In the first study, researchers measured mirror hand movements in 25 boys and girls aged 8-13 with ADHD and 25 children without the disorder. All of the children were right-handed. Each child was asked to successively tap each finger of one hand to the thumb. Any unintentional “mirror” movements in the opposite hand were recorded. Researchers found that children with ADHD showed twice as many mirrored hand movements as other children during the left-handed finger-tapping test. This difference was especially telling in boys with ADHD, who showed nearly four times as many mirrored hand movements as boys without the condition.
“The findings reveal that even at an unconscious level, these children are struggling with controlling and inhibiting unwanted actions and behavior," researcher Stewart Mostofsky, MD, of the Center for Autism and Related Disorders at the Kennedy Krieger Institute in Baltimore, says in a news release.
In the second study, researchers used transcranial magnetic stimulation (TMS) to activate brain cells with magnetic pulses and measure brain activity in 49 children with ADHD and 49 children without ADHD aged 8 to 12.
The researchers found that children with ADHD had problems with a brain-related inhibition of activity involved in motor function control of the hand. The degree of inhibition was 40% less in children with ADHD compared to children without the disorder. Children with ADHD also scored nearly 60% worse on motor development tests.
Researchers also found the degree of inhibition in the brain's motor control in the study was strongly related to the severity of ADHD symptoms reported by the participants’ parents.
“These studies are an important step toward understanding how ADHD affects communication between the brain and other parts of the body,” Jonathan W. Mink, MD, PhD, with the University of Rochester Medical Center in New York, writes in an editorial that accompanies the studies in Neurology. "The hope is that, ultimately, these studies and others will guide us toward development and testing of new therapies.”