Whether long-term marijuana smoking has robust effects for the human brain has been a matter of debate. In a paper in PNAS Silbey and colleagus from Dallas, Frisco, and Albuquerque have addressed this question using “multimodal measures in a large group of chronic marijuana” smokers. They claim that marijuna smokers the longer the more have decreasing gray matter in orbitofrotal cortex (OFC).
This part of the brain (citation from wikipedia:)is considered anatomically synonymous with the ventromedial prefrontal cortex. Therefore the region is distinguished due to the distinct neural connections and the distinct functions it performs. It is defined as the part of the prefrontal cortex that receives projections from the magnocellular, medial nucleus of the mediodorsal thalamus, and is thought to represent emotion and reward in decision making. It gets its name from its position immediately above the orbits in which the eyes are located. Considerable individual variability has been found in the OFC of both humans and non-human primates.
The authors are very cautious to attribute these changes to THC. They also found increased connectivity within the OFC and suggest that the OFC gray matter is more vulnerable to the THC effects.