In the largest imaging study of the human brain ever conducted—involving 1,896 14-year-olds—scientists have discovered a number of previously unknown networks that go a long way toward answering why some teenagers experiment with smoking or drugs and others don’t.
Robert Whelan, PhD, and Hugh Garavan, PhD, of the University of Vermont, along with a group of international colleagues, report that differences in these networks provide strong evidence that some teenagers are at higher risk for drug and alcohol experimentation—simply because their brains work differently, making them more impulsive. Their findings are published in the April 29 Nature Neuroscience.
This discovery helps answer a long-standing chicken-or-egg question about whether certain brain patterns come before drug use or are caused by it.
“The differences in these networks seem to precede drug use,” says Garavan, principal investigator of the Irish component of a large European research project, called IMAGEN, that gathered the data about 2,000 teens in the new study.
In a key finding, diminished activity in a network involving the orbitofrontal cortex is associated with experimentation with alcohol, cigarettes, and illegal drugs in early adolescence.
“These networks are not working as well for some kids as for others,” says Whelan, making them more impulsive. Faced with a choice about smoking or drinking, the 14-year-old with a less functional impulse-regulating network will be more likely to say, “yeah, gimme, gimme, gimme!” says Garavan, “and this other kid is saying, ‘no, I’m not going to do that.’”
Testing for lower function in this and other brain networks could, perhaps, be used by researchers someday as “a risk factor or biomarker for potential drug use,” Garavan says.
The researchers were also able to show that other newly discovered networks are connected with the symptoms of attention-deficit/hyperactivity disorder (ADHD). These ADHD networks are distinct from those associated with early drug use.
In recent years, there has been controversy and extensive media attention about the possible connection between ADHD and drug abuse. Both ADHD and early drug use are associated with poor inhibitory control. But the new research shows that these seemingly related problems are regulated by different networks in the brain—even though both groups of teens can score poorly on tests of their “stop-signal reaction time,” a standard measure of overall inhibitory control used in this study and other similar ones. This strengthens the idea that risk of ADHD is not necessarily a full-blown risk for drug use as some recent studies suggest.
The impulsivity networks—connected areas of activity in the brain revealed by increased blood flow—begin to paint a more nuanced portrait of the neurobiology underlying the patchwork of attributes and behaviors that psychologists call impulsivity as well as the capacity to put brakes on these impulses, a set of skills sometimes called inhibitory control.
Using a complex mathematical approach called factor analysis, Whelan and colleagues were able to fish out seven networks involved when impulses were successfully inhibited and six networks involved when inhibition failed—from the vast and chaotic actions of a teenage brain at work. These networks “light up,” Whelan says, in a functional MRI scanner during trials when the teenagers were asked to perform a repetitive task that involved pushing a button on a keyboard, but then were able to successfully stop—or inhibit—the act of pushing the button in mid-action. Those teens with better inhibitory control were able to succeed at this task faster.
But the underlying networks behind these tasks could not have been detectable in a “typical fMRI study of about 16 or 20 people,” says Whelan. “This study was orders of magnitude bigger, which lets us overcome much of the randomness and noise—and find the brain regions that actually vary together.”
“The take-home message is that impulsivity can be decomposed, broken down into different brain regions,” says Garavan, “and the functioning of one region is related to ADHD symptoms, while the functioning of other regions is related to drug use.
— Source: University of Vermont