Investigating objective markers of ADHD across development

Recent research has demonstrated the importance of endophenotypes, intermediate constructs between genotype and phenotype which index the risk for developing a particular disorder, in better defining developmental pathways to ADHD. This study involved an investigation of two promising candidate ADHD endophenotypes, intrasubject variability in reaction time (ISV RT) and infrared measures of micromovements, examining the potential of these endophenotypes in differentiating between ADHD and typically developing (TD) populations. All participants completed a diagnostic and cognitive assessment followed by an experimental task, the McLean Motion and Attention Test System (MMAT), in which data related to ISV RT and micromovements was collected. Results indicated significant differentiation between ADHD and TD samples according to a number of measures of micromovements, with small to medium effect sizes (Eta squared=.039-.059) in a child/adolescent sample as well as generally large effect sizes (Eta squared=.203-.270) in an adult sample. Building on previous work with child/adolescent populations, this study represents the first attempt to extend research on micromovements to adult populations, providing support for the utility of this endophenotype across the lifespan. Results related to ISV RT were inconsistent, as measures contributing to significant differentiation between ADHD and TD populations differed between child/adolescent and adult samples. Finally, correlations between movement variables and age illustrate that while indicators of hyperactive/impulsive movements decrease over time in both ADHD and TD samples, significant differences between ADHD and TD populations appear to be maintained throughout the lifespan. Implications for further research aimed at better defining the psychobiology of ADHD are discussed, noting the importance of the micromovement endophenotype in contributing to a broader understanding of the complexity of ADHD.