Increased reading speed for stories presented during general anesthesia

Background: In the absence of explicit memories such as the recall and reco gnition of intraoperative events, memory of auditory information played dur ing general anesthesia has been demonstrated with several tests of implicit memory, In contrast to explicit memory, which requires conscious recollect ion, implicit memory does not require recollection of previous experiences and is evidenced by a priming effect on task performance. The authors evalu ated the effect of a standardized anesthetic technique on implicit memory, first using a word stem completion task, and then a reading speed task in a subsequent study. Methods: While undergoing lumbar disc surgery, 60 patients were exposed to auditory materials via headphones in two successive experiments, A balanced intravenous technique with propofol and alfentanil infusions and a nitrous oxide-oxygen mixture was used to maintain adequate anesthesia. In the firs t experiment, 30 patients were exposed randomly to one of the two lists of 34 repeated German nouns; in the second experiment, 30 patients were expose d to one of two tapes containing two short stories. Thirty control patients for each experiment heard the tapes without receiving anesthesia, All pati ents were tested for implicit memory 6-8 h later: A word stem completion ta sk for the words and a reading speed task for the stories were used as meas ures of implicit memory. Results: The control group completed the word stems significantly more ofte n with the words that they had heard previously, but no such effect was fou nd in the anesthetized group. However, both the control and patient groups showed a decreased reading time of about 40 ms per word for the previously presented stories compared with the new stories. The patients had no explic it memory of intraoperative events. Conclusions: Implicit memory was demonstrated after anesthesia by the readi ng speed task but not by the word stern completion task., Some methodologic aspects, such as using low frequency words or varying study and test modal ities, may account for the negative results of the word stern completion ta sk. Another explanation is that anesthesia with propofol, alfentanil, and n itrous oxide suppressed the word priming but not the reading speed measure of implicit memory. The reading speed paradigm seems to provide a stable an d reliable measurement of implicit memory.