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The largest imaging study of the human brain ever revealed that some teenagers are at higher risk for drug and alcohol experimentation — simply because their brains work differently, making them more impulsive.

This finding of the study, which was conducted by Robert Whelan and Hugh Garavan of the University of Vermont, along with a large group of international colleagues, involving 1,896 14-year-olds, 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, who served as the principal investigator of the Irish component of a large European research project, called IMAGEN, that gathered the data about the teens in the new study.

In a key finding, diminished activity in a network involving the "orbitofrontal cortex" is linked 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.

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 ADHD (attention-deficit hyperactivity disorder). These ADHD networks are distinct from those associated with early drug use.

Both ADHD and early drug use are associated with poor inhibitory control — they're problems that plague impulsive people.

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.

The researchers 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.

"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.

Death among teenagers in the industrialized world is largely caused by preventable or self-inflicted accidents that are often launched by impulsive risky behaviors, often associated with alcohol and drug use. 

Additionally, "Addiction in the western world is our number one health problem," says Garavan. "Think about alcohol, cigarettes or harder drugs and all the consequences that has in society for people's health." Understanding brain networks that put some teenagers at higher risk for starting to use them could have large implications for public health.

The findings of the study was published Sunday in the journal Nature Neuroscience.