Fig. 5. Concentration-dependent effect of halothane, isoflurane, and desflurane on the amplitude of the flash-induced early cortical evoked response. The amplitude of the response was calculated as the difference between the most positive (maximum) and the most negative (minimum) peaks within 0 and 100 ms of the γ-filtered event-related potential in each trial. Bars represent averages from all animals. Each agent enhanced, rather than reduced, the amplitude of the early cortical response in a concentration-dependent manner. * Significant difference ( P ≤ 0.05) from waking control. When compared at equivalent low concentrations, isoflurane was the most effective of the three agents, because it significantly enhanced the response at 0.4–0.75 MACLR, whereas desflurane was the least effective, because it produced a significant increase in the early cortical response only at 1.3–1.75 MACLR. One MACLRis the minimum alveolar concentration at which the righting reflex is lost. At 1.8–2.4 MACLR, all three agents significantly augmented the early cortical response relative to waking baseline. The final augmentation at this level was not significantly different among the agents.