An examination of differences in the time course of oxazepam's effects on implicit vs explicit memory

The present study was designed to examine the effects of oxazepam on implicit vs explicit memory processes, as a function of this drug's time course. The effects of oxazepam (30 mg) or placebo on directly comparable tests of implicit memory (word stem completion) and explicit memory (cued recall) were examined at three time points: 100 min post-drug administration (prior to the theoretical peak plasma concentration of oxazepam; i.e. 'prepeak' condition), 170 min post-drug (close to theoretical peak; i.e. 'peak' condition) or 240 min post-drug (following theoretical peak: i.e. 'post-peak' condition). Sixty healthy volunteers were randomly assigned to either the drug condition or the placebo condition in a double-blind design and were tested on both memory tests at one of the three time points. In the 'pre-peak' condition, oxazepam impaired cued recall performance relative to placebo but did not impair priming. In the 'peak' condition, oxazepam impaired performance on both memory tasks. In the 'post-peak' condition, cued recall performance in the oxazepam group remained significantly impaired relative to placebo. However, oxazepam-induced impairments in priming were only marginal, suggesting that oxazepam-induced impairments in implicit memory processes begin to wane following theoretical peak drug concentrations. The fact that oxazepam-induced priming impairments were significant only when the word stem completion task was administered close to peak plasma concentrations, supports the hypothesis that benzodiazepines exert time-dependent effects on implicit memory processes. The results also support the theoretical distinction between implicit and explicit memory processes, since the directly comparable implicit and explicit tasks showed different impairment curves over time.