The "push-pull effect".

The purpose of this study was to prove or refute previous authors' suggestions that tolerance to +Gz is reduced when preceded by 0 Gz or -Gz. Six men and six women were subjected to one session of acceleration stresses that varied between -2 and +2.25 Gz on the NAMRL Coriolis Acceleration Platform (CAP). At the beginning and end of each session, we exposed the relaxed subjects to identical control segments that were comprised of +1 Gz for 30 s, followed by +2.25 Gz for 15 s, and then return to +1 Gz. Subjects were also exposed to three experimental segments that were comprised of 0, -1, or -2 Gz for 10 s, followed by +2.25 Gz for 15 s, and then return to +1 Gz. Subjects verbally reported any decrements in peripheral vision during exposure to +2.25 Gz. Blood pressure (BP) was reduced during each 15-s period at +2.25 Gz. The minimum BP was progressively lower during the 15-s period as the preexposure experimental conditions became more negative (+1, 0, -1, and -2 Gz). Episodes of peripheral vision loss increased as the preceding -Gz became more negative. BP during exposure to +Gz was significantly affected by the preceding 10-s exposure to -Gz, and is indicative of reduced +Gz tolerance. As this "push-pull effect" may result in unexpected incapacitation, it has important implications for aviation safety.

The influence of active versus passive head oscillation, and mental set on the human vestibulo-ocular reflex.

We compared passive (manual) whole body, and active head oscillation in normal human subjects attempting mentally to influence the vestibulo-ocular reflex (VOR). Our objective was to establish simple procedural guidelines for vestibular test procedures in clinical settings. Using a head-fixed target, both methods of oscillation yielded virtually zero gain. Using an Earth-fixed target, active oscillation gain was unity, while passive gain was slightly less than 1. Using an imagined Earth-fixed target in the dark, both active and passive gains were reduced considerably, but passive gain was reduced more. Using an imagined head-fixed target in the dark, VOR gain was near zero at low frequencies but increased as frequency increased. Again, passive gain was less than active gain. At frequencies above 1 Hz, VOR gain in all conditions approached a value between 0.7 and 0.9. We conclude that active and manual passive rotation are simple and effective methods to test the VOR, but emphasize that visual and mental influences must be carefully controlled.