In a study of 24 patients undergoing emergency cesarean sections, Lubke et al. used an electroencephalograpy-based bispectral index (BIS), recorded throughout the surgery, to measure hypnotic state during presentation of word lists to assess recall during “light” general anesthesia. For all patients, anesthesia consisted of a rapid-sequence induction with 4 mg/kg thiopental and 100 mg succinylcholine to facilitate intubation. After delivery of anesthesia, the nitrous oxide concentration was increased from 50 to 70%, with 0.2% isoflurane (end tidal) and 0.1–0.15 mg/kg morphine. Immediately after alterations of drug concentrations postdelivery, a program for word presentation was started. Two word lists of 16 words each in random order were played via headphones for each patient. Each word was repeated consecutively 12 times with a 2-s delay between repetitions.
As soon as patients began responding postoperatively, the research team conducted short, structured interviews regarding recall of pre-, intra- and postoperative events. After a short word stem exercise to demonstrate the test procedure, inclusion and exclusion parts of the word stem completion test were administered. Patients were asked to complete word stems, presented audibly and visually, with the corresponding word that had been played during anesthesia. If unable to recall the word from surgery, the patient was asked to fill in with the first word that came to mind (inclusion test). For the exclusion test, patients were asked to avoid words from surgery while completing word stems. This process dissociation procedure (PDP) allowed researchers to separate explicit memory from implicit memory.
The mean bispectral index during word presentation was 76.3. None of the patients had spontaneous recall of intraoperative events. However, patients were able to make correct inclusion–exclusion decisions (i.e. , patients had higher hit rates in the inclusion part of the test compared with base rate performance, and, in the exclusion part, hit rates were lower). These results indicate that explicit memory is not synonymous with conscious recall and, indeed, can occur in the absence of it.