Pot smokers say marijuana is a mind-expanding drug, but a new study conducted at The University of Texas at Dallas links heavy, long-term use of marijuana with smaller volume in the orbitofrontal cortex--a brain region associated with decision-making and addiction.

The same research shows that the brains of long-term users have greater connectivity in this region than do the brains of people who don't use pot, although this connectivity seems to disappear over time with prolonged use. The research also shows that the earlier an individual starts using marijuana, the more pronounced the brain abnormalities.

Whether these brain abnormalities cause any mental or emotional deficits isn't yet clear.

"The orbital frontal cortex is a key part of the brain's reward system/network and instrumental in our motivation, decision-making and adaptive learning," study leader Dr. Francesca Filbey, director of the university's Center for BrainHealth and an associate professor in the university's School of Behavioral and Brain Sciences, told The Huffington Post in an email. "As such, our finding that chronic marijuana users had smaller brain volume in the orbital frontal cortex, might manifest behaviorally making it difficult for them to change learned behavior."

For the study, Filbey and her colleagues used MRI scanners to compare the brains of 48 adults who had smoked marijuana three times a day for 10 years, on average, to the brains of 62 non-users.

While their findings are provocative, the researchers acknowledge that they do not prove that marijuana use directly causes changes in the brain--a point of view shared by Dr. Asaf Keller, a professor of anatomy and neurobiology at the University of Maryland School of Medicine, who was not involved in the study.

"As this is a retrospective study—and not a prospective one—it is impossible to determine whether individual differences in brain anatomy are related to genetic or environmental factors other than marijuana use," he told HuffPost Science in an email. "In sum, there is not indication that the anatomical differences in the brains of marijuana users are caused by marijuana use."

Keller has been critical of previous research linking casual marijuana use to changes in the brain.

Still, some researchers argue that this new study is an important step forward for marijuana research.

"This is important, well-conducted research that can serve as a reminder that marijuana use may not be without risks," Dr. Susan F. Tapert, a psychiatry professor at the University of California, San Diego, who was not involved in the study, told HuffPost Science in an email. "These findings point to the need for definitive longitudinal studies that assess future users prior to the onset of marijuana use, then again after use has started."