Young adults who smoked marijuana every day for three years during their teen years have an oddly shaped hippocampus and performed poorly on long-term memory tasks, according to a new study published in Hippocampus this week.
A team led by Northwestern’s Matthew Smith used MRI to map the brains of 97 participants: 44 healthy controls, 10 subjects with a history of marijuana use disorder, 28 schizophrenia patients with no history of substance use disorders, and 15 schizophrenia patients with a marijuana use disorder. Participants with past cannabis use disorder were in their early twenties during the study and had stopped smoking pot for two years; they all started using marijuana daily when they were between 16 and 17 years old for about three years.
All of the recruits took a narrative memory test designed to assess their ability to encode, store, and recall details from stories. "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," study co-author John Csernansky of Northwestern explains in a news release. Previous work have linked adolescent cannabis use with poor short-term and working memory, as well as the abnormal shape of sub-cortex structures. The hippocampus, on the other hand, plays a key role in long-term (or episodic) memory—the ability to remember life events.
Compared with controls, former users scored 18 percent worse on the long-term memory test, and the longer they used marijuana, the more abnormal their hippocampal shape appeared. The change in shape likely reflects structural damage to the hippocampus and its neurons and axons. Furthermore, young adults with schizophrenia who abused cannabis as teens performed about 26 percent more poorly on memory tests compared with young adults with schizophrenia who never smoked pot.
The study only looks at one point in time, so a much longer study is needed to confirm marijuana’s effects on the brain and memory impairment. Another recent MRI study with marijuana users, for example, failed to find any differences in the volumes in brain regions, including the hippocampus. Additionally, "it is possible that the abnormal brain structures reveal a pre-existing vulnerability to marijuana abuse," Smith adds. "But evidence that the longer the participants were abusing marijuana, the greater the differences in hippocampus shape suggests marijuana may be the cause."