Abstract
As we move through an environment the positions of surrounding objects relative to our body constantly change. Maintaining orientation requires spatial updating, the continuous monitoring of self-motion cues to update external locations. This ability critically depends on the integration of visual, proprioceptive, kinesthetic, and vestibular information. During weightlessness gravity no longer acts as an essential reference, creating a discrepancy between vestibular, visual and sensorimotor signals. Here, we explore the effects of repeated bouts of microgravity and hypergravity on spatial updating performance during parabolic flight. Ten healthy participants (four women, six men) took part in a parabolic flight campaign that comprised a total of 31 parabolas. Each parabola created about 20–25 s of 0 g, preceded and followed by about 20 s of hypergravity (1.8 g). Participants performed a visual-spatial updating task in seated position during 15 parabolas. The task included two updating conditions simulating virtual forward movements of different lengths (short and long), and a static condition with no movement that served as a control condition. Two trials were performed during each phase of the parabola, i.e., at 1 g before the start of the parabola, at 1.8 g during the acceleration phase of the parabola, and during 0 g. Our data demonstrate that 0 g and 1.8 g impaired pointing performance for long updating trials as indicated by increased variability of pointing errors compared to 1 g. In contrast, we found no support for any changes for short updating and static conditions, suggesting that a certain degree of task complexity is required to affect pointing errors. These findings are important for operational requirements during spaceflight because spatial updating is pivotal for navigation when vision is poor or unreliable and objects go out of sight, for example during extravehicular activities in space or the exploration of unfamiliar environments. Future studies should compare the effects on spatial updating during seated and free-floating conditions, and determine at which g-threshold decrements in spatial updating performance emerge.
Highlights
Gravity is critical for various physiological functions and goal-directed behavior
This study investigated the acute effects of weightlessness and hypergravity using parabolic flight maneuvers on spatial updating performance in humans
Spatial updating was assessed by a virtual 3D task that required participants to encode the identity and location of objects, memorize and update the egocentric object coordinates during a forward movement simulated by optic flow
Summary
Gravity is critical for various physiological functions and goal-directed behavior. The lack of gravity, i.e., weightlessness, leads to cardiovascular deconditioning, negative energy balance, bone and muscle loss, and sensorimotor impairments. The vestibular system senses linear and angular acceleration through signals from the otoliths and semicircular canals, and it drives various reflexes such as keeping gaze and posture when linear accelerations are changing. Interactions between the otoliths and semicircular canals critically contribute to spatial perception including selfmotion, orientation, and navigation (Cullen and Taube, 2017). During weightlessness gravity no longer acts as a fundamental reference, and the discrepancy between vestibular (including conflicts between otolith and semicircular canal information), visual, and sensorimotor signals can affect spatial abilities (Clément et al, 1989; McIntyre et al, 2001). Microgravity research has concentrated on posture, gaze, functional mobility, and spatial orientation, reporting misperceptions of visual orientation, depth and distance, and difficulties in shape recognition (Reschke and Clément, 2018). Whether the lack of gravity impairs spatial navigation performance and strategies is not well understood
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