Influence of gravitoinertial force level on the subjective vertical during recumbent yaw axis body tilt.

We tilted recumbent subjects at various angles about their yaw (foot to head) axis and had them indicate the direction of their subjective vertical and apparent head midline about the same axis. One set of tests was conducted during parabolic flight maneuvers where the background gravitoinertial acceleration varied from 0 to 1.8g. The blindfolded subjects (n = 6) were tested supine and at tilts of 60 degrees and 30 degrees left and right about their horizontal long body axis. They used a gravity neutral joystick to indicate their subjective vertical or their head midline continuously from the high force through the 0g portions of parabolas. In 0g, all subjects felt supine and oriented the joystick perpendicular to their body when indicating the subjective vertical. This points to strong influences of the symmetric somatic touch and pressure cues from the apparatus on orientation when the otolith organs are unloaded. In contrast to the settings in 0g, settings of the subjective vertical in 1g and 1.8g varied as a function of body orientation. However, the settings did not differ between 1g and 1.8g test conditions. Subjective vertical judgments were also made by subjects (n = 11) in the Brandeis slow rotation room, with the room stationary and rotating at a speed that produced a 2g resultant of gravitational and centrifugal acceleration. There were no differences between settings of the subjective vertical made in 1g and 2g. The similarity of 1g and hyper-g settings during recumbent yaw tilts, both in parabolic flight and in the rotating room, contrasts with the previously observed, strong influence which force levels above 1g have on settings of the subjective vertical during tilt of the body in pitch or roll. The findings for all three axes are consistent with a recently developed model of static spatial orientation.