[Conscious and unconscious acoustic perception during general anesthesia]

The possibility of processing sensory information during general anesthesia and the ability to recall it postoperatively is of major ethical, medical and even theoretical importance. Auditory stimuli especially are perceived intraoperatively and remembered postoperatively. Neuropsychological experiments indicate that sensory information can be processed and recalled both at a conscious and at an unconscious level. Therefore, we have to distinguish between explicit and implicit memory. Explicit memory is characterized by an active and conscious recall of space- and time-related events, i.e., episodes in a person's life. In contrast, implicit memory recalls passively and unconsciously without being related to space and time, i.e., language and general knowledge. Experimental results from amnesic patients indicate that these two memory systems work independently from each other. Even when explicit memory is grossly impaired the function of the implicit memory may still be completely preserved. Various studies on intraoperative awareness show that explicit memory is widely eliminated during general anesthesia. The incidence of conscious awareness that can be actively recalled postoperatively is reported to be 1-3%. In contrast, the implicit memory function can be partially preserved. When implicit memory tasks or hypnosis are employed, traces of unconscious memory of intraoperative auditory information can be shown in 20-30% of the patients. These observations are of important clinical relevance, because the unconsciously recalled information about the intraoperative procedure may have a negative influence on the patient's postoperative recovery and well-being. So far it is still not known which anesthetics most reliably suppress auditory perception and conscious and unconscious memory during the intraoperative period. Therefore, future studies should focus on several different points. The anesthetic state should be defined exactly and the functional state of the auditory modality should be monitored when auditory information is presented to the patients. The recollection of intraoperative events should be investigated using implicit memory tests, because these are regarded as more sensitive than explicit memory tests.