Move? Don’t move? Most children learn to control their actions with little difficulty, but children with attention-deficit/hyperactivity disorder (ADHD) often have trouble stopping “unnecessary” movements. Researcher Stewart Mostofsky and his colleagues at the Kennedy Krieger Institute in Baltimore have found that these children show different patterns of brain activity when inhibiting movement than do typically developing children.
“We’ve thought for quite some time that difficulty with response inhibition is associated with some of the core features of ADHD, including impulsivity, hyperactivity, and distractibility,” Mostofsky says. This research, published in the March 2008 issue of the Journal of Cognitive Neuroscience, supports that view, he says, and can point scientists toward a better understanding of the root causes of ADHD. [See also a related study.]
Mostofsky’s team studied 25 children with ADHD and 25 typically developing (TD) children between the ages of 8 and 13 years. The children were matched for IQ, and researchers used a battery of tests and behavioral measures to closely match children for the two groups. They excluded children with ADHD who had been diagnosed with other disorders of mood, conduct or anxiety. Twelve of the children with ADHD were taking stimulant medication to treat their condition; they stopped the drug 36 hours before the tests.
The children performed a simple “go/no-go” operation: They pressed a button when they saw a green spaceship. They did not press a button when they saw a red one. Most of the time, they saw the green signal, which established a tendency for an automatic, active response. When the occasional red signal appeared, the children had to switch rapidly from “moving” to “not-moving.”
The researchers used event-related fMRI (functional magnetic resonance imaging) to assess areas of heightened activity in the children’s brains while they acted or inhibited the action. Both sets of children performed equally well at the task, and the brain scans showed no significant differences between them on the “go” task.
Differences were dramatic, however, on the “no-go” task. Children with ADHD had less activation in a number of brain regions, especially in the pre-supplementary motor area (pre-SMA), a region of the frontal lobe known to be important for switching from one kind of motor response to another.
Mostofsky says that the pre-SMA normally guides movement that requires little thought—such as throwing a ball rather than aiming a ball. Typically developing children can rely on the pre-SMA to guide performance on the go-no go task, he theorizes, but children with ADHD may have to recruit additional brain circuits to stop themselves from acting.
“I don’t want anybody to misconstrue this to mean that the pre-SMA is the region of abnormality in ADHD,” Mostofsky says. He thinks that ADHD affects multiple brain circuits, but under these simple test conditions—where demands on memory and behavior are minimized—only abnormalities in premotor systems show up.
“This kind of work is important,” says F. Xavier Castellanos, director of Research at the New York University Child Study Center, “because it helps pinpoint the specific brain circuits (such as the pre-SMA) that may produce the symptoms of ADHD.”