Maia Szalavitz works to debunk the study we flagged yesterday:
The 20 marijuana-smoking participants, who took the drug at least once a week, were deliberately selected to be healthy. If they had any marijuana-related problems—or any psychiatric problems or other issues—they were excluded from participating.
Are you beginning to see what’s wrong? Although the pot-smoking participants showed brain differences in comparison to the controls who were also selected to be normal— both groups were normal! If the smokers had any marijuana-related problems or any type of impairment, they would not have been included in the first place. Therefore, the brain changes that the researchers found were—by definition—not associated with any cognitive, emotional, or mental problems or differences.
“I’m disappointed that scientists are still able to publish high profile papers that only look at neuroimaging without a behavioral endpoint,” says Carl Hart, an associate professor of psychology at Columbia University who was not associated with the research (Disclosure: he and I worked on a book project together). Hart compares the findings to brain differences found between the genders. “There are structural differences between men and women in certain areas,” he says, but they don’t predict differences in ability. “We don’t say this means women are impaired,” he adds.
German Lopez points out other limitations of the research:
[R]esearchers never analyzed if the detected differences persisted over time. Blood acknowledges the issue as one of the study’s limitations, but she cautions that it’s rare the brain fully reverses major changes. The study also didn’t identify whether marijuana caused the detected differences. It’s possible, for example, that people with different brain structures are more likely to use marijuana. But because the amount of marijuana use correlated with more variations in brain structure, Blood says she’s fairly confident the two factors are linked.
The Dish 