This could signify a more “immature pattern of brain development.”
Researchers from the Massachusetts General Hospital (MGH) decided to investigate the differences between young adult marijuana smokers and non-smokers when it comes to the brain’s response to social exclusion.
In the study, published in the journal Biological Psychiatry: Cognitive Neuroscience and Neuroimaging, the scientists looked at the brains of 42 young adults aged 18 to 25 at colleges in the Boston area — 20 who reported using marijuana two to four times a week and 22 who reported no recent marijuana use.
The participants were told that they were being studied for mental visualization ability, but really, the scientists were looking into their reactions to social exclusion with Cyberball, which is a recognized tool for investigating response to social rejection and ostracism.
The volunteers were told they would be playing a game of virtual “catch” with two other people, and they were asked to imagine the experience as vividly as possible.
However, they didn’t know that there were no other players and that the Cyberball system was programmed to vary the number of times the other “players” would throw the ball to the study participant. After a period in which the participant received the ball 75 percent of the time, there was a second period when the ball was never thrown to the participant. The last period included the participant in the game again.
"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," lead author Jodi Gilman, from the MGH Center for Addiction Medicine and assistant professor of Psychology in the Department of Psychiatry at Harvard Medical School, said in a press statement.
The results showed that activity in the brain region that is usually active during social rejection, called the insula, was reduced in young marijuana 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,” continued Gilman, “but we currently are unable to determine whether these differences in neural processing are a cause or a result of marijuana use."
Another brain region called the ventral anterior cingulate cortex (vACC) was also activated during the social exclusion exercise, and the researchers say there was a link between the amount of vACC activation and levels of peer conformity measured by tests taken before the Cybermall task — but this link was only seen in marijuana smokers.
The study authors note that a greater susceptibility to peer influence as seen in marijuana users could signify a more immature pattern of brain development.
However, the researchers also note that it’s hard to tell whether these neural responses would translate to the exact differences in social behavior in real-world situations — looking at the brain in a lab setting during a game of Cyberball is certainly different from experiencing social rejection in real life.
“That is definitely an area for future study,” said Gilman, “as is disentangling whether altered social processing contributes to or is a result of marijuana use.”