Studies of grey matter show more consistent differences, with evidence for bilateral hippocampus and amygdala volume reductions in adults with history of long-term cannabis use, where left hippocampal volume was inversely related to chronicity of exposure to cannabis (Yucel et al., 2008). In addition, a voxel-based morphometry study in young adults with first episode schizophrenia and history of adolescent marijuana use showed more prominent gray matter density and volume reduction in the right posterior cingulate cortex compared to their non-using counterparts (Bangalore et al., 2008). These changes may be influenced by ongoing neurodevelopment, particularly when exposure begins early. The functional implications of these differences appear disadvantageous, as marijuana-using teens show an increased susceptibility to depressive symptoms (Medina et al., 2007b) and poorer performance than non-users on neuropsychological tests of psychomotor speed, complex attention, story memory, planning, and sequencing ability, even after a month of sustained abstinence (Medina et al., 2007a).
