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Neuropsychological Deficits: Fact And Artifact About Marijuana Tests


neuropsychological marijuanaBy Mitch Earleywine, Ph.D State University of New York at Albany Chair, NORML board of directors

A new study claims to show small deficits on neuropsychological tests in college students who started smoking marijuana early in life. It might get a lot of press. Prohibitionists love to bang the drum of marijuana-related cognitive deficits, so I’d like NORMLites to know how to make sense of this sort of research. The recurring themes in this literature involve several alternative explanations that never seem to dawn on journalists. These results often arise from artifacts of the study rather than physiological effects of the plant. I’d like to focus on a few: other drug use, dozens of statistical tests, the incentives for performance, and the demands communicated by the experimenters.

The latest paper of this type is actually pretty good. Researchers studied over 30 people aged 18-20 who started using before age 17 (their average starting age was around 15) and who smoked at least 5 days per week for at least a year. They compared them to a comparable bunch of non-users. I hate to see 15-year-olds using anything psychoactive, even caffeine. Spending full days in high school with less than optimal memory functioning is no way to lay the groundwork for a superb life. I admit that I want these same people to grow up and be the next generation of activists, so feel free to call me selfish when I emphasize NORML’s consistent message: THE PLANT IS NOT FOR KIDS WHO LACK MEDICAL NECESSITY.

First, we have to keep other drug use in mind. Unfortunately, the marijuana group in this study got drunk more than 4 times as much in the last six months as the controls. Given what we know about binge drinking and neuropsychological functioning, it’s going to be hard to attribute any differences between these groups to the plant. It’s just as likely that any deficits stem from pounding beers. Studying cannabis users who aren’t so involved with alcohol would help address neuropsychological functioning much better.

In addition, we should always consider the number of measures in any study. Many of these neuropsychological tasks have multiple trials that can be scored multiple ways. The more statistical tests you run, the more likely it is that you’ll find a statistically significant difference by chance. It’s kind of like flipping coins. It’s rare to flip four heads in a row. But if you flip a coin a thousand times, odds are high that somewhere in the list of a thousand results will be four heads in a row. These investigators got 48 different test scores out of the participants. You’d expect at least 2 of them to be significant just by chance. They found differences on 14 different scores, suggesting that something’s going on, but we’re not sure which results are the “real” differences and which ones arose by accident. (That’s why we replicate studies like this.) And, as I mentioned, it might all be because of the booze.

We also have to consider incentives for performance. Most researchers bring participants to the lab for a fixed fee and ask them to crank out a bunch of crazy puzzles and memory assessments. It’s unclear why people would feel compelled to strain their brains. The authors of this study were kind enough to mention some relevant work by my friend (and former student) Dr. Rayna Macher. Dr. Macher showed that cannabis users respond best when you make the effort worth their while. She focused on people who used the plant at least four times per week for a year or more. She read one group some standard instructions for a memory test. The other group got the regular instructions plus an additional sentence: “It is important that you try your very best on these tasks, because this research will be used to support legislation on marijuana policy.”

As you’d guess, this simple sentence fired them up. Compared to cannabis users who didn’t hear that sentence, they performed better on 3 out of 10 measures. (You’d expect less than one difference by chance.) And compared to the non-users, the folks who got the incentive sentence did just as well on all the tests. For those who didn’t hear the incentive sentence, users did less well than non-users on 1 of the 10.

I know that prohibitionists are going to try to call this amotivation. (See my rant on that when you get a chance) I call it putting effort where it pays. But given what we know about how these studies can hamper the reform of marijuana laws, users everywhere should do their best on all tests whenever they get the chance.

Last but not least, we have to consider the demands communicated by the experimenter. Decades of data now support the idea that people often do what others expect them to do, especially if they believe the expectation, too. Another friend and former student, Dr. Alison Looby De Young, showed that these expectations are critical in studies of neuropsychological performance and cannabis. She gave a neuropsychological battery to men who had used cannabis at least three times per week for the last two years. One group of men read instructions that said that cannabis had no impact on their performance on these tests. Another group read instructions that said that cannabis was going to make them perform poorly. You guessed it, those men who heard they were going to flub the tests performed worse on 2 of the 4 tests. (You’d expect less than one difference by chance). As you might imagine, some laboratories communicate their expectations about cannabis and cognitive function subtly or not so subtly. Some participants are bound to behave accordingly. So what looks like a cognitive deficit is just an artifact of the laboratory environment where experimenters stare daggers at cannabis users.

In the end, I’m glad that researchers do this work, but these effects are too small and fleeting to justify prohibition. We already know that cannabis isn’t for healthy kids. People who get heavily involved with the plant early in life might not perform as well as those who never touch cannabis even if investigators control for other drug use, AND use a sensible number of tests, AND provide appropriate incentives, AND communicate a reasonable expectation.

But how many people should go to jail for that?

If you said, “None,” you’ve done an excellent job on an important cognitive test.

Source: NORML - make a donation


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Johnny Green


  1. stevenwilliamson on

    I AM 59 yrs old smoked Indian weed, Pot at 13. Served three years US ARMY, 1 Yr S. Korea. Cheech and Chong era. I graduated College, worked productively many years. As a medic I did see some drug use, aclohol abuse, ACID, Heroin and crank. I have had to strap people down in their bunk, and load a few Ambulances from 1973-1975. As far as marijuana use I would rather pass you a joint rather than fire you up with Morphine or Demoral. The nation needs to justify the Cartel making money instead of the free-enterprise system. I am also willing to par-take in any study that would help regulators make and mark true pot use. “refer Madness” scared the people out of there wits. “Loco-weed. Like too much alcohol moderation is the key.

  2. “Claims to show small deficits on neuropsychological function”
    All this does is show us anecdotal research as to the reason that, our Government is all screwed up. From this President on down to the person in a cubical in the basement of any Gov. agency. As we know, their actions speak volumes of a warped inconsistent thought process. (lets include State and Local elected individuals in the equation also)
    They all openly admit, (because it’s in fashion now to make such statements) as to their young indiscretion’s (lol) when it comes to the question asked “Did you ever use Pot” Obawand openly admits to his use as more than average and more than just cannabis. So to me, that explains it all. Like 8 points lower in I Q.. And he’s the President of The United States. And now he is diagnosed and documented with a neuropsychological defect.
    (This is posted in a jocularly fashion)

  3. Relax – take a toke – you’re with friends. Prohibitionists will be – because that’s what their expectation is. And the herd mentality of journalism reminds me of Lemmings and the inexorable call of the cliff.

  4. HOLY SMOKES… The so-called experimental group consumed alcohol at 4x the frequency as the control group??? That’s about as damning a confound as you can have in any empirical research, let alone research studying cognitive deficits.

    How did this ever make it through review to publication???

    Empirical research is all about explaining away the variance in the data. The people who ran this study should have made sure their control group was stratified with the experimental group to properly account for *other* independent variables that could effect the dependent variable, like how binge drinking clearly effects cognitive performance. Lots of teens who *only* drink — they’re out there.

    They never should have published knowing the experimental group consumed that much MORE alcohol than the controls. I think it would have made the results of the study a thousand times more interesting had they matched the drinking activity (if they had a control group that drank just as much) — they might have observed the cannabis group performed BETTER than the controls, not worse. Fact is, cannabinoids have been shown to encourage repair of neurological damage.

    I give you one of the Journal of Clinical Investigation’s most cited articles:
    And one I just heard of, recently:

    These articles expound the importance of stimulating CB1 and CB2 receptors in the brain to encourage neurogenesis — regrowing and repairing damage to the brain by directing as-yet unspecified stem cells to become the type of cell that would repair the damage and discouraging unhelpful inflammation of the damaged area. Real science. Not the sort of prohibitionist “throw it at the wall, see if it sticks” junk science the media clings to so dearly.

  5. When I hear real science speak on this subject, it soothes and quiets my mind.
    I have a google search tab set up to automatically display marijuana news stories of the the day, and this junk science story, that the good doctor just debunked, was ran by every major news outlet. Sigh, my blood pressure!

    Thank goodness that I can come to this site for some sanity.

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