Effects of extended hypoxia on night vision.

The effects of 16 d of sustained hypoxia (4300-m equivalent) on the dark adaptation threshold function were studied in 12 male subjects measured periodically (days 2,4,6,9,11,13,16 of exposure) over a 20-min test period for both red and green stimuli using a new computerized dark adaptometer. Comparison with sea level performance showed negligible elevations of thresholds for red response, but highly significant impairment of green response (p less than 0.00001) over almost the entire dark adaptation function. These losses peaked between the sixth and ninth day followed by little recovery, except at the eleventh day when the subjects descended briefly to 3200 m elevation. Impairments developed rapidly again upon return to the original higher altitude. The results differ from previous findings after shorter exposure periods, which showed only slight impairments of the early segment of dark adaptation. Implications of the results are discussed.