Advocates of any idea or substance discredit their stance when they dismiss out of hand studies that show negative aspects to what they advocate.  Yesterday*, a study was presented to Society for Neuroscience on the cognitive deficits  of marijuana users.

“Our data suggest that the earlier you begin smoking, the more marijuana you smoke and the more frequently you smoke [the greater the cognitive difficulty],” said Staci A. Gruber, director of the Cognitive and Clinical Neuroimaging Core at McLean Hospital and assistant professor at Harvard Medical School about marijuana use. According to an article on physorg.com and as reported on NPR this morning.

The study included 33 chronic marijuana smokers and 26 control subjects [all had similar education and income levels as per the NY Times] who did not smoke marijuana. They were given a battery of neurocognitive tests assessing executive function, including the Wisconsin Card Sorting Test, which involves sorting different cards based on a set of rules given. During the test, the rules are changed without warning and subjects must adjust their responses to the new rules.

The findings showed habitual marijuana users made repeated errors even when told that they were wrong. Users also had more trouble maintaining a set of rules, suggesting an inability to maintain focus. Early-onset users and those who used the most marijuana had the most trouble with the test, making more than twice as many errors and fewer correct responses than later-onset smokers.

The researchers, who included Mary Kathryn Dahlgren, Kelly A. Sagar and Megan T. Racine, all of McLean Hospital’s Brain Imaging Center, also performed functional MRI (fMRI) scans on the subjects while they completed tests of cognitive control and inhibition.

Marijuana smokers showed increased brain activation in a frontal area of the anterior cingulate cortex, a key region for attention, inhibition, and error processing, compared with control subjects. Interestingly, early-onset smokers activated a different part of that brain region compared to later-onset smokers, perhaps suggesting a neural change in response to marijuana exposure at an early age.

“Our results provide further evidence that marijuana use has a direct effect on executive function and that both age of onset and magnitude of marijuana use can significantly influence cognitive processing,” said Gruber.

NPR also reported that all pot smokers showed more problems than non smokers.

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Unfortunately, I can’t find a link to the abstract.