Research title: Competing core processes in attention-deficit/hyperactivity disorder (ADHD): Do working memory deficiencies underlie behavioral inhibition deficits?
Researchers: Matt Alderson, Mark D. Rapport, Kristen L. Hudec, Dustin E. Sarver, Michael J. Kofler
Published: Journal of Abnormal Child Psychology
The objective of this article was to test two competing theories of ADHD – the behavioral inhibition model described by Barkley (1997) and the functional working memory model developed by Rapport (2008).
Behavioral inhibition models of ADHD argue that a deficit in behavioral inhibition (BI), i.e, the ability to stop or withhold an ongoing response, is the fundamental deficit in ADHD and undermines the development of other important executive functions including working memory (WM). In other words, the well-documented WM deficits found in individuals with ADHD are secondary to a more fundamental problem with behavioral inhibition.
In contrast, the functional WM model of ADHD regards WM deficits as the central core component of ADHD and sees behavioral inhibition as a byproduct of WM processes rather a cause of deficits in WM.
To test these competing theories, 27 8 to 12 year old boys (14 with ADHD and 13 matched controls) completed different computer tasks that assessed both WM and behavioral inhibition. In the WM tasks, boys were required to correctly recall either a sequence of numbers/letters in reverse order (verbal WM) or the locations of a series of dots that were flashed on the screen (visuo-spatial WM). Different numbers of number/letters and dots were used on different trials so that children’s maximum WM capacity in each domain could be measured.
The Stop-Signal task was used to assess BI. In this task, participants are instructed to respond as quickly as they can when particular stimuli are flashed on a computer monitor, unless a ‘stop’ signal is delivered thru headphones immediately following the stimuli. By varying the time between when the ‘Go’ stimuli are presented and the ‘Stop’ signal comes on, the difficulty of inhibiting one’s response can be manipulated, i.e., the longer the delay the more difficult it becomes to inhibit or interrupt the ‘Go’ response. (Note – The intervals are very quick, on the order of 250 milliseconds.)
The analyses used to evaluate the competing theories are complex but were designed to directly test whether WM or BI represents a more fundamental deficit in ADHD. Findings clearly supported the central role of WM and the authors conclude their paper by stating “We believe that these findings support the functional working memory model of ADHD, and raise important concerns regarding the central role of behavioral inhibition in extant models of the disorder. In addition, these findings have potentially important clinical implications, and may help explain the inefficacy of cognitive therapies that target symptoms related to impulsivity/behavioral inhibition deficits”. They also argue that their findings support the value of developing interventions that directly target the training of WM.