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

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.

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 x s(-1) 2 min at sustained +3 Gz, return to +1 Gz at 1 G x 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.

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 sustained acceleration ranged from 0.6 to 3.3 mm.

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.