Studies on the central nervous system: visually evoked cortical responses in sheep

Several factors which were thought to affect the magnitude of flash evoked cortical potentials were examined in halothane-anaesthetised sheep. The proximity of the electrodes to the cortex and the patency of the eye which was contralateral to the recording electrodes had the greatest effect on the excursion distance of the visually evoked response. The choice of upper frequency filter (70 vs 700 Hz), the patency of the ipsilateral eye and closure of both eyelids resulted in relatively smaller reductions in excursion distance of the response. The importance of these findings in studies using evoked potentials for assessing sensibility and brain death in animals is discussed.