Pot Smoking Reduces Grey Matter but Increases Connectivity

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Marijuana use is indeed associated with changes in the brain, finds a new study published in the Proceedings of the National Academy of Sciences of the United States of America (PNAS). The brains of chronic marijuana users showed decreased mass in a region associated with decision-making and emotional processes, in comparison to the brains of non-users. However, that same region in marijuana-affected brains showed greater connectivity with other areas of the brain, perhaps a sign of the brain rewiring itself to adjust to the deficiency.

“The changes in connectivity may be considered a way of compensating for the reduction in volume,” said Francesca Filbey, the author of the study. “This may explain why chronic users appear to be doing fine, even though an important region of their brain is smaller in terms of volume.”

Researchers from the University of Texas at Dallas used several MRI imaging techniques in order to compare the brains of almost 50 marijuana smokers to those of 62 non-users who were matched according to criteria like age and sex – thus reducing the likelihood of conflicting results often produced by such testing when it uses different methodologies. The research also accounted for users’ tobacco and alcohol consumption.

All of the study’s “chronic” users were smoking marijuana three times daily on average. However, they were spread across a wide age range and had started using the drug at varying stages of life, which let researchers recognize changes throughout a lifespan without developmental biases interfering.

The research shows that marijuana users, in comparison to the control subjects, displayed less volume in the orbitofrontal cortex (OFC), the area of the brain that participates in decision-making, emotional responses and reward-and-punishment behavior. This region, however, displayed greater structural and functional connectivity with other areas. Scientists surmise that this could be an adaptive answer to the gradual reduction of mass in the OFC. Furthermore, the study determined that participants who began smoking earlier in life showed greater connectivity in the OFC. The greatest connectivity appeared at the beginning of use and decreased after 6-8 years of chronic use, though still greater than that of the non-users.

Researchers noted that the marijuana-users had a lower IQ score than the control subjects, but these findings showed no relation to change in brain mass.

Supporters of marijuana use have criticized the study as inconclusive when it comes assessing whether the changes observed are connected to negative performance outcomes, like mental agility or quality of life. Researchers readily conceded that the study did not determine whether pot smoking preceded the neurological changes or the opposite – a chicken or egg dilemma. Only long-term research can answer that type of question.

“It may be that these cannabis users are functioning in their daily lives in a manner that is indistinguishable from controls,” commented Paul Armentano of the National Organization for the Reform of Marijuana Laws, “in which case these imaging differences may hold little, if any, real-world significance.”