Various studies on marijuana usage have analyzed its effects on cognition, short-term memory loss and other brain activity. A 2015 study has confirmed that teenagers who regularly use marijuana and later quit may still experience some changes to their long-term memory. The study was published on March 12, 2015, in the journal Hippocampus and analyzed the data of 97 participants, some with a history of regular marijuana usage and others who did not use marijuana.
This study was led by Matthew J. Smith, assistant professor of psychiatry and behavioral science at Northwestern University Feinberg School of Medicine. The participants with a history of marijuana usage scored worse on long-term memory tests than their abstaining counterparts, even two years after dropping the habit. This was discovered via scans of the hippocampus, the part of the brain that stores and recalls long-term memories.
The active ingredient in marijuana is tetrahydrocannabinol, or THC. THC, along with up to 70 other cannabinoids, or compounds, targets the hippocampus, because that part of the brain is covered with cannabinoid receptors. "Due to the high level of receptors in the hippocampus that attract THC, I think it makes sense that we're seeing that that part of the brain looks different within someone who abused THC for several years as an adolescent," says Smith.
While none of the study participants reported using any other drugs besides marijuana, one interesting facet of this study is the inclusion of participants diagnosed with schizophrenia. Twenty-eight participants with schizophrenia had no history of marijuana usage, while 15 did. Research indicates that schizophrenic patients are known to have both short- and long-term memory loss and also are more likely to use marijuana at some point in their lives. Moreover, some research suggests that marijuana's ability to promote psychosis may actually encourage the onset of schizophrenia.
The memory tests involved reading a story and then being asked to recall specific details 20 to 30 minutes later. Psychologically healthy participants with a history of marijuana usage scored 18 percent worse on the memory tests than their nonsmoking counterparts, while schizophrenic participants scored 26 percent worse than schizophrenic participants who abstained. These effects were likely due to age, as the marijuana usage among participants happened between the ages of 16 and 17 years before the brain reached full development.
In addition to effects on memory, regular marijuana usage shrinks the orbitofrontal cortex of the brain, the area responsible for decision-making, causing a person to have limited judgment. This also causes a resistance to build up over time, causing the person to have to use more marijuana more frequently to achieve the same effects.
With the 2014 legalization of marijuana in some states, more research is being performed to study its effects on the brain, and scientists continue to find new information on how THC affects a person's thoughts and behaviors. However, unregulated marijuana usage, the most common kind, was not taken into account by Smith and his team, meaning that those who purchase marijuana on the street may not know exactly how much it affects their memory.
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