Memory for unconsciously perceived events: evidence from anesthetized patients.

Studies investigating memory for events during anesthesia show a confusing pattern of positive and negative results. To establish whether there are any consistent patterns of findings across studies, we conducted a meta-analysis of the data from 2517 patients in 44 studies. The meta-analysis included two measures of the effects of positive suggestions on postoperative recovery: (a) the duration of postoperative hospitalization and (b) the amount of morphine administered via patient-controlled anesthesia, as well as two measures of memory for specific information presented during anesthesia: (c) direct tests and (d) indirect tests. The meta-analysis indicated that positive suggestions presented during anesthesia have little or no effect on postoperative recovery. On the other hand, the meta-analysis showed that specific information is remembered following surgery, as long as testing is not delayed longer than 36 h. Studies of memory for events during anesthesia provide a useful avenue for exploring unconscious cognition.