Marijuana effects on human forgetting functions

Abstract

It has long been known that acute marijuana administration impairs working memory (e.g., the discrimination of stimuli separated by a delay). The determination of which of the individual components of memory are altered by marijuana is an unresolved problem. Previous human studies did not use test protocols that allowed for the determination of delay-independent (initial discrimination) from delay-dependent (forgetting or retrieval) components of memory. Using methods developed in the experimental analysis of behavior and signal detection theory, we tested the acute effects of smoked marijuana on forgetting functions in 5 humans. Immediately after smoking placebo, a low dose, or a high dose of marijuana (varying in delta9-THC content), subjects completed delayed match-to-sample testing that included a range of retention intervals within each test session (0.5, 4, 12, and 24 s). Performances (discriminability) at each dose were plotted as forgetting functions, as described and developed by White and colleagues (White, 1985; White & Ruske, 2002). For all 5 subjects, both delta9-THC doses impaired delay-dependent discrimination but not delay-independent discrimination. The outcome is consistent with current nonhuman studies examining the role of the cannabinoid system on delayed matching procedures, and the data help illuminate one behavioral mechanism through which marijuana alters memory performance.

Fig. 1. Mean (+ SD) percentage correct DMTS data for the 5 subjects and the group (+ SEM) at each dose

Each bar pattern represents a different dose, with multiple determinations at each dose. PLC indicates placebo; M1 indicates half placebo and half 2.2% Δ⁹-THC; M2 = 3.89% Δ⁹-THC. See text for dose administration details.

Fig. 2. Mean discriminability, calculated as logit p + 1, as a function of delay interval between sample and comparison stimuli

Each panel shows an individual subject, with the group mean (± SEM) in the bottom right panel. In each panel, each symbol represents the mean (± SEM) of a different dose with multiple determinations at each dose. Curves represent negative exponential forgetting functions fit to the data from each dose. ○ and solid curve indicate placebo; ▿ and dashed curve indicate half placebo and half 2.2% Δ⁹-THC (M1); □ and solid curve indicate 3.89% Δ⁹-THC (M2). Note different scaling on y axis for Subject 2896. See text for details on the calculation of logit p +1 and the negative exponential function.