Memory Goes Up in Smoke

The marijuana legalization movement has been picking up steam, with voters in Colorado and Washington passing laws to allow recreational use of the drug. But a new Northwestern study that links marijuana use to poor working memory suggests more research is needed on the detrimental effects of cannabis.

In a study of teens who were heavy marijuana users — smoking it daily for about three years — researchers found that the subjects performed poorly on working memory tests and had abnormal structures in their brains.

The subjects began using marijuana daily during mid-adolescence — a vulnerable time for neurodevelopment. The study occurred at least two years after they had stopped smoking marijuana.

A total of 97 subjects in their early to mid-20s participated, including 44 healthy controls, 10 otherwise healthy subjects who had used marijuana, 28 schizophrenia subjects with no history of substance abuse and 15 schizophrenia subjects who had a history of marijuana use.

Both groups with a history of daily marijuana use showed a pattern of poor performance on a series of tests of working, or short-term, memory, says study author Matthew Smith, an assistant research professor in psychiatry and behavioral sciences at the Feinberg School of Medicine. A poor working memory — the ability to remember and process information in the moment and, if needed, transfer it to long-term memory — predicts poor academic performance and everyday functioning.

Those with a history of marijuana use had similar patterns of shape abnormalities in the brain, but the pattern was more severe among marijuana users with schizophrenia. The study, one of the first to examine marijuana’s effect on the deep regions in the brain, also shows that chronic use of marijuana may contribute to changes in brain structure associated with schizophrenia and augment the disease’s underlying process.

Memory-related structures in the marijuana users’ brains appear to shrink and fold inward, indicating a possible decrease in neurons. The marijuana users’ brain abnormalities are correlated with poor working memory performance.

“The surprising finding,” Smith says, “is that we actually saw any shape differences, because one could hypothesize that after two years of abstinence the brain might start to heal itself. This evidence suggests that it may not necessarily do that.”

The shape differences in both groups of marijuana users correlate with the age of onset of marijuana use, Smith says, “so the younger someone starts using, the more abnormal the brain looks.” The findings suggest that these memory-related regions of the brain may be more susceptible to the effects of the drug if use starts at an earlier age.

“These participants started using when they were around 17 years old,” Smith says. “And we know for healthy brain development, that’s a critical period of life, the teenage years. Introducing marijuana — and probably any other drug or alcohol — into the brain at that age could have detrimental effects.”

In the United States, marijuana is the most commonly used illicit drug, and young adults have the highest — and growing — prevalence of use. Nearly 60 percent of Americans support legalization of marijuana. The study’s authors point out that decriminalization of the drug may lead to more widespread use.

“It seems that there’s not enough known to really make an informed decision” on legalization, given the possible negative effects of marijuana use, Smith adds. “More research is really needed to understand what’s going on.”

Because this cross-sectional study examined just one point in time, Smith says the brain abnormalities may have predated the marijuana use, so a longitudinal study is needed to determine if marijuana is responsible for the brain changes and memory impairment. The evidence that the younger a subject started using marijuana, the greater his brain abnormality indicates marijuana may be the cause. 

Smith and study co-author Hans Breiter (FSM85, 88), professor of psychiatry and behavioral sciences, are working on a follow-up study to look at what happens in the brain immediately after a persistent marijuana user stops using.

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