Dr. Gruber and her team found that on tests of "executive function" — higher-order brain skills that include planning and carrying out mental tasks — pot-users who started smoking before age 16 made twice as many mistakes as those who started later.
They also found that users who started younger tended to smoke far more than the later starters — three times as much pot, and twice as often — and that their brains "lit up" differently in the scanner, suggesting significant neural change.
This from McLean:
“We have to be clear about getting the message out that marijuana isn’t really a benign substance,” [Dr. Gruber] said. “It has a direct effect on executive function. The earlier you begin using it, and the more you use of it, the more significant that effect.”
The study included 33 chronic marijuana smokers and 26 control subjects 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.
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 to 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.
Dr. Gruber is the director of the Cognitive and Clinical Neuroimaging Core at McLean. Her team's findings reminded me of a frightening 2006 story I wrote at the Boston Globe. It began:
Researchers are offering new ammunition to worried parents trying to dissuade their teens from smoking marijuana: Evidence is mounting that for some adolescents whose genes put them at added risk, heavy marijuana use could increase the chances of developing severe mental illness psychosis or schizophrenia.
This week, the marijuana-psychosis link gained ground when two major medical journals reviewed the research to date and concluded that it was persuasive. In PLOS Medicine, an Australian public health policy specialist wrote that genetically vulnerable teens who smoke marijuana more than once a week "appear at greater risk of psychosis," while the British medical journal BMJ cited estimates that marijuana use could contribute to about 10 percent of cases of psychosis.
The study found that among those with this variant, smoking marijuana as teens increased their risk of psychosis in young adulthood nearly tenfold compared with those who did not smoke as teens. Those who smoked marijuana but did not have the gene variant incurred little or no added risk.
Dr. Gruber connected the dots between her findings and politics, pointing out that ballot measures in this month's elections had sought to legalize marijuana. (Sounds like she's against the idea.) My own takeaway was: By hook or by crook, keep your child away from pot at least until age 16...
This program aired on November 15, 2010. The audio for this program is not available.