Roger Knights points me to an article at a site called io9.com summarizing studies of memory retention after anesthesia. The studies show that, in general, people under sedation retain only the simplest memories of stimuli to which they were exposed while unconscious.
As readers of this blog know, some skeptics (notably anesthesiologist G. M. Woerlee) have argued that veridical observations in near-death experiences can sometimes be explained by anesthesia awareness – a condition in which a patient is partly awake on the operating table.
If complex memories cannot be retained by anesthetized persons, then this explanation would fail.
The article states:
One experiment involved an odd assortment of tests. Sometimes people were instructed to touch either their ear or their nose while under anesthesia. During a post-operative interview, they touched the suggested body part longer than the part that wasn't suggested, but only by a matter of seconds. Patients were also played tapes of words, all of which had the same first three letters. Later they were shown those letters, and asked to think of words starting with them. They generally picked more words from the list played during the operation than unlisted words. Finally, patients were played a list of nonsense words, either frequently or infrequently, during the operation. Afterwords, they were asked to guess the comparative frequency of the nonsense words. They recognized the words played more frequently than the ones played less frequently, but the frequently-played words were only recognized 62% of the time.
Anything more complicated than single words seems right out of the question. In a study of a group of patients, half of whom were played "The Three Little Pigs" during an operation and half of whom were played "The Wizard of Oz" during an operation, only 49% of the group guessed right.
Overall, it seems that softer tests of "learning" during anesthesia can get mildly positive results. On the other hand, "controlled studies using explicit memory tests" get "uniformly negative results."
Clearly, this degree of memory retention would not explain the detailed veridical observations reported by some NDErs. However, before we jump to conclusions, we need to keep in mind that the memory studies involved patients who had undergone ordinary anesthesia experiences. In other words, these subjects did not claim to have experienced anesthesia awareness. This is not surprising, since other studies have shown that anesthesia awareness is extremely rare, occurring in only 0.13% of procedures.
It's possible that the very small minority of patients who experience some degree of alertness under anesthesia do retain more complex memories than the average patient. I don't know if there is any way of testing this, since anesthesia awareness is so seldom encountered. For now, I guess, the jury is still out.