1 Introduction
It appears humans utilize a perceived and/or an internal representation of gravity in a variety of tasks, for example, catching, trajectory estimation, pointing, and body orientation estimation [
13]. Furthermore, we appear to be especially well adapted to earth gravity; humans can correctly estimate object trajectories and intercept objects, even when the trajectories are partially occluded, as long as the objects behave according to the familiar downwards acceleration of 9.81 m/s
2 (1 g) [
13,
20,
42]. Acting outside of earth gravity, however, can cause difficulties, as various tasks rely on the internal model of gravity calibrated at 1 g. For most of us, it is, of course, rather uncommon to experience earth-discrepant gravity in our everyday lives, as such experiences are typically reserved for spacecraft crew or researchers working with simulated gravity environments, such as parabolic flights or rotating rooms.
Virtual Reality (VR) offers everyone the capability to experience something resembling the aforementioned conditions, although its capabilities are mostly limited to audiovisual stimuli, as contemporary VR hardware cannot stimulate internal senses that are affected by true earth-discrepant gravity. The fact that it is impossible to simulate weightlessness or earth-discrepant gravity realistically has often been used as an example of the physical limits of VR technology; no system, no matter how advanced, can manipulate the true effects of gravity for our internal senses (e.g., References [
36,
37]). However, it has also been argued that the purpose of VR is not to generate perfect artificial stimuli but instead generate
illusions of alternate realities, such as earth-discrepant gravity [
37]. In any case, visual simulation of gravity has been found sufficient in many works of gravity-related research ranging from adaptation studies to astronaut training applications (see below).
Humans’ relationship with gravity has attracted a lot of research on various application areas of VR. For example, various training simulations of space-related zero-gravity tasks have been developed for VR (e.g., References [
5,
22,
26,
34]). Jiang et al. [
12] employed VR to study the effects of various color schemes on cognitive performance and emotion while simulating the effects of altered gravity conditions using tilted bed rest methodology (see, e.g., Reference [
14]). Aoki et al. [
2] studied visual information acquisition and spatial cognition using simulated zero-gravity in VR. VR and artificially manipulated gravity have also been used for rehabilitation training for patients with gait and balance issues [
27]. Brubach et al. [
4] used altered gravity to manipulate plausibility in a VR experiment. Jörges and López-Moliner [
13] argue that, in the future, humans are likely to gain increased exposure to earth-discrepant gravity conditions, either because of actual space travel, inhabiting foreign worlds, or because of various VR and
Augmented Reality (AR) systems that visually simulate altered gravity on earth for different purposes. Therefore, they consider it worthwhile to study the human capability to adapt to earth-discrepant gravity, as altered gravity conditions, both visually simulated and actual, significantly affect human performance in various tasks.
1.1 Adaptation to Earth-discrepant Gravity
The so-called “Prism adaptation” is an example of how the manipulation of the visual field can be used to recalibrate human visuomotor capabilities. In Prism adaptation, the manipulated in a way that a forward target; when the participant attempts to point or, for example, to throw objects at the target, the participant’s aim is similarly displaced. When pointing, an offset from the actual target location is directly observable when the participant’s hand is (partially) occluded and one cannot visually guide the hand to the target. After repeated attempts, the participant adapts to the visual displacement and gains the ability to hit the target. However, after the visual displacement is removed and the normal visual field is regained, the participant again temporarily lacks the capability to hit the target because of the adaptation that happened during practice. It appears that age can affect the adaptation effect as well as the time needed to readapt back to normal visual field [
1,
32].
Unlike in Prism adaptation, however, humans seem to somewhat resist adapting to altered gravity conditions. Previous studies suggest that humans possess a strong internal model of Earth’s gravity and linear acceleration [
24]. According to the survey by Jörges and López-Moliner [
13], our ability to adapt to different gravity would be limited, especially in VR. Slow adaptation can take place in outer space where both external and internal senses are accommodated equally, however, adaptation would be impossible in VR, where only audiovisual stimuli is available. They also argued that, in general, tasks with a pronounced motor component, such as point-to-point arm movement, allow for faster adaptation in actual earth-discrepant gravity than interception tasks [
8,
13,
19]. It should be noted, though, that several of the VR studies cited in the survey did not utilize immersive hardware similar to contemporary VR systems, but 2D projection walls, for example. In addition, the reported VR studies primarily used interception tasks, whereas studies with tasks having a pronounced motor component were performed in actual earth-discrepant gravity [
13]; this suggests VR studies with a pronounced motor component are needed.
McIntyre et al. [
23] investigated the capability of astronauts to intercept a vertically moving ball in space. The astronauts performed catching movements earlier in zero gravity than in normal gravity, which indicated using an internal model of gravity in addition to perceptual cues. Adaptation to zero-gravity did occur over the course of two weeks. Gaveau et al. [
9] investigated cosmonauts using point-to-point arm movement tasks before and after a spaceflight. They found evidence of the participants adapting their sensorimotor systems during the course of the spaceflight. Adaptation to decreased gravity led to overestimation of earth’s gravity upon return, as if their arm kinematics were optimized for increased gravity. They further theorized that abnormal vestibular and proprioceptive information could have caused them to feel abnormally high gravity, as the participants could sense an increase in gravity upon return to Earth.
Zago et al. [
41] used a projection system to estimate participants’ capability to intercept disappearing vertically downward moving targets with randomized laws of motion. They found that participants systematically expected objects to adhere to normal gravity and were surprisingly resistant to adapting to new models of gravity, although by training, the participants were able to adjust the timing of their motor activation. A follow-up study suggested that humans do not adapt to new models of gravity, but merely utilize the default earth gravity model and adjust central processing time when learning to intercept targets moving at constant velocity [
42].
Senot et al. [
35] utilized immersive stereoscopic VR and tasked participants to use a racket to intercept balls that were moving either in an upwards or downwards motion with either constant, accelerating, or decelerating velocity. Although the participants’ performance was best with targets moving in constant velocity, the authors found that participants systematically triggered the racket motion earlier with targets falling from above compared to when they were rising from below, suggesting that participants were anticipating downward moving targets to be influenced by gravity regardless of their true acceleration.
Ye et al. [
40] investigated task performance and adaptation in VR using four different tasks in different gravitational settings: striking a ball to hit the target, triggering a ball to hit a target, predicting the landing location of a projectile, and estimating flight duration of a projectile. The participants again showed a tendency to base their physics intuitions according to earth gravity; however, overall, they were able to adapt to the tasks presented in this study in terms of performance and accuracy.
Gravano et al. [
10] explored the use of mental imagery in adapting astronauts’ sensorimotor capabilities towards 0 g conditions by grasping, throwing, and catching an imaginary ball; their results suggest that the astronauts’ internal gravity models were updated even though the training was performed in normal gravity.
Cano Porras et al. [
6] studied simulated gravity and locomotion adaptation and found that visual-only gravity cues caused by virtual inclines in VR made participants to initially ignore body-based cues and adapt their locomotion as if they were walking on actual inclines. Gradually, the body-based cues took over, however.
Although humans are generally good at predicting object trajectories in motor tasks, it has been argued that humans generally have poor intuitions regarding physics and tend to make bad perceptual judgements based on physics [
17,
19]. Ullman et al. [
38] argued that humans interpret their physics judgements similarly to contemporary game engines; instead of precise computations, humans utilize shortcuts and “good enough” guesses to predict object trajectories. In VR, it helps if the virtual environment contains rich size cues to help gravity estimation [
19]. In fact, it could be that conflicting size cues, such as virtual characters whose size differs from the rest of the virtual scene, can confound gravity judgements [
29]. Because of the potential discrepancy between performance in motor tasks and perceptual tasks, La Scaleia [
19] investigated whether the internal gravity model only affects the former and not the latter. They used a VR system where a ball was first seen rolling and then falling in different simulated gravity conditions while the ball could be either visible or occluded through the free-fall phase. Participants’ perceptual judgements of naturalness, as well as their accuracy in intercepting the ball, were quantified. In the end, they concluded that the internal gravity model was utilized in both perceptual as well as interception tasks.
1.2 The Relationship between Scale and Gravity
Besides simulating earth-discrepant gravity, many VR applications utilize the manipulation of user scale, which can also affect the subjective perception of gravity, provided these applications simulate rigid body dynamics of objects. There are multiple reasons to manipulate users’ scale in VR: Applications, such as
multiscale Collaborative Virtual Environments (mCVE), allow different users to, for example, investigate architectural and medical visualizations from different scales and perspectives [
16,
44]. Scaling of VR users has also been utilized in, for example, locomotion [
18], remote collaboration [
28], and interior design [
43].
However, a realistic simulation of rigid body dynamics produces interesting perceptual effects when the scale of the VR user changes. For example, a human scaled down 10 times smaller would observe an object dropped at shoulder-height to hit the ground in roughly 0.17 s. At normal scale, dropping an object similarly would take 0.55 s to hit the ground, and at tenfold scale it would take 1.75 s. Therefore, in the eyes of the VR user, object motions appear as if gravity was 10 times stronger when the user is 10 times smaller, and similarly weaker if the VR user is scaled up, greatly affecting perceived accelerations as well as throwing distances [
29,
30]. The human internal model of gravity can thus pose challenges for VR users not only when simulating non-earthlike experiences, but also in mCVEs and other scale-varying applications that simulate gravity. Although, in single-user applications, the developer can always adjust simulated gravity to match the user’s expectations (provided physical accuracy is not needed), in mCVEs this would not be possible, since simulated gravity would not appear normal for simultaneous users coexisting at different scales. At very small scales, additional physical peculiarities come into play; for example, the work of Millet et al. has utilized VR for teaching nanophysics in robotics operations [
25].
The peculiarities of perceiving rigid body dynamics at various scales have been the focus of our previous studies. We studied the subjective realism of physics models at various scales and found that when interacting with physically simulated objects, participants tended to consider physics models where gravity was adjusted to match the participant’s scale (i.e., gravity was adjusted to be stronger when participants were taller and vice versa) to be more realistic [
30]. However, physics models in which gravity remained unaltered were perceived as unrealistic. This means the participants tended to use their own size as the metric when judging the realism of rigid body dynamics; a model in which objects would behave as if the participants were normal sized and the environment scaled instead was considered as the realistic one. Although it appears humans use their own body as the metric for scale among discrepant size cues in VR, there is some evidence that other virtual characters can disrupt this metric [
21,
29].
1.3 Study Overview
Whereas our previous work focused on the subjective perception of realism at 0.1× scale [
31], 10× scale [
31], as well as 0.2× scale [
29], in this work, we focus on adapting to gravity conditions similar to those experienced when the participant is at 0.2× scale. We predict that the internal model of gravity could be updated by exposing users to an interaction task that takes place under simulated earth-discrepant gravity in VR. More specifically, we focus on the task of throwing a ball under simulated hypergravity. Somewhat similar to Prism adaptation studies, as well as to the gravity adaptation study of Gaveau et al. [
9], we use the after-effects measured from the results of a pronounced motor task as evidence for adaptation. As for assessing the internal model of gravity, we rely on the human capability to predict partially occluded trajectories [
13,
20,
41]. Similarly to earlier studies involving trajectory estimation (e.g., References [
41,
42]), our measurement task uses a disappearing ball so the participants have to rely on their internal model of gravity when performing the throwing task. We choose 5 g as our simulated hypergravity, as this roughly corresponds to the subjective experience of gravity at 0.2 scale, which has been utilized in several earlier studies investigating scale (e.g., References [
29,
39]). We ran two studies mostly identical in content, with the second study serving as a replication of the first one to confirm the robustness of our findings. Our results suggest that our participants’ internal models of gravity were at least temporarily updated to some extent. We consider these results as a humble but promising step towards building applications that would allow training and adaptation towards altered gravity conditions using VR-simulated gravity.
4 Discussion
As both initial hypotheses were supported by our results in the original study (Study 1) as well as in the replication study (Study 2), this indicates that adaptation to higher gravity did occur among participants in this experiment. Not only were participants adapting to the throwing task in hypergravity more likely to have worse overall accuracy, but they also appeared to overshoot their throws more often compared to the normal gravity group. This pattern supports the idea that participants’ internal model of gravity had temporarily changed during the adaptation phase. We argue that there is evidence for this interpretation specifically, since we tested the adaptation
after exposure to hypergravity so the participants could not simply use other strategies to compensate for the loss of performance (similarly as in studies by Zago et al. [
41,
42], for example). This makes our findings somewhat similar to those made in Prism adaptation studies [
32], as well as the findings reported by Gaveau et al. [
9] in which cosmonauts’ internal gravity model was investigated and found to be altered by using a pronounced motor task upon return to normal gravity after exposure to earth-discrepant gravity. In our case, the participants did not appear to compensate after the return to normal gravity, but instead, their performance was congruent with the assumption that simulated gravity remained higher than normal, even though the participants were explicitly informed otherwise before beginning the measurement phase.
According to our exploratory analysis, it appears that returning visual feedback in the final throwing phase recalibrated the internal gravity model back to normal. Furthermore, when inspecting throw accuracies at the granularity of individual trials, it appears that the participants gained accuracy toward the end of the measurement phase, indicating that calibration back to normal gravity began to take place even before the visual feedback was reintroduced. Nevertheless, our findings contradict the argument of Jörges and López-Moliner [
13] that adaptation to earth-discrepant gravity would be practically impossible to achieve in VR. However, it should be noted that their survey mostly contained VR studies utilizing interception tasks and not pronounced motor tasks, such as in our study. Moreover, the quantitative change between gravity conditions (1 g–5 g) in our experiment was rather large compared to previous studies. In addition, Jörges and López-Moliner [
13] also argued that adaptation is quicker and easier in pronounced motor tasks, compared to catching and interception tasks. It is very much possible that if we had utilized these tasks in our study, then no adaptation would have been observed. However, our findings are in line with the results of Ye et al. [
40] as well as Gravano et al. [
10], which suggests that adaptation to tasks taking place in earth-discrepant gravity can be achieved even when training takes place in normal gravity. Our findings give further support to the idea that, in the future, it might be possible to use VR for gravity adaptation training, at least to some extent.
In our exploratory analysis, we also investigated the contribution of demographics (gender, age, video game experience, and VR experience) and subjective confidence to throwing accuracy. We did not, however, find that these factors would have been able to predict throwing accuracy in both studies. Overall, the gravity in which the adaptation phase took place was the best predictor for throwing accuracy in both studies, giving further support for our hypotheses. Additionally, our exploratory analysis regarding Study 2 did find that progression across trials in the measurement phase could predict throwing performance for the hypergravity group. This indicates that participants in the hypergravity group gradually regained their throwing accuracy throughout the phase.
4.1 Limitations
Study 1 had substantial limitations that undermined the reliability of its results. First, the throwing distance for the normal gravity group was exaggerated, indicating that participants in the normal gravity group would have had to exert a slightly bigger amount of force compared to the hypergravity group. Second, the bug that was related to throwing error computation subtracted 25 cm from the reported errors. The bug therefore artificially increased the reported accuracy of both groups regarding throws that landed outside of the target radius. The results that were less than 25 cm, however, became even more unreliable, since it is impossible to know whether these throws had originally landed outside of the target disc radius, or inside the radius, but to the other side of the center. Although we considered the removal of the results of Study 1 altogether, we ultimately decided the article would be more informative if we retained the result. The reason for this is partly that the mismatch in throwing force could have also introduced bias against our hypotheses, not in favor of them: If the normal gravity participants in Study 1 had to use more force in the adaptation phase compared to the hypergravity participants, then we assume this would bring the groups’ measurement phase results closer together, not further apart. Furthermore, the computation error in Study 1 affected both groups equally. The most significant evidence in favor of our results, however, is the fact that Study 2 was able to replicate the effects of the first study even after fixing both the 1 g throw distance and the throw error computation.
A limitation of both studies regarding the generalizability of their results is the fact that the studies were carried out in VR instead of actual hypergravity. Therefore, bodily cues that humans experience during actual abnormal gravity were missing. Furthermore, although we took steps to provide a coherent throwing experience, VR throwing is not the same as real throwing. Our participants did give an acceptable rating of 5 out of 7 for throwing realism in both studies (6 by normal gravity participants in Study 2), however, we cannot expect this simulation to be perfect. The purpose of the custom throwing model was to get throwing in VR to feel somewhat more natural and predictable compared to the Unity default throwing implementation. However, the error between real throwing and VR throwing is difficult to quantify, as many factors that affect throw length are not equated between simulated throwing using a VR controller and actually throwing a physical object. We also did not perform a formal comparison in which participants would have judged our method against the more common method of acquiring projectile velocity directly from a parent object at the time of release. Fortunately, however, the participants did not appear to consider our throwing model to be entirely unrealistic either. Other throwing models could be investigated in future replication studies, perhaps by even calibrating the throwing model for each participant individually.
6In both studies, there were a few individual participants who reported that they somehow kept throwing the ball slightly to the left from what they intended during the practice phase. We were not, however, able to reproduce any kind of bug or glitch that would unintentionally offset the ball’s trajectory when throwing. Though it was not formally quantified, we observed large differences in how long it took for participants to finish the initial practice stage that took place before the adaptation phase. This indicates individual differences in either physical throwing skill or how comfortable participants were with our particular implementation of throwing in VR. We did not observe any statistical difference in accuracy during the final phases of either study. Nevertheless, having a similar task already before adaptation and measurement would have given more evidence regarding equal throwing skills among participant groups. We, however, preferred that neither participant group had experienced the same exact throwing task before the measurement phase. We also acknowledge that participants within the normal gravity group had slightly more “practice” time in throwing due to the practice phase taking place in normal gravity. However, since we found the same pattern of results consisting of a large difference in the measurement phase and no difference in the final phase in both Studies 1 and 2 with different participants, we consider it very unlikely that our results could be explained by sampling bias related to differences in participants’ throwing abilities.
In both studies, there were some participants who strayed slightly from the experiment procedure in some way or experienced technical or other difficulties. If these issues took place either within the practice phase or adaptation phase, then we resolved the issues and carried on with the experimental procedure. However, if these issues took place in the measurement phase, then we resolved the issues but did not use data from that participant (we considered these cases as a breach of completing the study protocol as intended). Deviations from protocol included moving outside of the assigned area or failing to stick to the underhand throwing technique. In Study 2, there was one participant who appeared to keep throwing at the same strength after moving from hypergravity adaptation to the measurement phase. This participant’s data was most likely removed by our outlier threshold defined in the preregistration. In Study 2, there were two participants who experienced performance glitches; the data from these participants were removed, since we had a reason to suspect that these glitches affected throwing performance. After data removal, we collected data from new participants until we reached targets of 60 and 42 participants in Studies 1 and 2, respectively.
A phenomenon that could potentially affect throwing accuracy, especially in the adaptation phase, is the so-called distance compression effect, whereby egocentric distances in VR are generally underestimated [
7,
15,
33]. Although newer HMDs appear to be less prone to produce this effect, egocentric distances still appear to be estimated to be only 82% of actual distance [
15]. Using a virtual environment that is modeled accurately after a real, familiar place has been shown to mitigate the distance compression effect [
11]; therefore, we could have possibly placed the experiment, for example, in a familiar campus environment similar to our previous studies to help with distance perception [
30]. For this experiment, however, we chose to use a neutral and visually unfamiliar setting to eliminate any visual size cues that could bias participants’ perceptions toward either normal gravity or hypergravity (even the floor tiles were sized differently from what one typically sees in real-world architecture). Therefore, the distance compression effect could have potentially affected both of our participant groups in both studies. Replicating Study 2 with different types of environments would be advisable to investigate the potential confound caused by the distance compression effect.