From William Treurniet:

Re: Brain abnormalities seen in heavy pot smokers (June 2): Mu­rat Yü­cel et al. apparently concluded that certain brain structures were small because of the heavy pot use. Given the methodology reported in the article, such a conclusion is premature. Brain scans were not performed before the subjects began smoking pot, so changes in brain structure sizes were not measured. Only a correlation between brain structure size and pot use was observed, and that does not imply causality. People may just as well have become heavy pot smokers because they had smaller brain structures. Perhaps the pot somehow compensated for some psychological inadequacy due to the smaller hippocampus and amygdala, and that was why they became habitual users. Contrary to the reported conclusion, the results equally suggest that smoking pot may have had a positive effect on the heavy users.