Sensitivity of neurones in visual cortex (area 17) under different levels of anaesthesia

Abstract

Different levels of anaesthesia in cats surgically prepared under thiopentone sodium were induced by adding 0.2%, 0.8% and 2.2% halothane to 80% N₂O, 19% O₂ and 1% CO₂ mixture. Properties of visual cortical neurones were investigated under each anaesthetic level. 80% N₂O alone produced a combination of slow and fast wave components in the EEG (2nd stage of anaesthesia). Under this condition, many cortical neurones could be classified as “sustained” or “transient” on the basis of their firing pattern to a stationary bar, and sharp tuning curves for orientation and spatial frequency could be obtained, the cells having low contrast thresholds. As the halothane concentration was increased, the slow wave component of the EEG increased (3rd stage of anaesthesia), the tuning curves of cortical neurones to orientation and spatial frequency became less sharp, the contrast threshold was raised and classification into “sustained” and “transient” neurones became difficult. Finally the neurones became undrivable when the EEG showed continuous, large-amplitude slow waves (4th stage of anaesthesia). Thus findings made during the slow wave EEG patterns cannot reveal the full capabilities of cortical neurones.