To the Editor:
I read with great interest the erudite editorial that accompanied the article by Pham et al. 2by my colleague, Professor Lichtor.1In their article, Pham et al. 2found no evidence of implicit memory formation during anesthesia in children.
Without providing clear and concise answers, Lichtor1asks the following question: Is memory formation during anesthesia similar to what goes on during sleep? General anesthesia abolishes explicit or conscious memory except in the rare cases of awareness during anesthesia.3The evidence for memory formation beyond unconsciousness is controversial. It may occur only during light anesthesia, short of consciousness.4When it occurs, there is only evidence of perceptual, but not conceptual, priming.5As for sleep, Lichtor1notes there is evidence that it contributes to the consolidation of some types of explicit memory. There are also some reports that it enhances the learning of motor and perceptual skills.6
Lichtor1asks another question: Would patients have better postoperative control of pain and anxiety if therapeutic instructions are given both preoperatively and intraoperatively? My answer has to be negative. For patients to comprehend instructions during anesthesia, there must be conceptual priming, which does not occur during anesthesia—except at its lightest levels (e.g. , nitrous oxide, opioids, and muscle relaxants),7where conscious encoding of stimuli is still possible.5After a 1988 report that claimed improved recovery and reduced hospital stay for patients after surgery,8nearly all credible and controlled studies failed to replicate this finding or other beneficial findings relating to postsurgical analgesia, nausea and vomiting, cessation of smoking, and so on.9The suggestion by Lichtor1that anesthesia might be similar to sleep processes that facilitate memory consolidation cannot be true because anesthetics abolish memory by suppression of consolidation.10–12
Finally, although Hermann Ebbinghaus introduced many important ideas and methods for memory research (with himself as the sole subject) in the late 19th century, I would attribute the introduction of implicit or nonconscious forms of human memory to the literature at a much later date. The first suggestion that conscious or implicit memory exists was in 1957, when Scoville and Milner reported the case of patient H.M., who after surgery for epilepsy was unable to convert a new short-term memory into a permanent long-term memory.13He could, however, learn new motor skills without any awareness that he had ever before performed the tasks. Research into implicit memory exploded in the early 1980s.13
The above comments should not, however, obscure my agreement with my colleague's outstanding statement that, “although there are some similarities between anesthesia and sleep, general anesthesia is not really the same as sleep.”
University of Iowa, Iowa City. firstname.lastname@example.org