Young adults who used marijuana only recreationally showed significant abnormalities in two key brain regions that are important in emotion and motivation, scientists report. The study was a collaboration between Northwestern Medicine and Massachusetts General Hospital/Harvard Medical School. It is the first study to show casual use of marijuana is related to major brain changes. It showed the degree of brain abnormalities in these regions is directly related to the number of joints a person smoked per week. The more joints a person smoked, the more abnormal the shape, volume, and density of the brain regions.