Spatial Updating Depends on Gravity

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.

Keywords: parabolic flight; precuneus; spaceflight; spatial navigation; spatial updating; vestibular system; weightlessness.

Figure 1

The characteristic profile of a parabolic flight maneuver. At standard cruising altitude (about 12,000 ft) the aircraft is pulled up at a 47° angle, inducing a gravito-inertial acceleration (GIA) of 1.5–1.8 g, after which the engine’s thrust is limited to compensate air-drag, entering a phase of free-fall comparable to 0 g, and hence, weightlessness. This phase is completed by another phase of hypergravity before returning to 1 g again. After an initial test parabola, the maneuver is repeated a total of 30 times with 3–5 min breaks between parabolas and a longer (about 8 min) break after the 16th parabola.

Figure 2

Experimental setup in the Airbus A 310 Zero-G. Participants were buckled up in standard aircraft chairs with feet fixed to their ground floor with foot straps. The laptops were mounted to a plexiglass plate that was strapped to the participants’ upper legs that allowed them to maintain the same position throughout testing. Testing was performed during 15 parabolas, providing a total of 90 trials (30 trials during 1 g, 1.8 g, and 0 g, respectively). Photo credit: Novespace/ESA.

Figure 3

Experimental paradigm. Trials comprise an encoding phase (A), a delay phase (B), and a retrieval phase (C). Each trial started with a static presentation of the virtual environment and two objects located at two different positions during which participants had to memorize the location and identity of the objects. Next, all objects gradually sank into the ground until they completely disappeared. In the delay phase, participants either experienced a forward movement of 25 m or 45 m (updating trials) or remained at their position (static trials). In the subsequent retrieval phase, one of the two objects was presented in the center of the screen, and participants had to turn a 3D-arrow towards the object’s original position in the encoding phase.

Figure 4

Mean variable (A) and signed (B) pointing errors, and reaction time (RT; C) during 0 g, 1 g, and 1.8 g and different trial conditions (static, updating short, updating long). Variable pointing error was computed as the standard deviation of the signed pointing errors for each participant and each g-level and task condition using circular statistics. Data are estimated means and standard errors. Note that no contrasts were performed between task conditions for RTs because RTs were a logical consequence of task condition, i.e., the 3D arrow had to be moved a shorter angular distance for static and short updating trials compared to long updating trials (for details see “Behavioral and Statistical Analysis” in “Materials and Methods” section). *P < 0.05. **P < 0.01. ***P < 0.001. ^##P < 0.01 for 0 g vs. 1 g. ^###P < 0.01 for 0 g vs. 1 g. ^†P < 0.05 for 0 g vs. 1.8 g. ^††P < 0.01 for 0 g vs. 1.8 g. ^‡P < 0.05 for 1 g vs. 1.8 g.