Representative 30-s electroencephalogram recordings from individual animals. The horizontal black bars represent periods of electrical stimulation. (A) Electroencephalogram recorded from a rat with a ventral tegmental area (VTA) stimulation electrode during the awake state, before general anesthesia. (B) Electroencephalogram recorded from the same rat during isoflurane general anesthesia. VTA stimulation induced a prompt increase in electroencephalogram frequency. (C) Electroencephalogram recorded from a rat during propofol general anesthesia. VTA stimulation induced a prompt increase in electroencephalogram frequency. (D) Recording from a rat with a substantia nigra (SN) stimulation electrode during isoflurane general anesthesia. SN stimulation induced no obvious changes in the electroencephalogram.