Confirmation of an Inhibitory Control Deficit in Attention-Deficit/Hyperactivity Disorder

Journal of Abnormal Child Psychology,  June, 2000  by Russell Schachar,  Victor L. Mota,  Gordon D. Logan,  Rosemary Tannock,  Paula Klim

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Russell Schachar [1,3]

Victor L. Mota [1]

Gordon D. Logan [2]

Rosemary Tannock [1]

Paula Klim [1]

The objective of this study was to determine whether deficient inhibitory control distinguishes children with a diagnosis of attention-deficit/hyperactivity (ADHD) disorder, conduct disorder (CD), and comorbid ADHD + CD from normally developing children. Participants were rigorously diagnosed children (age 7 to 12 years) with ADHD (N = 72), CD (N = 13) or ADHD + CD (N = 47) and 33 control children (NC). We studied inhibitory control using the stop-signal paradigm, a laboratory task that assessed the ability to inhibit an ongoing action. The ADHD group had significantly impaired inhibitory control compared to NC, CD, and ADHD + CD children. These results indicate that children with ADHD have deficient inhibition as measured in the stop-signal paradigm and that ADHD occurring in the presence of ADHD + CD may represent a phenocopy of CD rather than a variant of ADHD.

KEY WORDS: Attention deficit hyperactivity disorder; conduct disorder; comorbidity; inhibition.

Attention-deficit/hyperactivity disorder (ADHD) is a common and impairing psychiatric disorder of childhood that affects approximately 3-5% of school age children in the general population (American Psychiatric Association, 1994; Szatmari, Offord, & Boyle, 1989) and 50% of children who are referred for clinic assessment (Offord et al., 1987). ADHD is defined by developmentally inappropriate and impairing levels of inattentiveness, impulsiveness, and hyperactivity commencing in early childhood.

In the early 1970s, Douglas (1972) proposed that attention deficit, rather than excessive activity, was the core abnormality of the disorder. However, deficits in basic or subordinate processes of attention such as memory and encoding have been difficult to identify (e.g., Douglas, 1988; Sergeant & Van der Meere, 1990). Instead, the focus of research and theory has shifted to a deficit in the executive processes that control subordinate cognitive processes, enabling them and directing them, turning them on and off (Barkley, 1997a,b; Logan, 1985; Logan, Schachar, & Tannock, 1997; Meyer & Keiras, 1997). Executive processes are involved in the management of the constant stream of sensory information competing for access to the processes controlling action and in decisions about the appropriateness and timing of action (Denckla, 1996). The effects of deficiencies in executive control processes may be subtle causing adjustment in the parameters of subordinate process; or they may be dramatic, causing cascading ef fects on subordinate and other executive control process throughout the system.

One particular executive process, inhibition, has been implicated as a potential locus of a core deficit in ADHD (Barkley, 1997a; Pennington & Ozonoff, 1996; Quay, 1997). Inhibition comes into play in situations requiring withholding or sudden interruption of an ongoing action or thought or in the suppression of information that one wishes to ignore. According to this theory, deficient inhibitory control impairs the ability of ADHD children to engage other executive-control strategies to optimize their behavior. The direct and cascaded effects of deficient inhibition affects working memory, self-regulation, internal speech, and "reconstitution" (i.e., the ability to reconstruct behavior). A deficit in inhibitory control means that individuals with ADHD act without thinking and therefore miss out on the benefits of these carefully considered control strategies.

Inhibitory control has been studied extensively using the stop-signal paradigm (Logan, 1994), which is a laboratory analog of a situation that requires rapid and accurate execution of a simple motor action and occasional and unpredictable, cessation of this action. The paradigm involves two concurrent tasks, a go task and a stop task. Typically, the go task involves a choice among stimulus and response alternatives (e.g., discriminating an X from an O). The object of the go task is to respond as quickly and accurately as possible. The stop signal, occurs unpredictably on occasion (typically on 25% of go-task trials), and involves presentation of a signal (typically a tone) that tells participants to completely stop their response to the go task on that trial.

Whether children are able to inhibit on a particular trial depends on the outcome of a race between the go and the stop processes: If the stop task response finishes before the go task response, children will inhibit their response to the go task (Jennings, van der Molen, Brock, & Somsen, 1992; Logan, 1985; Logan, 1994; Logan & Cowan, 1984; Logan, Cowan, & Davis, 1984; Ollman, 1973; Osman, Korablum, & Meyer, 1986). If the go task response finishes before the stop task response, children will fail to inhibit their response to the go task, responding much as they would if no stop signal had been presented. Thus, inhibitory control depends on the latency of two independent processes; the response to the go signal (go reaction time) and the response to the stop signal (stop-signal reaction time, SSRT). Poor inhibition could result from responding too quickly to the go signal or responding too slowly to the stop signal. The outcome of the race between the go and stop processes depends as well on the interval betw een the onset of the go signal and the onset of the stop signal (stop-signal delay). Short delay between the go and stop signals increases the probability of inhibiting and long delay increases the probability of responding.