Human Systems Integration Division @ NASA Ames

"INTRODUCTION: A luminous visual target in a dark hypergravity (Gz>1) environment appears to be elevated above its true physical position. This ""elevator illusion"" has been attributed to changes in oculomotor control caused by increased stimulation of the otolith organs. Data relating the magnitude of the illusion to the magnitude of the changes in oculomotor control have been lacking. The present study provides such data.
METHOD: Sixteen paid subjects were exposed to 1.0, 1.5, and 2.0 Gz on the human-rated centrifuge at NASA-Ames Research Center. The subjects viewed a small illuminated target in an otherwise darkened gondola, and adjusted the elevation of the target so that it appeared to be at their horizon; they also attempted to direct their gaze to the horizon, both in the dark and with the target illuminated. An ISCAN infra-red video camera system was used to the record the actual elevation of the subjects' eyes. RESULTS: Analyses of variance revealed that settings of the target (F=31.43, df 2/30, p<0.001), of the eyes when viewing the target (F=31.64, df 2/30, p<0.001), and of the eyes in total darkness when no target was viewed (F=29.87, df 2/30, p<0.001) all changed with Gz according to the same linear relationship of the form: Elevation = m (Gz) + b, where m ranged between -5.80 and -6.40 degrees per Gz, and b ranged between 3.61 and 4.84 degrees. The equation relating eye elevation and target elevation was simply: eye elevation = target elevation (r-square = .95).
CONCLUSIONS: These data strongly suggest that the elevator illusion is a direct result of changes in eye elevation, and that subjects misjudge the elevation of the target simply because they misjudge the elevation of their eyes in their heads."