Stimulant Treatment Reduces Lapses in Attention among Children with ADHD: The Effects of Methylphenidate on Intra-Individual Response Time Distributions

Abstract

Recent research has suggested that intra-individual variability in reaction time (RT) distributions of children with ADHD is characterized by a particularly large rightward skew that may reflect lapses in attention. The purpose of the study was to provide the first randomized, placebo-controlled test of the effects of the stimulant methylphenidate (MPH) on this tail and other RT distribution characteristics. Participants were 49 9- to 12-year-old children with ADHD. Children participated in a 3-day double-blind, placebo-controlled medication assessment during which they received long-acting MPH (Concerta®), with the nearest equivalents of .3 and .6 mg/kg t.i.d. immediate-release MPH. Children completed a simple two-choice speeded discrimination task on and off of medication. Mode RT and deviation from the mode were used to examine the peak and skew, respectively, of RT distributions. MPH significantly reduced the peak and skew of RT distributions. Importantly, the two medication effects were uncorrelated suggesting that MPH works to improve both the speed and variability in responding. The improvement in variability with stimulant treatment is interpreted as a reduction in lapses in attention. This, in turn, may reflect stimulant enhancement of self-regulatory processes theorized to be at the core of ADHD.

Reaction Time Variability in ADHD: A Review

“…In addition, results showed the effects of medication were of the largest magnitude and most consistent for RTV indicators, especially ex-Gaussian tau, compared to other performance indicators, such as RT speed and task accuracy. These results suggest that medication specifically reduces intermittent long RTs, which is consistent with other studies showing that medication reduces tau [100] and reduces the peak and skew of RT distributions [43]. Also, most studies using fast Fourier transform (FFT) analyses have demonstrated that stimulant medication attenuated [35] or normalized [103] long RT oscillations in individuals with ADHD compared with controls, although 1 study failed to replicate this effect [104].…”

“…Omission errors occur when a patient does not respond to a target stimulus and has been linked to attentional lapses [56], thus suggesting that long RTs, the primary cause of RTV, are due to inattention. Consistent with this interpretation, Spencer et al [43] observed that the effect of acute methylphenidate on RTV in a simple choice discrimination task was moderately correlated with the effect of the same medication in being able to reduce omissions on a separate continuous performance task.…”

“…In general, ADHD-related increases in RTV are attenuated, and may even normalize, with stimulant medication [35,41,43,[97][98][99][100][101][102]. In a recent investigation of medication effects on RTV, which included a range of cognitive tasks, Epstein et al [41] found that stimulant medication reduced RTVon a range of cognitive tasks.…”

“…Higher tau values are produced by RT distributions with a larger rightward skew or tail, consistent with a lapse of attention framework. Alternative metrics that do not require that ex-Gaussian assumptions have also been developed (for more detail, see Sabol et al [42] and Spencer et al [43]), and further work in this area will be critical for developing a consensus of RTV estimation. It should also be noted that 1 study, using ex-Gaussian parameters, reported significant differences, albeit with a small effect size (0.08) between ADHD and control groups in the fast portion of the RT distribution, which was interpreted as reflecting different causal mechanisms for the potentially multidimensional construct of RTV [44].…”

Understanding the Effects of Stimulant Medications on Cognition in Individuals with Attention-Deficit Hyperactivity Disorder: A Decade of Progress

“…The overall effect of medication was significant owing to faster SSRTs and lower RT variability, but the dose effect was not significant, suggesting that the maximum cognitive benefit may be elicited by a low dose of medication. Spencer et al (2009) evaluated the cognitive response of children with ADHD to stimulant medication based on an RT task of discriminating two stimuli (X or O) in a double-blind, dose-response, crossover study of placebo, low (0.9 mg/kg/day) and high (1.8 mg/kg/day) MP doses delivered by controlled-release formulations. Medication produced a significant reduction of the mode and SD from the mode (rather than the mean, which is correlated with SD) for the low as well as high doses, which was not owing to a speed-accuracy tradeoff, but instead reflected an overall increased efficiency of responding (faster and more accurate responses).…”