This decline in brain activity was associated with worse performance on working memory like math problems, where we retain and use information to perform tasks by mentally visualizing and manipulating information. Heavy users were considered young adults who had used marijuana more than 1000 times during their lifetimes while 10 to 999 times was labeled a moderate user and less than 10 times was considered a non-user.
The tasks tested working memory, reward, emotion, language, motor skills, such as tapping a finger to map brain control, relational assessment and theory of mind and the authors suggest a statistically significant effect on brain function during working memory tasks - unlikely to be due to random chance.
Because this is a cross-sectional study using functional MRI this paper is EXPLORATORY and should not be used for causal claims.

Since the effect was seen in both recent and lifetime cannabis users but less significant for the other tasks, this shows the downside to using statistical significance of something significant like colors of brain scans to create conclusions, and why journal editors should look critically at exploratory papers that invoke statistical significance.
During working memory tasks, the researchers found heavy cannabis use appeared to reduce brain activity in areas of the brain such as the dorsolateral prefrontal cortex, dorsomedial prefrontal cortex and anterior insula. These regions of the brain are linked to decision-making, memory, attention and emotional processing.
As governments continue to enable recreational marijuana use, the health effects will continue to be studied.
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