We studied EEG reactivity to visual stimuli during deep isoflurane (1.
5-2.05 vol% end-tidal concentration) anaesthesia. Twelve patients were
anaesthetized with isoflurane until burst suppression occurred in the
EEG. Red LED goggles were used to give visual stimulation of 60 flash
es, 4-ms duration each, at a frequency of 20 Hz. The stimuli, 3-s trai
ns of flashes, were given at random intervals. Both onset and offset o
f stimulation evoked bursts. The latency of visually evoked bursts was
comparable with long latency evoked potentials, which are known to be
related to cognitive processing. Our data showed that the central ner
vous system reacts strongly to photic stimulation during deep anaesthe
sia.