Tuesday, April 29, 2014

This is your brain on drugs: what a recent fMRI study can and can’t tell us about the effects of marijuana use

Two weeks ago (okay, I'm late to the party), news broke of a new study showing that the brains of casual marijuana users are different than those of non-users. The study was just published in the Journal of Neuroscience and can be found here.

The researchers used magnetic resonance imaging (MRI) to scan the brains of 40 young adults aged 18-25. 20 of those subjects were casual marijuana users and 20 were non-users. Controlling for other behaviors such as alcohol and tobacco use, the researchers found that marijuana use was correlated with changes to the shape, size, and density of particular areas of the brain. From the study:

“The results of this study indicate that in young, recreational marijuana users, structural abnormalities in gray matter density, volume, and shape of the nucleus accumbens and amygdala can be observed. Pending confirmation in other cohorts of marijuana users, the present findings suggest that further study of marijuana effects are needed to help inform discussion about the legalization of marijuana.”

The study generated a lot of media coverage, and, unfortunately, over-statements of the study’s actual implications for ongoing policy debates. For example, the Society for Neuroscience issued a press release for the study. The release, while titled with appropriate caution (“Brain Changes are Associated with Casual Marijuana Use in Young Adults”), relays unsupported claims from scientists regarding the ramifications of the study. One of the authors, Hans Breiter, is quoted as saying ““This study raises a strong challenge to the idea that casual marijuana use isn’t associated with bad consequences.” And Carl Lupica, a researcher from the National Institute on Drug Abuse who was not involved with the study, similarly suggests that “This study suggests that even light to moderate recreational marijuana use can cause changes in brain anatomy.”  

The problem is that the study doesn’t necessarily support such conclusions. The study’s findings, while intriguing and valuable, are still quite limited. For one thing, the study will need to be replicated. The subject pool of 40 is rather small. That’s not reason enough to dismiss the study -- much brain science research relies on small n studies, because MRIs are cumbersome and expensive, and one can find statistically significant results with small pools – but it is reason to be particularly cautious about the results pre-replication.

Second, correlation doesn’t equal causation. Law policymakers commonly ignore this important scientific concept, but even scientists sometimes get ahead of themselves and jump to conclusions not warranted by a study’s design. In this study, for example, it is quite possible that people who use marijuana have differently sized and shaped brains to begin with; for example, maybe their brains are simply wired to seek out more risky behaviors and that’s why they’ve decided to use an illicit substance. Since we don’t know the size and shapes of these brains before they started using marijuana, we can’t say which came first: the marijuana usage or “the structural abnormalities in gray matter density, volume, and shape of the nucleus accumbens and amygdala.”

Third, even if the study’s results could be replicated and even if they could (somehow) demonstrate a causal connection between marijuana use and brain structure, it’s not clear from this study anyway why we should care. To be sure, different areas of the brain are associated with different functions and I wouldn't want to tinker with the size, shape, or density of my brain. But the study’s author’s can’t yet say that the changes they observe in brain structure necessarily cause negative changes in behavior.  For example, some studies suggest that the nucleus accumbens might play a role in drug addiction. But it’s not clear whether that changes observed in this study are associated with (let alone cause) marijuana addiction or any other bad behavioral outcomes; indeed, the authors made a point of excluding “dependent” marijuana users from the subject pool.

Law and neuroscience is a very promising field. It is generating intriguing findings concerning important issues like culpability. But as the best in this nascent field know, there is still much to be learned about the brain. This study is an intriguing development and clearly worthy of more follow ups. I think research on the brain cold help us understand marijuana’s effects and put them in perspective with those of alcohol, tobacco, cocaine, etc. But for now, bold statements about the import of brain science for policy debates over marijuana seem premature.


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Hi Rob--

You're missing what I think is the key mistake in this study: the definition of casual use. The median use was 11.8 joints a week--and that's only for 4 or so days of smoking. Unless you're Snoop Lion or Willie Nelson, that's not casual use.

Posted by: W. David Ball | Apr 29, 2014 10:23:23 AM

David makes a good point flagging another potential problem with this study (and others like it): namely, what is “casual” versus “heavy” marijuana use? There are a number of prior studies examining “heavy” marijuana use and demonstrating a correlation between brain structure and “heavy” marijuana use. I took another look at a couple of those studies (cited in the present study) to see what was considered “heavy” use. The subjects in Yucel et al, 2008, for example, used 212 joints per month / 7 per day on average. I think we’d all agree subjects in that study were “heavy” users. But I think it’s also safe to say they represent only a small portion of the marijuana using population.
So how relevant are such results to the much larger population of “casual” users? That was the whole point of the present study (I think): to see whether “casual” use of marijuana might have similar effects on the brain. And on that score, I think David is right to call out the authors of the present study. One needs to defend a particular definition of “casual” and the authors don’t even attempt to do that.
To be sure, their subjects consumed far less marijuana than did subjects in the Yucel study. The subjects in the present study reported using on average (not median) 11.2 joints per week, 3.8 days per week, and 1.8 joints per session. (My figures are drawn from Table I; they are somewhat different from the ones David reports.) But whether this truly counts as “casual” use – per David’s comment– is quite debatable. If, say, 80 percent of users only smoke one joint per week, it’s hard to see the relevance of the study (or at least, what it adds to prior studies). But I don’t know enough about usage statistics to say the authors are clearly off the mark. I’d be curious if anyone knows where to find good statistics regarding the distribution of usage. In any event, it might be better just to avoid contestable labels like “casual” anyway. As this study points out, there seems to be a linear relationship between marijuana use and brain structure, so writing as if there are categorical differences between casual and heavy use may be misleading in the first instance. Instead, it seems the more one uses, the more one’s brain changes.

Posted by: Rob Mikos | Apr 29, 2014 11:33:04 AM

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