Diagnostic group differences and the separate and combined effects of methylphenidate and reinforcement on stop signal reaction time in children with ADHD compared to typically developing controls

This figure illustrates the cross-study comparisons that were made to address each of the hypotheses. The cells containing values in the lower portion of the figure represent the data included in the analyses for each specific hypothesis. Notes. ADHD = Attention-Deficit/Hyperactivity Disorder group; TD = typically-developing control group; SST = stop signal task; SSRT = stop signal reaction time; ST = Standard; RF = Reinforcement; Manip = Manipulation; H = hypothesis; Base = baseline;. Values in the cells represent mean (SD) SSRT for each condition included in the comparison. Error bars represent standard error of the mean (SEM). aFour participants from Study 1 were missing SST data from the Baseline Day. bThree participants with ADHD from Study 2 were missing SST data from the Reinforcement Day.