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Continuous changes in electroencephalographic topograms and auditory reaction time during simulated 21 ATA (atmospheres absolute) heliox saturation dives.

The present study was designed to investigate EEG changes during two simulated 21 ATA hyperbaric heliox saturation dives. Four professional male divers were attached to 13 electrodes over the scalp and compressed to 21 ATA by helium. Throughout the dives, EEGs were measured and stored on a magnetic FM tape recorder to calculate electroencephalographic topograms. Three patterns of slow wave increase were clearly distinguished by the electroencephalographic topogram during compression. The first pattern was caused by trains or bursts of theta waves which spread from the frontal region to the central region of the scalp. This pattern occurred at comparatively shallow depth and showed the slowest recovery during the bottom stage among three patterns. The second pattern found in two divers was caused by frontal midline theta bursts, which showed maximum activity at F2. The frontal midline theta bursts were augmented by a reaction time task. The third pattern was caused by vertex sharp waves which indicated a transition from sleep stage 1 to sleep stage 2. Neurophysiological meanings of these EEG patterns as well as the relationship between EEG changes and other indices were discussed.