While the Huffington Post was applauding a supposed drop in crime since marijuana was legalized in Colorado, a new study showed even casual pot use has dramatic effects on the brain.

"Smoking cannabis alters the size and shape of two key brain regions involved in emotion and motivation," one report said. 

"Even light use of the drug once or twice a week warps the brains of young adults, US scientists found."

Although the study was small -- just 20 potheads and 20 non-tokers -- the findings were simple: there's no such thing as "casual" use.

"Some of these people only used marijuana to get high once or twice a week," Professor Hans Breiter, one of the researchers from Northwestern University in Chicago. "People think a little recreational use shouldn't cause a problem, if someone is doing OK with work or school. Our data directly says this is not the case."

 "This study raises a strong challenge to the idea that casual marijuana use isn't associated with bad consequences," he said.

The scientists studied magnetic resonance imaging (MRI) brain scans of the 40 young people aged 18 to 25 years. They found major differences in two areas -- the nucleus accumbens and the amygdala, both linked to emotions and motivation. What's more, changes in the brain were directly linked to the amount of marijuana that was smoked.

Dr. Anne Blood of Harvard Medical School, a co-author, said: "These are core, fundamental structures of the brain. They form the basis for how you assess positive and negative features about things in the environment and make decisions about them."

The Washington Post said "the study is among the first to focus on possible brain effects in recreational pot smokers," and noted that a "federal agency helped pay for the work."

Which makes sense: Both President Obama and Attorney General Eric Holder are admitted pot smokers. No doubt they were looking for different results.