"The neurobiological underpinnings of motor delays and behavioral symptoms in ADHD are not well understood,” says Gilbert, who is also an associate professor of pediatric neurology at the University of Cincinnati College of Medicine.
"Our research provides more insight into the physiological measures of this disorder. We found an important biomarker for predicting ADHD symptoms and severity that offers a foundation for determining which children are at higher risk for severe and ongoing symptoms as they grow older.”
In a study led by Mostofsky, children with ADHD performed a finger-tapping task. Any unintentional "overflow” movements occurring on the opposite hand were noted. Children with ADHD showed more than twice the amount of overflow than typically developing children.
This is the first time that scientists have been able to quantify the degree to which ADHD is associated with a failure in motor control. In the second study, led by Gilbert, researchers further investigated motor control in children with ADHD further by measuring activity within the motor cortex, the part of the brain that controls voluntary movement.
Researchers used Transcranial Magnetic Stimulation (TMS) to apply mild magnetic pulses for brief durations to trigger muscle activity in the hand, causing hand twitches. Using TMS allowed researchers to measure the level of muscle activity and monitor the resulting brain activity, called short interval cortical inhibition (SICI), an important braking mechanism in the brain.
Overall, children with ADHD showed a substantial decrease in SICI, with significantly less inhibition of motor activity during the paired pulse stimulation compared to typically developing children.
The degree of inhibition in children with ADHD, measured by SICI, was 40 percent less than typically developing children. Furthermore, within the ADHD group, less motor inhibition (decreased SICI) correlated with more severe symptoms. The measure of SICI not only predicted motor impairment in ADHD children but also robustly predicted their behavioral symptoms as reported by parents. The findings suggest that reduced SICI may be a critical biomarker of ADHD.