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When Marijuana and Memory React

Study Links Cannabis Use to Poor Long-Term Memory

Marijuana is prominently featured in modern culture and in the news. Decriminalization is already leading to greater recreational and medicinal use, and the drug’s effects are still being studied. However, the latest study on the subject from Northwestern Medicine is the first to link abnormal changes in the hippocampus to poor long-term memory in former teen users.

The hippocampus, part of the brain’s temporal lobe, plays an important role in long-term, or episodic, memory, which includes the ability to remember autobiographical or life events. The Northwestern Medicine team, led by senior author John Csernansky, MD, Lizzie Gilman Professor and chair of Psychiatry and Behavioral Sciences at Northwestern University Feinberg School of Medicine, discovered the hippocampus to be oddly shaped in heavy cannabis users. The study also linked heavy marijuana use to poor long-term memory, with those users scoring 18 percent worse on memory tests.

Marijuana is the most commonly used illicit drug, and its use is highest — and growing — among young adults. The study tested individuals in their early 20s about two years after they stopped smoking marijuana. The participants had been heavy users — smoking daily — for about three years starting when they were between 16 and 17. The groups included healthy controls, participants with a history of marijuana use, participants with a history of marijuana use and schizophrenia, and participants with schizophrenia but not history of substance misuse. None of the subjects who used marijuana had a problem with any other drugs.

High-tech mapping and old school testing

Computerized programs developed by Northwestern Medicine with collaborators performed advanced brain mapping to examine the detailed and subtle differences in brain structure. In addition to fine mappings, the scientists administered a narrative memory test in which subjects listened to a series of stories for about one minute and were asked to recall as much as possible 20-30 minutes later. This test was intended to assess their ability to remember and recall details.

The research suggested that the longer teens used cannabis, the more abnormal the hippocampus would appear in adulthood. The findings imply that the brain becomes more susceptible to damaging effects the more chronic the use becomes. The scientists also found that young adults with schizophrenia who used marijuana performed 26 percent worse on memory tests than those who did not.

A track record and a path forward

Previous research by the same Northwestern Medicine scientists showed poor short-term and working memory performance in addition to other abnormally shaped brain structures. Abnormal shapes may suggest damage to the structures’ neurons, axons and the supportive environment.

“The memory processes that appear to be affected by cannabis are ones that we use every day to solve common problems and to sustain our relationships with friends and family,” said Csernansky.

Though it’s possible the abnormal brain structures may simply have revealed a pre-existing vulnerability to the effects of marijuana on memory, the Northwestern Medicine scientists believe the evidence suggests cannabis may be the cause. Because the study results examined one point in time, longitudinal research would be needed to show if marijuana is definitively responsible for the observed differences in brain and memory.