Using the Diffusion Model to Explain Cognitive Deficits in Attention Deficit Hyperactivity Disorder

Abstract

Slow, variable, and error-prone performance on speeded reaction time (RT) tasks has been well documented in childhood ADHD, but equally well documented is the context-dependent nature of those deficits, particularly with respect to event rate. As event rates increase (or, as the interstimulus intervals become shorter), RTs decrease, a pattern of performance that has long been interpreted as evidence that cognitive deficits in ADHD are a downstream consequence of a fundamental difficulty in the regulation of arousal to meet task demands. We test the extent to which this is a misinterpretation of the data that occurs when RT and accuracy are considered separately, as is common in neurocognitive research. In two samples of children aged 8-10 with (N = 97; 33 girls) and without (N = 39; 26 girls) ADHD, we used the diffusion model, an influential computational model of RT, to examine the effect of event rate on inhibitory control in a go-no-go task. Contrary to longstanding belief, we found that fast event rates slowed the rate at which children with ADHD accumulated evidence to make a decision to "no-go", as indexed by drift rate. This in turn resulted in a higher proportion of failed inhibits, and occurred despite increased task engagement, as reflected by changes in the starting point of the decision process. Thus, although faster event rates increased task engagement among children with ADHD, the increased engagement was unable to counteract the concurrent slowing of processing speed to "no-go" decisions. Implications for theoretical models of ADHD and treatments are discussed.

Keywords: ADHD; Arousal; Diffusion model; Drift rate; Event rate; Inhibition.

Figure 1

Anatomy of a reaction time. After a stimulus is encoded, the decision process starts at z, and moves in the direction of one of two boundaries (is the stimulus a word or non-word? Should I go or not go?). Drift rate (v) is the rate at which information accumulates towards a decision. Once the decision process has reached one of the boundaries, the corresponding motor response is initiated. How far apart the boundaries are (i.e. the boundary separation, or a) is an indication of the conservativeness of the response criterion. Wider separations indicate more conservative responding (i.e. more information is needed before a decision can be reached). Non-decision time (Ter) represents the time it takes to complete all non-decision processes, such as stimulus encoding and motor preparation. Reaction time = Decision Time + Non-decision Time. Trial-to-trial variability in drift produces RT distributions.

Figure 2

Hypothetical causes of inhibitory deficits in ADHD. a. Bias in drift where drift to no-go is slower than drift to go. ADHD dashed lines. b. ADHD start point biased to “go”

Figure 3

DDM parameters. SE bars displayed.