Attention-deficit disorder (attention-deficit/ hyperactivity disorder without hyperactivity): a neurobiologically and behaviorally distinct disorder from attention-deficit/hyperactivity disorder (with hyperactivity)

Abstract

Most studies of attention-deficit/hyperactivity disorder (ADHD) have focused on the combined type and emphasized a core problem in response inhibition. It is proposed here that the core problem in the truly inattentive type of ADHD (not simply the subthreshold combined type) is in working memory. It is further proposed that laboratory measures, such as complex-span and dual-task dichotic listening tasks, can detect this. Children with the truly inattentive type of ADHD, rather than being distractible, may instead be easily bored, their problem being more in motivation (underarousal) than in inhibitory control. Much converging evidence points to a primary disturbance in the striatum (a frontal-striatal loop) in the combined type of ADHD. It is proposed here that the primary disturbance in truly inattentive-type ADHD (ADD) is in the cortex (a frontal-parietal loop). Finally, it is posited that these are not two different types of ADHD, but two different disorders with different cognitive and behavioral profiles, different patterns of comorbidities, different responses to medication, and different underlying neurobiologies.

Figure 1

The scores for 4.5- to 15-year-old children on the number of items that can be held in mind (span) on the counting-span and spatial-span tasks. The data for the counting-span task are adapted from Crammond (1992) and those from the spatal-span task are adapted from Menna (1989). From “Normal development of prefrontal cortex from birth to young adulthood: Cognitive functions, anatomy, and biochemistry,” by A. Diamond. In Principles of Frontal Lobe Function, by D. T. Stuss and R. T. Knight (Eds.), 2002, London: Oxford University Press. Copyright 2002 by Oxford University Press. Reprinted with permission.

Figure 2

The scores for 8- to 15-year-old children with and without ADHD on the number of items that can be held in mind (span) on a visuospatial-span task. The number of participants in the study was 80. The linear regression lines are for children with ADHD (darker line) and the control group (lighter line). WM, working memory. From “Visuospatial Working Memory Span: A Sensitive Measure of Cognitive Deficits in Children With ADHD,” by H. Westerberg, T. Hirvikoski, H. Forssberg, and T. Klingberg, 2004, Child Neuropsychology, 10. Copyright 2004 by Psychology Press (www.psypress.co.uk/journals.asp). Reprinted with permission.