Neurophysiological and cognitive effects of smoked marijuana in frequent users.

RATIONALE: Previously, we reported that acute marijuana intoxication minimally affected complex cognitive performance of daily marijuana smokers. It is possible that the cognitive tests used were insensitive to marijuana-related cognitive effects.

OBJECTIVES: In the current study, electroencephalographic (EEG) signals were recorded as daily marijuana users performed additional tests of immediate working memory and delayed episodic memory, before and after smoking marijuana.

METHODS: Research volunteers (N=24), who reported smoking approximately 24 marijuana cigarettes/week, completed this study. Participants completed baseline computerized cognitive tasks, smoked a single marijuana cigarette (0%, 1.8%, or 3.9% (9)-THC w/w), and completed additional cognitive tasks; sessions were separated by at least 72-hours. Cardiovascular and subjective effects were also assessed throughout sessions.

RESULTS: Overall performance accuracy was not significantly altered by marijuana, although the drug increased response times during task performance and induced a response bias towards labeling "new" words as having been previously seen in the verbal episodic memory task. Marijuana reduced slow wave evoked potential amplitude in the episodic memory task and decreased P300 amplitude and EEG power in the alpha band in the spatial working memory task. Heart rate and "positive" subjective-effect ratings were increased in a (9)-THC concentration-dependent manner.

CONCLUSIONS: Relative to previous findings with infrequent marijuana users, the frequent users in the current study exhibited similar neurophysiological effects but more subtle performance effects. These data emphasize the importance of taking into account the drug-use histories of research participants and examining multiple measures when investigating marijuana-related effects on cognitive functioning.