Introduction
The perception of self-motion is a complex process that involves the integration of multiple sensory cues, particularly visual and vestibular information. When an individual experiences self-motion solely through visual cues, such as optic flow, the vestibular system indicates that the body remains stationary, potentially leading to biases in perception. These biases may be reduced when the precision of the vestibular cue is lowered, for example, by lying down or adapting to microgravity. However, this decrease in bias may be accompanied by a decrease in precision.
Experimental Design
To test this hypothesis, researchers conducted a study using a move-to-target task in virtual reality. Astronauts and Earth-based controls were shown a target at various simulated distances. After the target disappeared, forward self-motion was induced by optic flow. Participants were asked to indicate when they believed they had reached the target’s previously seen location. Astronauts completed the task on Earth (both supine and sitting upright) before space travel, early and late in space, and early and late after landing. Controls completed the experiment on Earth using a similar regime, with the supine posture used to simulate being in space.
Results
The study found that while variability was similar across all conditions, the supine posture led to significantly higher gains (target distance/perceived travel distance) than the sitting posture for astronauts pre-flight and early post-flight, but not late post-flight. No difference was detected between the astronauts’ performance on Earth and onboard the International Space Station (ISS), indicating that judgments of traveled distance were largely unaffected by long-term exposure to microgravity.
Discussion
The findings provide mixed evidence as to whether non-visual cues to travel distance are integrated with relevant visual cues when self-motion is simulated using optic flow alone. The study’s results support the hypothesis that postural changes in perceived self-motion may be caused by differences in vestibular sensitivity, although further examination is required.
The lack of significant differences in precision between sitting and upright postures suggests that other sources of error, such as variability in the processing of optic flow or the perception of distance, might dominate the total variability in the task, overshadowing any contribution of vestibular noise. Additionally, the study did not test any astronauts earlier than three days after their arrival on the ISS, allowing for the possibility that the vestibular system had already adapted to the new environment within that period.
Implications for Space Travel
Understanding how microgravity affects human perception is crucial for the development of safe, long-duration space travel. The study’s findings suggest that astronauts are unlikely to be exposed to dangers due to an unusual perception of traveled distance when in space, such as when operating sensitive equipment and machinery manually and in a visually guided manner in the absence of gravity.
While the study found inconclusive evidence regarding potential sex/gender differences in performance or reactions to the manipulations, it is unlikely that any such differences would be significant in the context of space travel.
Conclusion
This study investigated the influence of body posture on human perception of self-motion and distance. The results provide some evidence that the same amount of optic flow can elicit a greater sensation of having traveled further when supine compared to sitting upright, suggesting that optic flow is more effective at eliciting a sense of self-motion when supine. This finding supports the notion that visual and non-visual cues are at least partially integrated even when self-motion is presented only visually.
However, the study did not find any significant differences between performance on Earth and in the microgravity environment of the ISS, indicating that vestibular cues may play a minor role, if any, in the estimation of visually presented self-motion. Further research is needed to fully understand the complex interactions between visual, vestibular, and other sensory cues in the perception of self-motion and spatial orientation, particularly in the context of long-duration space travel.
