Effects of alertness and memory of a single dose of diazepam (10 mg) and the central stimulant methylphenidate (20 mg) were studied in healthy volunteers. It was questioned whether opposite effects of diazepam and methylphenidate are not only observed with respect to alertness but also with respect to memory. It was also questioned whether the two drugs equally affect the first (primacy) and last (recency) items in both the immediate and delayed recall of newly learned words. The experiment was performed in a double-blind, placebo-controlled way. 12 subjects were exposed to a subjective alertness scale and a verbal memory test: a 15-word test. Subjective alertness was found to be decreased after diazepam and increased after methylphenidate. Anterograde amnesia was found after diazepam in the memory test. More specifically, the primacy but not the recency effect was reduced during the immediate recall and both were reduced in the delayed recall. Methylphenidate had no effect on memory, however a ceiling effect might have obscured a putative drug effect. The results of a second experiment excluded this possibility. In all, the data demonstrate opposite effects of the two drugs on subjective alertness, suggesting opposite effects on vigilance. Opposite effects on memory were not established. This demonstrates that changes in alertness do not run in parallel with changes in memory. A scatter diagram, however, suggests a small effect of alertness on immediate recall. The effects of diazepam were also discussed in terms of the Atkinson and Shiffrin memory theory and it seems that diminished rehearsal processes are one of the key factors in explaining diazepam-induced amnesia.
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