Intra-individual reaction time variability based on ex-Gaussian distribution as a potential endophenotype for attention-deficit/hyperactivity disorder

Objective Intra‐individual variability in reaction time (IIV‐RT), defined by standard deviation of RT (RTSD), is considered as an endophenotype for attention‐deficit/hyperactivity disorder (ADHD). Ex‐Gaussian distributions of RT, rather than RTSD, could better characterize moment‐to‐moment fluctuations in neuropsychological performance. However, data of response variability based on ex‐Gaussian parameters as an endophenotypic candidate for ADHD are lacking. Method We assessed 411 adolescents with clinically diagnosed ADHD based on the DSM‐IV‐TR criteria as probands, 138 unaffected siblings, and 138 healthy controls. The output parameters, mu, sigma, and tau, of an ex‐Gaussian RT distribution were derived from the Conners' continuous performance test. Multi‐level models controlling for sex, age, comorbidity, and use of methylphenidate were applied. Results Compared with unaffected siblings and controls, ADHD probands had elevated sigma value, omissions, commissions, and mean RT. Unaffected siblings formed an intermediate group in‐between probands and controls in terms of tau value and RTSD. There was no between‐group difference in mu value. Conforming to a context‐dependent nature, unaffected siblings still had an intermediate tau value in‐between probands and controls across different interstimulus intervals. Conclusion Our findings suggest IIV‐RT represented by tau may be a potential endophenotype for inquiry into genetic underpinnings of ADHD in the context of heterogeneity.