Charles F. Darley

The contingent negative variation during a memory retrieval task.

Evoked potentials were recorded from the human scalp during performance of a memory retrieval task modeled after a paradigm originated by Sternberg (1966). Subjects were required to decide whether a probe digit was contained in a series of one to four target digits presented a few seconds before. The amplitude of the contingent negative variation (CNV) preceding the probe digit and the speed of CNV resolution after the probe varied as a function of target set size. CNV amplitude was greatest when the set size was one. The smaller the set size, the more positive the evoked potential 300 msec after the probe, regardless of whether a motor response was required.

Influence of marihuana on storage and retrieval processe in memory

Following presentation and immediate free recall testing of 10 20-word lists, 48 Ss were divided into two groups, one of which received an oral dose of marihuana extract calibrated to 20 mg of Δ1-THC and one of which received placebo. One hour later, all Ss were administered delayed recall, recognition, and order tests on the first set of words. Presentation of another set of 10 lists followed, and there were immediate recall and delayed recall, recognition, and order tests on these words. Performance of drug and placebo Ss did not differ significantly for any of the first delayed tests. However, the performance of drug Ss was poorer than that of placebo Ss on immediate recall, delayed recall, and delayed recognition of the second set of lists. We concluded that retrieval of information relevant to the occurrence or nonocurrence of an event was not affected by marihuana intoxication. Storage difficulties probably account for memory deficits due to the drug, and these difficulties appear to occur in the process of transferring information from short-term to long-term memory.

Marihuana and retrieval from short-term memory

Twelve subjects received an oral dose of marihuana extract calibrated to 20 mg of δ1-tetrahydrocannibinol on Day 1 of the experiment and performed a short-term memory task before and after administration of the drug. The subjects were then split into two groups, receiving either marihuana or placebo on the evenings of Days 1 to 4 and between two memory test sessions on Day 5. Placebo subjects showed little change in performance between the two test sessions on Day 5; however, results from Day 1 for all subjects and Day 5 for the drug group showed that reaction time increased from before- to after-challenge sessions. This increase in time under marihuana was explained as a change in encoding and/or response processes, rather than processes involved in the search of the memory store.

The effect of marihuana on tracking task perfomance

A 5-min paced contour tracking task was performed before and after receiving brownies containing either placebo or marihuana calibrated to a δ9-tetrahydrocannabinol content of 20 mg. The error patterns of 19 young male subjects who received placebo and 18 who received marihuana were compared. After marihuana there was an increase in total errors as measured by the standard deviation (P<0.01) and the mean deviation (P<0.02) error scores. Although marihuana is reputed to create a fluctuating effect, under the conditions of this experiment the variability of error scores between successive 15-second time periods in the marihuana group was not significantly greater than in the placebo group. In addition the marihuana deficit did not show significant time trends during the task.

Marijuana and memory intrusions.

Sixteen college-educated male subjects were tested on free-recall lists during intoxication with marijuana extract calibrated to 0.3 mg/kg delta-9-tetrahydrocannabinol and during placebo conditions. On each testing day subjects studied six lists using a regular overt rehearsal procedure and six lists using an association-overt rehearsal procedure in which they were to rehearse alound both list items and associations to those items. Both marijuana and the association-rehearsal procedure reduced the number of correct recalls and increased the number of intrusions (nonlist items which were incorrectly recalled as having been on the list to be learned). The intrusions were divided into three categories: a) words found on prior lists; b) associates spoken during the rehearsal; or c) totally new works not previously mentioned. Marijuana significantly increased the number of new intrusions; the association-rehearsal procedure did not. This result suggests that one of the effects of marijuana on cognitive functions in humans is to increase the number of intrusive thoughts and this may be the mechanism involved in some of the thought disorder observed with marijuana intoxication.

Marijuana effects on associations to novel stimuli.

Sixteen college-educated male subjects were given an object description task during placebo conditions and while intoxicated with marijuana extract cookies calibrated to 0.3 mg/kg delta-9-tetrahydrocannabinol, a dose within the range of usual social use. The task was scored for fluency, flexibility, elaboration, and uniqueness, all of which represent associational thinking and are considered to be components of creativity. Marijuana did not enhance any of these measures.

Marihuana effects on TAT form and content.

In a double-blind study, 72 normal male subjects were given either placebo or marihuana containing 20 mg. δ-9-tetrahydrocannabinol. Stories written to cards selected from the Thematic Apperception Test did not differ on hostile or sexual content scales between drug and placebo conditions, but 6 out of 10 scales specifically constructed to detect marihuana effects were successful at differentiating the two conditions. Under marihuana the stories had a timeless, non-narrative quality, with greater discontinuity in thought sequence and more frequent inclusion of contradictory ideas. Novelty of content was somewhat increased by marihuana, while relation to the picture, imagery, repetition, and closure were not significantly affected.