NEW ORLEANS -- The first study to evaluate working memory in persons with Attention Deficit Hyperactivity Disorder (ADHD) using positron emission tomography (PET) was reported by Emory University researchers at this week's Society for Neuroscience meeting.
Brain scans taken of men while they performed an effortful auditory arithmetic task showed precisely what one testing the working memory theory of basic brain organization would expect: efficient use of the prefrontal cortex in normal control subjects and much less use of that brain region in subjects with ADHD, reports Julie B. Schweitzer, Ph.D., assistant professor of Psychiatry and Behavioral Sciences at Emory University School of Medicine. Task-related neural activity was more diffuse among men with ADHD and appeared in the PET images to be most concentrated in the brain's occipital regions, areas associated with visual processing.
"Not surprisingly, the ADHD patients scored lower on the numeric task," Dr. Schweitzer says. "In very general terms, pinpointing those brain regions most affected -- or unaffected -- during the learning process shows us that persons with ADHD tend to integrate information in more visual and less auditory ways. The details of these types of findings will be of value to researchers designing new drugs targeted to affected brain regions and to educators designing teaching tools for children and adults with ADHD."
Working memory is a fairly new term that replaces and expands upon the concept of short-term memory. Researchers have theorized that working memory serves not only as temporary storage for new information, but also the active manipulation of this information. Part of the active process may involve inhibiting -- or forgetting -- certain knowledge.
Disruption of working memory may explain many of the everyday difficulties individuals with ADHD encounter, from coping with laborious reading to never remembering to complete one of multiple unfinished tasks.
The serial addition task performed by the 12 Emory study participants (six unmedicated men with ADHD, six men without ADHD) required that they integrate then inhibit new data. While listening to a tape-recorded voice recite a series of numbers, they were asked to add the second number spoken to the first, then the third number to the second without including (thus inhibiting or forgetting) the total of the first summation. For example, the correct response to the series 5 - 1 - 2 - 3 would be 6 - 3 - 5.
"ADHD has been recently conceptualized as a disruption of behavioral inhibition and its four dependent executive functions: working memory, self-regulation, internalization of speech and reconstitution (Barkley, 1997)," the researchers say in their abstract.
"Results from a (the current) PET activation study suggest that use of two subsidiary processes of working memory may be disrupted in ADHD: phonological loop and visuo-spatial sketchpad processes... Group differences may reflect a disruption in allocation of attention and use of a central executive system by subjects with ADHD, which in turn alters the use and efficiency of the subsidiary processes of working memory."
The Emory group is conducting further PET studies comparing working memory-related brain activation in subjects with ADHD prior to and after taking Ritalin. Ritalin enhances working memory, though researchers do not know how. The Emory team and others are seeking to better understand this process with functional brain scans.