The effects of intramuscular injections of diazepam (0.3 mg/kg) and scopolamine (8 mug/kg) on memory processes and subjective moods were studied in 36 volunteers. Subjects (Ss) were tested in groups of four in a double blind procedure with treatments distributed according to a Latin square design. Lists of words were presented to Ss who were then tested with an immediate free recall test prior to drug administration. Following injection delayed free recall and recognition tests were given. Subsequently two sets of lists were presented separately and tested in the same fashion. Two of the lists in the last set were composed of words falling into distinct categories. Memory was additionally analyzed by testing immediate recall of digit sequences and employing a visual recognition test. Subjective moods were evaluated with a rating questionnaire. Both diazepam and scopolamine impaired memory functions although the action of the latter drug was more pronounced and prolonged. The deficit appeared to be in the storage process leaving retrieval processes unaffected. Scopolamine in addition interfered with organizational processes. Subjectively, scopolamine also produced a larger sedative effect than diazepam.

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A total of 120 healthy volunteers were randomly assigned to four treatments (placebo, 0.1, 0.2, and 0.3 mg/kg) and three testing times (7 AM, 1 PM and 7 PM). Immediate and delayed free recall of word lists revealed consistent decreases in performance as oral diazepam dose increased from 0.1, 0.2, to 0.3 mg/kg. Paradoxically, as the dose increased, the number of predrug list words recalled also increased. A serial number-learning task displayed a pattern of delayed improvement of acquisition as the dose increased. Response times in a semantic-categories task were prolonged as the dose increased. Parallel recovery functions were observed for all doses and tasks. Full recovery after a single administration of 0.1, 0.2, and 0.3 mg/kg doses was estimated to occur after 3.5, 4.5, and 5.5 h, respectively. Several analyses were consistent with the view that acquisition and not retrieval was impaired by diazepam. There were no circadian interactions with the effects of the drug.

Seventy volunteers were injected with diazepam (0.3 mg/kg), scopolamine (8 mug/kg), or placebo, followed 70 min later by another injection of physostigmine, physostigmine and methscopolamine (in case of diazepam treatment), or placebo. Physostigmine was given in two doses, 16 and 32 mug/kg; methscopolamine, 8 and 16 mug/kg. Subjects (Ss) were tested in groups of 5 in a double blind procedure with treatments distributed according to a Latin square design. Prior to treatment, Ss heard a series of lists of words, followed by an immediate recall test. Following the first injection, delayed free recall and recognition tests were given. The second drug was then injected, followed by a presentation of another two sets of lists which were tested similarly. Subjective feelings were also evaluated with a rating questionnaire. Diazepam and scopolamine did not affect recall of information which had been learned prior to drug injection. However, both drugs impaired the learning or acquisition of new information. Physostigmine, especially in its high dose, antagonized most of the memory deficits produced by scopolamine while those of diazepam remained. This is a strong indication that scopolamine acts centrally through an anticholinergic mechanism while diazepam may act through a different system.

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Although diazepam (Valium) reduces learning and memory of information presented after administration (anterograde amnesia), in some cases it improves retention of predrug information (retrograde facilitation). Three experiments examined the magnitude and the conditions for producing retrograde facilitation and tested three hypotheses about the cause of memory enhancement. Differential effort and enhanced consolidation explanations were rejected in favor of a reduced interference interpretation. Improvement in predrug memory occurs because poor postdrug learning reduces the amount of new information available to interfere with prior learning.