The core cognitive deficit of ADD is in working memory.

ADD appears to be an instance of childhood-onset “dysexecutive syndrome.” ADD and ADHD are characterized by dissociable cognitive and behavioral profiles, different patterns of comorbidities, different responses to medication, and different underlying neurobiological problems. The points I make in this paper include the following: many individuals currently diagnosed with the inattentive subtype of ADHD appear to be misdiagnosed. The term ADHD should be reserved for when hyperactivity is present (as the term implies), regardless of whether inattention is also present. Not only is “ADHD without hyperactivity” (ADHD of the predominantly inattentive type) an awkward locution, but it also tries to squeeze ADD into a box in which it does not belong. I join the growing chorus of those who argue that attention-deficit/hyperactivity disorder (ADHD) of the “truly” inattentive subtype (what I will call “attention-deficit disorder” ) is a different disorder from ADHD where hyperactivity is present (e.g., Barkley, 2001 Cantwell, 1983 Carlson, 1986 Carlson & Mann, 2000 Goodyear & Hynd, 1992 Hynd, Lorys, Semrud–Clikeman, Nieves, Huettner, & Lahey, 1991 in particular see the outstanding paper by Milich, Balentine, & Lynam, 2001). 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. It is proposed here that the primary disturbance in truly inattentive-type ADHD (ADD) is in the cortex (a frontal–parietal loop). Much converging evidence points to a primary disturbance in the striatum (a frontal–striatal loop) in the combined type of ADHD. Children with the truly inattentive type of ADHD, rather than being distractible, may instead be easily bored, their problem being more in motivation (under-arousal) than in inhibitory control. It is further proposed that laboratory measures, such as complex-span and dual-task dichotic listening tasks, can detect this. 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. Most studies of attention-deficit/hyperactivity disorder (ADHD) have focused on the combined type and emphasized a core problem in response inhibition.

Add vs adhd medication