We 59 also looked at response inhibition in marijuana users after 28 days of monitored abstinence, as compared to matched controls. Participants were excluded for any neurological problems or Axis I diagnoses other than cannabis abuse or dependence. The study used the same go/no-go task described above, and although marijuana users performed similarly as controls, they exhibited increased activation on inhibition (“no-go”) trials in right dorsolateral prefrontal cortex, bilateral medial frontal cortex, bilateral inferior and superior parietal lobules, and right occipital gyri. On “go” trials, marijuana users had increased activation in right prefrontal, insular, and parietal cortices (p<.05, clusters >943 µl). More response during “no-go” trials related to worse neuropsychological performance (e.g., impulsivity, complex attention, cognitive flexibility, planning). Neuropsychological indicators of impulsivity were in turn linked to more medial temporal and less anterior cingulate response in marijuana users (p<.05). Differences remained even after controlling for lifetime and recent alcohol use. This suggests that marijuana users have increased brain processing effort during an inhibition task despite showing intact task performance, even after 28 days of abstinence. Such increased neural processing effort to achieve inhibition may predate the onset of regular use, or result from it.
