Research title: A meta-analysis of working memory impairments in children with attention-deficit/hyperactivity disorder.
Researchers: Rhonda Martinussen, Jill Hayden, Sheila Hogg-Johnson, Rosemary Tannock
The objective of this study was to conduct a meta-analysis of research on working memory (WM) impairments in children with ADHD. In a meta-analysis, researchers combine results from multiple studies to provide an overall assessment of findings in a given research area. By pooling results across studies, findings specific to individual studies are minimized so that a broader, and often more accurate, understanding can be obtained.
This meta-analysis was based on 26 studies comparing WM performance in children with ADHD to WM performance in typically developing children. Most studies focused on children younger than 13 and the majority matched children on age and gender. In all but 2 studies, children on medication had discontinued it for at least 24 hours prior to testing.
Separate meta-analyses were conducted for 4 components of WM: 1) Verbal storage; 2) Verbal Central Executive; 3) Visuo-spatial Storage; and, 4) Visuo-spatial Central Executive. These components come from the prominent theory of WM developed by cognitive psychologist Aaron Baddeley. The verbal and visuo-spatial storage components are responsible for the short-term storage and rehearsal of visual and visuo-spatial information respectively. The Central Executive component(CE) is thought to be an attentional controller responsible for oversight and coordination of the 2 storage systems. Functions of the CE are believed to include focusing attention, dividing attention among concurrent tasks, and providing an interface between working memory and long-term memory.
Meta-analyses were conducted computing separately for each WM component; the number of available studies varied by component, ranging from 7 for Spatial CE to 16 for Verbal Storage. Results indicated that WM in children with ADHD was impaired relative to comparison children on all 4 components. Differences in the verbal domain were moderate in magnitude while those in the visuo-spatial domain were large. Group differences in WM were larger in those studies that controlled for reading disability or language impairments.
The authors conclude by noting that WM deficits compromise children’s academic achievement and demonstrating that “…children with ADHD exhibit moderate to large deficits in multiple components of WM has important implications for clinicians.” Specifically, they suggest that academic problems in children with ADHD may result from WM deficiencies rather than being a direct consequence of inattention and/or hyperactivity-impulsivity as is commonly thought. They also suggest that specific academic accommodations tailored to address problems related to WM deficiencies may be important for reducing academic impairments in children with ADHD.