Casual marijuana use can change user's brains, according to a new government-funded study. 

Hans Breiter of Northwestern University led the research, which examined a total of 40 people, half of whom consume marijuana. The small sample size has some observers questioning the validity of the findings. 

New MRI scans were used to compare the brains of people who stated they smoked marijuana once a week or more to those in the control group. The team noticed something unusual about the nucleus accumbens, an area associated with pleasure. Breiter and his team found the areas to be larger in those who smoked cannabis than in those who did not. 

"This study suggests that even light to moderate recreational marijuana use can cause changes in brain anatomy," Carl Lupica from the National Institute on Drug Abuse said. He was not involved in the study.

The research did not examine behavior of marijuana smokers or the control group. Breiter and his team did not have MRI's of the subjects taken before the study began. This has led some observers to question if marijuana use caused a change in brain structure, or if the difference in neurophysiology made subjects more prone to smoke the substance. 

Dr. Sunil Kumar Aggarwal, an associate member of the New York Academy of Medicine and a resident physician at a large medical center, leads a group of pain management investigators and is a contributor to the Huffington Post. Aggarwhal quickly responded to the study on Facebook, releasing some early comments from his team of medical researchers. 

"[I]t's not clear in any of these types of studies if you are actually measuring the effects of cannabis or the effects of stigma, threat, and stress related to its use," wrote one researcher.

"There isn't any information in the actual brain scans to suppose that this is a 'disruption' of function. That's an interpretation that reveals a bias built into the study," stated another member of Aggarwal's team. 

The new study was funded by the National Institute on Drug Abuse (NIDA), Counterdrug Technology Assessment Center, the Office of National Drug Control Policy, and the National Institute of Neurological Disorders and Stroke. 

Papers establishing the Counterdrug Technology Assessment Center state "[t]he Center shall operate under the authority of the Director of National Drug Control Policy and shall serve as the central counter-drug technology research and development organization of the United States Government."

Studies funded by groups with stated interests in the outcomes of the studies are often the subject of special scrutiny, as in cases when petroleum companies fund investigations into climate change.

Another member of Aggarwhal's team reminds the public it is "important for people to understand the importance of conflicts of interest in research, and to remember that they need to keep ideas of cannabis use in a context of its relative effect in comparison to other substances."  

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