Teens with attention-deficit/hyperactivity disorder (ADHD) and teens who start using cigarettes, drugs or alcohol tend to share at least one personality trait: impulsiveness, experts say.
But a new brain-imaging study of nearly 1,900 14-year-olds finds that the brain networks associated with impulsivity in teens with ADHD are different compared to those who use drugs or alcohol.
What that finding suggests is that multiple underlying mechanisms drive impulsivity -- in other words, the impulsivity that leads kids to blow off their homework and the impulsiveness that drives kids to take a drag off a joint aren't the same, neurologically speaking.
"The behavior of the two groups might look the same, but it's driven by different brain networks," said lead study author Robert Whelan, a postdoctoral research fellow at the University of Vermont.
Moreover, the findings, published in the April 29 online issue of Nature Neuroscience, could suggest that the brain is primed to push some teens -- but not others -- toward substance abuse.
ADHD is a neurobehavioral disorder marked by excessive levels of activity, inattention and impulsiveness beyond what's normal for a child's age.
People with ADHD are at higher risk of substance abuse and alcoholism. The explanation was thought to lie in the lack of self-control or inability to curb impulses that are part of the disorder, Whelan said.
But the brain-imaging study suggests that from a neurological standpoint, ADHD and substance use may not be nearly as closely tied as previously believed, Whelan said.
In the study, researchers used data from an ongoing study of European teens who underwent brain imagining tests every two years starting at age 14. The youths were asked about symptoms of ADHD and if they had tried alcohol, cigarettes or other drugs.
While having their brains scanned, the students took a test used to measure self-control, or inhibition: Participants were told to press a button when they saw a right or a left arrow flash on a screen, but not to press the button when the arrow pointed up.
The kids with ADHD and those who had tried various substances didn't perform any worse on the test of self-control than other kids. However, researchers did find distinct patterns of brain activity in ADHD and in kids who'd tried alcohol, cigarettes or drugs while taking the test.
Among the kids who had tried alcohol, cigarettes or other drugs (mostly marijuana), scans showed different patterns of brain activity in the right inferior frontal gyrus and in the orbital frontal cortex compared to teens who had abstained. Prior research has found the inferior frontal gyrus is involved with inhibition. For example, people with head injuries that damaged that area of the brain have problems with inhibition, Whelan said, while the orbital frontal cortex has been implicated in drug use.
Even teens who reported having only tried a drink or two by age 14 showed a different pattern of activity in the orbital frontal cortex, suggesting the brain differences aren't caused by the substances, but are already present and play a role in what drives certain teens to experiment with alcohol and others to abstain, Whelan said.
In the teens who had symptoms of ADHD, different networks lit up during the self-control test. Kids with ADHD symptoms showed differences in the bilateral frontal lobe and the basal ganglia.
"The fact that we found there were different networks lends credence to the argument that ADHD and substance abuse are not so tightly coupled," Whelan said.
Dr. Lukshmi Puttanniah, director of child and adolescent psychiatry at Lenox Hill Hospital in New York City, said the study strongly suggests that impulsiveness can have many underlying explanations.
"It's adding to a body of knowledge that the fundamental thing underlying both ADHD and substance use is difficulty controlling impulses," Puttanniah said. "Some people thought that whatever neurobiological pathway that underlies it is common between ADHD and substance abuse. But what this study shows is the neurobiological pathways underlying the impulsivity of ADHD and substance use disorders are actually distinct."
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