Spatial Orientation in simulated and real weightlessness: Where is 'up'?
Institute of Physiology and Anatomy, German Sport University Cologne, Germany
Key words: egocentric, allocentric, gravicentric, spatial orientation, intuitive subjective reference frame, simulated weightlessness, water immersion, parabolic flight, subjective vertical
Introduction On Earth, spatial orientation is determined using three reference frames which are featured through stability, omnipresentation and intuitive perception: the direction of gravity, visual stimuli with intrinsic orientation (e.g. houses, trees etc.), and the own body (e.g. body longitudinal axis – head "up“; feet "down“). The absence or the incongruence of such reference frames during spaceflight might therefore challenge astronauts’ spatial orientation and thus lead to performance errors with potentially negative impact on mission success. While latter studies explicitly instructed their participants to orient by gravity, we now explore participants’ intuitive subjective reference frame without referring to a particular reference frame in simulated (water immersion) and real weightlessness (parabolic flights). Methods: Simulated weightlessness Twelve participants (5 female, 7 female) were placed into four angles of pitch tilt (upright 0°, tilted back 45°, supine 90°, head-down tilt 135°) while they were strapped on a plate. The allocentric cues (i.e. their vision of the surrounding room) were eliminated. Participants were tested in two different conditions: water immersion (on 5 m depth) (WET) – as simulated weightlessness – and laboratory condition (LAB) – as a control condition. They were ask to (1) adjust the pitch angle of a displayed 3D tree until the tree appeared in accordance to the instruction that “…leaves are at the top and roots are at the bottom” and (2) to move their index finger on a imagined line from a starting position vertically “up” (and back to the starting position). Real weightlessness 14 participants (6 female, 8 male) were tested on Earth and in weightlessness (parabolic flight). They looked into a cylinder, which blocked their vision of their surroundings, so that they only saw a switch surrounded by a luminous ring. The switch could be flipped into any direction within the full 360° range. It provided no visual cues about “down”, but the experimenter could add visual verticals. Participants were asked to flip a series of switches into the "OFF" position. Results: Simulated weightlessness When participants were tilted about their pitch axis, analyses about the slopes (in terms of both tasks) revealed bimodal distributions for LAB and unimodal distributions for WET condition. Furthermore, about FINGER task, we found 2 egocentric groups – a 90° shift. MANOVA indicated, that gravicentric responses overestimated in action task – but underestimated in perception task. However, about TREE and FINGER, we find no significant differences between WET and LAB condition. Results: Real weightlessness There were only slight differences about the slopes in regard to the visual modifications. However, these yielded no distinct bimodal distributions – neither in 0 G nor in 1 G. Thus, MANOVA showed significant differences about visual modifications and also about joystick positions. Further analysis about the conditions revealed no differences. Holistic Conclusion Our findings conclude that there is an evidence for the intuitive subjective vertical in humans. We further conclude that it depends on response type, the availability/weighting of reference frames, and the preferred reference frame. Altered conditions might have no influence on the accuracy and precision about tasks. Summarizing that, it is difficult to generalize findings on spatial orientation in terms of the intuitive subjective reference frame.