‘Teenagers who smoke cannabis for just three years could be damaging their long-term memory, researchers have warned. Participants in a study who used the drug daily for around three years in their teens had an abnormally shaped hippocrampus—a region of the brain vital to memory by the time they were in their early 20s, they also performed around 18 per cent worse in long-term memory tests than individuals who had never touched the drug.
The results were uncovered using sophisticated brain-mapping scans taken two years after they stopped smoking cannabis. Professor John Csernansky, from Northwestern University in the US, who co-led the research, said: “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”. The study is one of the first to suggest that abnormally shaped brains in heavy cannabis users is directly related to memory impairment. The longer the participants had been exposed to cannabis the more misshapen their hippocampus appeared on scans.
This could mean brain regions related to memory may be more susceptible to the effects of the drug the longer the abuse occurs. fIn total, 97 people took part in the study, including some who started smoking cannabis daily between the ages of 16 and 17, and continued for around three years. At the time of the study, they had been cannabis-free for a around two years.
The scientists used new computer software to fine-map MRI scans of the hippocampus. Beforehand participants had taken a memory test in which they listened to a series of stories for around one minute before recalling as much of the content as possible 20 to 30 minutes later.
Results of the memory test were correlated with the scans and cannabis use for each individual. Lei Wang, a senior author of the study and assistant professor of psychiatry and behavioural sciences at the university, said: ‘Advanced brain mapping tools allowed us to examine detailed and sometimes subtle changes in small brain structures.”
The study also found that young adults with schizophrenia who abused cannabis in their teens performed about 26 per cent worse on memory tests than young adults with schizophrenia who had never smoked cannabis. Previous research by the same team has linked poor short-term and working memory performance to abnormal shapes of three other brain regions, the striatum, globus pallidus and thalamus.
Coauthor Dr. Matthew Smith, whose study is published in journal Hippocampus, said: ‘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. It is possible that the abnormal brain structures reveal a pre-existing vulnerability to marijuana abuse. But evidence that the longer the participants were abusing marijuana, the greater the differences in hippocampus shape suggests marijuana may be the cause.”