Fig. 4.  Anesthetics slow θ rhythm. Spectrograms illustrating the concentration-dependent slowing of θ rhythms by halothane (A ), isoflurane (B ), and nitrous oxide (N²O) (C ). Exploratory activity was absent in rats exposed to isoflurane concentrations above 0.5 minimal alveolar concentration, but animals continued to explore spontaneously under concentrations of halothane approaching 1.0 minimal alveolar concentration. (A ) Spectrograms based on 47, 3, 6, and 14 segments for control and halothane 0.26, 0.49, and 1.06%, respectively. (B ) Spectrogram for immobility under 0.69% isoflurane demonstrating the absence of a θ peak during immobility. Based on 106, 14, 5, and 284 segments for control and isoflurane 0.3%, 0.69% exploring, and 0.69% immobile, respectively. (C ) Spectrograms based on 14, 43, and 4 segments for control, and 30 and 60% N²O, respectively. (D ) Summary of all experiments. The effect of three anesthetics on θ frequency is plotted as a function of anesthetic concentration. Lines represent linear regression fits and are bracketed by 95% confidence intervals. To facilitate comparison, anesthetic concentrations are expressed as multiples of EC⁵⁰amnesia for contextual fear conditioning (FC) (see Fig. 1) and x-values were scattered to reveal overlying data points for control and N²O 30%.