Source: Alice G. Walton, "The Brain May Compensate For Heavy Pot Smoking -- Until It Can't Anymore," Forbes, 11 Nov 2014.
"The research, published in Proceedings of the National Academy of Sciences, found that an area in the front of the brain called the orbitofrontal cortex (OFC) is smaller in people who smoke three joints a day than people who never touch the stuff. But what's interesting about the new study is that it also found greater connectivity in the white matter throughout the brain, suggesting that the brain is trying to compensate in some way – at least for a while. After years of heavy pot use, the study also found, the brain can no longer continue this compensation, and the effects begin to disappear."
"In previous research, we have observed that changes in the orbital frontal cortex are correlated with greater use of marijuana and behavioral difficulties in participants' personal and professional life."
"What we did find is that the younger an individual was when they started using marijuana the greater the brain changes appeared to be."