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

Abstract

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.

Trial registration: ClinicalTrials.gov NCT03471585.

Keywords: Cannabis; Context reinstatement; Episodic memory; False memory; Memory illusion; THC.

Conflict of interest statement

Financial Disclosures

All authors report no biomedical financial interests or potential conflicts of interest.

Copyright © 2019 Society of Biological Psychiatry. Published by Elsevier Inc. All rights reserved.

Figures

Figure 1.

Schematic of experiment and the MS-Doss task. Participants encoded object-scene stimuli approximately 2.5 hours after capsule administration followed by a memory test for the objects two days later. Objects could be the same pictures from encoding (Targets), similar exemplars of an object from encoding (Similar Lures), and novel exemplars (Dissimilar Lures). Additionally, context reinstatement was manipulated by pairing the object with the original scene from encoding (Reinstated) or switched to a different scene (Switched).

Figure 2.

Hit and false alarm rates as a function of drug condition and context reinstatement (a) and memory accuracy (p(“old”|Target) - p(“old”|Sim. Lure)) and lure discrimination index (p(“similar”|Sim. Lure) - p(“similar”|Dis. Lure)) as a function of drug condition, averaged over context condition (b). Paired scatter points in (a) highlight the significant effect of context reinstatement across drug conditions (asterisks indicate p < .001). Paired scatter points in (b) highlight the THC-induced impairments of specific perceptual details in memory (asterisks indicate p < .05). Horizontal bars and error bars within violin plots are means and 95% CIs, respectively.

Figure 3.

False alarm rates (p(“old”|Sim. Lure)) as a function of drug condition and context reinstatement conditionalized on their item-context semantic congruency (based on each participant’s judgments at encoding). Paired scatter points highlight the significant effect of context reinstatement. Single and double asterisks indicate moderate and strong effects of context reinstatement, respectively. Horizontal bars and error bars within violin plots are means and 95% CIs, respectively.