WASHINGTON – Cannabis consumers who start using marijuana in their teens may be altering the hardwire hubs that route signals within the brain. When it comes to cognitive tasks like working memory and inhibitory control, though, regular cannabis consumers test just the same as non-consumers.
Those are some of the findings of a new study conducted by Peter Manza, Dardo Tomasi, and Nora Volkow of the National Institute on Drug Abuse (NIDA). Volkow is the longtime director of NIDA.
Manza presented the study earlier this month at the Society for Neuroscience’s annual meeting in Washington, D.C. The full study has yet to be published, but Manza spoke about the results and presented an abstract at the conference.
Data from the Connectome Project
Drawing from 441 participants in the Human Connectome Project, the researchers compared 30 regular cannabis consumers, ages 22-35, with a control group of 30 non-consumers of similar age. For the purposes of the study, “regular consumption” was defined as having consumed cannabis more than 1,000 times in a lifetime.
Researchers found no significant differences in cognitive outcomes (like cognitive speed and working memory) between regular cannabis consumers and non-consumers.
Researchers found that the regular cannabis consumers experienced more negative emotions than non-consumers. The younger they were when they first consumed cannabis, the worse the feelings they reported.
The study did not find any significant differences in cognitive outcomes between regular cannabis consumers and non-consumers. Those cognitive measures included processing speed, working memory and inhibitory control.
Researchers did find a correlation between underage cannabis consumption and local functional cognitive density (lFCD), a measure of local network connectivity in a given region of the brain. Cannabis consumers who started using cannabis at age 21 or older exhibited nearly identical brain connectivity levels as those who had never used cannabis.
Age of Onset Makes a Difference
MRI scans revealed differences in consumers who began using cannabis under the age of 21. The younger the participants were when they started using cannabis, the greater the difference in lFCD compared to lifetime non-consumers.
The researchers are not sure if cannabis caused the negative emotions, because they did not test for causation. The study showed only that regular cannabis use, brain activity and negative emotions may be associated, said Manza. He added: “We don’t know what’s causing what.”
The researchers also don’t know if the participants already were experiencing negative emotions before they started consuming cannabis regularly, Manza cautioned, nor if their feelings were solely associated with regular cannabis use (and not other lifestyle factors). The researchers did not examine THC-potency levels of the cannabis participants used, which Manza acknowledged can be related to emotions.
Manza, a post-doctoral fellow at the National Institutes of Health, speculated that the study’s findings may be due to cannabis’ impact on brain dopamine levels when it is consumed regularly. Impaired dopamine levels have previously been revealed in psychopathology associated with cannabis abuse, the researchers reported, including a higher risk of psychosis.
NIDA is now considering assessing what happens when regular consumers withdraw from habitual use, Manza said, in addition to examining the relationships between regular cannabis use, brain activity and dopamine release.
Researchers organized the user and control groups at host site Washington University (St. Louis) to mitigate the influence of variables including age, sex, and alcohol and other substance use. Participants’ median age was 29-30 and two-thirds of participants were male (a ratio that Manza said reflects the regular cannabis user population.)
The team plans to publish this study in a scientific journal, Manza said.