Effects upon the EEG of psychometric performance during deep dives in helium—Oxygen atmosphere

Abstract

A study of EEG modifications and psychometric performances was performed during two dives in hyperbaric chambers with a helium—oxygen breathing mixture (pO 2 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. An increase in theta activity was observed in the anterior regions while the subjects performed certain psychometric tests (number ordination; symbol recognition double word; digital dexterity). This increase, which could reach 700% of the values recorded during periods of rest preceding or following the tests, appeared at 350 m and deeper. These modifications could be due either to deficits in cerebral oxygenation, to perturbed cerebral blood flow, or to alterations in energetic or metabolic processes caused by hyperbaric conditions. These alterations appeared during repetitive tasks which necessitated constant effort and attention.