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

Abstract

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 (NCohort 1 = 214 and NCohort 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.

Figure 1

(adapted from Ratcliff & McKoon, 2008). Diffusion model parameters are depicted for a hypothetical single trial. Drift rate (v) is the rate at which information accumulates towards a decision boundary, as reflected by the average slope of the line. It is determined by speed of information processing and “noise” unrelated to the decision processes (which is represented by the hypothetical jagged deviations from the average slope shown in the Figure). Larger values of v indicate faster processing. Boundary separation (a) indicates the conservativeness of the response criterion with wider separations indicating more conservative responding. Finally, non-decision time (Ter) includes all non-decision processes, such as stimulus encoding and motor preparation. Larger values of Ter indicate longer non-decisional processing times.

Figure 2

(adapted from Ratcliff & McKoon, 2008). Hypothetical RT distributions for children with and without ADHD. If the drift rate for children with ADHD is “x” degrees slower than controls, the slowdown results in large changes in the tail of the distribution (Z), but only small changes at the leading edge (Y).