Thankfully, I'm only late to the party by 4 days. I wanted to save this post for 4/20 for obvious reasons.
On April 16 in the Journal of Neuroscience, a study was published by Gilman et al. entitled "Cannabis Use Is Quantitatively Associated with Nucleus Accumbens and Amygdala Abnormalities in Young Adult Recreational Users." Using MRI scans on 20 young adults (age 18-25) who qualified as casual recreational marijuana users and 20 non-using controls who were matched for gender, age, ethnicity, education and handedness, the researchers tested for three things (to quote from the abstract for brevity): (1) gray matter density using voxel-based morphometry, (2) volume (total brain and regional volumes),
and (3) shape (surface morphometry). The researchers also controlled for alcohol use and cigarette smoking.
Their basic conclusions were that there is an association between casual marijuana use and density of gray matter in the left nucleus accumbens as well as the amygdala. The density, as well, correlated with higher use of marijuana as reported by the subjects.
Let us keep in mind, first, that this was a cross-sectional study. As Ryan Smith suggests: "While the correlative relationship
reported here is statistically strong, a longitudinal study design is
necessary to make the causative claims throughout the first 29 paragraphs
and abstract of this manuscript."
While I partially sympathize with this statement, as correlation does not equal causation (nothing new, right?), I think the researchers have reason to believe that the association is causal, although as they suggest at the end of the study, it has not been conclusively verified. It has already been established that cannabis use is associated with working memory impairment, but there was still a need to establish a potential association between cannabis use and the neural circuitry. As such, we should see this new study in the context of another study published in Schizophrenia Bulletin by Smith et al. entitled "Heavy marijuana users have abnormal brain structure, poor memory." This study from December examined heavy marijuana users as opposed to casual users and found that there is an association between heavy use of cannabis and brain abnormalities as well as poor performance on memory tasks.
Thus, Ryan Smith's objection to the test may be seen as unnecessary. While a longitudinal study would effectively falsify or validate the causal relationship, showing that degree of use is correlated with degree of abnormality/impairment is good enough for supposition (I say this as a matter of opinion because, the way I see it, it seems less likely that having more problems causes you to smoke more weed. I think degree would be unrelated in such a relationship).
However, there are other objections to be made, namely relating to the funding the research received. Funding parties include the National Institute of Drug Abuse and the Office of National Drug Control Policy. This suggests potential funding bias, although practically we cannot make such an assertion. I would honestly be surprised, however, that this study was biased by its funding given the context of established associations between cannabis and cognitive impairment/brain abnormalities.
Although, that's the politics of science, and at this point the only thing that can be done is replication of the findings; but to close, that's not exactly a hard thing to do, considering that this study - to a smaller degree - replicated the results of previous studies. When will it end?
Thank you very much for reading.
*For supplementary reading on marijuana's negative health effects, I highly recommend this infographic from Healthline.*
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Gilman, J., Kuster, J., Lee, S., Lee, M., Kim, B., Makris, N., van der Kouwe, A., Blood, A., & Breiter, H. (2014). Cannabis Use Is Quantitatively Associated with Nucleus Accumbens and Amygdala Abnormalities in Young Adult Recreational Users. Journal of Neuroscience, 34 (16), 5529-5538 DOI: 10.1523/JNEUROSCI.4745-13.2014
Meier, M., Caspi, A., Ambler, A., Harrington, H., Houts, R., Keefe, R., McDonald, K., Ward, A., Poulton, R., & Moffitt, T. (2012). Persistent cannabis users show neuropsychological decline from childhood to midlife. Proceedings of the National Academy of Sciences, 109 (40) DOI: 10.1073/pnas.1206820109
Smith, M., Cobia, D., Wang, L., Alpert, K., Cronenwett, W., Goldman, M., Mamah, D., Barch, D., Breiter, H., & Csernansky, J. (2013). Cannabis-Related Working Memory Deficits and Associated Subcortical Morphological Differences in Healthy Individuals and Schizophrenia Subjects. Schizophrenia Bulletin, 40 (2), 287-299 DOI: 10.1093/schbul/sbt176