Δ9-Tetrahydrocannabinol (THC) impairs visual working memory performance: a randomized crossover trial.

THC, the main psychoactive component of cannabis, impairs visual working memory—the ability to temporarily hold information in mind. In two double-blind, randomized crossover experiments, healthy adults performed a visual working memory task after taking oral THC (7.5 or 15 mg) or a placebo. THC reduced working memory performance (effect size d = 0.65), increased self-reported mind wandering, and decreased the accuracy of participants' awareness of their own task performance. The findings suggest that THC disrupts working memory both by increasing mind wandering and by reducing metacognitive monitoring. The study also demonstrated that short task durations and small sample sizes can mask such drug effects.

Study at a glance

Abstract

With the increasing prevalence of legal cannabis use and availability, there is an urgent need to identify cognitive impairments related to its use. It is widely believed that cannabis, or its main psychoactive component Δ9-tetrahydrocannabinol (THC), impairs working memory, i.e., the ability to temporarily hold information in mind. However, our review of the literature yielded surprisingly little empirical support for an effect of THC or cannabis on working memory. We thus conducted a study with three main goals: (1) quantify the effect of THC on visual working memory in a well-powered sample, (2) test the potential role of cognitive effects (mind wandering and metacognition) in disrupting working memory, and (3) demonstrate how insufficient sample size and task duration reduce the likelihood of detecting a drug effect. We conducted two double-blind, randomized crossover experiments in which healthy adults (N = 23, 23) performed a reliable and validated visual working memory task (the "Discrete Whole Report task", 90 trials) after administration of THC (7.5 and/or 15 mg oral) or placebo. We also assessed self-reported "mind wandering" (Exp 1) and metacognitive accuracy about ongoing task performance (Exp 2). THC impaired working memory performance (d = 0.65), increased mind wandering (Exp 1), and decreased metacognitive accuracy about task performance (Exp 2). Thus, our findings indicate that THC does impair visual working memory, and that this impairment may be related to both increased mind wandering and decreased monitoring of task performance. Finally, we used a down-sampling procedure to illustrate the effects of task length and sample size on power to detect the acute effect of THC on working memory.