Reaction Time Variability in Children With ADHD Symptoms and/or Dyslexia

This study investigated the nature of reaction time (RT) variability in children with ADHD symptoms and/or dyslexia. While previous research has established that children with ADHD often show slower and more variable responses, it remains unclear whether this variability stems from a generalized slowing of processing, frequent attentional lapses, or deficits in response preparation. To address this, the authors used an ex-Gaussian modeling approach to analyze RT distributions from a Stop Signal task. This method decomposes RT data into three parameters: mu (the mean of the Gaussian component), sigma (the standard deviation of the Gaussian component), and tau (the exponential component representing the tail of slow responses).

The researchers found that children with ADHD symptoms displayed significantly higher values of sigma compared to typically developing (TD) controls. This indicates increased variability throughout the Gaussian portion of their RT distribution. Interestingly, there was no significant difference in mu or tau between the groups, suggesting that the variability was not driven by a generalized slowing of responses or an excess of abnormally slow responses (attentional lapses). Furthermore, children with dyslexia performed similarly to TD controls across all parameters, providing evidence against the hypothesis that shared processing speed deficits underlie the comorbidity of ADHD and dyslexia.

The results suggest that the intra-individual variability characteristic of ADHD is not a monolithic phenomenon. Instead, the specific type of variability observed—in this case, increased sigma—points toward deficits in response preparation or motor timing rather than simple fluctuations in effort or attention. By distinguishing between these components, the study clarifies the cognitive mechanisms involved in ADHD and suggests that task demands, such as the need for precise motor timing in Stop Signal tasks, play a critical role in how ADHD-related deficits manifest.