Working memory and information processing in ADHD: Evidence for directionality of effects

Objective: Children with ADHD demonstrate impaired performance on a wide range of neuropsychological tests. It is unclear, however, whether ADHD is associated with many neurocognitive deficits or whether a small number of impairment(s) broadly influence test performance. The current study tests competing model predictions regarding two candidate causal mechanisms in ADHD: information processing speed and working memory. Method: A well-characterized sample of 86 children (M_age = 10.52, SD_age = 1.54; 34 girls; 64% Caucasian/Non-Hispanic) with ADHD (n = 45) and without ADHD (n = 41) completed eight fully crossed experimental tasks that systematically manipulated working memory (BF₁₀ = 1.80 × 10⁹³) and information processing speed (drift rate; BF₁₀ = 7.61 × 10⁶). Results: Bayesian mixed-model ANOVAs indicated that increasing working memory demands produced significant reductions in information processing speed (drift rate; BF₁₀ = 5.82 × 10⁹⁶). In contrast, experimentally reducing children's information processing speed did not significantly change their working memory performance (BF₁₀ = 1.31). ADHD status interacted with the working memory manipulation, such that the ADHD and non-ADHD groups showed equivalently high accuracy under the encoding-only conditions (BF = 3.45) but differed significantly under high working memory conditions (encoding + recall; BF = 19.58). Importantly, however, ADHD status failed to interact with (a) the working memory manipulation to differentially affect information processing speed and (b) the information processing speed manipulation to differentially affect working memory performance (all BF > 4.25). Conclusions: These findings indicate that top-down executive control exerts significant effects on children's ability to quickly process information, but that working memory deficits and slowed information processing speed appear to be relatively independent impairments in ADHD.