Magnetic resonance imaging (MRI) was used to examine structural differences in the hippocampus, an area of the brain crucial to intact memory functioning. Participants were classified as: (1) light to non-drinkers (≤1 drink per month, ≤ 1 lifetime marijuana use episode), (2) heavy drinking adolescents (history of consuming 4/5+ drinks in a day), and (3) heavy marijuana users who also engaged in heavy episodic drinking. Manual tracing techniques were employed by reliable raters, and revealed that heavy drinkers had smaller left hippocampal volumes (p<.01), while marijuana+alcohol users had similar volumes as controls 32. Additionally, greater alcohol abuse/dependence severity was associated with smaller left hippocampal volumes, a finding that supported previous animal models 33. Heavy drinkers showed significantly different patterns of hippocampal asymmetry (p<.05; smaller left than right hippocampal volumes) compared to light-drinking youths, with an asymmetry ratio linked to memory performance. For controls, greater right than left hippocampal asymmetry correlated with better verbal learning (p<.05), but not in user groups 34 (see Figure 1).
