Effects of lorazepam and oxazepam on perceptual and procedural memory functions.

Rationale: Lorazepam has been found to consistently impair performance on both episodic and perceptual priming tasks, whereas other benzodiazepines have shown perceptual priming to be preserved. However, it has recently been postulated that benzodiazepines may exert time-dependent effects on implicit memory processes after research findings indicated some benzodiazepines, other than lorazepam, impair performance on priming tasks when tested at the time of peak plasma concentration level after benzodiazepine administration.

Objectives: To compare time-dependent effects of lorazepam and oxazepam on implicit memory tasks, specifically perceptual priming and procedural learning.

Methods: Thirty-three healthy female undergraduates were randomised to one of three time groups (pre-peak, peak, post-peak) and administered placebo, 2.5 mg lorazepam, and 30 mg oxazepam, in counterbalanced order, at 1-week intervals. Assessments included word-stem completion (perceptual priming) and rotary pursuit (procedural learning) tasks.

Results: At all time intervals, lorazepam but not oxazepam significantly impaired perceptual priming but procedural learning was preserved under both drugs.

Conclusions: These findings are consistent with previous research showing a differential effect of lorazepam in impairing perceptual memory but the notion that benzodiazepines exert time-dependent effects on implicit memory processes was not supported.