(CBS) - Even casual marijuana use may alter neural pathways in the brain, according to a small study published in the Journal of Neuroscience. The researchers say they've turned up evidence of changes in the brain, which may be a possible sign of trouble ahead for recreational users.
The young adults who volunteered for the study were not dependent on pot, nor did they show any sign of marijuana-related problems in their daily lives.
"What we think we are seeing here is a very early indication of what becomes a problem later on with prolonged use," things like lack of focus and impaired judgment, said Dr. Hans Breiter, a study author.
Longer-term studies will be needed to see if such brain changes cause any symptoms over time, said Breiter, of the Northwestern University Feinberg School of Medicine and Massachusetts General Hospital.
Previous studies have shown mixed results in looking for brain changes from marijuana use, perhaps because of differences in the techniques used.
The study is among the first to focus on possible brain effects in recreational pot smokers, said Dr. Nora Volkow, director of the National Institute on Drug Abuse. The federal agency helped pay for the work. She called the work important but preliminary.
The 20 pot users in the study, ages 18 to 25, said they smoked marijuana an average of about four days a week, for an average total of about 11 joints. Half of them smoked fewer than six joints a week. Researchers scanned their brains and compared the results to those of 20 non-users who were matched for age, sex and other traits.
The results showed differences in two brain areas associated with emotion, motivation and reward processing -- the amygdala and the nucleus accumbens. Users showed higher density than non-users, as well as differences in shape of those areas. Both differences were more pronounced in those who reported smoking more marijuana.
"Some of these people only used marijuana to get high once or twice a week," Breiter said in a press release. "People think a little recreational use shouldn't cause a problem, if someone is doing OK with work or school. Our data directly says this is not the case."
Volkow said larger studies are needed to explore whether casual to moderate marijuana use really does cause anatomical brain changes, and if so, whether that leads to any impairment.
The current work doesn't determine whether casual to moderate marijuana use is harmful to the brain, she said.
Murat Yucel of Monash University in Australia, who has studied the brains of marijuana users but didn't participate in the new study, said in an email that the new results suggest "the effects of marijuana can occur much earlier than previously thought." Some of the effect may depend on a person's age when marijuana use starts, he said.
Another brain researcher, Krista Lisdahl of the University of Wisconsin-Milwaukee, said her own work has found similar results. "I think the clear message is we see brain alterations before you develop dependence," she said.
In recent years, marijuana has gained popularity as the drug has become more accepted and even viewed as harmless -- or therapeutic -- to users. A study published in March from the White House Office of National Drug Control Policy and the RAND Drug Policy Research Center, found marijuana use has risen by 30 percent from 2006 to 2010.
A Feb. 25 study published in the International Journal of Drug Policy found that about 10 percent of high school students who would otherwise be at a low risk for picking up a pot-smoking habit -- such as those who don't smoke cigarettes, students with strong religious beliefs and those with non-marijuana smoking friends -- say they would use marijuana if it was legal.