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. 
 
 Advertisement 
 
 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. 
 
 Get The Weekender in your inbox: The Globe's top picks for what to see and do each weekend, in Boston and beyond. Sign Up Thank you for signing up! Sign up for more newsletters here 
 
 “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. 
 
 Advertisement 
 
 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. 
 
 Advertisement 
 
 “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 ||||| A new study links casual marijuana use with significant changes to parts of the brain. 
 
 A marijuana plant flourishes under grow lights at a warehouse in Denver. (Photo: 2010 photo by Ed Andrieski, AP) 
 
 Using marijuana a few times a week is enough to physically alter critical brain structures, according to a new study published Tuesday in The Journal of Neuroscience. 
 
 "Just casual use appears to create changes in the brain in areas you don't want to change," said Hans Breiter, a psychiatrist and mathematician at the Northwestern University Feinberg School of Medicine in Chicago, who led the new study. 
 
 There is actually very little research on the potential benefits and downsides of casual marijuana smoking — fewer than four times a week on average. 
 
 In his study, done in collaboration with researchers at Harvard University, scientists looked at the brains of 20 relatively light marijuana users and 20 people who did not use it at all. All 40 were college students in the Boston area. 
 
 The study found volume, shape and density changes in two crucial brain areas — the nucleus accumbens and the amygdala — involved with emotion and motivation and some types of mental illness. "This is a part of the brain you do not want to mess around with," Breiter said. 
 
 The more marijuana the students smoked, the more their brains differed from the non-users, the study found. 
 
 The brain continues to develop well into the 20s, and even into the 30s, said Breiter, who is concerned about the long-term impacts of marijuana use on the developing brain. 
 
 Staci Gruber, an associate professor of psychiatry at Harvard Medical School, who was not involved in the research, said Breiter's findings are consistent with her own, although she has focused on somewhat heavier users. 
 
 "There have been a growing number of studies that suggest that marijuana use in emerging adults is associated with differences in brain structure and cognitive abilities," said Gruber, also the director of Cognitive and Clinical Neuroimaging Core at McLean Hospital outside Boston. "I'm not saying (pot smoking) is analogous to shooting heroin or cocaine, but it's also not quite the benign substance people thought it was." 
 
 Responding to a study that found a decline in IQ points among people who used marijuana regularly, Francis Collins, the director of the National Institutes of Health, told USA TODAY recently that people should be more aware of these potential brain impacts. 
 
 "Perhaps it would be better if ... there was a little bit more recognition of that particular consequence," he said. 
 
 Gregory Gerdeman, a biologist and neuropharmacologist at Eckerd College in St. Petersburg, Fla., said he has no reason to doubt the new study's findings but worries generally about marijuana research funded by federal agencies, like the Office of National Drug Control Policy, which is charged with limiting drug use. (The research was supported by grants from the National Institutes of Health as well as the Office of National Drug Control Policy and Northwestern Medicine's Warren Wright Adolescent Center.) 
 
 "If you're getting money from the drug czar's office, that money's not going to continue if you don't end up publishing something that at least supports the general story of the danger of drug abuse," Gerdeman said. 
 
 He said it doesn't surprise him that heavy pot smoking might make it difficult for students to reach their intellectual potential. But still, he said, "if it were my child, even with this study, I'm more comfortable with young people having a casual marijuana habit than drinking regularly." 
 
 Read or Share this story: http://usat.ly/1kv51Lv
– 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.