Bad News for Long-Term Pot Smokers
The short-term effects of smoking weed are obvious: An increase in giddiness, an insatiable desire for Doritos, and a casual acceptance of one's loserhood. But, after the high wears off and the smoke clears, are there lasting effects upon the brain? This has been a contentious issue for many years; competing studies claim different results.
A new study published in the very high quality journal Proceedings of the National Academy of Sciences (PNAS) claims to measure three major statistically significant brain alterations caused by marijuana use. It's mostly bad news.
Reduced Brain Volume
MRI scans were performed on the brains of a group of very heavy marijuana smokers. These subjects averaged roughly three joints per day and had been smoking on average roughly nine to ten years. MRI scans were also performed on a second group of non-smokers with otherwise nearly identical characteristics.
A computer algorithm processed both sets of images, dividing up the areas of the scan into gray matter (the main masses of neurons), white matter (nerve pathways which connect areas of gray matter), and cerebrospinal fluid. The computer then used some heavy math to compare the amount of each tissue in the brains of the smokers and non-smokers.
The analysis yields three very interesting conclusions. The first and biggest: the smokers had a significant reduction in the volume of gray matter in the orbitofrontal cortex (OFC). The OFC is located just behind the eye on each frontal lobe of the brain. Studies find that this area seems to be involved with correlating sensory inputs with rewards (such as a particular taste or touch being associated with a good or bad experience) and plays a part in decision-making.
Most significantly, the OFC seems to function "in controlling and correcting reward-related and punishment-related behavior." Losses in this area might not come as a surprise to those with friends who partake regularly.
Increased Connection (Initially)
There is a small positive effect of cannabis use. The brain matter calculations also show that there is an increased functional connectivity between the OFC and the rest of the brain. In their words, "greater network recruitment is engaged to compensate for OFC liability." The white matter nerve connections (forceps minor) between the OFC and the rest of the brain appeared to be strengthened to help it combat loss of volume. This is measured by looking at how fluid flows through these areas (fractional anisotropy, FA); the greater the disturbance in fluid, the better the nerve connections are working.
However, the good news only lasts for a while.
Decreased Connection (Eventually)
After a few years, subjects who continued to smoke heavily eventually lost this increased OFC connectivity. The first few years of use saw connectivity increase up to a limit; additional years of smoking saw it decrease back to initial levels, and smokers of a decade and more saw on average a net loss in OFC connectivity. So, if you smoke long enough, you'll both decrease the volume of the OFC and break down its connection to the rest of the brain.
This study has two messages. Heavy usage of marijuana has mixed and complicated effects on the brain. OFC volume shrinks, but at first it increases connections to the rest of the brain to compensate. The second message is unequivocal: long-term heavy use both shrinks and cuts off the OFC area of the brain.
But, I doubt this study will change any minds. It's always 4:20 somewhere, potheads.