Neurophysiological and cognitive effects of smoked marijuana in frequent users

Abstract

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.

Conflict of interest statement

The authors declare that, except for income received from our primary employer, no financial support or compensation has been received from any individual or corporate entity over the past three years for research or professional service and there are no personal financial holdings that could be perceived as constituting a potential conflict of interest.

Copyright 2010 Elsevier Inc. All rights reserved.

Figures

Figure 1

Top Panels: Overall accuracy performance on the episodic memory task as a function of Δ9-THC concentration and time. Middle Panels: Percentage of accurate identification of old words on the episodic memory task as a function of Δ9-THC concentration and time. Bottom Panels: Percentage of accurate identification of new words on the episodic memory task as a function of Δ9-THC concentration and time. Error bars represent one SEM. Overlapping error bars were omitted for clarity. An * indicates that the 1.8 and 3.9% Δ9-THC conditions significantly differed from the placebo condition (p < 0.05). An § indicates that the 3.9% Δ9-THC condition significantly differed from the placebo condition (p < 0.05). An † indicates that the 1.8% Δ9-THC condition significantly differed from the placebo condition (p < 0.05).

Figure 2

Top Panels: Percentage of accurate responding on the working memory task during the low- (left) and high-load (right) versions as a function of Δ9-THC concentration and time. Bottom Panels: Mean amount of time participants required to complete the working memory task during the low- (left) and high-load (right) versions as a function of Δ9-THC concentration and time. Error bars represent one SEM. Overlapping error bars were omitted for clarity. An * indicates that the 1.8 and 3.9% Δ9-THC conditions significantly differed from the placebo condition (p < 0.05). An § indicates that the 3.9% Δ9-THC condition significantly differed from the placebo condition (p < 0.05). An † indicates that the 1.8% Δ9-THC condition significantly differed from the placebo condition (p < 0.05).

Figure 3

Top Panel: Evoked potentials relative to a 200 ms pre-stimulus baseline at parietal-occipital site POz during the Presentation and Recognition phases of the episodic memory task, before (dotted line) and 15 min after (solid line) smoking. Amplitude of the slow wave decreased after smoking the 3.9% Δ9-THC cigarette, and amplitude of the P150 decreased after smoking both active Δ9-THC concentration cigarettes relative to placebo. Bottom Panel: Mean slow wave amplitude as a function of Δ9-THC concentration and time. Slow wave amplitude was often lower in the 1.8% Δ9-THC than placebo condition, but the change from pre-smoking baseline was significantly different from placebo only 15 min after smoking the 3.9% Δ9-THC cigarette. Error bars represent one SEM. Overlapping error bars were omitted for clarity. An § indicates that the 3.9% Δ9-THC condition significantly differed from the placebo condition (p < 0.05).

Figure 3

Top Panel: Evoked potentials relative to a 200 ms pre-stimulus baseline at parietal-occipital site POz during the Presentation and Recognition phases of the episodic memory task, before (dotted line) and 15 min after (solid line) smoking. Amplitude of the slow wave decreased after smoking the 3.9% Δ9-THC cigarette, and amplitude of the P150 decreased after smoking both active Δ9-THC concentration cigarettes relative to placebo. Bottom Panel: Mean slow wave amplitude as a function of Δ9-THC concentration and time. Slow wave amplitude was often lower in the 1.8% Δ9-THC than placebo condition, but the change from pre-smoking baseline was significantly different from placebo only 15 min after smoking the 3.9% Δ9-THC cigarette. Error bars represent one SEM. Overlapping error bars were omitted for clarity. An § indicates that the 3.9% Δ9-THC condition significantly differed from the placebo condition (p < 0.05).

Figure 4

Top Panel: EEG spectral power at midline frontal site Fz averaged over all task and resting conditions, before (light line) and 15 min after (dark line) smoking. Power in the theta and beta bands decreased after smoking both active Δ9-THC concentration cigarettes relative to placebo. Bottom Panel: Mean theta band power as a function of Δ9-THC concentration and time. Error bars represent one SEM. Overlapping error bars were omitted for clarity. An * indicates that the 1.8 and 3.9% Δ9-THC conditions significantly differed from the placebo condition (p < 0.05). An § indicates that the 3.9% Δ9-THC condition significantly differed from the placebo condition (p < 0.05).

Figure 4

Top Panel: EEG spectral power at midline frontal site Fz averaged over all task and resting conditions, before (light line) and 15 min after (dark line) smoking. Power in the theta and beta bands decreased after smoking both active Δ9-THC concentration cigarettes relative to placebo. Bottom Panel: Mean theta band power as a function of Δ9-THC concentration and time. Error bars represent one SEM. Overlapping error bars were omitted for clarity. An * indicates that the 1.8 and 3.9% Δ9-THC conditions significantly differed from the placebo condition (p < 0.05). An § indicates that the 3.9% Δ9-THC condition significantly differed from the placebo condition (p < 0.05).

Figure 5

Ratings of “Good Drug Effect” (left) and heart rate as a function of Δ9-THC concentration and time. Error bars represent one SEM. Overlapping error bars were omitted for clarity. An * indicates that the 1.8 and 3.9% Δ9-THC conditions significantly differed from the placebo condition (p < 0.05). An § indicates that the 3.9% Δ9-THC condition significantly differed from the placebo condition (p < 0.05).