Brain-imaging studies of people with attention-deficit/hyperactivity
disorder (ADHD) are moving beyond historic concentration on the frontal lobe
to examine activity in other regions of the brain as well as considering the
medication status of the subjects.
Originally, brain researchers studying ADHD looked at that area because
people with ADHD exhibited behavior like those with frontal-lobe lesions.
Later, they began to look at the frontal subcortical regions and related
areas, and now research has expanded its scope further still.
For instance, when subjects were given tests of attention, functional MRI
images of persons with ADHD showed less activation than controls in the
posterior parietal attention system, noted Leanne Tamm, Ph.D., Vinod Menon,
Ph.D., and Allan Reiss, M.D., of the Department of Psychiatry and Behavioral
Sciences at the Stanford University School of Medicine, in the June
American Journal of Psychiatry.
The researchers administered a visual oddball task, a test of directed
attention, to 14 right-handed adolescent boys. The oddball task requires
pressing one button when the subject sees a circle (the standard stimulus,
presented more frequently) on the test screen or a second button when a
triangle (the "oddball" stimulus, presented less often) appears.
Twelve healthy boys were tested as controls. The two groups were closely
matched in cognitive and academic functioning. Reaction times and errors
committed were recorded.
The ADHD group pressed the wrong button more often than the control group,
but reaction times did not differ. However, the ADHD subjects showed less
activation in the bilateral association cortex, the right precuneus, and the
thalamus. Pressing the wrong button—an error of commission—
indicates impulsivity, while the differentially activated brain regions reveal
their difficulty in shifting attention and making alternative responses.
"These findings suggest a critical role for these brain regions in
accurate target detection and task performance, which may underlie known
deficits in directed attention in individuals with ADHD," wrote Tamm and
her colleagues. "[I]t is important to reconsider the notion of ADHD as
primarily a disorder of the frontal-striatal function and consider the role of
the parietal attention system in the behavioral phenotype of ADHD."
Behavioral inhibition has generally been the focus of ADHD brain research,
and examining other areas of the brain and other behaviors, while not new,
still needs to be encouraged, said Eve Valera, Ph.D., an instructor in the
Department of Psychiatry at Harvard Medical School. "If you don't look
at other regions, you won't find anything there," said Valera, who is
now studying the cerebellum in adults with ADHD.
Two other recent studies explored differences in several areas of brain
activation in two groups of children with ADHD: those who took stimulant
medications and those who didn't, since medication status has been suggested
as a confounding factor. The first study found that underactivation in
children and adolescents never treated with stimulants appeared not only in
the prefrontal cortex but also in the parietal and temporal cortices.
"Despite similar task performance, medication-naïve boys with
ADHD showed hypoactivation in task-specific brain regions compared with
healthy subjects in two tasks: reduced activation in rostral mesial prefrontal
cortex during the go/no go task and in right hemispheric inferior prefrontal,
temporal, and parietal brain regions during the switch task," wrote Anna
Smith, Ph.D., of the Department of Psychiatry at the Institute of Psychiatry
at King's College, London, and colleagues.
Another study found that, unlike healthy controls, subjects with ADHD did
not activate the anterior cingulate cortex during unsuccessful inhibitions.
The difference occurred in treatment-naïve subjects, indicating an effect
not caused by past stimulant treatment. That may mean, wrote Steven Pliszka,
M.D., of the Department of Psychiatry at the University of Texas Health
Science Center in San Antonio, and colleagues, "that when ADHD subjects
make an error or face conflict, the anterior cingulate cortex fails to
adequately adjust cognitive control for the demands of the task." They
also found an increase in left ventrolateral prefrontal cortex activity on
unsuccessful inhibitions, unlike ADHD subjects.
Knowing that some imaging results are not artifacts of prior treatment is a
help to researchers, because, said Valera, it is hard to find child research
subjects who have ADHD and are not medicated.
Besides the need to replicate current studies, Valera sees three major
opportunities for imaging research in ADHD. For one thing, future studies
should include more girls, she said. "Data for girls seem congruent, but
we don't know for sure."
Also, imaging the same subjects over time would improve understanding of
developmental changes in children and might reveal whether there were
cognitive abnormalities at some times but not at others. Finally, researchers
need to look for abnormalities in the connections among brain regions by
studying networks of activation, she said.
"Parietal Attentional System Aberrations During Target
Detection in Adolescents With Attention Deficit Hyperactivity Disorder:
Event-Related fMRI Evidence" is posted online at<http://www.ajp.psychiatryonline.org/cgi/content/full/163/6/1033>.▪