Underestimation of self-tilt increases in reduced gravity conditions

Area of Science:

• Human physiology
• Vestibular system research
• Space adaptation

Background:

• On Earth, large self-tilt angles in the roll plane cause subjective visual vertical (SVV) measurements to show a bias toward the longitudinal body axis.
• This indicates a systematic underestimation of self-tilt in normal gravity conditions.

Purpose of the Study:

• To test the hypothesis that self-tilt underestimation increases in partial gravity.
• To investigate if this underestimation is more pronounced at lower gravity levels.

Main Methods:

• Subjective visual vertical (SVV) was measured in parabolic flight across three partial gravity levels (0.25g, 0.50g, 0.75g).
• Participants adjusted a projected dot array to perceived vertical while in a tiltable seat (0-45 degrees roll).
• Body and head roll angles were measured using inertial measurement units.

Main Results:

• Regression analysis revealed significantly decreased slopes of SVV setting with decreasing G-levels.
• This demonstrates an increased bias of SVV toward the longitudinal body axis in reduced gravity.
• Average regression slopes were 0.95 (0.75g), 0.84 (0.50g), and 0.63 (0.25g).

Conclusions:

• Reduced gravity conditions significantly increase the underestimation of roll self-tilt.
• The findings support the hypothesis that self-tilt is underestimated more at lower gravity levels.