Effects of Cannabis on Memory & Cognitive Function

The effects of cannabis on tests of short-term memory are well-established. Miller reported "The single, most consistently reported behavioural effect of cannabinoids in humans is an alteration of memory functioning."

Most human studies of the effect of cannabis on memory have used digit span or verbal recall tests where dose-related decrements in performance have been demonstrated, including errors of omission (correct responses missing) or intrusion (incorrect responses made). However, these appear to represent errors in detail rather than the main factual elements in narrative, and the recall of previously-learned material does not appear to be affected.

Leweke et al found no effect of cannabis on recall of "negative" emotionally charged words compared to other words, although "positive" words were recalled more frequently than expected. In a thorough review of the cognitive effects of cannabis, Solowij concluded that cannabis may affect the acquisition of information due to interruption of attention and concentration, and may interfere with transfer from short-term to long-term storage.

There is dispute as to the duration of any cannabis effects on memory and performance. In a study of 3 regular marijuana smokers, Heishman et al found no subjective intoxication or tachycardia 24 hours after marijuana administration, although there remained impairment on arithmetic and recall tasks, concluding "These preliminary results suggest that marijuana can adversely affect complex human performance up to 24 hours after smoking.". A 1985 study by Chait et al had suggested the existence of a "hangover" syndrome after "significant changes were observed on two subjective effects scales and on the time production task after active, but not placebo, marijuana" roughly 9 hours after smoking, although other measures were unaffected. However, in a later study of 12 regular users by Chait, testing subjective ratings and a battery of psychomotor and cognitive tests the morning after use, found few statistically significant differences, these being of negligible magnitude including counterintuitive results, and conceded "marijuana smoking was not associated with a "hangover" syndrome similar to those reported after use of alcohol or long-acting sedative-hypnotics."

In a separate study, Heishman et al reported "Significant memory impairment was observed on a forward and reverse digit span task, and performance was impaired on the digit symbol substitution task by the high, but not low, dose of marijuana. Performance on a divided attention task was not affected by marijuana.", also finding that users would titrate the quantity of smoke of lower and higher strength marijuana by taking smaller puffs, inhaling less and for shorter duration with the higher potency material.

Varma et al, studying 26 Indian cannabis users, reported "Compared with a control group, the cannabis users were found to react slowly in perceptuo-motor tasks, but not to differ in intelligence or memory tests. The users suffered disability in personal, social and vocational areas and indicated higher psychoticism and neuroticism scores." The same team earlier reported a study of regular users averaging 150mg THC per day, finding "the cannabis users were found to react more slowly, to be poorer in concentration and time estimation, to have higher neuroticism and greater perceptuo-motor disturbance".

In a study of cognitive function among 12 male volunteers, Hooker & Jones reported "A significantly greater number of short story omissions and intrusions occurred in delayed free recall after marijuana. Immediate and sustained attention, controlled retrieval from semantic memory, and speed of reading and naming colors were not affected." Miller et al reported "Free recall was reduced in a dose related manner by the drug, but recognition memory was unaffected.".

Tinklenberg et al found oral marijuana (0.3mg/kg THC) to have no effect on fluency, flexibility, elaboration, uniqueness, associational thinking or creativity. Individual differences in ability are clearly of major importance in determining performance, Miller et al reported "Recall performance following marijuana intoxication was positively related to level of recall performance in the placebo condition.". In a separate study, the same team reported "marijuana significantly decreased immediate and final free recall but only slightly influenced recognition memory."

Melges required volunteers to consume 20mg THC in a short "fast" period (10 min) or long "slow" period (45min), and found "In fast conditions, THC induced greater difficulties with tracking information over time, greater disruptions of self-other interpersonal perceptions, and more persecutory ideation that did alcohol or placebo. Similar but less marked differences were found in the slow conditions. ...changes in tracking difficulties, self-other metaperspectives, and persecutory ideation were substantially and significantly correlated."

In a study of the effects on long term memory, Darley et al reported "Marijuana had no effect on recall or recognition performance. These results do not support the view that marijuana provides access to facts in long-term storage which are inaccessible during non-intoxication. " The presence of retrieval cues does not appear to affect recall of word lists

Summary:

Studies have consistently demonstrated significant impairment of short-term memory for number sequences or verbal recall, although not for major events or factual elements of narratives. The effects of cannabis are usually subtle, and although statistically significant, the magnitude of any impairment tends to be dose-related and subject to tolerance of the drug"s effects, on the personal characteristics of the user, and on the circumstances in which the drug is taken.

Cannabis may increase the likelihood of minor errors of time perception, verbal recall and similar factors, but not affect recall of important actions observed or experiences undertaken. In a regular user of cannabis, acute intoxication by the drug would have little effect on performance.

Cannabinoids appear to disrupt short-term memory by interfering with the filtering of information, such that a greater volume of information reaches consciousness, overwhelming the ability to store, or prioritise such information for storage in memory.