*The following is excerpted from an online article posted on PsychCentral.
A new study shows that the age at which an adolescent starts using marijuana affects which parts of the brain will be affected.
Researchers at the Center for BrainHealth at the University of Texas at Dallas found that study participants who began using marijuana when they were 16 or younger had brain variations that indicate arrested brain development in the prefrontal cortex, the part of the brain responsible for judgment, reasoning, and complex thinking.
In contrast, those who started using after age 16 showed the opposite effect, demonstrating signs of accelerated brain aging, according to the study, which was published in Developmental Cognitive Neuroscience.
"Science has shown us that changes in the brain occurring during adolescence are complex. Our findings suggest that the timing of cannabis use can result in very disparate patterns of effects," said Francesca Filbey, Ph.D., principal investigator. "Not only did age of use impact the brain changes, but the amount of cannabis used also influenced the extent of altered brain maturation."
For the study, the researchers analyzed MRI scans of 42 heavy marijuana users; 20 participants were categorized as early onset users with a mean age of 13.18, while 22 were labeled as late onset users with a mean age of 16.9.
According to self-reports, all the participants, who ranged in age from 21 to 50, began using marijuana during adolescence and continued throughout adulthood, using at least once a week.
According to Filbey, in typical adolescent brain development, the brain prunes neurons, which results in reduced cortical thickness and greater gray and white matter contrast. Typical pruning also leads to increased gyrification, which is the addition of wrinkles or folds on the brain’s surface.
However, in this study, MRI results reveal that the more marijuana early onset users consumed, the greater their cortical thickness, the less gray and white matter contrast, and the less intricate the gyrification, as compared to late onset users.
This indicates that when participants began using marijuana before age 16, the extent of brain alteration was directly proportionate to the number of weekly marijuana use in years and grams consumed.
In contrast, those who began using marijuana after age 16 showed brain changes that would normally manifest later in life: Thinner cortical thickness, and stronger gray and white matter contrast.
"In the early onset group, we found that how many times an individual uses and the amount of marijuana used strongly relates to the degree to which brain development does not follow the normal pruning pattern," she said.
"The effects observed were above and beyond effects related to alcohol use and age. These findings are in line with the current literature that suggest that cannabis use during adolescence can have long-term consequences."