Overall, changes in brain functioning in adolescents differ by substance use pattern. Research has shown that heavy drinking during adolescence can lead to decreased performance on cognitive tasks of memory, attention, spatial skills, and executive functioning. These behavioral ramifications of heavy alcohol use may emerge as a consequence of the reduced volume of important brain structures (e.g., hippocampus), compromised quality of white matter, and abnormalities in activation during cognitive tasks. Studies have also shown that marijuana use during adolescence can result in decreases in cognitive functioning, particularly learning and sequencing scores. In integrating and interpreting the results of adolescent marijuana studies from our laboratory, it is important to note that the groups are generally equivalent on task performance, and therefore the underlying brain responses in controls and users can be largely assumed to represent activity to the same mental action. Corresponding marijuana-related changes in cognition may be related to increases in gray matter tissue volume, decreases in white matter microstructural integrity, and increases in neuronal activation during cognitive tasks.
