Comparing the effects of stimulation and propofol infusion rate on implicit and explicit memory formation

Doubt remains about the conditions under which learning persists despite anaesthesia. This study investigated the relative importance of dose of anaesthetic and stimulation for learning during propofol infusion before surgery. Thirty-six patients were randomly assigned to three groups. Group 1 received two word lists (category examples and nonsense words) during infusion of propofol to a target concentration of 2 microg ml(-1). Groups 2 and 3 received the word lists during infusion of propofol 5 microg ml(-1). Group 2 received nonsense words before tracheal intubation and category examples during intubation; Group 3 heard category examples before and nonsense words during intubation. Bispectral index was recorded as a measure of depth of sedation/anaesthesia. We assessed explicit memory on recovery using a structured interview and a recognition test. We assessed implicit memory using a category generation test and a preference rating task. To establish baseline, a control group of 12 patients completed the category generation test without receiving the category examples during anaesthesia. Overall, there was no evidence for learning during propofol infusion, though the category generation task showed a trend towards more implicit memory for words presented during intubation than during anaesthesia. We conclude that learning does not occur during anaesthesia without surgery.