Effects upon the EEG of psychometric performance during deep dives in helium--oxygen atmosphere

A study of EEG modifications and psychometric performances was performed during two dives in hyperbarbic chambers with a helium--oxygen breathing mixture (pO2 maintained at 0.4 ATA). The depths attained were 500 m for the first experiment (Sagittaire II) and 610 m for the second (Physalie VI). Four subjects were studied. EEG modifications that could be considered as high pressure nervous syndrome began at 300 m. They were characterized by an increase of slow activity (particularly theta), depression of fast activity and transformation of the waking EEG into one resembling that of stage I sleep. The psychometric tests revealed a diminution of sensorimotor performance in all subjects (10--20%). Performance in intellectual tests varied according to subject and experiment, and did not fall more than 15% below normal values.