fMRI response during spatial working memory in adolescent marijuana users : what is the relationship to recency of marijuana use?

Marijuana is commonly used in adolescence, yet the impact on the developing brain is unclear. Working memory impairments have been observed in adult marijuana users after recent use, but may remit after a month of abstinence. The differential effects related to recent use and abstinence have not been delineated in adolescents. To address this question, three studies examined functional magnetic resonance imaging (fMRI) brain response during spatial working memory (SWM) among adolescents. Adolescent brain development may be influenced by heavy marijuana use, yet the neural underpinnings of SWM have not been well described in adolescents. Study 1 investigated fMRI response to SWM across normal adolescent development. Participants were 49 youths ages 12 - 17 without histories of neurological or psychiatric disorders. Results demonstrate the emergence of left prefrontal activity and superior-to-inferior shift in localization of parietal response with increasing age, suggesting that younger teens utilize more rote spatial rehearsal, while older teens rely more on spatial storage and verbally-mediated strategies. Study 2 evaluated fMRI response during SWM among 15 heavy marijuana using adolescents after 28 days of verified abstinence relative to 17 non-abusing controls, ages 16 - 18 years old. Marijuana users demonstrated decreased right prefrontal and increased right superior parietal response relative to controls, which could suggest greater reliance on spatial strategies and less general executive control among marijuana users. These results were observed after 28 days of abstinence, suggesting persisting differences in brain functioning among heavy marijuana users. Study 3 characterized the differential residual and persisting changes in neural activation patterns associated with adolescent marijuana use by examining fMRI during SWM among adolescent marijuana users after recent use or after one month of abstinence. Participants were 15- to 18-year-olds: 13 marijuana users who used in the week before scanning, 13 marijuana users who were abstinent for 27 - 60 days before scanning, and 18 demographically similar controls. Recent users demonstrated increased medial/left superior frontal and right parietal response relative to abstinent users, which could suggest greater neural effort for inhibitory control and spatial rehearsal. Although cross-sectional, results may indicate a shift in neural processing strategies through early abstinenc