Acceleration effects on pupil size with control of mental and environmental factors.

Background: Using still photography and cinematography, early studies reported that pupils dilate immediately on exposure to acceleration and the dilation persists during sustained +Gz. Due to the lack of control for mental and environmental factors and poor sampling rate in early techniques, we re-examined the effects of increased acceleration on pupil size in the human centrifuge. Methods: Eight volunteers were dark adapted for 20 min by fixating on a 2.5 mm LED disk, 1 m away in otherwise complete darkness. Pupil diameters and horizontal and vertical eye movements were recorded binocularly in real time using an eye-tracking and pupillometry system. The recording sequence consisted of 1 min at +1 Gz, followed by transition to +3 Gz at 0.1 G . s -1 , 2 min at sustained +3 Gz, return to +1 Gz at 1 G . s -1 , and finally maintained at +1 Gz for another minute. During this sequence, the subject's heart rate was also monitored and the subject was instructed to subtract 2 from 1000 to maintain mental alertness. Results: All subjects showed a significant decrease in pupil diameter (p < 0.05) and increase in heart rate (p = 0.02) during +3 Gz and regained the baseline level on returning to +1 Gz. The maximum decrease in pupil diameter during sustained acceleration ranged from 0.6 to 3.3 mm. Conclusion: Our findings suggest that pupils constrict during hypergravity. It appears that the reflex regulation of pupil size is attributed to parasympathetic action through fibers originating in Edinger-Westphal's nucleus in the mesencephalon, which receives direct input from the otoliths.