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Study: Heavy pot smoking shrinks gray matter

Longtime, chronic marijuana use causes significant abnormalities in brain function and structure – and perhaps lower IQ – according to a major new study conducted in part at the Mind Research Network on the campus of the University of New Mexico.

Chronic users tend to have a smaller volume of gray matter in a part of the brain associated with decision making and addiction and a marked increase in a function called brain connectivity, which researchers said may be an attempt by the brain to make up for the decreased gray matter volume.

Distinguished research professor Vince Calhoun of the University of New Mexico and a scientist with the Mind Research Network, indicates the area of the brain that loses volume in chronic marijuana users. He was one author of a new study on the effects of long-term pot use carried out at UNM and the University of Texas at Dallas. (Greg Sorber/Albuquerque Journal)

Distinguished research professor Vince Calhoun of the University of New Mexico and a scientist with the Mind Research Network, indicates the area of the brain that loses volume in chronic marijuana users. He was one author of a new study on the effects of long-term pot use carried out at UNM and the University of Texas at Dallas. (Greg Sorber/Albuquerque Journal)

The study – the first of its kind and the most significant to date – was published Tuesday in the Proceedings of the National Academy of Sciences. It was funded by the National Institute of Drug Abuse and comes at a time some policymakers in Albuquerque and elsewhere are pushing to legalize marijuana use.

Using three different MRI techniques to analyze the subjects, Dr. Vince Calhoun, a distinguished research professor of electrical and computer engineering at UNM, worked closely with colleagues from the Center for BrainHealth at the University of Texas at Dallas.

The study’s lead author was Dr. Francesca Filbey, director of Cognitive Neuroscience Research in Addictive Disorders at the Center for BrainHealth at UT Dallas. She was formerly with Albuquerque’s Mind Research Network, where the study began several years ago.

The MRI scans all were taken in Albuquerque, Calhoun said. The analysis was completed in Dallas and Albuquerque.

The study clearly shows the effects of longtime marijuana use on the brain, said Calhoun, also the executive science officer and director of Image Analysis of the MRN, a nonprofit, independent institute based on the UNM campus.

What is not known, he said in an interview Tuesday, is whether those effects are lasting or if a cessation of marijuana use ultimately would mean a return to a more normal brain.

The effects seem to depend to a degree on the age of a user when he or she begins smoking pot and how long it is used.

For the project, Filbey, Calhoun and other researchers studied 48 long-term adult marijuana smokers and 62 gender- and age-matched nonusers as a control group. The control group accounted for such potential biases as gender, age and ethnicity. The authors also controlled for tobacco and alcohol use.

The users who participated in the study consumed the drug three or more times a day for an average of 10 years, Calhoun said, adding, “That’s chronic.”

The study found that pot smokers have measurably less volume in the orbitofrontal cortex of the brain’s frontal lobe, and the evidence strongly suggests a relationship with marijuana and the length of time the drug was consumed.

The findings indicate gray matter in the front of the brain may be more vulnerable than white matter to the effects of THC, the main psychoactive ingredient of marijuana.

The MRIs also revealed that youthful and regular marijuana use prompts greater structural and functional connectivity in the brain. The greatest increases in connectivity appear when someone first begins using marijuana, and the more the drug is used, the study found, the greater the connectivity.

Distinguished research professor Vince Calhoun of the University of New Mexico and a scientist with the Mind Research Network in Albuquerque, helped find conclusive evidence that chronic pot use affects the brain's structure and function and possibly IQ. (Greg Sorber/Albuquerque Journal)

Distinguished research professor Vince Calhoun of the University of New Mexico and a scientist with the Mind Research Network in Albuquerque, helped find conclusive evidence that chronic pot use affects the brain’s structure and function and possibly IQ. (Greg Sorber/Albuquerque Journal)

The increase in connectivity may be a compensatory function of the brain to make up for the decrease in volume of the orbitofrontal cortex, Calhoun said.

Brain connectivity, which is crucial to processing information, refers to connections, dependencies and interactions among various and distinct units of the nervous system. Although the increased connectivity begins to decline after six to eight years of continued use, pot smokers still exhibit more intense connectivity than healthy nonusers, a finding that may explain why chronic, long-term users “seem to be doing just fine” – despite the smaller brain volumes, Filbey said.

Until now, studies on the long-term effects of marijuana on brain structures have been largely inconclusive due to limitations in methodologies, she said.

“While our study does not conclusively address whether any or all of the brain changes are a direct consequence of marijuana use, these effects do suggest that these changes are related to age of onset and duration of use,” Filbey said.

Cognitive tests showed that the users had slightly lower IQ scores compared to the control groups, but the IQ differences are not necessarily related to the brain abnormalities. No causative or direct correlation was made between IQ deficits and the decrease in volume of the affected portion of the brain.

Speaking as a scientist, Calhoun said, he would like to see a longitudinal study – one that looks at marijuana users over a significant period of time – to see if the structural and cognitive changes are permanent.

The study does not address whether consuming marijuana is dangerous. Nor would Calhoun give an educated opinion on legalizing pot for recreational use or medical marijuana.

“I’ll leave that to the policymakers,” he said. “That’s a very hard question to answer.”

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