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Underestimation of self-tilt increases in reduced gravity conditions.

BACKGROUND
During large angles of self-tilt in the roll plane on Earth, measurements of the subjective visual vertical (SVV) in the dark show a bias towards the longitudinal body axis, reflecting a systematic underestimation of self-tilt.


OBJECTIVE
This study tested the hypothesis that self-tilt is underestimated in partial gravity conditions, and more so at lower gravity levels.


METHODS
The SVV was measured in parabolic flight at three partial gravity levels: 0.25, 0.50, and 0.75 g. Self-tilt was varied amongst 0, 15, 30, and 45 deg, using a tiltable seat. The participants indicated their SVV by setting a linear array of dots projected inside a head mounted display to the perceived vertical. The angles of participants' body and head roll tilt relative to the gravito-inertial vertical were measured by two separate inertial measurement units.


RESULTS
Data on six participants were collected. Per G-level, a regression analysis was performed with SVV setting as dependent variable and head tilt as independent variable. The latter was used instead of chair tilt, because not all the participants' heads were aligned with their bodies. The estimated regression slopes significantly decreased with smaller G-levels, reflecting an increased bias of the SVV towards the longitudinal body axis. On average, the regression slopes were 0.95 (±0.38) at 0.75 g; 0.84 (±0.22) at 0.5 g; and 0.63 (±0.33) at 0.25 g.


CONCLUSIONS
The results of this study show that reduced gravity conditions lead to increased underestimation of roll self-tilt.