Decomposing attention-deficit/hyperactivity disorder (ADHD)-related effects in response speed and variability

Objective: Slow and variable reaction times (RTs) on fast tasks are such a prominent feature of attention-deficit/hyperactivity disorder (ADHD) that any theory must account for them. However, this has proven difficult because the cognitive mechanisms responsible for this effect remain unexplained. Although speed and variability are typically correlated, it is unclear whether single or multiple mechanisms are responsible for group differences in each. RTs are a result of several semi-independent processes, including stimulus encoding, rate of information processing, speed-accuracy trade-offs, and motor response, which have not been previously well characterized. Method: A diffusion model was applied to RTs from a forced-choice RT paradigm in two large, independent case-control samples (N_{Cohort 1} = 214 and N_{Cohort 2} = 172). The decomposition measured three validated parameters that account for the full RT distribution and assessed reproducibility of ADHD effects. Results: In both samples, group differences in traditional RT variables were explained by slow information processing speed, and unrelated to speed-accuracy trade-offs or nondecisional processes (e.g., encoding, motor response). Conclusions: RT speed and variability in ADHD may be explained by a single information processing parameter, potentially simplifying explanations that assume different mechanisms are required to account for group differences in the mean and variability of RTs.