Quantifying dose-dependent changes in visual flash-induced evoked response during halothane and isoflurane anesthesia

We investigated a dose-dependent modulation of flash-induced event-related potentials (ERP) and resting potentials (RP) recorded from rat primary visual cortex in waking and halothane- and isoflurane-anesthetized rats. Our results show that slow oscillations in flash-induced afterdischarge (FAD) range of frequencies and gamma (20-60 Hz) oscillations are present at rest during waking and light-to-moderate anesthetized states. While the temporal occurrence of these oscillations appears random during rest, FAD and gamma oscillations appear entrained following flash stimulation. During gradual increase in anesthetic dose, FAD is gradually suppressed between 0.1% and 0.4% of both halothane and isoflurane. However, FAD suppression is consistently stronger for isoflurane than for halothane. On the other hand, gamma oscillations are gradually enhanced between 0.1% and 1.5% of both halothane and isoflurane. This enhancement is consistently stronger for isoflurane than halothane. At the anesthetic dose of 1.5%, gamma power is abolished with halothane but is still present with isoflurane. Since gamma power is enhanced at anesthetic doses at which consciousness is lost (0.4-0.8%), our results support the view that reduction in gamma power is not correlated with the loss of consciousness.