No implicit memory under propofol-alfentanil-anesthesia: The lexical decision task

Objective: Indirect memory tests have revealed partially preserved memory functions in amnesic patients in spite of the inability of these patients to consciously access memory traces in direct tests. The aim of the study was to evaluate, whether implicit learning is resistant to adequate levels of anesthesia. Material and Methods: Twenty patients undergoing routine surgery were randomly exposed to one of two audiotapes via headphones during anesthesia with propofol- and alfentanil. The audiotapes, each of them containing repeated lists of 50 German nouns, were presented for 30 minutes beginning after skin incision. A control group of 20 trauma patients listened to the tapes without anesthesia. Implicit memory was tested 6-12 hours after the first stimulus presentation using a lexical decision task: A list containing the 50 old words from the tape, 50 additional new words, and 100 nonsense words was flashed rapidly in the middle of a video-monitor. For each stimulus, subjects had to decide whether it was a word or a nonsense word by pressing one of two buttons. The measures of interest were the reaction times of the old versus new words. Results: We found a significant decrease in reaction times for the old words in the control group indicating implicit memory. The patients, who were exposed to the tapes during anesthesia, did not show decreased reaction times to old words thus arguing against an implicit memory effect during this type of anesthesia (p < 0.015 between groups). Conclusion: While the control group showed a robust reaction time effect, the study group, that was exposed to the stimuli during the operation, did not show reaction time differences, hence no implicit memory effect. The study shows that depth of anesthesia appeared to be adequate.