Researchers have combined MR with a virtual reality game of catch to compare brain activity among young marijuana users and non-users who are not being thrown the ball.

The study, which was published in the March issue of Biological Psychiatry: Cognitive Neuroscience and Neuroimaging, shows how marijuana use may reduce activity in the insula - a part of the brain that is usually active during social rejection - for young users.

"While we know that peer groups are one of the most important predictors of marijuana use in young adults, we know very little about the neural correlates of social rejection in those who use marijuana," said Jodi Gilman, Ph.D., of the Massachusetts General Hospital Center for Addiction Medicine, and lead author of the paper, in a statement.
The study enrolled 42 young adults between ages 18 and 25 from Boston-area colleges. Of those participants, 20 reported using marijuana two to four times a week while the remaining 22 had not recently used marijuana. The researchers told them they would be playing an online game of Cyberball — a computerized tool for investigating response to social rejection — to assess mental visualization ability.

In fact, however, the participants were simply playing the game — essentially "catch" — with a computer that was programmed to vary the number of times the participant was thrown the ball. At first, the participants received 75 percent of the throws, then they received no throws for a while, and then they began receiving throws again.

During the Cyberball game, MR images were taken on three brain regions — the anterior insula, the ventral enterior cingulate cortex (vACC), and the orbitofrontal cortex (OFC).

They found that the OFC did not show significant activation during the task, vACC was active during exclusion for marijuana users and non-users alike, and the anterior insula was only significantly active among the non-users.

"The unexpected reduction in insula response may indicate that marijuana users are less conscious of social norms or have reduced ability to reflect on negative social situations, but we currently are unable to determine whether these differences in neural processing are a cause or a result of marijuana use," said Gilman.

The researchers say the relationship between peer rejection and drug use are complicated. Previous studies have shown that young adults and adolescents who use marijuana are more likely to develop learning problems or become dependent on the drug than their older counterparts, and that social isolation might make a young person more likely to engage substances, particularly tobacco.

"While we believe this study does indicate that the neural response to social exclusion is different in marijuana users compared to non-using controls," Gilman explained, "it is hard to speculate whether that translates to actual differences in social behavior in real-world situations. That is definitely an area for future study, as is disentangling whether altered social processing contributes to or is a result of marijuana use."

MRI Homepage