This paper explores the changes in cognitive function which occur as someone "loses consciousness" under anesthesia. Seven volunteers attempted a categorization task and a within-list recognition test while inhaling air, 0.2% isoflurane, and 0.4% isoflurane. In general, performance on these tests declined as the dose of anesthetic was increased and returned to baseline after 10 min of breathing air. A measure of auditory evoked responding termed "coherent frequency" showed parallel changes. At 0.2% isoflurane, subjects could still identify and respond to (...) category exemplars but showed impaired short-term memory function. Electrical stimulation at 0.4% isoflurane, intended to mimic the arousing effects of surgery, had a small, beneficial effect on performance. A mean of 63% of category exemplars was identified at this stage, but recognition memory for those exemplars was at chance on recovery. There was no evidence for learning of words presented at 0.8% isoflurane. (shrink)