Recently a study came out by the Center for BrainHealth at the University of Texas in Dallas about marijuana use and the association with a smaller orbital frontal cortex. They found that the orbital frontal cortex decreased in size among heavy marijuana users, along with an increased connectivity in the forceps minor and orbitofrontal cortex network. The study also noted a correlation between lowered IQ and cannabis use.
I paid close attention to the study, and noted some things about it that bothered me.
In this study, we have a control group of 62 compared to a group of 48 cannabis users. Of those 48 cannabis users, 27 uses cannabis exclusively. The sample size of control vs. exclusively cannabis users is quite small. The average IQ among the control group was 110, while the average among the exclusively cannabis users was 104. Many articles about the study feature the IQ results prominently, such as this one from the L.A. Times, (http://www.latimes.com/...) but the group itself says that there's not enough data to support a a connection between a smaller OFC and a lower IQ.
Cognitive tests show that chronic marijuana users had lower IQ compared to age-and gender-matched controls but the differences do not seem to be related to the brain abnormalities as no direct correlation can be drawn between IQ deficits and OFC volume decrease.How the data was acquired
The data gathered for this study was done using MRI, which stands for magnetic resonance imaging. A MRI measures tissue structures in the brain by jiggling the hydrogen atoms present in the water of the tissue. The authors counted the gray matter voxels in the MRI of the control group and compared them to the voxels of the cannabis users and exclusively cannabis users. (Voxels are essentially pixels - a consistent measurement used in MRI.) However, cannabis acts upon two receptors in the human body - CB1 controls intoxication, which is what gets you high. CB2 controls inflammation and can reduce addictive behaviors in the brain. Reducing inflammation would reduce water content in the tissues of the orbito-frontal cortex, resulting in a smaller looking group of voxels on the MRI scan.
Gray matter is notoriously hard to get an image of, because of its lowered water content compared to white matter. A 3T MRI is the industry standard for best results, but they would have been better off using a 7T MRI, especially if they're basing this study on counting gray matter voxels.
A dysfunctional orbito-frontal cortex
The orbito-frontal cortex provides stimulus-reward associations, and suppresses negative emotions. Dysfunction of the orbitofrontal cortex leads to behavioral problems such as obsessive-compulsive disorders, trichillomania, aggression, attention-deficit hyperactivity disorder, impulse control problems, Tourette's syndrome, and depression. These problems can lead to drug addiction/substance dependence. The later stages of Alzheimer's disease may be impacted by altered connectivity of OFC systems.
If the OFC was truly dysfunctional, these behaviors would be seen more often in marijuana users. However, this is quite the opposite. Marijuana users are less depressed, less stressed, and not at all aggressive. So there's two conclusions to take from all this: either the OFC is really shrinking in marijuana users meaning that we don't understand what the function of that part of the brain is, or that the OFC isn't shrinking, and the imaging techniques used to count the voxels of the gray matter are at fault.
Increased brain connectivity
Here's the really interesting thing - marijuana use increases structural connectivity in the human brain in all four nodes of the brain, such as the bilateral temporal lobe and the bilateral OFC.
Although increased structural wiring declines after six to eight years of continued chronic use, marijuana users continue to display more intense connectivity than healthy non-users, which may explain why chronic, long-term users “seem to be doing just fine” despite smaller OFC brain volumes, Filbey explained.What does this mean? If the OFC is actually shrinking, marijuana use helps grow neurons in the parts of the brain surrounding that area. If the OFC isn't shrinking, but increased neural connectivity allows the brain to better communicate with itself, then the brain as a whole is a better tuned organ.
All in all, this is a fascinating study, but deeply flawed.