By Dr. Christopher Tokin

Babies instinctively clutch on fingers and seek out their mother's voice.  For many, it takes little effort to understand the difference between laughter and anger.

But how instinctive is a teen's desire to snort a line of coke?

In the largest functional brain imaging study ever performed, researchers report findings that suggest that poor impulse control is pre-wired in some individuals. Specifically, they say that they have identified specific brain networks linked to impulse control and drug addiction - and that these differences exist even before an individual is exposed to drugs or alcohol.

To determine this, the researchers used a scan called a functional MRI, which allowed them to examining how different parts of the brain work together in real time. They "peeked into" the brains of nearly 1,900 14-year-olds while they asked them to perform repetitive tasks, and then measured their ability to stop mid-task.  Called "stop-signal reaction time," it is a measure that is used to gauge inhibitory control. Patients who abuse drugs or alcohol perform poorly on this test.  So do children with ADHD.

"These networks are not working as well for some kids as they are for others," says Dr. Robert Whelan of the University of Vermont, the lead researcher in this investigation.  He explains how they were able to break down how different brain networks were involved with specific types of impulses.

Interestingly, they were able to identify teens who had prior exposure to alcohol, nicotine, or other illicit drugs and were able to identify specific brain patterns associated with early experimentation with these substances.  Furthermore, teens with poor impulse control but no prior substance use had brain images similar to those who had already admitted use.

The findings suggest that there may be an opportunity to identify teens at risk before they indulge. "While identifying those at greatest risk of addiction is a complex process with many different factors involved, identifying brain networks specific to impulse control represents the first step" says Whelan.

According to the National Institute of Health, more than 40 percent of U.S. high-school seniors report drinking alcohol, 21 percent have used marijuana, and 8 percent have used Vicodin unrelated to a medical condition.

The study also looked at brain images of teens suffering from ADHD.  Two million American children are affected with ADHD, and a disproportionate number become alcohol or drug abusers.  The cause-and-effect literature regarding ADHD and substance abuse is mixed.   Many with ADHD also suffer with various other psychiatric disturbances, such as depression, bipolar disorder or conduct disorder, increasing their substance abuse risk.  Both ADHD and substance abuse have impulse-control issues at their core.

In agreement with prior studies, both adolescents with a history of ADHD or a history of alcohol or drug use had poor impulse control scores.  But researchers found that the brain networks activated in teens with ADHD were different than the ones associated with early drug use.

"This suggests that these two conditions may be unnecessarily tightly coupled together," says Whelan.  This knowledge may help guide doctors' treatment strategies.

So have we finally discovered what causes addicts to be addicts?  While researchers say we can't entirely set aside environmental and social factors such as good parenting, peer-pressure, or stress, the findings do suggest that for some, the pull toward risky behavior is stronger than for others.

"None of us has a perfect genetic make-up, and risk factors for addiction are etched into our DNA," says Dr. Paul Thompson, professor of neurology at UCLA.  "But the risk of addiction is a mix of genetic liability, our life experiences, and bad luck.

"None of these alone will make us addicts," he says, "but in aggregate, they can be overwhelming."