Marijuana and memory: effects of smoking on storage.

Two experiments were conducted to test the hypothesis that smoked marijuana interferes with short-term retention by disrupting the storage phase of the memory process. In the first experiment, information was presented after smoking, and recalled at a later time; in the second experiment, information was presented prior to smoking and recalled after smoking. Ten male students who were experienced users participated in three conditions in each experiment: control (no smoking); placebo (THC-delta-9-free marijuana); and marijuana (= 20 mg THC-delta-9). In Experiment 1, storage occurred after administration of marijuana and both recall and certain aspects of recognition were decreased; in Experiment 2, storage was completed prior to administration of marijuana and no differences in either recall or recognition performance distinguished the marijuana-smoking from the other groups. Experimental studies of the effects of cannabis on perceptual and cognitive function have related these effects to dose, task, and degree of experience with marijuana.1–6 Short-term memory performance is among the most consistently and uniformly affected.3, 7, 10 Subjective accounts by users indicate that memory is adversely affected, and they report occasions when they cannot remember the beginning of their sentences or the word spoken just previously. Short-term memory is typically conceived as divided into stages of acquisition, storage, and retrieval of information. Deficits in memory tasks may be attributed to changes at any one of these stages. Some recent studies suggest that the retention difficulty after marijuana is in the storage phase.7–9 In a previous study we found that even after smoking high doses of marijuana (equivalent to 22.5 mg THC-Δ-9) subjects demonstrated perfect recall of consonant trigrams (e.g. DKG) immediately after each trigram was presented, suggesting that the material was perceived and retrieved without difficulty.9 However, when recall of a trigram was delayed for six seconds, retention decreased significantly in comparison to the placebo and low-dose conditions (7.5 mg THC-Δ-9). The greater the delay between presentation of an item and its recall, the greater the deficit, suggesting two explanations, both locating the deficit in the storage phase: (1) either there is a direct effect of the drug on storage procedures—the mechanism responsible for this effect is unknown; or (2) attention cannot be sustained after administration of marijuana. Storage necessitates attention for rehearsal (encoding) of material; otherwise it decays rapidly and cannot be easily retrieved. If then, storage is disrupted, information introduced immediately after smoking will be affected and recall of this material at a later time will reflect inadequate encoding or rehearsal procedures. On the other hand, materials learned prior to smoking should be recalled as well by subjects who have just smoked as by subjects who have not smoked. That is, all subjects should be receiving and storing the materials when not under the influence of marijuana, and if retrieval is not affected by marijuana, smoking should not interfere with retention. To investigate these hypotheses, information was presented after smoking and recalled at a later time; and in a second experiment, the same information was presented prior to smoking and recalled after smoking. Abel7, 8 performed a similar experiment but did not specify the THC-Δ-9 content of the marijuana cigarettes and did not use placebo controls (or employed the less desirable procedure of using different subjects as controls.) He found that marijuana affected recognition processes such that subjects were less able to discriminate between items they had seen and those they had not seen a short time before.