PAGE THE PROBLEM Legibility of cockpit instruments can be degraded by strong vestibular stimulation. The purposes of this experiment were to clarify the mechanisms involved in the differential visual suppression of nystagmus during pitch-forward and pitch-backward stimulation, to ascertain the influence of luminance level on visual performance, and to ascertain habituation effects induced by the experimental conditions. FINDINGS* Subjects positioned on their sides were oscillated sinusoidally

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... .04 Hz, peak velocity :b90 deg/'sec) about an Earth-vertical axis. Initilaly, nystagmus slow phase velocity was'about equal during the forwardand backward-pitch halves of the stimulus cycle in darkness; but when subjects tracked a dimly illuminated air•craft instrument, nystagmus slow phase v elocity during forward pitch was about ten times that during backward pitch. Consequently, tracking errors were much greater during forward pitch. Change In luminance level from 0.01 ft-I to 1 .0 ft-I produced small, statistically significant decrements in nystagmus slow phase velocity and substantial improvements in tracking performance. Following this part of the experiment, nystagmus was again recorded in darkness. There was a differential decline in slow phase velocity, the slow-phase-down response showilng signi fi cantlJy greater decline. Sti mulIus-response ; phase relations were also altered for the slow-phase-down response, but were unaltered for the slow-phase-up response. It is proposed that ir~i'eractions between eyelid and eyeball movements caused different frequencies of upbeating and downbeating nystagmus which, in turn, produced different visual suppression of nystagmus slow phase velocity in the two halves of the stimulus cycle. The asymmetric visual suppression may have contributed to the asymmetric habituat'on of the two reactions. ACKNOWLEDGMENTS The authors wish to express their appreciation to G. T. Turnipseed, D. J. Gripka, and J. W. Norman for their assistance with experimental equipment and in collecting and analyzing experimental data. Thanks are also due to LCDR R. S. Kennedy for comments on interdependence of eyelid and eyeball movement. The findings in this report are not to be construed as an official Department of the Army position, unless so designated by other authorized diocuments. ii