Δ⁹-Tetrahydrocannabinol During Encoding Impairs Perceptual Details yet Spares Context Effects on Episodic Memory

Background: With the growing acceptance of cannabis use, it is crucial to understand the drug's effects on episodic memory accuracy and distortion. We investigated the impact of the administration of Δ⁹-tetrahydrocannabinol (THC), the main psychoactive constituent of cannabis, on a context-based memory illusion. Methods: In a double-blind, placebo-controlled, within-subjects design, healthy infrequent cannabis users (N = 24) memorized object pictures that were superimposed over scenes (e.g., gray cat on beach) after pretreatment with placebo or THC (15 mg oral). Two days later under sober conditions, memory for the object pictures was tested by asking participants to discriminate between previously seen objects or perceptually similar lures (e.g., different gray cat). Context reinstatement was manipulated by presenting objects on their original or different scenes (e.g., beach or forest). Results: THC impaired memory for perceptual details of objects compared with placebo, and the context illusion was obtained in each condition: context reinstatement increased high-confidence false recognition along with correct recognition of previously seen objects. Although THC did not interact with these context effects overall, post hoc analyses showed that THC magnified the context illusion when objects were semantically congruent with their encoding contexts but abolished the context illusion when objects were incongruent with their encoding contexts. Conclusions: These results are consistent with the hypothesis that THC impairs the encoding of specific object information more than item-context associations. As a result, THC may spare the distorting effects of context reinstatement on memory. In fact, THC may increase these distorting effects under conditions when objects are semantically congruent with context.