Abstract:

Distance compression in virtual reality (VR), which will lead to a distortion of fine manipulation in practical application, depicts that people tend to underestimate the spatial distance of visual stimuli in virtual environments. The apparent perceived differences between virtual and real environments break the immersive experiences and lower users' acceptance. Therefore, it is crucial to ameliorate the distance compression to increase the fidelity and ultimately promote the wider application of VR. Capitalizing on the fact that distance compression is a multiple modality phenomenon and occurs for auditory and visual stimuli, researchers reported that the distance judgment in VR would get more accurate when the positions of auditory and visual stimuli were incongruent. However, it is unclear to what extent the incongruency is to get effective amelioration. In this study, we aimed to completely examine the effect of the auditory stimulus on distance compression in VR. We presumed that the larger the incongruency was, the better amelioration obtained.

We used the HTC Vive Pro to render the virtual environment and the build-in headphone to present auditory stimulus. A total of 30 participants were recruited to perform a distance judgment task. In Experiment 1, we first controlled the presence or absence of the auditory stimulus. We also varied the egocentric distance of visual stimulus (3 m, 4 m, 5 m). Then, in Experiment 2, we controlled the incongruency of the audio-visual condition, that is, the exocentric distance between auditory and visual stimuli (0.5 m, 1 m, 1.5 m, 2 m). Each block consisted of 30 consecutive trials, wherein the reference visual stimulus was presented at the beginning 5 s. Participants were asked to judge whether the following adjust stimulus was nearer or farther than the reference stimulus. The egocentric distance would be adjusted according to the response of the prior trial. The egocentric distance of the adjust stimulus in the last trial was referred to as the ultimate distance judgment for the initial reference stimulus.

For Experiment 1, we performed a two-way repeated analysis of variance (ANOVA) on the distance compression rate, which was calculated by subtracting the perceived egocentric distance from the physical egocentric distance and then dividing it by the physical egocentric distance. The within-subject factors included the egocentric distance of reference stimulus (3 m, 4 m, and 5 m) and the presence of the auditory stimulus (audio-visual condition and visual-only condition). The result of Experiment 1 is shown in Figure 1. We found that the distance compression rate under the audio-visual condition was marginal significantly smaller than that under the visual-only condition, F(1,29) = 4.05, p = 0.054, η_p² = 0.12. In contrast, the main effect for the egocentric distance of reference stimulus and the interaction were not significant (p > 0.05). Then, we performed a paired one-sided t-test to compare the distance compression rates for audio-visual and visual-only conditions at different levels of the egocentric distance of reference stimulus. We found that the distance compression rate for the audio-visual condition was smaller than that for the visual-only condition at egocentric distances of 4 m (difference = 1.8%, t = −1.587, p = 0.062) and 5 m (difference = 1.6%, t = −1.85, p = 0.037), but not at an egocentric distance of 3 m (p = 0.307).

For Experiment 2, we performed a two-way repeated ANOVA on the distance compression rate. Since the same participants were recruited for Experiment 1 and Experiment 2, the results of the audio-visual condition in Experiment 1, that is, an exocentric distance of 0 m, were included in the current analysis. The within-subject factors included the egocentric distance of reference stimulus (3 m, 4 m, and 5 m) and the exocentric distance between auditory and visual stimuli (0 m, 0.5 m, 1 m, 1.5 m, and 2 m). The result of the distance compression rate is shown in Figure 2. We found that the main effect for the exocentric distance between auditory and visual stimuli was significant, F(4,116) = 8.29, p < 0.001, η_p² = 0.22. In contrast, the main effects for the egocentric distance of reference stimulus and the interaction were not significant (p > 0.05). Then, a Tukey multiple comparison test was performed after pooling the results of the three egocentric distances of the reference stimulus. We found that the distance compression rate decreased with the increment of the exocentric distance. Furthermore, it could reach a marginally significant or significant difference when comparing any two exocentric distances at least 1 m apart. Finally, we fitted a linear curve for the relationship between the exocentric distance between auditory and visual stimuli and the distance compression rate: distance compression rate = −0.024 × exocentric distance + 0.056. The slope was significant (p = 0.008), indicating the distance compression rate was negatively correlated with the exocentric distance and could be ameliorated at a pace of 2.4% for every 1 m. The adjusted R² was 90.7%.

We reported the effect of auditory stimulus on the distance compression in VR. Based on our results, we highly recommended presenting the auditory and visual stimuli simultaneously in the time domain and a minimum of 1 m apart in the space domain to ameliorate the distance compression in VR.

Key words: virtual reality, distance compression, auditory and visual stimuli, egocentric distance, exocentric distance