There’s no denying the effect marijuana and other drugs have on cognitive abilities. However, when it comes to marijuana, scientists are at odds over whether or not these effects are good or bad. A recent study conducted at the Center for BrainHealth at the University of Texas at Dallas has revealed that chronic marijuana use’s effect on the brain can depend heavily on age of first use and duration of use.

"We have seen a steady increase in the incidence of marijuana use since 2007," Dr. Francesca Filbey, director of the Cognitive Neuroscience Research in Addictive Disorders at the Center for BrainHealth, said in a statement. "However, research on its long-term effects remains scarce despite the changes in legislation surrounding marijuana and the continuing conversation surrounding this relevant public health topic."

Filbey and her colleagues recruited 48 adult marijuana users and 62 non-users who were matched by age and gender. Researchers determined that marijuana users consumed the drug around three times per day while controlling for tobacco and alcohol use. Magnetic resonance imaging (MRI) techniques were used to determine cognitive functioning abnormalities and brain structure of long-term marijuana users.

Chronic marijuana users had smaller brain volume in the area of the brain associated with addiction, orbitofrontal cortex (OFC), but also showed signs of increased brain connectivity. Although chronic users also scored a lower IQ on cognitive tests, the research team discovered no correlation between a lower IQ score and decreased brain volume in the OFC.

"To date, existing studies on the long-term effects of marijuana on brain structures have been largely inconclusive due to limitations in methodologies," Filbey added. "While our study does not conclusively address whether any or all of the brain changes are a direct consequence of marijuana use, these effects do suggest that these changes are related to age of onset and duration of use."

Findings also showed that regular marijuana use at a younger age was associated with greater functional and structural connectivity. Increases in connectivity are at their greatest at the beginning of marijuana use. Even when increases in structural wiring starts to decline after six to eight years of chronic use, marijuana users still displayer higher connectivity compared to non-users. This explains why, despite lower OFC brain volume, long-term marijuana users “seem to be doing just fine.”

"What's unique about this work is that it combines three different MRI techniques to evaluate different brain characteristics," said Dr. Sina Aslan, founder and president of Advance MRI, LLC and adjunct assistant professor at the University of Texas at Dallas. "The results suggest increases in connectivity, both structural and functional that may be compensating for gray matter losses. Eventually, however, the structural connectivity or 'wiring' of the brain starts degrading with prolonged marijuana use."

Source: Filbey F, et al. Proceedings of the National Academy of Sciences (PNAS). 2014.