Is vertical postural orientation in weightlessness correlated with the subjects' perceptual typology?

It was proposed to investigate whether the central nervous system is able to select the most appropriate reference frame for controlling subjects' vertical orientation in response to severe environmental constraints such as microgravity and support perturbations. The question addressed was whether the subjects, regardless of their perceptual typology, might be able to use the same visual reference frame to control their vertical orientation. Fifteen subjects were asked to perform a sit-to-stand (STS) task under microgravity. Subjects were seated on a chair with their feet fixed to a horizontal or forwards tilted support (20°). When the support was tilted the subjects' ankles were placed at an angle of 110°. Two main findings emerged from this study. First, in all the experimental conditions, the results obtained showed that the subjects' controlled their postural orientation fairly correctly but not very precisely, since a bias was systematically observed in their head and trunk orientation. In other words, changes in their ankle angles and the absence of gravity only slightly disturbed their vertical body orientation, and when these two constraints were applied concomitantly, their effects were not cumulative. Secondly, contrary to our initial hypothesis, the subjects' postural orientation performances were not correlated with the perceptual typology. All the subjects preferentially used the visual reference frame to control their vertical postural orientation. The present findings confirm that the proprioceptive dominance of visual independent (VI) subjects enhances their ability to select the most appropriate reference frame to deal with inter-sensory disturbances such as those consisting here of microgravity combined with proprioceptive perturbations imposed at ankle level.