Increased intra-individual reaction time variability in attention-deficit/hyperactivity disorder across response inhibition tasks with different cognitive demands

This study investigates whether intra-individual variability (ISV) in reaction time (RT) serves as a stable endophenotype for ADHD. Specifically, the researchers examined if the profile of RT variability in children with ADHD changes when task demands—specifically working memory load—are manipulated using two different Go/No-go tasks.

Researchers compared 57 children with ADHD and 83 typically developing (TD) controls, aged 8–13. Participants performed a simple Go/No-go task (minimal working memory demand) and a complex Go/No-go task (higher working memory demand). The team used two advanced statistical techniques: ex-Gaussian analysis to decompose RT distributions into normal (Gaussian) and exponential (lapses of attention) components, and Fast Fourier Transform (FFT) to analyze the periodicity of these response patterns.

Children with ADHD consistently demonstrated higher ISV than TD controls across both tasks. Ex-Gaussian analysis revealed that this variability was driven by increases in both the normal component (sigma, reflecting response preparation) and the exponential component (tau, reflecting intermittent lapses of attention). These deficits were robust regardless of the task's cognitive complexity. However, FFT analysis showed that the periodicity of these attentional lapses was task-dependent: children with ADHD showed increased power in lower frequency bands during the simple task, while TD children exhibited different spectral power patterns during the complex task, suggesting that TD children may better adapt their response timing to meet higher cognitive demands.

The findings suggest that increased ISV is a stable, core feature of ADHD that persists across varying levels of cognitive demand. By identifying that ADHD-related variability stems from both impaired response preparation and periodic attentional lapses, this study supports the use of ISV as a reliable intermediate endophenotype, which could help refine diagnostic criteria and improve our understanding of the neurobiological mechanisms underlying ADHD.