Does Marijuana Affect Your Cortex?

A recent study investigated for changes in the shape and size of the cortex.

Posted Nov 11, 2019

Sadly, there have been only a few attempts to determine an answer to that question during the past 20 years. The majority of these studies used sophisticated imaging methods to determine changes in the thickness of the cortex due to cannabis use. Unfortunately, the results of past studies have been confusing and inconsistent.  

One study found increased thickness in some areas and reduced thickness in other areas, while a second investigation reported no difference in cortical thickness between cannabis users and control subjects. Another study concluded that the degree of thickness variation was related to how often a person used cannabis. Significant changes in cortical thickness, if they exist, might underlie subtle changes in personality or impaired thinking abilities.

Why should anyone expect that marijuana would affect cortical thickness? The answer is related to the critical role that the brain’s own endogenous cannabis system plays in the control of brain cell growth and differentiation. These brain processes, especially those occurring in the cortex, underlie our ability to think, feel, and remember. Thus, finding an answer to this question is important to understand the effects of marijuana use on brain function.

One recent investigation (Eur Neuropsychopharmacol 29:257, 2019) published this month investigated the effects of marijuana in a large group of adults on three important measures of the cortex shape and size. They examined the thickness of the cortex across the entire brain, the surface area of each cortical region and a measure that they called the gyrification index. The cortex is covered in hills and valleys and each hill is associated with a specific brain function. The gyrification index is a measure of the amount of cortex buried within the valleys as compared with the amount of cortex located on the hills. The degree of gyrification of the cortex is related to intelligence. Humans have a very high gyrification index and we should avoid doing anything that negatively affects our index.

These scientists made an important decision in the design of their study, they divided their 261 subjects into different groups to compare the effects of marijuana in people who were dependent on daily cannabis, those who were non-dependent (such as weekend recreational users) and those who were not using cannabis. The average age of the subjects was about 27 years; most of the subjects were male (males abuse cannabis more often than females) and they all had similar IQ scores, brain size, and reported alcohol use. The subjects were recruited from around the world, including Amsterdam, Barcelona, and Australia. The majority of cannabis users reported daily use for six years, on average.

This large multinational, well-powered study found no evidence of cortical alterations of any kind. No significant changes in cortical thickness, cortical surface areas, and gyrification index in relation to cannabis use, level of dependence, or even age of initiation of cannabis use were discovered. The study had one important limitation. The subjects were mostly young adults; thus, the scientists could not determine whether there were any individual changes following long-term marijuana use.

Thus, it will be important in the future to conduct similar investigations to determine the influence of genetic vulnerability on changes in cortex shape and size. Finally, it is imperative to recognize that the lack of detectable changes in these measures of surface morphology does not rule out the possibility of important changes in specific neural circuits or brain chemistry. These critical studies remain to be performed. Stay tuned.


© Gary L. Wenk, Ph.D. is a Professor of Psychology & Neuroscience at The Ohio State University and Medical Center, and the author of “The Brain: What Everyone Needs to Know” (2017) and “Your Brain on Food,” 3rd Edition, 2019 (Oxford University Press).