Fig. 3. γ-Aminobutyric acid (GABA) levels, electroencephalographic (EEG) power, and neck electromyographic (EMG) power during wakefulness and anesthesia. GABA levels were decreased by isoflurane ( A and B ): Isoflurane significantly (* P < 0.001, Mann–Whitney U test) decreased GABA levels when administered after wakefulness ( A ). GABA levels also were significantly (* P < 0.02, Mann–Whitney U test) decreased relative to wakefulness when isoflurane delivery preceded wakefulness ( B ). Electroencephalographic power was increased by isoflurane ( C and D ): Compared with spontaneous wakefulness ( C ) or postanesthesia wakefulness ( D ), power of the frontal cortex electroencephalogram in the 0.5- to 4-Hz (delta), 5- to 8-Hz (theta), and 11- to 29-Hz (sigma and beta) bands was significantly (* P < 0.01, t test with Bonferroni correction) increased during isoflurane anesthesia. Neck muscle tone was decreased by isoflurane ( E and F ): Expressed as percent of electromyographic power during wakefulness, neck muscle activity was significantly (* P < 0.001) reduced by isoflurane when anesthesia followed ( E ) or preceded ( F ) wakefulness.