Pop culture always depicts “stoner” types as bumbling, forgetful types as though to suggest marijuana inhibits brain function. Those images may not be too far off the mark, says a study by the Northwestern University Feinberg School of Medicine: Teens who smoke weed regularly display poor long-term memory.
Heavy marijuana use, defined as daily consumption for at least three years, has physical effects on the brain itself: The hippocampus, associated with long term (or episodic) memory, is typically misshapen when compared with teens who abstain from the drug (or at least smoke it less).
“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 senior author John Csernansky, MD, Lizzie Gilman Professor and chair of Psychiatry and Behavioral Sciences.
The changes were observed in subjects in their early 20s, two years after they stopped smoking marijuana. In tests, those subjects scored about 18% worse than similar-aged adults who never used marijuana so heavily.
Science has long suspected a link between marijuana use and memory damage. The drug has also been believed to alter the growth of the hippocampus – in fact, both suspicions have been confirmed in previous studies. What makes this one special is that it’s the first to link the memory performance with the shape of the hippocampus, and then tie both to marijuana abuse.
The rest of the brain isn’t immune, either – previous research by Northwestern found that structures in the sub-cortex, associated with short-term and working memory, showed signs of being misshapen as well.
“Both our recent studies link the chronic use of marijuana during adolescence to these differences in the shape of brain regions that are critical to memory and that appear to last for at least a few years after people stop using it,” said lead study author Matthew Smith, PhD, assistant professor in Psychiatry and Behavioral Sciences.
To test their theories, scientists used computer programs that performed detailed mappings between MRI scans of multiple individuals’ brains. Subjects were then given a memory test that involved listening to a story for one minute and then recalling as much information as possible 20 to 30 minutes later.
Included in the 97-person study were subjects with schizophrenia, both those who abused marijuana and those who did not. It turns out pot use is even more detrimental for those subjects, as they scored 26% worse on the tests than schizophrenia sufferers who had not abused weed.
Though the study suggests that marijuana’s active ingredients cause damage to the brain’s neurons, axons and supportive environments, as of now there’s only correlation. It could be, for instance, that subjects with deformed brain structures are predisposed to marijuana abuse.
The results may throw a wrench in nation-wide efforts to legalize and/or decriminalize marijuana. Already the most popular drug among teenagers, the researchers worry that legalization could increase access.