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

This study investigates whether the increased intra-individual reaction time variability (ISV) observed in children with ADHD is a stable trait or if it fluctuates based on cognitive task demands. Specifically, the authors examine whether ADHD-related variability persists across tasks with varying working memory loads and whether this variability stems from specific patterns of response preparation or intermittent lapses in attention.

Researchers recruited 140 children (57 with ADHD, 83 typically developing controls) to complete two Go/No-go tasks: a simple version with minimal cognitive demand and a complex version requiring working memory to guide inhibitory control. The authors utilized two advanced statistical techniques to analyze reaction time (RT) data: ex-Gaussian modeling, which separates RT into normal (Gaussian) and slow (exponential) components, and Fast Fourier Transform (FFT), which identifies the periodicity of response fluctuations across different frequency bands.

Children with ADHD demonstrated significantly higher ISV than controls across both tasks, regardless of the level of cognitive demand. Ex-Gaussian analysis revealed that this variability was driven by elevations in both the normal component (sigma), reflecting impaired response preparation, and the exponential component (tau), reflecting infrequent, unusually slow responses. While the overall pattern of variability was stable, FFT analysis showed that the periodicity of these attentional lapses differed by task demand, suggesting that the neural mechanisms underlying these lapses are sensitive to the context of the task.

The findings support the hypothesis that increased ISV is a robust, intermediate endophenotype of ADHD. By demonstrating that this variability is not merely a byproduct of task difficulty but a consistent feature of the disorder, the study highlights the importance of looking beyond mean reaction times to understand the neurobiological inefficiencies—specifically in the pre-SMA and related circuits—that characterize ADHD.