[Vsiual evoked potentials and excitability cycle during simulated diving at -610 m in oxygen-helium atmosphere (Physalie VI)].

Visual evoked potential (VEPs) and visual excitability cycles (VECs) were studied during compression in an oxygen--helium breathing mixture, up to a pressure of 62 ATA, equivalent to a depth of 610 m in sea water (Physalie VI). Records were taken at atmospheric pressure (control responses in confined helium--oxygen atmosphere) and at different depths during compression and decompression; recording was always performed at the same hours; subjects were lying with closed eyes. Isolated or double flashes were delivered every 1.2 sec, from a stroboscopic lamp located in the tank, at a maximum distance of 20 cm from the eyes. Responses were recorded with hook electrodes left in situ throughout the experiment (mid-vertex occipital derivation). VEP and VEC changes occurred in three stages: (1) Between surface and -435 m, components II-V of the VEP decreased in amplitude, while component VI increased. The after-discharge, and the PEV to the second of a pair of stimuli were facilitated. (2) Below 435 m, latencies and amplitudes of waves II-V increased, while those of wave VI decreased. After-discharge and response to the second stimulus were reduced. (3) During decompression up to -120 m, VEPs and VECs again changed, in a way similar to compression from 0 to -435 m. VEP and VEC changes, which are thus opposite during compression, may depend upon speed of compression, although the latter factor is probably not alone. Other factors, like pressure itself or the nature of the gas mixture may well intervene. It is likely that the observed modifications are the result of various factors interacting at various depths.