The Boston scientists found that the degree of brain changes appeared to be directly related to the amount participants smoked per week. 
 
 Young adults who occasionally smoke marijuana show abnormalities in two key areas of their brain related to emotion, motivation, and decision making, raising concerns that they could be damaging their developing minds at a critical time, according to a new study by Boston researchers. 
 
 Other studies have revealed brain changes among heavy marijuana users, but this research is believed to be the first to demonstrate such abnormalities in young, casual smokers. 
 
 The Boston scientists also found that the degree of brain changes appeared to be directly related to the amount participants smoked per week. 
 
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 Researchers did not study whether those changes were linked to corresponding declines in brain function, but lead author Jodi Gilman, a psychology instructor at Harvard Medical School and a brain scientist at Massachusetts General Hospital, said such abnormalities in young brains are reason for concern. 
 
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 “This is when you are making major decisions in your life, when you are choosing a major, starting a career, making long-lasting friendships and relationships,” Gilman said. 
 
 The findings, published Wednesday in the Journal of Neuroscience, come amid an increased debate about the long-term effects of marijuana, as a growing number of states legalize the drug for medicinal and recreational use. 
 
 Forty Boston-area young adults aged 18 to 25, many from Boston University, were selected for the study. Researchers used scans to measure the volume, shape, and density of two regions of the brain — the nucleus accumbens and the amygdala. 
 
 Half of the group said they used marijuana at least once a week, and the other 20 had not used the drug in the past year, and reported using it less than five times in their life. 
 
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 Among the group that did smoke, the median use was about six joints per week. 
 
 Scans revealed that the nucleus accumbens was larger in marijuana users, compared with nonusers, and its alteration was directly related to how much the person smoked. The nucleus accumbens is a hub in the brain that is involved with decision making and motivation. Structural changes were also seen in the amygdala, which is involved with emotional behavior. 
 
 These changes, Gilman said, may be evidence that the brain is forming new connections that encourage further drug use, “a sort of drug learning process.” The study did not address whether the brain changes are permanent. 
 
 The results are similar to animal studies that show when rats are given THC, the mind-altering ingredient in marijuana, their brains also form new connections, indicating an adaptation to the unnatural level of reward and stimulation from marijuana. 
 
 Other scientists not involved in the study say its small size makes it hard to extrapolate to the general population. But they also said the findings may help explain what happened to the brains of participants in other marijuana studies that demonstrated behavioral and functional changes, but did not use scans to identify potential brain abnormalities. 
 
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 “Anything that underscores that there may be structural changes in the brain [from marijuana use] is important,” said Dr. Staci Gruber, an associate psychiatry professor at Harvard Medical School and a director of brain imaging at McLean Hospital. 
 
 Gruber’s studies of marijuana smokers have focused on those with longer, more chronic use and have found that those who started smoking at earlier ages, while still in their teens, are less able to perform certain reasoning and decision-making tasks, compared with those who started later in life. 
 
 Stuart Gitlow, president of the American Society of Addiction Medicine, said the Mass. General study provides much-needed “hard evidence” of brain changes that appear to match the changes in cognitive skills — thinking and reasoning — that other researchers have demonstrated in marijuana studies. 
 
 “We’ve known that people who use marijuana when they’re younger tend to have cognitive abnormalities, but this gives us direct evidence,” he said. 
 
 “It’s fairly reasonable to draw the conclusion now that marijuana does alter the structure of the brain, as demonstrated in this study,” Gitlow said, “and that structural alteration is responsible, at least to some degree, for the cognitive changes we have seen in other studies.” 
 
 Earlier research has shown different brain changes linked to alcohol or other drug use, such as cocaine. 
 
 Dr. Hans Breiter, a coauthor of the Mass. General study, said there are still many unanswered questions about the potential long-term effects of these various chemicals, especially if people use more than one drug. One of his earlier studies, for instance, showed that the amygdala region of the brain shrank with cocaine use, while the new marijuana study suggests an increase. 
 
 “Most drug users use more than one drug,” said Breiter, a professor of psychiatry and behavioral sciences at Northwestern University Feinberg School of Medicine. 
 
 “Cocaine users use opiates, and most marijuana users also drink,” he said. 
 
 Kay Lazar can be reached at Kay.Lazar@globe.com ||||| Experimental mice have been telling us this for years, but pot-smoking humans didn't want to believe it could happen to them: Compared with a person who never smoked marijuana, someone who uses marijuana regularly has, on average, less gray matter in his orbital frontal cortex, a region that is a key node in the brain's reward, motivation, decision-making and addictive behaviors network. 
 
 More ambiguously, in regular pot smokers, that region is better connected than it is in non-users: the flow of signal traffic is speedier to other parts of that motivation and decision-making network, including across the superhighway of "white matter" that connects the brain's hemispheres. 
 
 The researchers who conducted the study speculate that the orbital frontal cortex's greater level of "connectedness"--which is especially pronounced in people who started smoking pot early in life--may be the brain's way of compensating for the region's under-performing gray matter. Whether these "complex neuroadaptive processes" reverse themselves when marijuana use stops is an important unanswered question, they added. 
 
 The new findings, reported Monday in the journal PNAS, confirm findings about chronic marijuana use from rodents. But scientific evidence in humans has been more mixed. 
 
 Even now, however, the authors of the study acknowledge that they cannot discern whether a pot smoker's smaller orbital frontal cortex is the cause or the result of chronic marijuana use. A 2012 study found that subjects with a smaller orbital frontal cortex at age 12 were more likely to start using marijuana by age 16, suggesting that deficits in this crucial region may predispose one to substance-abuse behaviors. 
 
 This study, conducted by researchers from the University of Texas' Center for Brain Health and the Albuquerque-based Mind Research Network, did not follow subjects over time, so it is at a disadvantage in showing cause and effect. Instead, it compared 48 "chronic" marijuana users (at least four times a week over the past six months) with 62 non-using control subjects who were matched for age and gender with the using group. Subjects were an average age of 28 to 30 years old. 
 
 Researchers noted that the IQ of the marijuana-using group was significantly lower than that of the non-using group--not a finding of the study, but an incidental factor that might be indirectly linked to marijuana use. 
 
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– Even casual use of marijuana could be messing up young people's brains at a time of life when they need to make major decisions, a new study finds. Researchers say that people aged 18 to 25 who used marijuana at least once a week were found to have abnormalities in the parts of the brain linked to emotion, decision-making, and motivation, the Boston Globe reports. While it's not clear whether the changes are linked to decreased function, "this is a part of the brain you do not want to mess around with," the lead researcher warns. The casual tokers had an average of six joints a week, and the changes appeared to be more pronounced the more that was smoked. The changes in young brains are reason to worry, the lead researcher says, because "this is when you are making major decisions in your life, when you are choosing a major, starting a career, making long-lasting friendships and relationships." A neuropharmacologist not involved with the study says that while he doesn't doubt the results, he is wary of marijuana studies like this one that were funded by federal agencies tasked with limiting drug use. "If it were my child, even with this study, I'm more comfortable with young people having a marijuana habit than drinking regularly," he tells USA Today.